nuckolls.bob(at)aeroelect
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 Posted: Wed Jul 22, 2009 6:11 pm Post subject: Autopilots vs. "The Right Stuff" |
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At 11:21 AM 7/21/2009, you wrote:
| Quote: | Yes, with the wondrous capabilities and reliability of today's computer controlled machines, be assured that nothing can go wrong...go wrong...go wrong....go........
I seem to recall reading about several accidents caused by the erroneous actions of computer controlled servo systems; actions that couldn't be detected nor corrected by the pilots. Admittedly, these were the result of faulty assumptions being built into the computer's database by humans. I guess it comes down to "who (or what) do you trust?" |
Or designing for failure tolerance. Accidents
that follow servo system failures are generally
situations where the pilot did not have his/her
hand on the stick while too close to the ground.
I've had a/p systems hiccup . . . and it's
never pleasant. I wouldn't shoot a coupled
approach without having a hand on the wheel
and a thumb on the disconnect switch. You
need to unhook the autopilot to land so you
can be ready to punch it off when you break out
and/or punch it off when and if it strays.
I worked a FMEA on a Baron accident where the
pilot was in a climb and punched altitude hold
as he approached his assigned level. The autopilot
immediately grabbed pitch and held the desired
altitude. At the same time, it started driving
trim to take force out of the pitch servo.
Unfortunately, the down-pitch relay stuck and
continued to trim down while the a/p servo worked
to keep pitch level. When the pitch servo ran out
of authority, the aircraft nosed down. The pilot
was then aware of a problem. He correctly deduced
a difficulty in the automatic flight control
system and punched the a/p disconnect button.
The pitch servo relaxed and the airplane was
now trimmed nearly full nose down trim. Forces
on the wheel needed to keep the airplane level
would have been high. He was so busy trying
to fly the airplane that he probably didn't notice
the trim situation. This was a smoking hole event
where all of the wreckage ended up in a single
divot.
In years since we've installed pitch authority
limiters in servos (no reason for the a/p to
be more capable than a robust pilot), trim in
motion annunciation signals, and software based
monitors that watch for diverging forces.
The accident was a simple, analog, single channel
autopilot that had a connection to the standard
factory electric trim. The a/p had no way to spot
the stuck relay but was capable of standing off
the effects of divergent forces until they were
too large for the pilot to physically handle
when he told the autopilot to buzz off.
Microprocessors, highly integrated flight
control systems, and lessons-learned have largely
eliminated such events. The forces and rates
in a stepper motor implemented wing leveler
can never over-stress a pilot or an airframe.
Bob . . .
[quote][b]
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