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Fifty years ago I was flying
P2V-5s in the Navy, (twin-engine patrol bomber) and there was a runaway
'Varicom" on takeoff in another squadron somewhere that caused excessive nose-up
both pilots were unable to overcome... went straight up and came straight
down. The Navy and Lockheed came up with a remedy.
Varicam was a trim surface in
between the stabilizer and the elevator, powerful enough to balance the big
Fowler flaps.
The emergency procedure was to
roll the plane into a 45-60 degree bank and put full flaps down, both of
which demanded more 'nose up', of which there was already a plethora.
And the circuit breaker was pulled, while both pilots pushed forward.
Then an enlisted-crewdog struggled under about 2Gs to find the hand crank, made
his way back into the tail, stuck the hand crank up into the
overhead de-energized trim motor and cranked like hell to lower the Varicam back
to neutral.
But the point is, full flaps
and a steep turn should help if the trim runs away 'up'. If it goes
'down'? Reflex the flaps and hope you don't have to roll inverted
periodically to maintain altitude until ... hmmm... any
suggestions?
fwiw
Terrence
L235/320 N211AL
----- Original Message -----
Sent: Saturday, June 28, 2008 11:06
PM
Subject: [LML] Re: runaway trim
I used to fly Canberras, which had a powerful electric trim motor that
changed the horizontal stab angle of attack. There were several fatal
accidents before it was found that the trim could run away, and the elevator
did not have enough authority to overcome the hardover trim. We then
fitted two microswitches that had to be pressed simultaneously, which fixed
it. As we have heard, it can happen.
The obvious thing to check is that you can overcome a runaway trim motor
in flight. If you cannot, then reduce the trim authority, most easily by
reducing the size of the trim tab. Beyond that, I am installing a
circuit breaker near the trim controller so that I can pull it and cut off all
power.
Runaway trim frightens the hell out of me. It can be very hard, or
impossible, to manage. We need to make sure that we can deal with it by
checking it out in flight and making it controllable. Just my
opinion.
Jerry Fisher
On Sat, Jun 28, 2008 at 5:42 PM, < marv@lancair.net> wrote:
Posted for "Abe
Gaskins" < abe@mail.mgmindustries.com>: In my Legacy I
had autotrim installed. I am empirical proof that a runaway trim
condition can happen. Upon attempting to reach Sun-n-Fun this year I was
over flying Chattanooga and I had a progressive runaway trim pitch up
condition. Trim went to the stops. I found the condition managable
though I did have to push forward with heavy input in order to maintain
control. OK in VFR but scary and potentially leathal in IMC on a dicey
approach. Abe Gaskins N272AG --
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