Autopilots and Automatic Flight Control System Troubleshooting and Maintenance

Closeup,high,detailed,view,on,engine,power,control,and,otherAutopilots and especially Flight Control Systems are critical system components. They often require maintenance too, just as airframes and engines need inspection and servicing. They sometimes wear out and fail. They are sometimes used improperly by the pilot/operator leading to perceived poor performance too. Some of the more common instances we encounter for Avionics Support are covered here.

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Intermittent problems are often the most difficult to troubleshoot and address. Imagine getting a pilot/operator report such as “Carl says problem intermittently happens while transitioning from cruise to approach mode but only when it is sunny and the temperature on the ground is at least 55 degrees but otherwise works great when Bob or Jane are flying”. Obviously, this is something that is not easily duplicated on the ground in the hangar as it will work fine when technicians look at it.

Even with some pilot/operators providing lots of details, possibly even including example videos taken in operation highlighting the issue, Technicians often need to conduct a safety check and perform an evaluation functional check flight themselves to get a true feel of all parameters that induce the issue.

Pilot Misuse Issues

Take the example above – Carl reports a problem with some good details, but the system is fine when Bob or Jane is flying. Is it possible Carl has more experience with this autopilot / AFCS and more in-tune when something does not seem to be working correctly? Is it possible Bob and Jane are more experienced pilots and are better at keeping the aircraft in trim during major flight transitions which autopilots / AFCS can have a hard time keeping up with if simply engaged as Carl may be doing?

In this case, there may be nothing wrong with the autopilot at all. It is highly possible to be misuse or an expectation issue. We find this very common on transition from cruise to approach, vectoring if in the improper mode selection, too steep of an approach course intercept angle, intercepting glide path too late into the approach, or even variations in aircraft operation such as putting the landing gear down before flaps or putting the flaps down before landing gear. Lots of variables that are often pilot induced.

Servo Motor Issues

There are several varieties of autopilot servo motors, with some designs being more robust and reliable than others. But at the end of the day, a servo is a servo and is prone to wear and age just as any electromechanical item is. Analog servos often require troubleshooting. Some newer digital-based servos provide built-in-test-equipment (BITE) capabilities and can even report difficulties in an error log.

The good news is servos are typically designed in a manner so that an internal electromechanical failure does not induce a flight control jamming issue. We find clutch failures, motor brush deterioration, aging/infrequent use, and even digital logic lock-up (computer failure) to be the most common factors. Keep in mind that some servos are hard mounted in the airframe in such a manner that removal/replacement involves control cable rigging.

Airframe Flight Control Rigging Issues

Autopilots and Flight Control systems interface directly with the primary flight controls. This means that proper aircraft rigging is critical to aircraft safety, proper operation, and system performance. We often get operator complaints of laggy system response. While well enough to hand-fly, or in some cases what has always seemed normal to the aircraft pilot/operator, we often find the entire aircraft flight controls being slightly out-of-rigging which exacerbates poor autopilot performance.

Another contributing factor is servo installation – an improperly installed unit can have excess friction binding or in some cases even mechanical rubbing on adjacent components. While rare, we have seen it happen from systems installed by other entities. Often these matters are either not noticed or miniscule when the system is first installed but over time wears more and more leading to either a more mechanically visual issue or wear enough to the point it can be felt by the pilot through flight controls.

The last item, and one of the most common, is servo bridle cable clamp slippage and/or improper tensioning. Bridle cables, which are spooled to the servo, attach to the aircraft primary control cable so that the main control cable can be moved back and forth by the servo. If the bridle cables loosen, either due to improper installation, slippage, or even airframe fuselage contraction and expansion due to temperature fluctuations, it can cause the system to be slow in response to commanded inputs. This often results in the aircraft having pitch and roll oscillations in flight when the autopilot / AFCS is engaged.

Wiring Issues

We do see intermittent wiring issues with autopilots and AFCS. Improper original installation, while working for a while, can eventually lead to intermittent continuity due to either chafing or vibration. We find this to be most common in trim and disconnect switches on the yoke as they are subject to vibration, being bumped into, and stretchy coil-cords from yoke to panel being a high wear item.

While rare, it is possible for a system to even be mis-wired but not caught on the ground until an error is seen in flight. This is why it is critical to have autopilot / AFCS maintenance performed by a trusted and competent provider that not only knows how to install/maintain but how to operate the system as well.

Software glitch issues

Many will recall the Boeing 737 MAX fiasco induced by improper pitch-trim commands. Improper sensory input or computer/software logic inducing un-commanded pitch-trim action can be fatal – all factors that sometimes have nothing to do with the pilot or installer. A very recent example is a service bulletin issued by a major well known aviation electronics brand: in summary, an original halt entire system use or disable the pitch-trim servo until further notice. Eventually the manufacturer was able to fix the glitch via a software update.


Autopilots and AFCS can often provide years of trouble-free low maintenance use. But if not, the results can be from minor frustration all the way to a fatal catastrophe. It is critical to be cognitive of how the systems are supposed to function, any associated recommended routine servicing/inspection, and having the right team to help you in that process.

Did you know that the Avionics Department at First Flight Aviation has tremendous experience servicing, troubleshooting, installing, and even flying these systems? Let our Experts assist you in maintaining, updating, integrating, or educating you on your specific needs. The Avionics Team can be reached from the company website: Contact Us

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