Given the recent discussion on this, and the fact that I am now
in flight test with N437RP (Lancair IV-P), I conducted a runaway trim test
yesterday.
As background, I have set the trim travel to provide a moderate amount
of pitch down trim, but substantial up trim …as would be required for
landing. I also installed a locking toggle switch in the center of the panel,
just below the radio stack, that switches the trim from left to right stick
control. This switch has a center OFF position that completely removes power
from the trim system.
I started the in-flight trim test at 120 KIAS. Full down trim
was controllable with moderate stick back pressure. I judged this to be manageable
for a reasonable period of time given the forces involved—estimated at 10
pounds or so, but not measured.
The full up trim, however, required significantly more force to
maintain level flight. I would estimate 25 pounds. The force was great enough that
I could hold it with my left hand, but could not operate the coolie hat trim
switch with my thumb and maintain the pressure at the same time. I rolled into
a 70+ degree left bank to ease the forward stick pressure, and even though I
applied significant power almost immediately, the airspeed dropped to 110-100
before I could control it. This was a real attention getter; enough so
that I trimmed it back to normal and came home to think it over on the ground!
A runaway up trim in my airplane will require immediate and positive action to
retain control of the airplane. I want to also see if I can/should limit the
trim travel in this direction. I don’t think I am using all of the
available nose up trim during normal landings, but have not explored the full
CG range yet. If possible, I will limit the travel to the max required for “normal”
landing, and try this test again.
Bob
From: Lancair Mailing
List [mailto:lml@lancaironline.net] On Behalf Of Gary Casey
Sent: Sunday, June 29, 2008 12:07 AM
To: lml@lancaironline.net
Subject: [LML] Re: Runaway Trim
If the trim motor fails it will certainly just stop moving -
can't move without power. But moving back upstream, if one of the relays
fail closed there is a runaway condition, and a relay can stick either open or
closed. Further, the stick switch can fail open or closed. Granted,
it is more likely for one of these components to fail open than closed, but the
severity of the failure is far, far worse if closed than open. In my
case, if a stick switch fails closed I have to recognize this and switch to the
other stick immediately. If the relay fails closed I have to find and
pull the breaker. It would be nice to have a "master" trim
on-off switch. In my case there are coolie-hat switches and I worry that
something could fall under one of the switches, jamming it in position.
On my list of things to modify are, in order of priority:
1. Reduce the travel of at least the elevator trim tab
so that it will produce only the minimum-required down trim. Full up-trim
is probably manageable, but full down-trim might not.
2. Eliminate the relay array by wiring the two sticks
in parallel. The only disadvantage of this is that if the two pilots trim
in opposite direction simultaneously the breaker will pop (at least settling
the disagreement :-).
3. Going to a two-switch arrangement as mentioned
below by Robert. The certificated planes i have flown have this
arrangement for exactly this reason: If one switch sticks closed power
will be interrupted by the other switch when released. If the opposite
trim direction is then selected the breaker will blow. Intuitive and fail-safe.
4 And I've also thought about putting 4 switches on
the stick. Instead of a pair of SPDT momentary-contact switches use 4
SPST momentary-contact switches. The two on the front of the stick are to
trim down and the up-trim switches are on the back. My coolie hat is on
the back and I've had a couple of times when doing a go-around that I was
unable to simultaneously push hard enough and operate the trim switch so I had
to operate the trim switch with the other hand. The theory is that when
pushing forward the index finger is free and when pulling back the thumb is
free to move the appropriate switch.
From: Dennis Johnson [mailto:pinetownd@volcano.net]
Sent: Thursday, June 26, 2008 9:51 PM
To: lml@lancaironline.net
Subject: Runaway Trim
My understanding from talking with the RAC guys, who make the trim
components I installed in my Legacy, is that stuck (runaway) trim is
unlikely. (Actually, they said it was impossible, but I find it hard to
use that word.)
A runaway trim seems unlikely to me too. There was one
GlaStar incident a few years back that was attributed to such a
failure while in the pattern. That incident resulted in two
fatalities, and a design change to the GlaStar trim authority. Remembering
that accident and having experienced the muscle required to overcome the
mis-trim on my ES-P even when slowed to 90 kt, I figured I could either install
a mechanical backup (not practical) or try an electrical approach.
To eliminate runaway some installations have two buttons that must
be pressed simultaneously to activate either
direction. That eliminates runaway but not single-direction
failure. In my failure mode it was not a runaway failure: I was able to
trim nose-up but not nose-down. This new controller should allow me to
reverse the previous direction bypassing the control stick buttons. So if
after climb out I again could not trim nose-down I would be able to press
the reverse button and reduce stick pressure. At least - that's the
theory.
Robert M. Simon
ES-P N301ES