Partisan Autopilots

September 18, 2008 by Bruce Landsberg

A while back we discussed blank avionics screens and how dependent we’ve become on the hardware (Nothing can go wrong…). There was a recent Aviation Safety Reporting System ( NASA ASRS) report on a  wild and crazy autopilot (AP) that went berserk on a Piper Mailbu.

“WE HAD JUST STARTED TO RECEIVE VECTORS FOR THE ILS/DME RWY X APCH INTO ZZZ. THE AUTOPLT SUDDENLY PITCHED THE ACFT NOSE DOWN ABOUT 30 DEGS. WE WENT THROUGH OUR ASSIGNED ALT OF 3000 FT DOWN TO ABOUT 2400 FT. (AIRSPD WAS AROUND 165 KTS.) BEFORE I COULD GET THE AUTOPLT DISENGAGED, THE PLANE SUDDENLY PITCHED UP VIOLENTLY AND THEN PITCHED DOWN AGAIN WITH ENOUGH FORCE THAT EVERYTHING WAS FLYING AROUND THE COCKPIT AND CABIN.”

In a subsequent discussion with the pilot, he said he attempted to manually over ride the autopilot before hitting the red disengage button on the control yoke.  Rule number one on APs is to know at least three ways to retake command. This will be by yoke switch, panel switch, and circuit breaker – at a minimum.

To put it in political terms, APs are extremely partisan – they will always trim against you when engaged in a tug of war. For every pound of force you apply, they will counter. When you finally cut it off, the aircraft will be totally out of trim, and if you’re holding the barely equal and opposite force to balance, it will take strength, time ,and altitude to get things back to equilibrium.

I’m a big believer in APs and that they are essential for single pilot flight in significant IMC. But as with the glass, we need to be ready and able to discipline an unruly crew member and fly to a safe landing.

Bruce Landsberg
Senior Safety Advisor, Air Safety Institute

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12 Responses to “Partisan Autopilots”

  1. Bob Ferguson Says:

    Bruce,
    You are 100% correct about the autopilot fighting you in a pitch malfunction situation. It is absolutely imperaitve that pilots know how to disable their autopilots immediatly and safely. I always tell them that the aircraft master switch is a positive disconnect. Everything will shut down except the engine(s) but they can turn it back on immediately and they will have control of the aircraft again. That is a last resort situation and should not be used in lieu of the normal things you already mentioned in your blog. Keep up the good work,
    Bob

  2. David L. Hughes Says:

    In the Malibu, at high altitudes, the a/p can be using maximum nose up trim to either achieve or maintain assigned altitude.

    An a/p disconnect in that condition can lead to a nose high critical attitude which must be corrected manually and immediately.

    If not corrected, a stall will occur rapidly.

    Recovery of the stall, requires strong forward pressure.

    If recovery from the initial stall is not immediate, the secondary stall is even more exciting.

    Every Malibu pilot should experience a full nose up trim, nose high, climb power, critical attitude recovery under the guidance of an experienced flight instructor.

    Dave
    601228 CFI

  3. Manny DeReyes Says:

    Sometimes before the pilot has an inkling that the AP is acting-up, it surprizes you. It would be a thought if the manufacturers put in a trim-in-motion alert or clicker to advise you what “it” is doing and when. I advocate at the first sign of trouble, looking at your trim indicator, then punching-off the AP to deal with the problem. This has 2 effects; one is it stops a potential runaway condition; two, is if caught early enough, it prevents heavy forces from occurring in pitch or roll. Looking at the trim tells you what you may expect. It is also worth it to look at any aircraft changes that may have caused it; a possible change in configuration, icing, split flap, aileron trim problem, wing heavy condition from unbalanced fuel, etc. Nose down at cruise or descent is a big problem, nose up at slow speed is a greater one. Banking recovery keeps near 1 g on the airframe, fuel and oil systems.

    MD 577496

  4. scott sulentich Says:

    bruce,
    i actually flew my seneca to bob ferguson in tulsa to have him check my a/p. he says all is well, but…. i have had instances in which “it” seems to fly to the beat of it’s own drummer. this obviously decreases my ifr comfort level. assuming bob is correct, and i have every reason to believe he is, could a malfunctioning flight director be to blame? can an avionics shop diagnose this?
    thanks,
    scott

  5. Bruce Says:

    Scott…..

    Good question and beyond my level of technical knowledge. You need to consult with a VERY good avionics shop.

    Thanks for the comment ……Bruce

  6. Marty Coddington Says:

    Hi Bruce. Back in my charter pilot days I sometimes flew doctors out into the boondocks to conduct clinics. It was easy when the weather was nice but as you can imagine, if the airport did not have weather reporting and things were marginal, it was a different story. We used a straight Navajo for this work. On a day when everything had gone perfect, we were on the return trip to home plate VFR @ 5500. The a/p was engaged on “altitude hold”. Suddenly the plane pitched up to what seemed like vertical. It sucked the blood out of my head and I remember feeling a bit incapacitated. I could not gain any useful information from the flight instruments as they were a blur. For some reason my peripheral vision took over or perhaps I turned my head to the side where I instantly learned of my unusual attitude. I pushed forward on the wheel as hard as I could and eventually brought the nose level. Now it was apparent that the plane was way out of trim and I turned the wheel for what seemed like an eternity. I commented to the doctors that this was the worst clear air turbulence I’d ever experienced. Back at base I told the harrowing story to one of our senior pilots and the Director of Maint and their response was “you mean you did not have your right calf leaning up against the trim wheel? ALL Navajo pilots guard against this by keeping in contact with the wheel!” So, perhaps ALL Navajo pilots really don’t do that but on that day one more was added to the list who do. They went on to say that “runaway trim” was not unusual and that the a/p doesn’t know the trim is running away but it does know when the forces needed to hold an altitude exceed a certain number of pounds and then it disengages. It seems my plane was trimming nose up for some time and when the force was reached, the a/p decided to exit the argument which left me pointed at the heavens.I flew another 500 hours in Navajos, mostly Chieftains, and never had a repeat. Not many solutions here but perhaps someone out there is driving a Navajo and has not heard of the “calf” technique.

  7. Douglas Drummond Says:

    The first responder to this situation, Bob Ferguson said: “I always tell them that the aircraft master switch is a positive disconnect. ” Well it is, but, if the A/P relays have stuck, it may re-engage when the master is reset. I heard this story from my first flight instructor back in 1974 when I was a pre-solo student — it happened to him in a Piper Arrow. He went around, put the gear down, and then switched off the master when on final. In his case, the A/P was stuck in a heading hold mode, so he flew the pattern using the heading bug on Directional Gyro.

    When I am in a Piper, or other airplane with A/P or power trim, I ALWAYS make sure I know where the appropriate breaker is located.

  8. Cary Alburn Says:

    My experiences with autopilots, good or bad, are relatively few, since the airplanes I’ve flown over the years that had them, I tended not to use them or to use them only for a few minutes while I attended to some other chore, like chart reading. But one bad experience soured me on relying on them.

    I was flying an overly equipped Mooney 231, which had a King flight director/HSI and a 3 axis autopilot with altitude hold. On an IFR but in VMC flight across eastern Wyoming late at night, the autopilot suddenly commanded a quick roll to the right and at a pretty steep bank, disconnected, the nose dropped, and the airplane continued to roll to a very steep bank, all before I reacted and righted the airplane. I lost enough altitude that ATC called and asked what was going on.

    So far as I know, I didn’t do anything to encourage Otto to do what it did, but Otto certainly discouraged me from doing any coupled approaches, in that airplane or any other since. Murphy: What can go wrong, will.

    Cary

  9. George Horn Says:

    In a Cheyenne II at FL 240 northwest of DFW, high-speed cruise, a windshield suddenly FULL of terrain! We were pitched so violently DOWN (about 25-degrees) that the passenger cut his scalp on the bulkhead/stringer above/thru the headliner. A Delta heavy had passed westbound 20 minutes prior at FL 240, according to FTW center.
    Autopilot? or Upset?
    Either way, keep seatbelts fastened TIGHT and practice a rapid-disconnect occasionally. Disconnect the AP FIRST rather than attempt to overcome it’s ability to trim further out-of-trim.

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