NW-Bound
Give me a museum and I'll fill it. (Picasso) Give me a forum ...
- Joined
- Jul 3, 2008
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- 35,712
.. Turkish Air flight 1951 crashed short of Amsterdam because a faulty radio altimeter caused the autopilot to reduce the throttles under the mistaken belief that the plane was already on the ground...
Forgot to comment on this. Bad design!
If radar altimeter is used for such a function, it should have been dual. For autoland, you need dual radar altimeters for the autopilot anyway, which is more critical than the autothrottle. The system I worked on used something much simpler: wheel spin-up sensors and gear strut compression switches. The chance of them failing simultaneously is null. Plus, if they indicate "gear compressed" and "wheel spinning" while in-flight before landing, the computer knows that they are bad, and inhibits some modes...
Having brought this up, I now have to add something or people say I post misleading information.
I am sure that the designers did do FMEA on the use of the single radar altimeter for the throttle retarding function. Presumably, they argued that it was benign, as the throttle movement rate was designed to be only so fast (to give pilots time to react), and if it was bad, pilots would have caught it. That assumption was proven wrong.
The last time I worked on commercial aircraft was more than 30 years ago. Back then, to prevent a crash from any kind of failures like the above, the procedure was for both pilots to have one hand on the throttle levers. This way, they could sense if the autothrottle was screwing up during this critical phase of the landing. And in the case of a go-around, they had their hands where they could shove it up if the autothrottle failed to do it.
Perhaps the pilots in that accident did not follow the procedure, or they did and still could not detect the throttle movement. Whatever it was, the design was weak, and the human backup procedure failed to work as intended.
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