Articles, Technology

Autopilot Troubleshooting

Posted on Apr 4, 2020 by SuperUser Account

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One common type of post that appears on the COPA forums is "My autopilot does ________ when I do ______. What's a quick solution?" Often this kind of post is answered with members suggesting "Mine was fixed by bending the trim tab" or "Have your shop adjust the ______" or "You need to replace the _____".

The autopilot is one of the most complex systems in the airplane. It involves electronics, the mechanical components of the flight controls and rigging, and electrical actuators to apply force to the flight controls.

Pitch trim problems

Pitch Trim 101: There are two trimming devices.

  1. A Pitch Motor located in the tail controls gross trim adjustments by changing the levers in the tail.
  2. A Pitch Servo under the rear baggage area does fine tuning by tugging on the control cables. These work together. The Servo has pressure sensors that say to the computer 'I need More'. It gets more by turning on the pitch motor. If the motor does not relieve the pressure in a few seconds then a audio beeping is heard and a flashing trim with arrow is displayed until the pressure is reduced.

Pitch Motor failure occurs if the motor will not operate or if the motor will not operate at a low startup voltage. When we hit the hat switch for manual trim we bypass the AP computer and give it full power which makes the motor move fast. In AP mode the AP ramps up the voltage to get the desired effect. If the motor starts late it can cause a delay and shows up as an error or other flight problems. When both devises work right the plane get completely trimmed so when you release the AP you have no stick pressure or jerk.

The AP computer can make the motor look bad. In my case my computer had a blown fuse in the motor drive circuit. Replacing the fuse did not cure the problem as the drive transistor was fried too.

Testing: Try reseating the AP. You can do a ground check to see if the AP is driving the Pitch Motor in the tail. Without starting the engine activate the AP and set it for a climb or decent (open door). The pitch servo will move the stick, thats normal. Fight the stick while it's moving to see (hear) the motor running in the tail. What you are doing is exceeding the pressure switch in the servo to tell the computer to activate the Pitch Motor. You can hear the same motor by hitting the hat switch but remember you are checking the AP. Don't confuse the sound of the Aileron trim cartridge in the wing with the tail. Your shop may be able to open the AP and check the fuse. The fuse could be good and the drive circuit could still be bad. Voltage checks can be done easily as there is an access panel in the tail under the elevator. There is also a relay that switches between the AP computer and the manual hat switch that can fail. Change or swap this relay to check it.

Autopilot roll oscillation (wiggly stick)

This is very common in Cirri, but not normal. Before you worry, check a few things:

  1. Is your rudder exactly centered with the airplane on the ground (look at the forward end of the rudder cap). (It should be within 0.1")
  2. Were you flying with more than a 10 gallon imbalance between the tanks at the time? (If not, try again with balanced fuel.)
  3. Is the "ball" centered exactly in normal cruise flight? (if not, you probably have a rigging problem)

Any of the above can contribute to a roll oscillation.

Also perform the simple Aileron trim test which is:

  1. Trim airplane for straight & level and hold that position.
  2. Insert FULL aileron trim in one direction and then let go and measure how long it takes to reach a 30 degree bank.
  3. Re-establish straight & level, and repeat step 2 for the opposite direction. If the roll trim is perfect, the two times are the same. A small difference is OK.

This procedure will tell you if the roll trim tab is mis-set. Do not, however, adjust the trim tab unless you answered questions 1 and 2 correctly.

General recommendations

Autopilot troubleshooting is very complex. Don't let anyone "tweak" with the airplane unless they are following the Cirrus autopilot troubleshooting checklist.

Don't get caught up in the "maybe it's the _______" game. Don't "try" replacing the _______ because someone else reported that worked for them. Don't try to point your shop in the right direction. (If the shop needs YOUR help, they're in over their heads!) Cirrus has published a detailed autopilot troubleshooting guide. There is a reason the autopilot troubleshooting documentation is 48 pages long - this is a tough job.

The reality is that the autopilot is a very complex system that needs a highly skilled and disciplined approach to troubleshooting. Many Cirrus Service Centers simply don't have the ability to do this - even though they are quite capable of competently replacing parts and fixing less complex problems.

It's very important that you take a by-the-book approach to solving your autopilot problem. If your SC/avionics shop has already gone through the entire troubleshooting procedure and come to a dead end, they should get Cirrus involved. If they have not, they are wasting your time and money.

If your aircraft is still under warranty, your SC needs to get Cirrus approval for going through your aircraft step-by-step until they find the problem and fix it. Under warranty. Do not assume Cirrus will pay for "trying" things based on your suggestion unless they approve ahead of time.

You just need a shop that has the willingness to RTFM (Read The Freaking Manual) and capability to do the work. If that's not the shop you are using, Cirrus can point you to one that can.

A/P Disconnects (DFC90)

Autopilot disconnects in cruise flight are very often caused by worn out servos, typically the roll servo. If the start-up voltage for the servo is higher than 2.2 Volts the DFC90 autopilot will not work properly in many cases. The roll servo will have to be changed.

  • Remove left wingtip, attach to wing structure with zip ties (this way you do not have to open all electrical connections)
  • Remove left aileron
  • Remove inspection cover in front of left aileron
  • Remove and replace roll servo

This work should be done by an experienced A/P, because the correct installation of both the servo and the aileron (safety wires) is critical.

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