|
I have a Legacy with a CH Products stick grip with "hat switch" for trim
and RAC servos and an RAC relay between the hat switches and the trim
servos. I'm stubborn by nature and I'm still unwilling to agree that
runaway trim is a realistic concern in my Legacy. I'm not saying that's
true in other airplane types or in other Legacies with different trim
systems.
Just to clarify, "runaway trim" means, to me, that the trim moves without
command from either pilot. Either the trim starts moving on its own or it
continues moving after the trim switch is released.
In contrast, "stuck trim" means, to me, that the trim servo fails to move
when the trim switch is activated.
I had stuck trim (not runaway trim), which I believe is
a common failure mode and each of us with trim systems like mine
should confirm that the airplane is flyable if the trim sticks in
any reasonably likely position. Mine stuck when adding power to
execute a missed approach after a practice ILS in clear weather. Lots of
nose up and lots of stick pressure, but the airplane was easily controllable at
all times. But it did get my attention at a busy time while cleaning up
the airplane for the missed approach.
By the way, maybe I'm just slow, but if my situation had
been runaway trim instead of stuck trim, I don't think I could have
reacted in time to find the right circuit breaker and pull it out
to stop the trim before it moved all the way to the stop. I
don't remember, but I may have had both hands on the stick, which would have
made pulling the breaker even tougher.
Here's why I don't think runaway trim is a realistic concern for my Legacy.
1. I have examined the hat switch and am confident that the switch
handle ("hat") is unlikely to become stuck in any position.
2. If the electrical contacts in the hat switch stick closed, I
believe that physically moving the switch in the opposite direction will
unstick the contacts. That's not a certainty, but the risk seems to
be too low for me to worry about.
3. If the trim relay sticks closed, I believe that moving the
trim switch in the opposite direction will exert enough force to break the relay
open. I haven't tested this, but RAC says they have and that it
will.
4. I have physically examined my wires in the trim circuit and
believe that a wiring fault that causes runaway trim is unlikely.
5. The typical measures taken to prevent runaway trim add
complexity and parts count, reducing overall reliability of the trim system,
making stuck trim more likely.
If any of these assumptions are wrong, please tell me where my mistake
is! Runaway trim can be an attention grabbing experience and I don't want
to be ignoring that risk based on false assumptions. Steve's experience is
troubling because it could indicate that my conclusion is wrong. Once I
can talk to him and discover the root cause, I may change my conclusion.
One last thing. My conclusion is based on how I
have defined the level of risk that is acceptable to me and understand
that reasonable people can find the exact same risk to be unacceptable.
I'm not saying that runaway trim in my system is impossible, just that
its probability is low enough that I'm not going to worry about it.
Just like flying over the mountains in a single engine airplane, which I did
this weekend from Northern California to Portland, Oregon. Wow, what a
smoky trip from all the wildfires in California! On the flight to
Portland Saturday morning, I couldn't see the ground until Eugene. And
when I landed back home this afternoon, the airplane is covered
with gritty material. But other than that, it was a fun trip.
Dennis Johnson
Legacy, 145 hours
|