Troubleshooting Altitude Hold

TexasGyro | 24 September, 2015 11:52

The most common autopilot/flight director complaint is Altitude Hold. There are multiple systems within an aircraft the can affect altitude hold operation. There is the static system and all its plumbing, Air Data Computer or Altitude Can, pitch servo and capstan, trim servo, flight director computer, vertical gyro, and cable tensions.

This is written with 3 axis gyro based autopilot’s in mind, not rate based autopilots (S-Tec), although some of the same principles may still apply. This assumes that the wiring is good, but by following the steps you could determine where to start checking wires if that was the problem.

To know which system is causing the problem you should ask yourself the following questions?

Is there a pitch problem in any other mode of operation?
Does the problem only happen at higher airspeeds/altitudes?
Have you flown through precipitation recently?
Do all of your airspeeds and altimeters agree with each other?
In a dual flight director equipped aircraft does it do it on both sides?
Is my electric trim working properly?

The first thing to look at is the problem in altitude hold mode only, or does it appear in other modes?

In straight and level flight with everything trimmed out nicely (preferably in smooth air), engage the autopilot without any FD modes engaged. Do you see the same problem as you saw in altitude hold mode?

YES: The problem is not the static system, air data computer or the flight director computer. With the autopilot still engaged as described above, gently press against the yoke to affect and attitude change and see if the trim wheel moves in the opposite direction.

YES: Our problem isn’t the trim servo
NO: Does manual electric trim work?

YES: We have narrowed it down to the pitch servo, autopilot computer, or vertical gyro. How does the pitch servo effect trim operation you ask? In most systems the pitch servo provides a feedback to the autopilot computer letting it know how much force is being applied to the control surface. The autopilot then decides how much help to give the servo with offsetting trim. So if the feedback loop is bad, then no trim and porpoising occurs. If it is the vertical gyro then you might notice oscillations on your pilot’s horizon, or possibly slow erection. If you have two identical vertical gyros then try swapping them and see if your problem goes away, otherwise you might have to find an instrument shop to check the autopilot output of your VG. Determining if the problem is the pitch servo or the autopilot computer is a bit of a crapshoot without the proper breakout equipment. In my experience it is usually the computer.

NO: Bad trim servo

NO: Our problem is the pitch servo, AP computer, vertical gyro, Flight Director Computer, or Air Data Computer. For the pitch servo, AP computer, or vertical gyro see above. Do all other vertical modes operate normally i.e. Glideslope or VS?

YES: The altitude hold can is likely bad

NO: The Flight Director Computer is Likely bad

If you have flown threw precip, or left the aircraft outside during a monsoon, then you could have water in your static system. I have seen water cause all kinds of weird problems in an aircraft’s static system. Sometimes the problem will only arise at certain attitudes, or when below freezing. Typically you will see a variance between the two static systems (if so equipped) if there is a water/ice problem. If your aircraft is equipped with static systems drains, then have a qualified technician drain it for you and see if your problem goes away. If it doesn’t have drains then you might have to remove the main static line coming behind your instrument panel and blow compressed air through the system to dry it out. Make 100% sure that no components are left in the system prior to blowing air in, or serious damage could occur. Primary altimeters, airspeeds and Vertical Speeds as well as Air data computers, encoders, autopilots and standby instruments should all be out of the loop. If the problem seems to only happen at higher altitudes then the water could be freezing and creating a partial or total block of the line.

True story: I had a customer bring in a 414A complaining about the altitude hold on his 800B IFCS. We did a ground check of the system and didn’t find any problems, but went ahead and removed his altitude can and sent it for repair. The repair facility cleaned and aligned the can, we reinstalled it and he flew away. Several weeks later he was back complaining about the same problem. Again we did a ground check of the system and found no problems. We removed his pitch and trim servos as well as his attitude gyro and tested them. A few weeks later he was back, same problem. This time I enlisted the aid of a test pilot and without the customer on board we did a shakedown flight. Try as we may we couldn’t find a single problem with his autopilot, so I asked the pilot to come fly with me. We got to altitude and flew for 30 to 40 minutes without a single hiccup. I asked him to return to the airport as it didn’t look like we were going to see a problem. It was at this time the he informed me that it only happened when he got up and went to the back of the airplane to use the bathroom (single pilot and he was typically the only person on board). I guess no one ever explained CG to him.