Air Accidents Investigation Branch
Aircraft Accident Report No. 1/92 - (EW/C1165)
Report on the accident to BAC One-Eleven,
G-BJRT over Didcot, Oxfordshire on 10 June 1990
| Registered Owner and Operator: | British Airways Plc |
| Aircraft: Type: | BAC One-Eleven |
| Aircraft Model: | Series 528FL |
| Nationality: | British |
| Registration: | G-BJRT |
| Place of accident: | Over Didcot, Oxfordshire |
| Latitude: 540 34' North | |
| Longitude: 0010 10' West | |
| Date and Time: | 10 June 1990 at 0733 hrs |
| All times in this report are UTC |
The accident was notified by Southampton Airport Air Traffic Control
to the Department of Transport on Sunday 10 June 1990 and the
Air Accidents Investigation Branch (AAIB) began an investigation
the same day. The following participated in the investigation:
Mr D F King, Principal Inspector of Air Accidents (Engineering)
Mr R St J Whidborne, Senior Inspector of Air Accidents (Operations)
Mr S R Culling, Senior Inspector of Air Accidents (Engineering)
Mr R J Vance, Senior Inspector of Air Accidents (Flight Recorders)
The investigation was assisted by:
Mr I J Weston, Air Traffic Control (ATC) Investigations, Safety Regulation Group, Civil Aviation Authority (CAA)
Dr A J F MacMillan, Royal Air Force (RAF) - Rapid Decompression
Mr R Green, Aviation Medicine - Human Factors
The accident happened when the aircraft was climbing through 17,300
feet on departure from Birmingham International Airport en route
for Malaga, Spain. The left windscreen, which had been replaced
prior to the flight, was blown out under effects of the cabin
pressure when it overcame the retention of the securing bolts,
84 of which, out of a total of 90, were of smaller than specified
diameter. The commander was sucked halfway out of the windscreen
aperture and was restrained by cabin crew whilst the co-pilot
flew the aircraft to a safe landing at Southampton Airport.
The following factors contributed to the loss of the windscreen:-
A safety critical task, not identified as a 'Vital Point', was
undertaken by one individual who also carried total responsibility
for the quality achieved and the installation was not tested until
the aircraft was airborne on a passenger carrying flight.
The Shift Maintenance Manager's potential to achieve quality in
the windscreen fitting process was eroded by his inadequate care,
poor trade practices, failure to adhere to company standards and
use of unsuitable equipment, which were judged symptomatic of
a longer term failure by him to observe the promulgated procedures.
| Injuries | Crew | Passengers | Others |
| Fatal | - | - | - |
| Serious | 1 | - | - |
| Minor/none | 1 | - | - |
The pilot's windscreen was missing and one securing bolt was found
in the window frame, this had retained a portion of the rubber
seal and a metal bush from the windscreen. The bolt was not new
and its countersunk head had pulled through the windscreen. The
aircraft window frame was checked for distortion and found to
be satisfactory.
Other damage to the aircraft consisted of:-
The High Frequency (HF) aerial, stretching from a forward position
on the top of the fuslage to a fitting close to the tailplane
bullet, was missing and the fittings damaged. There was a dent
on the top left side of the fuselage approximately 3 inches long
about 3 feet above the overwing emergency exit and a scratch on
the top left side of the fuselage. Minor damage to several items
on the flight deck.
There was no other damage.
| 1.5.1 | Commander | Male, aged 42 years |
| Licence | Airline Transport Pilot's Licence | |
| valid until 13 November 1999 | ||
| Instrument rating | valid until 16 January 1991 | |
| Route check | valid until 30 September 1990 | |
| Safety procedures | last check 23 October 1989 | |
| Medical | last examination 14 March 1990 Class One no limitations | |
| Height: 1.67 metres. Weight: 70 kg | ||
| Flying experience | ||
| Total | 11,050 hours | |
| On type | 1, 075 hours | |
| Last 28 days | 19 hours | |
| Last 90 days | 96 hours | |
| 1.5.2 | Co-pilot | Male, aged 39 years |
| Licence | Airline Transport Pilot's Licence | |
| valid until 24 June 1991 | ||
| Instrument rating | valid until 19 November 1990 | |
| Route check | valid until 8 July 1990 | |
| Safety procedures | last check 9 October 1989 | |
| Medical | last examination 20 December 1989, Class One, no limitation | |
| Flying experience | ||
| Total | 7,500 hours | |
| On type | 1,100 | |
| Last 28 days | 58 hours | |
| Last 90 days | 169 hours |
| 1.5.3 | Cabin crew | Purser | Male, aged 37 years |
| No 2 | Male, aged 29 years | ||
| No 3 | Male, aged 36 years | ||
| No 4 | Female, aged 33 years |
All Safety and Emergency procedure checks had been completed in
the current year.
| 1.6.1 | General information | |
| Manufacturer | British Aircraft Corporation (BAC) Ltd | |
| Type | BAC One-Eleven Series 528FL | |
| Registration | G-BJRT | |
| Serial number | BAC 234 | |
| Date of manufacture | 1977 | |
| Registered owner | British Airways Plc | |
| Total airframe hours | 37,724.07 hours | |
| Certificate of Airworthiness | Transport Category (Passenger) expires 16 March 1992 | |
| Hours to next check | 41 hours |
| 1.6.2 | Aircraft weights and centre of gravity | |
| Maximum Take-off Weight Authorised | 44,000 kg | |
| Dry Operating Weight | 25,818 kg | |
| Zero Fuel Weight | 32,925 | |
| Payload | 7,197 kg | |
| Take-off fuel | 9,980 kg | |
| Actual Take-off weight | 42,905 kg | |
| Maximum landing weight | 39,460 kg | |
| Actual landing weight (1) | 40,725 kg |
Note: 1 Fuel state on landing at Southampton was 7,800 kg, therefore
fuel used during the flight was 2,180 kg.
1.6.3 General description
The BAC One-Eleven 500 series is a twin-engined, passenger aircraft
powered by Rolls Royce Spey turbofans. The fuselage is pressurised
and air-conditioned; 8,000 feet conditions being obtainable at
35,000 feet, under which conditions the pressure differential
is 7.5 psi.
The pilots' windscreens are of five-ply glass/polyvinyl-butyl
construction, the innermost (glass) laminate being low-tempered
to form a splinter shield in the event of a bird strike. Windscreen
heating is applied, primarily to improve the impact resistance
of the windscreen at low outside air temperatures. The windscreen
is not designed on the 'plug' principal, where cabin pressure
effectively contributes to holding it in place, but is fitted
from the outside of the aircraft and is secured by means of 90
countersunk bolts, also fitted from the outside. The large number
of bolts are required to prevent leakage of pressurised air through
the window seal but the force of internal air pressure could be
satisfactorily resisted by far fewer bolts.
1.7 Meteorological information
1.7.1 Synoptic situation
High pressure existed to the west of Ireland with a light northerly
flow over the Didcot area There was a possibility of broken Stratus
with a base at 600 feet and scattered Altocumulus with base at
12,000 feet and tops at 15,000 feet with a thin layer of Cirrus
above 25,000 feet. Visibility was about 10 kilometres. At 18,000
feet the wind was 360° at 17 kt and the air temperature was
minus 17°C. The freezing level was at 9,000 feet.
1.7.2 Actual conditions at Southampton
The 0720 hrs observation at Southampton Airport included the following:-
Wind: 350°/12 kt. Visibility:- 8,000 metres in haze. Temperature:-
plus 15°C.
Not relevant
1.9.1 ATC assistance
At the time of the accident the flight was receiving an Air Traffic
Area Radar Control Service from the Bristol sector of LATCC on
a frequency of 132.80 MHz. The flight came under the control of
Southampton Zone on frequency 131.00 MHz at 0744 hrs. A transcript
of ATC recorded transmissions from the onset of the emergency
is reproduced at Appendix A.
The co-pilot made a 'Mayday' call and declared that the aircraft
had suffered an emergency depressurisation and was descending
to FL100 on a heading of 195°M. The controller acknowledged
receipt of the 'Mayday' call from BA 5390 but did not attempt
to establish if the aircraft could still receive his communications
and, although he alerted his Chief Sector Controller (CSC), took
no further action since he was waiting for further information
about the emergency. He continued to operate the sector as if
no emergency existed, accepting further aircraft onto the frequency
with no attempt to off-load traffic or minimise radiotelephony
activity. However, fortunately there was no conflicting traffic
and the CSC had advised the neighbouring sectors of the emergency
descent and told the LATCC watch supervisor and the RAF Distress
and Diversion Cell about the emergency call. Just prior to the
handover to Southampton, BA 5390 was descended to an altitude
of 4,000 feet in error rather than FM0 as had been co-ordinated,
despite the Bristol Sector Controller not being aware of the airfield's
QNH. This difficulty was resolved when the flight was transferred
to the Southampton Zone Controller who had been alerted to the
possibility of the aircraft landing there and had taken alerting
action following a telephone call from LATCC.
The co-pilot did not select the special purpose Secondary Surveillance
Radar transponder code (7700) to indicate an emergency condition
but retained the code that had been already ailocated to the flight.
This accorded with the United Kingdom Aeronautical Information
Publication RAC 7-4 which states : '....if the aircraft is already
transmitting a code and receiving an air traffic service that
code will normally be retained.'
1.9.2 ATC handling of emergencies
Guidance to controllers on the handling of emergency traffic is
contained in the MATS Part 1 paragraph 5.1.7 which states:-
'Emergency aircraft - Selection of controlling agency
On receipt of information which indicates that an aircraft is
in an emergency, the controller must decide whether or not to
transfer the aircraft to another agency. The choice of agency
will depend upon the circumstances and no hard and fast rules
apply. The following guidance material wrn help controllers to
make this decision:
Retaining Control
If the controller can offer immediate assistance the aircraft
should normally be retained on the frequency. If necessary impose
a radio silence on other aircraft or transfer them to another
frequency.
Alternatively it may be nrore expedient to transfer the emergency
aircraft to a discrete frequency, particularly if a radio silence
would endanger other traffic.
The aircraft will have to be retained on the original frequency
if it is unreasonable to ask the pilot, or if he is not prepared,
to change frequency. The controller may be able to relay instructions
and information from other units to the pilot.
Transferring Control
If a controller considers that another unit may be able to give
more assistance than he can himself, and in the circumstances
it is reasonable to ask the pilot to change frequency, he shall
either;
(a) Consult the Air Traffic Control Centre Supervisor and transfer
the aircraft according to his instructions, or
(b) Alert the nearest suitable unit and transfer the aircraft
to a common frequency, giving assistance to that unit as required.
Before transferring aircraft, controllers should obtain sufficient
information from the pilot to be convinced that the aircraft will
receive more assistance from another unit. If a change of frequency
is desirable the pilot must be instructed to revert immediately
if there is no reply on the new frequency. Controllers should
then listen out on the original frequency until the aircraft is
known to be in two way communication.'
1.9.3 ATC training
An ATC service in the United Kingdom may be provided only by a
person who holds an Air Traffic Controller's licence with the
appropriate rating made valid at the ATC unit at which the service
is to be provided. The Air Navigation Order authorises the grant
of licences to persons who demonstrate their knowledge, experience,
competence, skill and physical and mental fitness to the satisfaction
of the CAA. The CAA publication CAP 160 details the evidence which
must be furnished, the examinations which must be passed and other
requirements which must be met before licences, ratings, validations
and endorsements are issued.
An applicant for a licence is required to demonstrate his or her
knowledge and skill by passing examinations at two levels:-
a. Rating. The ability to provide a particular type of ATC service
(eg aerodrome control, area control or area radar control).
b. Validity of a Rating. The ability to provide an ATC service
at a particular place. This includes the ability to operate equipment
(eg radar) when it is used to provide the service.
The Bristol Sector Controller had completed an approved course
and examination for the issue of an Area Procedural and Area Radar
rating at the National Air Traffic Services (NATS) College of
Air Traffic Control (CATC) in May 1985 and was then posted to
LATCC for validity training. This was successfully completed and
led to the rating being validated on the Bristol Sector position.
Prior to the mid 1980's the Area Radar rating examination had
included an emergency exercise. Both the CATC and the ATC Licencing
Branch informally agreed that the inclusion of an aircraft emergency
during the examination placed undue emphasis on the emergency
and worked against assessing the examinee's ability to handle
routine traffic situations. In order to overcome this problem,
an agreement was reached between the College and ATC Licensing
Branch that the emergency would be removed from the examination
but that appropriate training for such events would continue to
be given. The Bristol Sector Controller on duty at the time of
the emergency had undertaken his course in 1985 but the precise
content of his course could not be established as the records
of courses conducted at that time were not available.
This situation is believed to have continued until 1988 when the
ATC Licensing Branch was removed from NATS and placed within the
CAA Safety Regulation Group, eventually becoming part of the Air
Traffic Services Standards Department (ATSSD). Due in part to
that change, the CATC, which remained within NATS, was required
to submit to annual inspections by the ATSSD so that approved
courses might continue. In contrast to other ATC courses which
have a published syllabus (CAP 390 - ATC Training Manual) no such
publication is made for Area Procedural/Area Radar Courses. As
the CATC was the only establishment to provide such courses, individual
syllabuses were agreed between ATS SD and the College. No mention
of practical emergency training is given in this syllabus for
area radar nor in the course approval which was given after the
ATSSD inspection in 1989. The syllabus did require certain parts
of MATS Part 1 relating to emergency training to be covered, but
instructors took a wider view and also tended to discuss the handling
of emergency situations during theoretical lessons. The instructors,
however, found it more difficult to incorporate emergency situations
into routine practical exercises as they found it was likely to
disrupt the learning process. Such training tended to be injected
at a relatively early stage of the course with little opportunity
for later consolidation. Therefore, the course manager was allowed
to omit certain emergency situations. As a consequence, training
in practical emergencies could be reduced to such an extent that
it was non-effective. As the syllabus did not require practical
emergency instruction, the CATC management did not inform ATSSD
where such training was not given. ATSSD was not aware that such
decisions had been taken and believed the situation remained as
per the agreement following the removal of emergencies from the
examination. Once a student leaves the College there appears to
be no requirement to undergo any emergency training or periodic
appraisal on emergency procedures in order to maintain an Area/Area
Radar validated rating.
The single concrete runway, 02/20, at Southampton Airport is 1,723
metres long. The landing distance available on runway 02 is 1,650
metres. A VOR/DME (SAM 113.35 MHz) is located on the airfield
which is at an elevation of 44 feet above mean sea level.
1.11.1 Cockpit Voice Recorder (CVR)
A Fairchild Model A1OO four channel CVR was fitted and a satisfactory
replay of the 30 minute audio record was obtained. Channel allocation
was
| Channel 1 | Cabin Address |
| Channel 2 | Co-pilot's hot microphone |
| Channel 3 | Pilot's hot microphone |
| Channel 4 | Cockpit area microphone |
The rapid decompression caused no discernible change to the signal
on the area microphone channel but it was clearly audible on both
crew hot microphone channels.
1.11.2 Universal Flight Data Recorder (UFDR)
A Sundstrand UFDR was fitted. A satisfactory replay was obtained
from the following recorded parameters:- Indicated Airspeed, Altitude,
Heading, Normal acceleration, Flap position, Pitch attitude, Roll
attitude, No 1 engine P7, No 2 engine P7, VHF transmit discrete.
Recorded data showed the aircraft climbing at 300 kt Indicated
Airspeeed (IAS) through 17,300 feet at the time of the loss of
the windscreen. As the control column was pushed forwards, probably
due to the movement of the commander through the windscreen frame,
the aircraft pitched 60 nose down and banked 250 to the right.
When the co-pilot took control and closed both throttles, the
speed was allowed to increase to 340 kt as the aircraft descended
at 4,600 feet per minute to FL110. On reaching this level the
speed was reduced to 266 kt with a further decrease to 163 kt
as flaps were extended according to the normal operating schedule
and then power was applied to maintain this height and speed.
The time elapsed from the depressurisation to level flight at
FL110 was 148 seconds.
1.12 Wreckage and impact information
The aircraft was brought to rest on the runway and electrical
power turned off The aircraft was towed off the runway and parked.
1.12.1 Examination of the left windscreen and attaching bolts
The windscreen was found near Cholsey, Oxfordshire, along with
the windscreen outboard corner post fairing strip and some associated
bolts.
Of the 90 bolts used to attach the windscreen to the aircraft,
11 had remained in the windscreen and 18 were found loose nearby;
one had remained in the aircraft window frame.
Twenty-six of the bolts recovered with the windscreen were new
bolts identified against the British Standard as having the part
number A211-8C. The remaining four bolts recovered were re-used
bolts identified as having the part number A211-7D. The Illustrated
Parts Catalogue (1PC) specifies that the attaching bolts should
be part number A211-8D. The specifications for these bolts are:-
| Part No | Shank length (inches) | Diameter (inches) | Thread Size |
| A211-8D | 0.8 | 0.1865-0.1895 | 10 UNF |
| A211-8C | 0.8 | 0.1605-0.1639 | 8 UNC |
| A211-7D | 0.7 | 0.1865-0.1895 | 10 UNF |
| UNF = Unified Fine | UNC = Unified Coarse | ||
The bolts engage with 10 UNF Kaylock floating anchor nuts mounted
on the inside of the windscreen frame. The replacement windscreen
had been installed with 84 bolts (A211-8C) whose diameters were
approximately 0.026 of an inch below the diameters of the specified
bolts but of the same thread pitch, and six bolts (A211-7D) which
were of the correct diameter, but 0.1 of an inch too short.
The left windscreen had been changed during the night shift of
the 8/9th June 1990 and the accident flight was the first since
that installation. Eighty of the bolts which had attached the
old windscreen were recovered from the work area during the investigation,
and 78 of these were identified as A211-7D, the remaining two
being A211-8D. The old windscreen, which had been fitted four
years earlier, before the aircraft had been acquired by British
Airways, had therefore been primarily attached by bolts which
were 0.1 of an inch shorter than those specified.
1.13 Medical and pathological information
Not relevant.
There was no fire.
Following the loss of the left windscreen and subsequent decompression
of the fuselage, the commander found himself half way out of the
aircraft through his windscreen aperture. He recalls the impression
of lying on his back against the upper surface of the flight deck
exterior and, realising that he was still able to breathe, he
concentrated on this until he assumes he lost consciousness. He
regained consciousness after the aircraft had landed and when
he was being recovered by fire and ambulance staff inside the
flight deck prior to be being placed on a stretcher and taken
to hospital.
The co-pilot and the crew members who were holding on to the commander
had individually reached the conclusion that his survival was
highly improbable in the extreme conditions to which he was exposed.
They were considerably reassured when, at a late stage in the
descent at about 3,000 feet, the commander started to kick his
legs.
The aircraft was not fitted with an automatic presentation oxygen
system in the cabin and this was not required to be fitted under
the original requirements for the issue of the aircraft's Certificate
of Airworthiness. Therapeutic oxygen was available in the cabin
and consisted of 18 sets of facemasks and four portable oxygen
cylinders. The oxygen system supplied gaseous oxygen to the crew
and passengers if decompression occurred and for therapeutic purposes.
Oxygen cylinders were mounted underfloor in the forward fuselage
in the electrics bay. From the cylinders the oxygen was piped
through in-line filters to the control panel in the flight deck
right hand console. For therapeutic supply, an outlet from the
double pressure regulator connected to an isolation valve (normally
closed) and thence to a ring main which served twinflow sockets
in selected passenger service panels. With the crew shut off valve
and passenger isolation valve open, oxygen was obtained by connecting
a therapeutic mask to an outlet point. Therapeutic masks were
stowed in the aft stowage compartment. Immediately following the
loss of pressurisati on, the No 2 steward went and sat in seat
20D whilst donning the mask of a portable set that was stowed
nearby. Oxygen masks were available to the flight deck crew but
the co-pilot elected not to don his mask since he realised that
the aircraft would soon reach FL100 (see paragraph 1.17.7 below).
He also did not want to impede his ability to communicate with
the other crew members who were holding on to the commander.
1.16.1 Trials of 8 UNC and 10 UNF countersunk head bolts with
10 UNF anchor nuts
During the course of the investigation British Airways carried
out a simulation of the window fitting procedure to determine
the torque that could be applied to 8 UNC countersunk head bolts
fitting into 10 UNF Kaylock type anchor nuts. A 24 anchor nut
test piece was used as follows:
To determine the torque at thread slip of twenty 8 UNC bolts in
10 UNF Kaylock nuts. This was found to be in the range of 1 to
7 lbf in.
To determine the torque required to engage the bolt in the locking
mechanism of the nut, four 10 UNF bolts were fitted in 10 UNF
Kaylock nuts. This torque was found to be in the range of 10 to
11 lbf in.
A further, more representative test was carried out in the presence
of AAIB using a BAC One-Eleven in which 32 bolts (A21 1-8C) were
used to fasten a window and seal in an aircraft. In this test
torque figures ranging between 0 and 12 lbf in were achieved before
the threads slipped.
A third test was carried out using some of the anchor nuts removed
from G-BJRT to ensure that no unforeseen effect could have made
the G-BJRT window unrepresentative; ten 8 UNC bolts were fitted
and these slipped at torques ranging from 0 to 6 lbf in.
The combined results using 8 UNC bolts in 10 UNF Kaylock nuts
showed a maximum torque of 12 lbf in and an average of 4.7 Ibf
in at thread slip.
It was noted that the thread range 4 UNC to 1/2 inch UNF, commonly
used on aircraft bolts, contains three adjacent pairs of sizes
with similar thread pitches which allow the smaller bolt to engage
in the larger Kaylock nut.
1.16.2 Examination of the torque limiting screwdriver used to
fit the windscreen
Tests on a similar torque limiting screwdriver to that used to
fit the windscreen showed that at a low setting (5 lbf in) the
feel of the screwdriver clutch slipping was indistinguishable
from the feel of an 8 UNC thread slipping in a 10 UNF anchor nut.
At a higher setting (15 lbf in) a more pronounced click was felt
as the screwdriver clutch released.
The actual torque limiting screwdriver used had a high level of
residual friction (typically 7 lbf in at a setting of 20 lbf in)
after the set value had been achieved and was therefore taken
to the manufacturer for examination in the presence of AAIB and
British Airways. The torque limiting screwdriver employed a cam
plate with three lobes to retain three ball bearings which were
displaced against the action of a spring to release at the set
torque. Once released, the drive shaft carrying the ball bearings
rotated through a third of a revolution until the balls reindexed
against the cam. Thus, in use, the torque should build up to the
set value, slip and reduce to a residual value whilst the balls
move across the constant radius section of the cam to the next
indexing position.
The residual torque was confirmed as being high at a value of
approximately 30 per cent of the torque set, rather than the usual
value of between 5 and 10 per cent. Subsequent discussions with
the manufacturer disclosed that the specification for the grease,
used in the assembly of the torque limiting screwdriver, had been
changed approximately five years ago because of problems of the
grease breaking down with age. At this time retrospective action
for those torque drivers already sold was considered impractical
because of the large numbers involved and the lack of information
about their location. The screwdriver under test was at
least five years old and strip examination revealed that the excessive
friction was caused by deterioration of the old specification
grease. No significant wear was evident on the cam or the ball
bearings, and when rebuilt with the correct grease the torque
limiting screwdriver performed satisfactorily.
The high residual torque occurred after the set value had been
achieved (ie 20 lbf in) and did not affect the torque at
which the screwdriver operated. The residual torque would not
have been felt before the set torque was reached.
1.16.3 Special checks called for on windscreen bolts after the
accident
Before the diameter of the replacement bolts had been established
British Airways issued an instruction to be carried out on all
its BAC One-Elevens before the next flight, to remove every fourth
bolt from the No 1 left-hand and No 1 right-hand windscreens to
check for correct length.
Throughout the British Airways fleet of BAC One-Elevens two aircraft
failed the check, having a total of 41 short bolts (A211-7Ds).
A similar check was carried out on the four BAC One-Elevens belonging
to another airline and two aircraft failed the check, having a
total of 107 short bolts.
When the smaller diameter bolts were identified in the detached
window British Airways called for a 100 per cent visual inspection
of bolt head diameter; this check utilised the fact that the smaller
bolt head had 27 per cent less area than the head of the correct
bolt. All the aircraft passed the check.
1.17.1 Certification of Airworthiness of Aircraft
1.17.1.1 Type Certification of the BAC One-Eleven
The BAC One-Eleven Model 500 was type certificated to British
Civil Airworthiness Requirements (BCAR) Section D in 1970 which
calls up duplicate inspections after certain safety critical maintenance
operations. However the glazing elements of windscreens are not
identified as principal structural elements, nor does the application
of this duplicate inspection philosophy attempt to cover possible
safety critical situations caused by servicing errors.
There are no airworthiness requirements for aircraft windows to
be fitted from the inside (plug type).
The BAC One-Eleven windscreen was designed to be secured with
countersunk head bolts to British Standard A211-8D. This British
Standard specifies that the British Standard number and the bolt
part number shall not be applied on the bolts, but shall be clearly
marked on the labels of parcels of bolts.
| No label, no contents | 46 |
| No label, contained stock | 25 |
| Labelled, no contents | 68 |
| Labelled, contained stock | 269 |
The last category was further broken down showing that:-
In 251 drawers the majority of the contents were as the label,
(163 drawers contained solely the contents described on the drawer
label).
In 18 drawers the majority of the stock was wrongly labelled,
(in 9 drawers none of the contents were as described on the drawer
label).
The uncontrolled nature of this carousel had been recognised by
some British Airways personnel, who had reported the problem informally.
There was no record of this problem in the QMDRs at Birmingham,
a system specifically designed to receive reports of this nature.
1.17.3.6 Quality Assurance Practice Training
The initial training for QMP consisted of 1.1/2 days of external
training for middle management who provided ad hoc training to
foremen and supervisors in the local area, based on a standard
package consisting of a video plus viewfoils. The foremen in turn
were required to train the subordinate grades.
Continuation training in QMP was carried out as and when required.
The Audit Team, through sampling of QMP awareness across the Company
in June 1988, identified a shortfall; the Quality Forum directed
each Department to carry out QMP training, and an illustrated
'Guide to QMP' was produced. A further QMP survey in January 1989
identified that improvements had been achieved but that a lack
of comprehension still existed. At the time of the accident, action
to remedy this was still under discussion.
b. The Birmingham Exposition
Product Samples were required from Birmingham on a monthly basis
and prior to each aircraft Certificate of Airworthiness renewal.
They were carried out by the Station Maintenance Manager and a
nominated Shift Maintenance Manager. The quality monitoring schedule
for the Product Sample is at Appendix D.
The completed Product Sample proforma were distributed to the
Area Maintenance Manager, the British Airways Quality Forum and
some to the CAA.
A British Airways Engineering Department procedure stated that
the Area Maintenance Managers were responsible for maintaining
the established quality targets with respect to the following:-
Technical Despatch Reliability
Acceptable Deferred Defect levels
Repetitive Defects
Air Safety Reports
Significant Technical Defects sent for investigation E1022's)
Product Samples
QMDRs
Quality Audit Reports
Technical Log entries.
c. Continuous Monitoring Reports from Birmingham
British Airways literature circulated amongst engineering staff
stressed the need for an open reporting system using QMDRs. Over
a 39 month period, ending in April 1990, 36 QMDRs were raised
on local issues at Birmingham. Eleven of these were as a result
of the monthly Product Samples, and the other 25were raised
by the British Airways employees, of whom approximately a quarter
had been active in the system. The Area Maintenance Manager stated
that there was less of a need to complete QMDRs as some faults
could be identified and actioned immediately as he had control
of the Birmingham engineering budget.
d. Product Samples from Birmingham
British Airways produced ten copies of Product Samples carried
out at Birmingham, seven of these related to the period before
the accident and were carried out during work packages involving;
Acceptable Deferred Defect Clearance, Base Checks and Modifications,
and Ramp Checks. The seven pre-accident product samples raised
a total of 65 deficiencies which were of a minor nature.
The CAA produced six copies of Product Samples carried out at
Birmingham before the accident; three of these duplicated copies
provided by British Airways, and the additional three, from early
1989, were similar in content to the others.
e. British Airways Quality Audit at Birmingham
Paperwork audits of the Engineering function at Birmingham to
assess the use of and adherence to monitoring procedures, required
under QMP procedures, were scheduled and performed at six monthly
intervals. A physical audit of the Birmingham station, in the
form of a two day visit, was last carried out prior to the accident
by a representative of the British Airways Quality Audit Unit
on 15/16 June 1988, when it was reported that the engineering
facility was to a high standard. Seven observations were raised
relating to minor, non-aircraft, matters.
f. CAA Supervisory Visit to British Airways Engineering at Birmingham
One of the duties of the CAA's Flight Operations Inspectorate (FOI 7) was (at the time of the accident) to carry out supervisory visits to survey the engineering services provided by British Airways at Birmingham in support of their Air Operator's Certificate. The last FOI 7 visit, an 'Air Operator's Certificate:
Supervision of Operator's Line Maintenance Station', before the
accident took place on 22 June 1989 , followed a proforma schedule,
lasted for approximately half a day and did not detect any significant
engineering problems.
1.17.3.7 Use of E1022 Procedure at Birmingham
Over the same 39 month period in which 36 QMDRs were raised, 365
E1O22s were raised at Birmingham.
1.17.4 Fitting the windscreen
1.17.4.1 History of the shift
The Shift Maintenance Manager arrived at work in the offices under
the International Pier 45 minutes earlier than his shift start
time in order to allow himself time to catch up with the paperwork
and establish the shift work content; this included three significant
defects, routine items and various minor cabin defects.
A Supervisory Aircraft Engineer and a further Licenced Aircraft
Engineer, normally part of the shift, were not available that
night and, although the work outstanding remained the same, the
Friday night shift was routinely not supported by the four man
night shift supplement because there was reduced scheduled flying
on Saturday and Sunday. The shift consisted of:-
The Shift Maintenance Manager
1 Licenced Aircraft Engineer
1 unlicenced engineer airframe/engines
1 Supervisory Aircraft Engineer (Avionics)
1 Avionics engineer.
The engineers were directed to their tasks whilst the Shift Maintenance
Manager carried on with the administration and the task of entering
the contents of the aircraft technical logs into the computer.
At about midnight, the Shift Maintenance Manager spent some time
with the Licenced Aircraft Engineer on a steering defect and the
completion of this coincided with the arrival of a Tunisair Boeing
737 which the shift had to handle. As none of the engineers had
Boeing 737 experience the Shift Maintenance Manager carried out
the pre-departure inspection and the refuelling in conjunction
with the Licenced Aircraft Engineer to give him experience. All
this activity took place at various locations around the airfield
and was co-ordinated using radio.
The departure of the Tunisair Boeing 737 at around 0145 hrs coincided
with the meal break, which the Shift Maintenance Manager spent
working on administration whilst he ate his sandwiches. After
the break he directed his two airframe engineers onto a galley
water leak on one of the BAC One-Eleven aircraft which needed
rectifying before the aircraft departed the following morning.
Although there was no operational requirement for G-BJRT the next
day, the Shift Maintenance Manager knew that the oncoming morning
shift were also depleted and that an aircraft wash had been booked,
using overtime, at 0630 hrs the following morning. Whilst no external
pressure had been put on him, he was aware that the previous week
the wash team had been brought in on a similar basis and not used.
In order to achieve the windscreen change during his shift and
have the aircraft ready for the wash team, he decided to carry
out the windscreen change himself.
The aircraft was located in the No 2 bay, off the Eastern Apron
on the other side of the airfield, and was parked tail into the
hangar with the nose by the doors. In retrospect the Shift Maintenance
Manager could not recall exactly what the weather was, but thought
that it was raining; in which case he would have closed the doors,
leaving a few feet between the nose of the aircraft and the doors.
The windscreen change was carried out between approximately 0300
hrs and 0500 hrs on the Saturday morning.
1.17.4.2 Procedures used
British Airways statistics show that 12 No 1 windscreens, left
or right, had been changed on their BAC One-Eleven aircraft over
the last year, and a similar number the year before. The Shift
Maintenance Manager had carried out about six windscreen changes
on BAC One-Eleven aircraft whilst employed by British Airways.
Maintenance Manual
The Shift Maintenance Manager glanced briefly at the Maintenance
Manual as he had not changed a windscreen for about two years
and wanted to refresh his memory. This check confirmed his impression
that it was a straightforward job with no apparent difficulties.
b. IPC
The IPC was available on a microfiche reader, but was not used
to identify the part number of the bolts to be replaced, consequently
a stock check, using TIME, to assess the availability and location
of replacement bolts was not carried out. The Shift Maintenance
Manager justified this omission by saying that he was quite satisfied
that the bolts that he had removed were the correct bolts, and
that it would take so much time to find the correct numbers in
the IPC that he did not feel justified in using the IPC in the
circumstances of the job in question.
The page of the IPC for the 528 series aircraft shows a sketch
of the pilot's No 1 windscreen and the adjacent DV window, but
only illustrates one bolt - that in the DV window, which is an
A21l-7D. The components for the pilot's No 1 window are listed
in the text, along with several alternative modification states,
and its bolts are defined as 'attaching parts' and are identified
as A211-8Ds. The IPC for the 510 series, in contrast, is very
clear in identifying the correct bolts.
The bolts actually fitted to the defective windscreen were, in
the main, A211-7Ds, the bolts illustrated as applicable to the
DV window. That is bolts of the correct diameter but 0.1 of an
inch shorter than those specified.
c. Bolt selection
The Shift Maintenance Manager removed the windscreen with the
aid of the Avionics Supervisor, who also disconnected the electrical
connectors of the screen heaters. The bolts were 'on condition'
items, and as some of the paintfilled bolt heads had been damaged
during removal, and others showed signs of corrosion, the Shift
Maintenance Manager decided to replace them and took one of the
bolts to the store to identify it by comparison with those held
in the carousel. The carousels were under the control of a storeman
and had drawers which were clearly labelled with a location code
to which engineers were directed, after entering the part number
into the adjacent stores computer terminal.
Because of their small head size the bolts do not carry individual
identification, but the Shift Maintenance Manager accurately matched
the removed bolt by going through several trays, and comparing
the removed bolt with the drawer contents. He then identified
the part number of the bolt as A21l-7D by looking at the stores
issue note in the drawer (the windscreen should have been fitted
using A21 1-8Ds). The Stores Supervisor, who had been in the job
for about 16 years, informed him that A211-8Ds were used to fit
that windscreen, but did not press the point. The Shift Maintenance
Manager decided that as A21l-7D bolts had come Out, he would replace
them with bolts of the same size.
The minimum stock level in the carousel for A211-7D bolts was
50, but there were only four or five bolts in the drawer (when
checked by the AAIB the following Monday it contained four). The
Shift Maintenance Manager drove to the unsupervised carousel underneath
the International Pier, taking the removed bolt with him. The
drawers in this carousel were labelled with the part number of
the contents, although the labels were old and faded. The ambient
illumination in this area was poor and the Shift Maintenance Manager
had to interpose himself between the carousel and the light source
to gain access to the relevant carousel drawers. He did not use
the drawer labels, even though he now knew the part number of
the removed bolt, but identified what he thought were identical
bolts by placing the bolts together and comparing them. He also
picked up six A211-9Ds, thinking that the attachment of the outboard
corner post fairing strip would need longer bolts.
The old seal was found to be serviceable, so the new windscreen,
which weighed 60 pounds, was manoeuvred into position and the
electrical connections made.
d. Torque loading of the bolts
The aircraft manual calls for a torque of 15 lbf in to be applied
to the bolts, which are then retorqued to 5 lbf in after 100 flying
hours. The Shift Maintenance Manager's experience told him that
many of the bolts would be found up to three turns loose during
the retorque procedure, so he decided to increase the initial
torque to 20 lbf in.
The British Airways toolstore at Birmingham held a calibrated
dial indicating torque wrench to cover the range of 5 to 120 lbf
in, but the retorque requirement of 5 lbf in was at the bottom
of the range and the dial indicating torque wrench was not considered
suitable for this task. Two calibrated torque checking gauges
were available at Birmingham to allow engineers to confirm the
wrench accuracy.
The calibrated dial-indicating torque wrench was not available
on the toolboard that night, but the Stores Supervisor had recently
acquired from British Airways at London, on his own initiative,
a torque limiting screwdriver specifically for the windscreen
task, but on receipt it was found to be out of calibration date
and it was therefore not cleared for use. It was not the company
policy at Birmingham to allow the engineers to adjust torque wrenches
as and when required, but rather to have the wrenches adjusted
in a standards room and then issued for use at that specific setting.
It was therefore the intention of the Stores Supervisor to have
it set in the London standards room before issue, but, in the
absence of any suitable alternative, the storeman set this screwdriver
to the figure of 20 lbf in requested and gave it to the Shift
Maintenance Manager, who checked the setting using both torque
checking gauges.
The Shift Maintenance Manager used a 1/4 inch bi-hexagonal socket
to hold the No 2 Phillips screwdriver bit onto the speedbrace
used to run the screws down into the countersinks. The socket
did not have any means, such as a spring clip, to retain the screwdriver
bit, consequently the Shift Maintenance Manager found that during
the two-handed operation of using the speedbrace the bit fell
out several times and he had to descend from the safety raiser
(mobile staging) and retrieve it from the floor. To overcome this
problem when using the same V4 inch bi-hexagonal socket with the
torque limiting screwdriver, he held the screwdriver in his right
hand and used his left hand to hold the bit in the socket Additionally,
to reach most of the bolts with both hands from the safety raiser,
he had to stretch across the nose of the aircraft, outside the
safety rail provided by the safety raiser. This situation was
exacerbated by the fact that the safety raiser was incorrectly
positioned alongside the aircraft. His left hand obscured his
view of the bolt head, and the need to stretch removed the operation
from his direct vision.
He fitted the windsreen using 84 of the bolts collected from the
International Pier carousel and obtained a similar feel from the
torque limiting screwdriver for each one; a feel that met his
expectations. When he came to the outboard corner post fairing
strip he realised that the A21l-9D bolts were too long, descended
from the staging and retrieved and refitted the six old bolts
that he had removed with the fairing.
The new bolts that he had fitted were in fact A211-8C bolts -
one size down in diameter but with the same thread pitch as those
specified and within 0.050 of an inch in length to the A211-7D
bolts removed from the window. The bolts engage in 10 UNF 'Kaylock'
floating anchor nuts; the self locking action is the result of
part of the nut being an elliptical shape prior to the insertion
of the bolt. Some of the anchor nuts were attached directly to
the inside of the aircraft window frame and some were carried
on strips, themselves attached to the window frame. The outboard
corner post fairing strip interposed an additional thickness and
required A21l-8D bolts, and these were specified for the attachment
of the whole windscreen, even though in the majority of locations
approximately five threads would be visible below an anchor nut
fastened directly to the frame when used with an A21l-8D bolt.
The amount of thread in safety would be reduced when used with
the backing strips and the outboard corner post fairing.
e. Missed cues
The safety raiser used by the Shift Maintenance Manager did not
give easy access across to the centreline of the aircraft, and
he had to stretch over the aircraft nose to accomplish the task.
Due to the inadequate access to the job and the obscuring effect
of his left hand the Shift Maintenance Manager was not in a position
to observe that the bolt thread was slipping in the anchor nut
thread, instead of the torque limiting screwdriver allowing its
shaft to remain stationary while the handle rotated. However,
the bit and socket would have continued to rotate in his left
hand.
The window was finished in primer and had countersunk holes for
the bolts; an A21l-8C bolt head sits significantly further below
the surface of the window, down in the countersink, than does
an A21 1-8D bolt head, leaving an annulus of unfilled countersink
which is easily discernible when viewed under good conditions.
This excessive annulus of unfilled countersink was not seen.
When the bolts were being fitted to the windscreen centre column,
the bolts from the right hand window, the heads of which filled
the countersinks, were close to those of the left hand window,
and, although painted, the difference is perceptible under normal
circumstances. The Shift Maintenance Manager missed this difference
in depth of the bolt heads in the windscreen centre column. (See
photograph in Appendix E).
When fitting the outside corner post fairing with the six bolts
previously removed from it, the Shift Maintenance Manager failed
to notice the difference in torque achieved or the difference
in countersink fit of the bolt heads between the old and new bolts.
The following night the Shift Maintenance Manager carried out
another windscreen change, this time a right hand one. The job
had been set up before he arrived and he noticed that the bolts
were A21l-8Ds. He recalled fitting A21l-7D bolts the previous
night, but he rationalised that the aircraft were old and of differing
modification states and the previous night he had an aircraft
modification standard requiring A211-7D bolts and that night he
had an aircraft requiring A211-8D bolts.
f. Documentation
The documentation used to report and clear the defect stated:-
| DEFECT SYMPTOM | ACTION TAKEN |
| SYSTEM Port Windscreen | |
| During cruise darkening & bubbling noted in small area on bottom LH port windscreen. Q.R.H. drill carried out | Windscreen replaced. A>S>R> Actioned F/Check satis |
| Signed by Reporting Captain | Signed by engineer (subject to the following declaration) |
| THE WORK RECORDED ABOVE HAS BEEN CARRIED OUT IN ACCORDANCE WITH THE REQUIREMENTS OF THE AIR NAVIGATION ORDER FOR THE TIME BEING IN FORCE AND IN THAT RESPECT THE AIRCRAFT/EQUIPMENT IS CONSIDERED FIT FOR RELEASE TO SERVICE | |
Note:
Q.R.H Quick Reference Handbook.
A.S.R Air Safety Report, raised by the captain. The Shift Maintenance
Managers action was to clear the defect
F/Check Functional check of the windscreen heating system.
During the course of the investigation a number of visits to the
operator's engineering facility at Birmingham were made, the Shift
Maintenance Manager who changed the windscreen was interviewed
and informal interviews conducted with the other maintenance managers
in order to provide a context for the actions of the engineer
who undertook the windscreen replacement task. Subsequently these
managers provided written signed statements, mostly concerned
with the range of issues raised at the interviews.
The overriding impression given by the Maintenance Managers was
that morale was high and that they were proud of their record
in meeting the task and of the way that they went about it.
The Shift Maintenance Managers did not criticise the shift system,
however the potential problems associated with sleep deprivation
and circadian effects were acknowledged and various strategies
were cited to cope with the situation.
During the initial part of the investigation the Shift Maintenance
Manager who carried out the windscreen fit did not appear to grasp
the lack of care that his actions implied. He co-operated fully
in the investigation and, when shown the full list of errors and
omissions that he had made, offered an explanation for each individual
action.
The Area Manager was aware of the pressures to produce aircraft
that the Shift Maintenance Managers worked under, and continually
stressed that there were other objectives besides maximising the
work throughput on the shifts.
Four of the six Maintenance Managers who subsequently gave written
statements raised the issue of the large numbers of E1022 forms
originated at Birmingham and concluded that these indicated their
concern for quality and general standards.
One Maintenance Manager stated that he felt that the QMP system
was in its infancy at the time of the accident but that the E1022
process was well known. He went on to say that the staff at Birmingham
felt more comfortable with the E1022 system because they knew
exactly how it worked and they knew that they would get a response.
Another Maintenance Manager also concluded that when he returned
damaged parts through the E1022 system he had direct contact with
the development engineer by telephone and his requests were actioned
without them being channelled through a third party. The E1022
system was therefore more effective, the QMPs took longer to action
and were, in his opinion, clearly for non-urgent quality lapses.
1.17.6 Human factors
1.17.6.1 Personal details
The person who fitted the windscreen was a Shift Maintenance Manager
holding authorisations on BAC One-Eleven, Boeing 737, Boeing 757,
HS 748 and with transit authorisations on L-1011 Tristar, Boeing
747 and a CAA licence holder for airframe and engines on the Viscount.
His experience included 10 years in the RAF, followed by 23 years
with British Airways. He appeared to be a mature, dedicated engineer
who was well respected by flightcrew and engineers alike. No domestic
or financial distractions were identified, either by British Airways
management, the Behavioural Psychologist engaged by the AAIB who
interviewed him or the AAIB Inspectors; the Shift Maintenance
Manager denied any such problems.
He had been on leave over the period of the last night shift carried out by his shift and so the Friday/Saturday night shift during which the windscreen was fitted was his first night work for approximately five weeks. He had had a normal nights sleep the previous night and had gone to bed at about 1730 hrs, and had slept for 1.1/2 hours, getting up at 2030 hrs. He said that he would have been ha>
The Shift Maintenance Manager made limited use of a fairly weak
prescription for reading glasses, but did not habitually use them
at work and was not wearing them when making the bolt selection.
His record with British Airways has been reported as exemplary
and he had received commendations during this period.
1.17.6.2 Behavioural Psychologist's Report
A Behavioural Psychologist interviewed the Shift Maintenance Manager
who carried out the windscreen fitting task and was present during
AAIB interviews with him and informal interviews with the other
Maintenance Managers. His report is included at Appendix F.
1.17.6.3 Ophthalmologist's Report
The Shift Maintenance Manager was examined by a consultant in
ophthalmology who concluded that his eyes were normal with full
central fields and normal ocular muscle balance. He had full stereoscopic
vision and his intra-ocular pressures were normal. However he
was presbyopic and for this he needed glasses for close work,
and his own half-eye reading glasses were perfectly adequate for
his needs.
If he were to read small print or figures without his reading
glasses, he would have difficulty. With his reading glasses and
in good lighting, he would have no problems.
1.17.6.4 Relationship between Serious Accidents and Near Misses
Two analyses of groups of accidents and incidents occurring in
industrial situations have shown that for every serious accident
there can be between 400 to 600 near misses. These figures indicate
that, in an industrial context, degraded standards may exist for
some time before a serious accident occurs or the situation becomes
apparent to an independent observer.
The experience of accidents involving aircraft maintenance shows
that an accident usually occurs as a result of a series of errors,
and that the probability of the accident occurring is low compared
with the probabilities of the individual failures in the chain
of events leading to it. The incorrect installation of the windscreen
resulted from a sequence of contributory events (para 1.17.4.2),
any one of which, if identified and eliminated from the chain
could have prevented the windscreen loss.
1.17.7 The effects of rapid decompression
In an attempt to analyse and quantify the dynamic forces and physiological
effects caused by the loss of the windscreen, all the available
data was presented to the Aircrew Systems Division of the RAF
Institute of Aviation Medicine, RAE, Farnborough.
The conclusions drawn suggested that the critical factors affecting
the survivability of all the aircraft occupants were the time
of decompression and file final cabin altitude. Those affecting
the commander were the time of decompression and the final altitude
of exposure, together with the low temperature and the aerodynamic
forces to which he was exposed during the remainder of the flight.
Calculation provided that the duration of the decompression was
likely to have been in the region of 1.13 to 1.46 seconds, and
this was supported by the duration of the rapid changes of aircraft
attitude. The maximum cabin altitude, achieved during this time
period, depended upon the interaction between the ram effect of
the outside airflow and the airflow provided by the internal pressurisation
systems. Analysis suggested that this was unlikely to have been
greater than 13,000 to 13,500 feet which, when followed by the
descent profile flown, would not have promoted sufficient hypoxia
to impair either the passengers or the crew.
The forces acting upon the commander, to project him through the
windscreen aperture, were a function of the differential pressure
between the inside and outside of the cabin and are calculated
as having a force of approximately 5,357 pounds (depending upon
his exact proximity to the aperture). This would be quite adequate
to drive a person weighing 70 kg from his seat and through the
aperture, whereafter the ram effect of the airstream would pin
him to the fuselage and seriously impair movement.
1.18 New investigation techniques
None.
The crew were faced with an instantaneous and unforeseen emergency.
The combined actions of the co-pilot and cabin crew successfully
averted what could have been a major catastrophe. The fact that
all those on board the aircraft survived is a tribute to their
quick thinking and perseverance in the face of a shocking experience.
Up to the time of the loss of the windscreen, the flight had proceeded
uneventfully and in accordance with the company's normal procedures.
It was quite in order for the flight crew to release their shoulder
harnesses once they were established in the climb and, for reasons
of comfort, the commander loosened his lap strap as he neared
the cruising phase of the two and a half hour flight to Malaga.
Therefore, when the left windscreen was blown out, it was not
surprising that the commander, who was very lightly built, was
drawn partially through the windscreen aperture. It is not certain
what prevented his complete egress from the aircraft but, since
the No 2 steward later bad to free his legs from a position between
the control column and the flight deck coaming, it is likely that
he had been restrained by his legs during the initial stage of
the emergency. Later, he was restrained simply by the efforts
of the No 2 steward who was holding on to both of his legs.
The co-pilot immediately took control of the aircraft and was
able to establish a rapid descent despite the disorientating effects
of the dramatically transformed cockpit environment coupled with
a push over and right roll. It was fortunate that he was an experienced
pilot with more than 1,000 hours experience of flying the BAC
One-Eleven aircraft. Thus he was able to handle the aircraft on
his own and complete the normal operating procedures from memory
without the assistance of another pilot. He alone was faced with
a double emergency, namely rapid decompression and incapacitation
of the handling pilot. He rejected the idea of donning his oxygen
mask in favour of being able to shout instructions to his cabin
crew. In the event, this was probably sensible but if the depressurisation
had occurred at a greater height, say above 20,000 feet, it would
have been imperative for him to don the oxygen mask to avoid incapacitation
to the extent that he could not fly the aircraft.
2.2.1 The selection and use of the wrong bolts
The windscreen was lost because it had been secured by bolts,
the vast majority of which were of an incorrect diameter. The
windscreen fitting process was characterised by a series of poor
work practices, poor judgements and perceptual errors, each one
of which eroded the factors of safety built into the method of
operation promulgated by British Airways:-
a. During the fitment of the windscreen to G-BJRT the Shift Maintenance
Manager was confronted with certain situations which made his
job more difficult
Incorrect bolts, A211-7D had been used in the previous installation
Insufficient stock of A211-7D bolts, incorrect but demonstrably
adequate, existed in the carousel in the bay stores at the Eastern
Apron.
Nevertheless, problems of this type are not unusual and cannot
be used to explain the subsequent chain of events which led to
the loss of the windscreen.
b. A number of procedures were ignored and some poor trade practices
followed:-
The IPC, available to identify the required bolts' part number
was not used
The stores TIME system, available to identify the stock level
and location of the required bolts, was not used
Physical matching of old and new bolts by touch and eye was attempted,
leading to a mismatch with bolts from the International Pier carousel
Arbitrary choice of A21 1-9Ds to fit through the corner fairing
took place
An increase in bolt torque over that stated in the Maintenance
Manual was used.
c. Non conformity with British Airways standards was also demonstrated:-
An uncontrolled torque limiting screwdriver was set up outside
the Calibration Room.
d. Use of unsuitable equipment took place:-
A bi-hexagonal bit holder was used leading to occasional loss
of the bit and covering of the bolt head during the torquing process
A Safety Raiser which provided inadequate access to the job was
used.
e. A number of cues were either ignored or missed:-
The warning from the Storekeeper that A211-8D bolts were required
did not influence the choice of bolts
The amount of unfilled countersink left by the small boltheads
was not recognised as excessive
The increased amount of unfilled countersink with the new bolts,
compared to the flush fitting of adjacent, correctly sized bolt
heads in the windscreen centre column, went unnoticed
The difference in torque and the amount of countersink remaining
unfilled between the new bolts and old bolts used in the corner
fairing went unnoticed
The use of, as he thought, A211-7Ds when using A211-8Ds the next
night was not questioned
The difference between the bolt thread stripping in/through the
nut and the torque limiting screwdriver 'breaking' was not recognised
even though the bi-hexagonal socket and screwdriver bit, located
by his left hand, were still rotating. However, the high residual
torque of the particular screwdriver resulted in a less positive
'break' and, although the break torque was never achieved with
the 8 UNC bolts, it was when setting the screwdriver and when
installing the fairing. This screwdriver, on reaching the set
torque may have felt more like the thread stripping than would
one with a more 'snappy' break.
2.2.2 The windscreen replacement task
The windscreen is part of the aircrafts pressurised hull and is
attached from the outside by 90 bolts. It may be the only critical
item on the aircraft that was susceptible to failure through the
chain of circumstances listed above, in that:-
a. Its replacement required the renewal of the majority of the
bolts in the judgement of the Shift Maintenance Manager.
b. The wrong diameter bolts engaged with the anchor nuts, and
had the same thread pitch.
c. The bolts were not special to type items needing a part number
to identify and obtain replacements, but were general use items,
obtainable from an uncontrolled carousel.
d. The windscreen was not designed on the plug principle such
that internal air pressure would hold it in place, but was fitted
from the outside.
e. The windscreen replacement was not designated a 'Vital Point'
task, therefore no duplicate inspection was required.
f. The Shift Maintenance Manager was the only person whose work
on the night shift was not subject to the review of a maintenance
manager.
The windscreen may therefore have been unique in that it alone,
of all the critical components, could have accommodated the errors
which occurred during its fitment, to expose them so dramatically
the first time that the windscreen was called upon to resist cabin
pressure. Had it been any other item, the selection of the wrong
bolts may have been unmistakably apparent during the fitting process,
or the subsequent failure may not have been so obvious or traumatic.
2.2.3 Relevant British Airways' Procedures
2.2.3.1 AGS dispensing
The use of unsupervised dispensers for aircraft general spares
is a recognised and necessary part of aircraft engineering practice.
Small units can rarely afford to keep a full-time storekeeper
to administer a dispenser, or even a store, and good trade practice
has to be relied upon. Before the Shift Maintenance Manager went
to the unmanned carousel he knew the part number of the bolts
he was seeking, and although they were too short, similar bolts
had held the old windscreen in place for four years. Despite the
poor segregation, labelling and lighting, the selection of the
wrong bolts cannot be explained by the carousel system.
2.2.3.2 Work by Shift Maintenance Managers
During the course of his duties the Shift Maintenance Manager
reviewed the work of his shift, this review augmented the self
certification task required of the engineers by British Airways'
maintenance policy. Once he had decided to carry out rectification
work himself, he withdrew from the active supervision of the rest
of the shift. The task of the windscreen installation was not
designated a 'Vital Point' and consequently no duplicate inspection
was called for and none took place, nor was the work of the Shift
Maintenance Manager subject to review by another manager.
Thus the Shift Maintenance Manager had no backstop with any chance
of detecting his errors. Errors that were made more likely by
the sleep deprivation and circadian effects associated with the
end of a first night shift.
2.2.4 Quality Assurance
2.2.4.1 Application of Self Certification to Aircraft Engineering
The adoption of self certification systems within manufacturing
industry has typically resulted in savings, mainly arising through
reduction in scrap and in the achievement of higher manufacturing
efficiency. Nevertheless, at the end of the production line the
product is normally still tested, before being despatched. Some
aircraft maintenance tasks which may be undertaken using self
certification procedures do not allow for the testing of the end
product before it is flown.
It could be argued that the concept of self certification suffers
from the drawback that the expectations of the engineer are such
that he is unlikely to detect an error of his own making; the
application of self certification reduces the level of inspection
and supervision.
It is recommended that the applicability of self certification
to aircraft engineering safety critical tasks following which
the components or systems are cleared for service without functional
checks, should be reviewed by the CAA. Such a review should include
the interpretation of 'single mal-assembly' within the context
of 'Vital Points' and the requirements which include a waiver
making the definition of 'Vital Points' non-mandatory for aircraft
with a Maximum Take-Off Weight Authorised of over 5,700 kg which
were manufactured in accordance with a Type Certificate issued
prior to 1 January 1986.
2.2.4.2 Feedback
A fundamental requirement of any management process is a feedback
loop to detect the success or failure of the system, and two types
of feedback are available - a formal feedback through auditing/monitoring
activities and an informal feedback through free discussion amongst
engineers discussing their work problems in an open forum.
Some feedback was generated by the monitoring of a series of performance
parameters which were airline parameters with quality overtones
rather than parameters capable of giving a comprehensive picture
of the engineering quality built into tasks. The crucial element
missing was direct assessment of the standards used by the Shift
Maintenance Managers to perform their tasks.
Whilst literature circulated by British Airways stressed the need
for open reporting through QMDRs, a number of the Maintenance
Managers indicated that they felt more comfortable with the E1022,
Ground Occurrence Report Form, with which they were particularly
familiar, finding it a more direct and responsive reporting system.
The findings at Birmingham are consistent with the British Airways
Audit Team sampling of QMP awareness in 1988 and a further QMP
survey in 1989 which identified that a lack of comprehension still
existed. At the time of the accident action to remedy this was
still under discussion within British Airways.
The E1022 system was well established and understood when QMP
was introduced three years before the accident. The statements
of the Birmingham Maintenance Managers indicate that at least
some of them still prefer, and may use, the E1022 system in instances
when a QMDR might be more appropriate. The list of circumstances
under which an El 022 is to be used appears to overlap into procedural
areas which might be thought of as the domain of the QMP system.
Some evidence of a quality problem within the British Airways
engineering unit at Birmingham is provided by the failure of the
unit to use the Continuous Monitoring system to report some of
the problems seen during investigation of the windscreen fitment:
The poor labelling and segregation of parts in the uncontrolled
carousel under the International Pier
Inadequate access available to certain areas of the aircraft
from the work platform
Inadequate tools to achieve some specific torque loading
Windscreen attachment bolts found loose at the 100 hour re-torque.
It is recommended that British Airways review their Quality Assurance
system and the relative roles of ElO22s and QMDRs be clarified
and that they continue to educate and encourage their engineers
to provide feedback from the shop floor.
At the time of the accident a physical audit of the Birmingham
base was about due according to the QMP schedule. The British
Airways Quality Audit Unit had last visited Birmingham two years
before the accident over a two day period and were satisfied with
the engineering standards.
It is recommended that British Airways should review the Product
Sample procedure with a view to achieving an independent assessment
of standards and conduct an in-depth audit into the work practices
at Birmingham.
2.2.4.5 CAA Supervisory Visits
The CAA supervision of the engineering functions of operators,
away from their main bases, is undertaken by FOI 7, and the British
Airways engineering facility at Birmingham was given a half-day
visit approximately a year before the accident. The visit, in
view of the time allocated, was necessarily superficial and only
likely to have picked up gross discrepancies.
It is recommended that the CAA should review the purpose and scope
of the FOI 7 Supervisory Visit.
| 4.1 | The CAA should examine the applicability of self certification to aircraft engineering safety critical tasks following which the components or Systems are cleared for service without functional checks. Such a review should include the interpretation of 'single mal-assembly' within the context of 'Vital Points' and the requirements which include a waiver making the definition of 'Vital Points' non-mandatory for aircraft with a Maximum Take-Off Weight Authorised of over 5,700 kg which were manufactured in accordance with a Type Certificate issued prior to 1 January 1986. |
| 4.2 | British Airways should review their Quality Assurance system and the relative roles of E1022s and QMDRs be clarified and they should continue to educate and encourage their engineers to provide feedback from the shop floor. |
| 4.3 | British Airways should review the need to introduce job descriptions/terms of reference for engineering grades including Shift Maintenance Manager and above. |
| 4.4 | It is recommended that British Airways should review the Product Sample procedure with a view to achieving an independent assessment of standards and conduct an in-depth audit into the work practices at Birmingham. |
| 4.5 | The CAA should review the purpose and scope of the FOI 7 Supervisory Visit. |
| 4.6 | The CAA should consider the need for the periodic training and testing of Engineers. |
| 4.7 | The CAA should recognise the need for the use of corrective glasses, if prescribed, in association with the undertaking of aircraft engineering tasks. |
| 4.8 | The CAA should ensure that, prior to the issue of an ATC rating, a candidate shall undergo an approved course which includes training in both the theoretical and practical handling of emergency situations. This training should then be enhanced at the validation stage and later by regular continuation and refresher exercises. |
D F KING
Inspector of Air Accidents
Air Accidents Investigation Branch
Department of Transport
January 1992
The Civil Aviation Authority's response to these Safety Recommendations
is contained in CAA Follow-up on Accident Reports (FACTAR) No.1/92,
to be published coincident with this report.
