AOPA Pilot Magazine - January 2017 - 87

P&E

PROFICIENCY

The other autopilot failure
Taming runaway trim
B Y J A C K L I P S C O M B A N D D AV I D J AC K K E N N Y
AUTOPILOTS ARE USEFUL, especially in IMC, but neither they nor the instruments that guide them are
foolproof. Pilots who train with traditional gyroscopic
instruments are taught that if a gauge that feeds attitude or position information to the autopilot drops
offline, whether from its own internal failure or that
of the vacuum system, the autopilot will continue to
follow those increasingly erroneous inputs until either
the pilot or its G switch disconnects it. Unusual attitudes progressing to loss of control can result. That's
just one reason that using an autopilot is no excuse for
dropping your scan.
However, there's another failure mode that's even
more insidious-and potentially even more dangerous.
It affects systems that pair a primary pitch servo with
automatic electric trim. Turning on the autopilot
energizes solenoids that mechanically engage the
drive gear on each servo with a driven gear on the
pulley that moves the bridle cables connected to the
corresponding flight control.

When the autopilot commands a change in pitch-
say, because the pilot selects a higher altitude-the
pitch servo energizes to move the elevator. A separate
circuit in the pitch servo detects the torque on the pitch
servo's shaft and activates the pitch trim servo in the
appropriate direction to trim off the control pressure,
just as the pilot would do while flying by hand. Once
the control pressure is neutralized, the torque on the
pitch servo is removed, which deactivates the trim
servo. The pitch servo itself stays engaged as long as
the autopilot is on.
The problem arises if the pilot disconnects the
autopilot while also putting forward- or back-pressure
on the yoke-as one would naturally do to counter an
uncommanded altitude excursion. The control pressure may prevent the pitch servo from mechanically
disengaging, even though the bars disappear from the
flight director, the autopilot disconnect annunciator
lights up, and by every indication the autopilot's been
turned off. Worse, the pressure of the elevator control
www.aopa.org/pilot AOPA PILOT | 87



http://www.aopa.org/pilot

Table of Contents for the Digital Edition of AOPA Pilot Magazine - January 2017

https://www.nxtbook.com/nxtbooks/aopa/pilot_202405
https://www.nxtbook.com/nxtbooks/aopa/pilot_202404
https://www.nxtbook.com/nxtbooks/aopa/pilot_202403
https://www.nxtbook.com/nxtbooks/aopa/pilot_202402
https://www.nxtbook.com/nxtbooks/aopa/pilot_202401
https://www.nxtbook.com/nxtbooks/aopa/pilot_202312
https://www.nxtbook.com/nxtbooks/aopa/pilot_202311
https://www.nxtbook.com/nxtbooks/aopa/pilot_202310
https://www.nxtbook.com/nxtbooks/aopa/pilot_202309
https://www.nxtbook.com/nxtbooks/aopa/pilot_202308
https://www.nxtbook.com/nxtbooks/aopa/pilot_202307
https://www.nxtbook.com/nxtbooks/aopa/pilot_202306
https://www.nxtbook.com/nxtbooks/aopa/pilot_202305
https://www.nxtbook.com/nxtbooks/aopa/pilot_202304
https://www.nxtbook.com/nxtbooks/aopa/pilot_202303
https://www.nxtbook.com/nxtbooks/aopa/pilot_202302
https://www.nxtbook.com/nxtbooks/aopa/pilot_202301
https://www.nxtbook.com/nxtbooks/aopa/pilot_202212
https://www.nxtbook.com/nxtbooks/aopa/pilot_202211
https://www.nxtbook.com/nxtbooks/aopa/pilot_202210
https://www.nxtbook.com/nxtbooks/aopa/pilot_202209
https://www.nxtbook.com/nxtbooks/aopa/pilot_202208
https://www.nxtbook.com/nxtbooks/aopa/pilot_202207
https://www.nxtbook.com/nxtbooks/aopa/pilot_202206
https://www.nxtbook.com/nxtbooks/aopa/pilot_202205
https://www.nxtbook.com/nxtbooks/aopa/pilot_202204
https://www.nxtbook.com/nxtbooks/aopa/pilot_202203
https://www.nxtbook.com/nxtbooks/aopa/pilot_202202
https://www.nxtbook.com/nxtbooks/aopa/pilot_202201
https://www.nxtbook.com/nxtbooks/aopa/pilot_202112
https://www.nxtbook.com/nxtbooks/aopa/pilot_202111
https://www.nxtbook.com/nxtbooks/aopa/pilot_202110
https://www.nxtbook.com/nxtbooks/aopa/pilot_202109
https://www.nxtbook.com/nxtbooks/aopa/pilot_202108
https://www.nxtbook.com/nxtbooks/aopa/pilot_202107
https://www.nxtbook.com/nxtbooks/aopa/pilot_202106
https://www.nxtbook.com/nxtbooks/aopa/pilot_202105
https://www.nxtbook.com/nxtbooks/aopa/pilot_202104
https://www.nxtbook.com/nxtbooks/aopa/pilot_202103
https://www.nxtbook.com/nxtbooks/aopa/pilot_202102
https://www.nxtbook.com/nxtbooks/aopa/pilot_202101
https://www.nxtbook.com/nxtbooks/aopa/pilot_202012
https://www.nxtbook.com/nxtbooks/aopa/pilot_202011
https://www.nxtbook.com/nxtbooks/aopa/pilot_202010
https://www.nxtbook.com/nxtbooks/aopa/pilot_202009
https://www.nxtbook.com/nxtbooks/aopa/pilot_202008
https://www.nxtbook.com/nxtbooks/aopa/pilot_202007
https://www.nxtbook.com/nxtbooks/aopa/pilot_202006
https://www.nxtbook.com/nxtbooks/aopa/pilot_202005
https://www.nxtbook.com/nxtbooks/aopa/pilot_202004
https://www.nxtbook.com/nxtbooks/aopa/pilot_202003
https://www.nxtbook.com/nxtbooks/aopa/pilot_202002
https://www.nxtbook.com/nxtbooks/aopa/pilot_202001
https://www.nxtbook.com/nxtbooks/aopa/pilot_201912
https://www.nxtbook.com/nxtbooks/aopa/pilot_201911
https://www.nxtbook.com/nxtbooks/aopa/pilot_201910
https://www.nxtbook.com/nxtbooks/aopa/pilot_201909
https://www.nxtbook.com/nxtbooks/aopa/pilot_201908
https://www.nxtbook.com/nxtbooks/aopa/pilot_201907
https://www.nxtbook.com/nxtbooks/aopa/pilot_201906
https://www.nxtbook.com/nxtbooks/aopa/pilot_201905
https://www.nxtbook.com/nxtbooks/aopa/pilot_201904
https://www.nxtbook.com/nxtbooks/aopa/pilot_201903
https://www.nxtbook.com/nxtbooks/aopa/pilot_201902
https://www.nxtbook.com/nxtbooks/aopa/pilot_201901
https://www.nxtbook.com/nxtbooks/aopa/pilot_201812
https://www.nxtbook.com/nxtbooks/aopa/pilot_201811
https://www.nxtbook.com/nxtbooks/aopa/pilot_201810
https://www.nxtbook.com/nxtbooks/aopa/pilot_201809
https://www.nxtbook.com/nxtbooks/aopa/pilot_201808
https://www.nxtbook.com/nxtbooks/aopa/pilot_201807
https://www.nxtbook.com/nxtbooks/aopa/pilot_201806
https://www.nxtbook.com/nxtbooks/aopa/pilot_201805
https://www.nxtbook.com/nxtbooks/aopa/pilot_201804
https://www.nxtbook.com/nxtbooks/aopa/pilot_201803
https://www.nxtbook.com/nxtbooks/aopa/pilot_201802
https://www.nxtbook.com/nxtbooks/aopa/pilot_201801
https://www.nxtbook.com/nxtbooks/aopa/pilot_201712
https://www.nxtbook.com/nxtbooks/aopa/pilot_201711
https://www.nxtbook.com/nxtbooks/aopa/pilot_201710
https://www.nxtbook.com/nxtbooks/aopa/pilot_201709
https://www.nxtbook.com/nxtbooks/aopa/pilot_201708
https://www.nxtbook.com/nxtbooks/aopa/pilot_201707
https://www.nxtbook.com/nxtbooks/aopa/pilot_201706
https://www.nxtbook.com/nxtbooks/aopa/pilot_201705
https://www.nxtbook.com/nxtbooks/aopa/pilot_201704
https://www.nxtbook.com/nxtbooks/aopa/pilot_201703
https://www.nxtbook.com/nxtbooks/aopa/pilot_201702
https://www.nxtbook.com/nxtbooks/aopa/pilot_201701
https://www.nxtbook.com/nxtbooks/aopa/pilot_201612
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https://www.nxtbook.com/nxtbooks/aopa/pilot_201609
https://www.nxtbook.com/nxtbooks/aopa/pilot_201608
https://www.nxtbook.com/nxtbooks/aopa/pilot_201607
https://www.nxtbook.com/nxtbooks/aopa/pilot_201606
https://www.nxtbook.com/nxtbooks/aopa/pilot_201605
https://www.nxtbook.com/nxtbooks/aopa/pilot_201604
https://www.nxtbook.com/nxtbooks/aopa/pilot_201603
https://www.nxtbook.com/nxtbooks/aopa/pilot_201602
https://www.nxtbook.com/nxtbooks/aopa/pilot_201601
https://www.nxtbook.com/nxtbooks/aopa/pilot_201512
https://www.nxtbook.com/nxtbooks/aopa/pilot_201511
https://www.nxtbook.com/nxtbooks/aopa/pilot_201510
https://www.nxtbook.com/nxtbooks/aopa/pilot_201509
https://www.nxtbook.com/nxtbooks/aopa/pilot_201508
https://www.nxtbook.com/nxtbooks/aopa/pilot_201507
https://www.nxtbook.com/nxtbooks/aopa/pilot_201506
https://www.nxtbook.com/nxtbooks/aopa/pilot_201505
https://www.nxtbook.com/nxtbooks/aopa/pilot_201504
https://www.nxtbook.com/nxtbooks/aopa/pilot_201503
https://www.nxtbook.com/nxtbooks/aopa/pilot_201502
https://www.nxtbook.com/nxtbooks/aopa/pilot_201501
https://www.nxtbook.com/nxtbooks/aopa/pilot_201412
https://www.nxtbook.com/nxtbooks/aopa/pilot_201411
https://www.nxtbook.com/nxtbooks/aopa/pilot_201410
https://www.nxtbook.com/nxtbooks/aopa/pilot_201409
https://www.nxtbook.com/nxtbooks/aopa/pilot_201408
https://www.nxtbook.com/nxtbooks/aopa/pilot_201407
https://www.nxtbook.com/nxtbooks/aopa/pilot_201406
https://www.nxtbook.com/nxtbooks/aopa/pilot_201405
https://www.nxtbook.com/nxtbooks/aopa/pilot_201404
https://www.nxtbook.com/nxtbooks/aopa/pilot_201403
https://www.nxtbook.com/nxtbooks/aopa/pilot_201402
https://www.nxtbook.com/nxtbooks/aopa/pilot_201401
https://www.nxtbook.com/nxtbooks/aopa/pilot_201312
https://www.nxtbook.com/nxtbooks/aopa/pilot_201311
https://www.nxtbook.com/nxtbooks/aopa/pilot_201310
https://www.nxtbook.com/nxtbooks/aopa/pilot_201309
https://www.nxtbook.com/nxtbooks/aopa/pilot_201308
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https://www.nxtbook.com/nxtbooks/aopa/pilot_201305
https://www.nxtbook.com/nxtbooks/aopa/pilot_201304
https://www.nxtbook.com/nxtbooks/aopa/pilot_201303
https://www.nxtbook.com/nxtbooks/aopa/pilot_201302
https://www.nxtbook.com/nxtbooks/aopa/pilot_201301
https://www.nxtbook.com/nxtbooks/aopa/pilot_201212
https://www.nxtbook.com/nxtbooks/aopa/pilot_201211
https://www.nxtbook.com/nxtbooks/aopa/pilot_201210
https://www.nxtbook.com/nxtbooks/aopa/pilot_201209
https://www.nxtbook.com/nxtbooks/aopa/pilot_201208
https://www.nxtbook.com/nxtbooks/aopa/pilot_201207
https://www.nxtbook.com/nxtbooks/aopa/pilot_201206
https://www.nxtbook.com/nxtbooks/aopa/pilot_201205
https://www.nxtbook.com/nxtbooks/aopa/pilot_201204
https://www.nxtbook.com/nxtbooks/aopa/pilot_201203
https://www.nxtbook.com/nxtbooks/aopa/pilot_201202
https://www.nxtbook.com/nxtbooks/aopa/pilot_201201
https://www.nxtbook.com/nxtbooks/aopa/pilot_201112
https://www.nxtbook.com/nxtbooks/aopa/pilot_201111
https://www.nxtbook.com/nxtbooks/aopa/pilot_201110
https://www.nxtbook.com/nxtbooks/aopa/pilot_201109
https://www.nxtbook.com/nxtbooks/aopa/pilot_201108
https://www.nxtbook.com/nxtbooks/aopa/pilot_201107
https://www.nxtbook.com/nxtbooks/aopa/pilot_201106
https://www.nxtbook.com/nxtbooks/aopa/pilot_201105
https://www.nxtbook.com/nxtbooks/aopa/pilot_201104
https://www.nxtbook.com/nxtbooks/aopa/pilot_201103
https://www.nxtbook.com/nxtbooks/aopa/pilot_201103_test
https://www.nxtbook.com/nxtbooks/aopa/pilot_201102
https://www.nxtbook.com/nxtbooks/aopa/pilot_201011demo
https://www.nxtbook.com/nxtbooks/aopa/pilot_201011
https://www.nxtbook.com/nxtbooks/aopa/pilot0308
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