Among all the things that can break in an airplane, autopilots are the most perplexing and potentially expensive, short of the engine.For years, they labor without complaint, dutifully holding heading and minding altitude. Then all of a sudden, the pitch control goes a little crazy or the needle tracking gets sloppy. Maybe its just a corroded connection or a tired servo. Then again, maybe not. Autopilots have a deserved reputation for being difficult and expensive to repair. At some point, you have to ask yourself if the box is worth fussing with one more time or replacing.
The truth is that late model autopilots-the Bendix/King KAPs and KFC or the Century 41s-are almost always worth repairing because replacing them is expensive and you wont get additional reliability. With older models-Pipers AutoControl series and so on-that may not be the case.
Many autopilots have characteristic failure modes. If your hangar mates Century 41 had a trim switch failure and you have the same vintage autopilot, don’t be surprised to have the same problem.Heres a short list of the bad things that can happen to popular autopilots and what to do when its your turn, plus a primer on autopilot basics.
How They Work
In general, there are two types of autopilots: Attitude autopilots and rate-based autopilots. Attitude APs use an attitude gyro with electric pick offs for roll and pitch sensing. Obviously, the attitude gyro is the critical component.
How is your attitude gyro driven? Is it electric or vacuum? Both types are used. Consider your plight, then, if youre in IMC with autopilot engaged and the vacuum pump dies.
If the system has heading track mode-as most do-the heading command signal from the directional gyro or HSI directs the autopilot to steer the aircraft to a specified heading.
Navigation tracking information is generated by a common nav source, such as a VOR/LOC receiver and or GPS or loran receivers output to the autopilot computer, which contains the electronic circuitry necessary to interpret autopilot command signals.
These command signals are sent as control voltages to the servos, which actually move the control surfaces.
Attitude-driven systems have a rep for smooth and precise rides but can be expensive to maintain because of high component costs and the fact that they have more parts. The bill will be even higher if you have a flight director.S-Tec has championed and done we’ll in the market with the rate-based autopilot design, in which a turn coordinator is responsible for roll outputs. An internal rate gyro sends a signal to the flight computer providing standard rate turn command for the roll axis.
The DG/H S I is responsible for heading track, just as in the attitude system. An absolute pressure transducer plumbed into the aircrafts static system handles the pitch axis and/or altitude hold function, sending a signal through the flight computer and then to the servo. Some rate-based autopilots have just pitch control, others have both pitch and altitude hold.
In general, field experience suggests a turn coordinator will outlast an artificial horizon, thus making a rate-based system cheaper to maintain and to install.
Some argue that a rate-based system doesnt offer as smooth a ride, since the turn coordinator bounces in turbulence. This is debatable and may have to do with the condition of the instrument.
A rate-based system probably offers more redundancy, since a vacuum failure has no effect on the electric turn coordinator operation. On the other hand, if the TC crumps, youre still toast.
And When They don’t
As with most aircraft systems, lack of use can promote component and overall system failures. Take for example an early model Piper Cherokee with even the most basic wing leveler autopilot.
We see the following scenario often: A customer purchases an airplane and advises that the autopilot is inoperative. After a few qualifying questions, we determine that the aircraft has been idle for a long period. Now that its back in the air, the autopilot wont even engage. Would it work if it had been used regularly? Perhaps. Advice: If you have an autopilot, use it.
Misuse and abuse is another occasional cause of autopilot failures. Landing the aircraft with the altitude hold engaged on a regular basis is definitely hard on the pitch servo and trust me, we have seen this happen more than once. The same applies for forcing the airplane into rolls with heading or wing leveling engaged.
Your AP no doubt came with an operators manual, which is required in a POH supplement. Review it now and again, especially with regard to limitations and ground-test modes. Ground test is there for a reason and may explain anomalies.
It will also explain what many owners consistently don’t know: Which instruments on the panel are tied into autopilot operation and which arent. This knowledge is critical to understanding how your system may behave if it fails and it could save your butt in an emergency.
Characteristic Failures
Heres a short list of bad things that can happen to popular autopilots and what to expect when its your turn. Attitude Driven: All of the systems manufactured by Century Flight Systems (and Piper Autocontrol) are vacuum-driven attitude systems. System components are almost always identical but model numbers are slightly different.
Usually, the only differences between Century and Autocontrol components relate to STC applicability. Basically, its a numbers game but its important and sometimes sticky when trying to replace older system components.
It might not be as easy as you think to swap an Autocontrol component with a component stamped with a Century data tag, due to STC applicability.
The single most common failure of a vacuum-driven system is related to attitude gyro performance. A sluggish gyro will cause the AP to command correspondingly sluggish pitch and roll control and often, these gyros don’t fail at once but over a period of time.
Pay particular attention to the attitude gyro at engine start. Does it take longer than used to erect? Is it sluggish rolling into and out of turns? Its not unusual for an attitude gyro to show these symptoms intermittently before failing entirely.
Replacement with an overhauled or new gyro is often the first step. If there’s doubt, try a loaner before spending money chasing other components or wiring.
Older systems such as the Century 21 and 41, for example, have complex wiring harnesses that connect to and from the gyros, servos and flight computers. With age, these get stressed and chaffed and create intermittent failures.
The harness assemblies arent cheap to replace and finding the problem-if it is the wiring-is time consuming. Recent Century harnesses seem more rugged. Some of the more advanced Century systems utilize multiple computer/amplifiers, which can lead to mode failures in automatic pitch trim operation and nav tracking/glideslope.
Trim problems: If the airplane has electric pitch trim, the control-wheel mounted switch is connected to the autopilot through a harness assembly that can be time consuming and expensive to fix. It may be better to replace the harness assembly when replacing the switch, just to head off a wiring failure later.
And the electric trim system, with its servo and accessories, is often a major portion of the autopilot system. Servos arent a common failure item, compared to gyros, but they don’t last forever.
An older servo may show signs of failure by sluggish control response, causing wing rock or oscillation and possibly blowing through the selected heading while in heading track mode.
Some shops say servos should be routinely overhauled ahead of failure but I think this is a bad idea, and costly to boot. You could sink more money into the servos than the system is worth. I would be cautious of a shop that recommends this approach when all seems to function satisfactorily. Servo failures, while not necessarily rare, arent the most common fault, either.
Rate Based
Rate-based systems from S-Tec seem reliable and often easier to troubleshoot because of their simplicity. Theyre more likely to fail hard, not even engaging if there’s a problem.