• Any other proof of second order vibrations?

Following these tests, the strain guaged blade 
sets were installed in an engine of a Boeing 
707 flying testbed.   The ensuing flight tests 
showed that the vibratory response in flight 
was considerably stronger than that apparent 
during the ground tests.   When using -3C1 
rated climb power, a response was observed at 
altitudes avove 10,000 feet which was at its 
most severe at altitudes between 25 and 30,000 
feet; vibratory stress levels of up to about 
80% of the endurance limit were observe

(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.

• Report doesn't explain weakness of hypothesis.