From: jcmorris@mwunix.mitre.org (Joe Morris)
Subject: Re: Stupid Pilot Trick Awards
Organization: The MITRE Corporation
Date: Fri, 31 Jul 1992 13:31:50 GMT

kerry@Ingres.COM (Kerry Kurasaki) writes:

>About 5 years ago, I heard a story of some biz jet that crashed due
>to a thrust reverser that deployed [assymetrically] in-flight.  With
>the right (or left) one hanging out, the plane was thought to be
>uncontrollable.

>Now comes the NTSB to test this hypothesis.  They get another biz jet
>(same make and model) and arrange to have it modified so that a
>thrust reverser can be deployed assymetrically in-flight.  They fly
>the same profile, and trigger the device.

>To me, this sounds exactly like the joke of the software engineer
>whose brakes fail but somehow manages to stop his car from going off
>a 3000 foot cliff.  What does he say?  "Lessee, let's try this again
>and see if the same thing happens!"

The idea of a flight test isn't that unreasonable, as long as the thrust
reverser was rigged so that it could be restowed in flight.  Sort of
like your car being tested to see if the driver can actually stop it
before it runs off the cliff -- but with a safety line which will
snub it before the wheels actually reach the edge.

A recent report on the L'Express commuter crash last year had an example
of exactly this type of test.  One prop was found at essentially zero
degrees pitch (i.e., no thrust and very little drag to slow the prop; 
just the opposite of a feathered prop).

The teardown of the engine revealed that the splines which link the
prop command to the prop controller in the hub were completely worn
down, allowing the prop pitch to change without regard to commands
from the cockpit.  There were no survivers and the airplane was not
equipped with a CVR, so the theory was that with one prop delivering
thrust and one offering nothing but drag the crew had no way to control
the airplane.

To test this theory, they rigged the prop of a similar airplane with 
deliberately damaged splines, then flew it to see what would happen.
The prop was spiked to develop proper thrust in the early stages of
the flight; the spike could be removed in flight to see what the
result would be when the prop drifted out of control.  For a variety
of reasons, the low range of the drive was artificially limited to
22 degrees.

When the spike was released, the prop *slowly* drifted to lower pitch
angles (which might have delayed recognition of the failure by the
crew); more to the point, when it hit the 22 degree limit the **very**
experienced pilot -- who was expecting the failure -- reported severe
control problems.  No attempt was made to try a flight test with a
lower blade pitch; the other evaluations were done in simulators in
which no pilot was able to make a safe landing given the particular
failure mode.

My point here is that deliberately failing a component on a flight
is sometimes the only way to find out for sure what could have happened
on an accident flight.  Sometimes that isn't possible without putting
the test airplane (and its crew!) in jeopardy, but often the danger
can be minimized by either limiting the scope of the failure or
by allowing the failure to be "unfailed".

This is much the same as factory test pilots use to try out a new
airplane design.  Next time you go to a lecture where Bill Kershner is
speaking, ask him about his days as test pilot for Piper, and
the test flights for the Apache (??? I'm not completely sure) where
successive flights to find the proper value of Vd were run with
increased speeds of 2 mph per flight until some new manager came along
and ordered a much larger increment...which almost cost the life of
the test pilot.  (If I recall Bill's story, that was just after he
left...)

Joe Morris

"If you want to avoid Vd, stay out of dives."  - Bill Kershner
Newsgroups: rec.aviation