Learjet 55 Goes down in Philadelphia

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fubar57

General
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Nov 22, 2009
The Jungles of Canada
Bears all the hallmarks of Somatogravitic Illusion. (the pilot perceives the aircraft is climbing steeply and over-corrects with excessive nose down pitch).
 
Highly unlikely that an AH failure on its own would cause this. Coupled with Spatial disorientation, such as Somatogravic Illusion seems like a high probability.

Disagree. Artificial Horizon malfunction in IMC with hand flown operating pilot is absolutely classic scenario for a steady and controlled roll manoeuvre that leads to CFIT. In contrast Somatogravic problems often cause disorientation, initially in Pitch. This aircraft deviated in roll, to a spiral dive, classic AI failure result, absolutely most likely to cause this.
Unfortunately, few crews can pick-up the problem, because often the old mechanical gyro instrument can suddenly suffer a degredation that does not cause the FAIL flag to show. In fact, even more modern mechanical repeater instruments can have indication failures. The operating pilot in IMC will not usually pick up a partial AI failure, unless a warning flag is shown by the instrument, the pilot flies the instrument.
Of course, crews must cross scan the other AI and the Standby AI. Unfortunately, few crews do this at a rate that would mitigate the disaster we see. To assist crews with this awful scenario, automatic Instrument Comparator systems that detect mismatch of ATTITUDE indications more than around 3 degrees of Roll were introduced, but this was mainly in large airliners. The comparator warning caused crew Bold Face actions of cross scanning instrumentation to determine the failed instrument. Checklist proceedures were also included.
In the simulator, training with unannunciated AI failures in Roll almost always led to spiral dive LOC. However, with anunnciated instrument or comparator warnings, safe flight was usually maintained.
Unfortunately, an INU fault giving incorrect AI indication caused the fatal total loss of the B747 at Stanstead England 22 December 1999. In that accident, the fault was anunnciated with aural and fail flag warnings but, the crew failed to follow the proceedures and the operating pilot followed the incorrect roll and flew it into the ground in IMC.

Eng
 
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