In a message dated 10/3/2007 6:02:04 P.M. Central Daylight Time,
VTAILJEFF@aol.com writes:
For those of you that have STEC autopilot systems please read SAIB
60-21 attached below. The STEC autopilots use the TC for "attitude" reference.
Their sales literature tell you how much safer this is over the attitude
reference systems other manufacturers use (like King). Unfortunately, STEC
does not tell you that if the TC ground wire becomes disconnected or the TC
loses the ground signal the AP will still operate by slowly rolling the
aircraft one way or another (albeit without an attitude reference). This can
pose a big problem in the cockpit if you are IMC (the AP "ready" light is on
but the TC is failed) and can cause a pilot to lose control of the
aircraft. It is my opinion that this has been the cause of at least two fatal
accidents and one serious accident. The survivors of the serious accident --
two instrument rated pilots related how confusing it can be to fly with the AI
going one way and the TC going the other way.
Unfortunately, STEC is currently fighting one of these
cases Federal Court in NJ where I have testified against them and has
chosen to not inform its customers of this potentially deadly problem.
Jeff,
I read it and here is another scenario (I believe I already related this to
you).......
S-Tec has failed (an interesting word) to inform the using community of
another failure with similar failing results. I have an S-Tec 50 and
the pitch control board was messed up during a flight thru a very electrically
active cloud in 2001. One of the consequences was damage to a circuit
that is the control computer tach test for the TC, a 10 VDC
presence on a certain pin - the failure provided a constant 10
VDC on the tach test pin so that even if the TC circuit breaker was pulled (the
ultimate power reference signal deletion), the tach test was valid and the AP
considered the non-existent lateral management signal from the TC to be
valid. S-Tec checked this board twice and confirmed that it was OK
(apparently their test procedures do not test very many failure modes)
before they finally admitted there was a problem.
Activating the AP without a valid signal from the TC results in a slow
spiral (or roll if altitude hold is engaged). I have written about this
condition before on the LML and
REQUIRED PRE-FLIGHT TEST:
0) Before any possible flight in IFR conditions;
1) Pull the TC circuit breaker.
2) Master on, Run the AP thru the test sequence.
3) If the "ready" indicator illuminates - You got a problem
baby!!! When my circumstance was finally resolved (replacing the
pitch control board) they had already seen a few problems like this.
Go ahead, ask how I discovered this failure
mode .......................
Note, along with the new pitch control board came a sheet with 38 (?)
possible modifications, each involving up to 6 components on the pitch control
board to deal with pitch control torque and sensitivity problems - Isn't it
nice that Tru-Trak has these critical parameters externally adjustable?
Scott Krueger
AKA Grayhawk
Lancair N92EX IO320 SB 89/96
Aurora, IL
(KARR)
Darwinian culling phrase: Watch
This!