back Chris Johnson, Index.

Formal Proof Techniques

A ground test engine was fitted with a set of 
fan blades, some of which were fitted with 
strain guages.   This fan blade set was 
selected so that all the blades had the same 
natural frequency of their second order bending 
mode.   This test showed that, under these 
conditions, there was an excitation of a non-
synchronous vibratort response but it did not 
produce vibratory stress levels greater than 
50 of the endurance limit. 

(Page 58 from UK Air Accident Investigation Branch
report on the accident to Boeing 737-400 G-OBME near
Kegworth, Leicestershire on January 8th 1989.