AFCS Problems

A phrase familiar to software people goes like this: Garbage in equals garbage out (GIGO). This means that if the input data or the software itself was bad then the output of the program will be incorrect. If you haven’t already freaked out thinking this is an article about software, you might be wondering how this applies to the automatic flight control system (AFCS) in your aircraft, especially if it’s an older analog system that has no software.

In reality, GIGO applies to many things beyond computers, including life, but we’ll limit this discussion to AFCS problems.

When we’re talking about automatic flight control systems we’re talking about autopilots (APs), which provide stabilization and hands-off flight control, and flight directors (FDs), which provide navigation steering commands to the flight instruments and can usually be coupled into the autopilot for fully automatic flight path control.

To do the job properly all automatic flight control systems rely on various line replaceable units (LRUs), from the flight control computer (FCC) itself to the sensors that supply the input data. Gyros, AHRS, air data sensors/computers, accelerometers, servo position devices, navigation radios, and other LRUs provide specific inputs to drive the various functions of the system. Therefore if a particular function of the system isn’t working, many AMTs’ typical courses of action is to first replace the flight control computer (though this might be premature). If the new computer doesn’t solve the problem, then it’s time to look at the particular sensor(s) that feed that particular function and replace that LRU also. While this procedure may solve the problem, there will be a time, sooner or later, when you run out of boxes to replace and now you’re faced with having to dig deeper to find the true cause. What do you do now?

If the problem isn’t the LRUs then you would rightfully suspect it’s probably somewhere in the appropriate wiring. So you get out your wiring diagrams and multimeter, and start ringing out the wires, checking for continuity, short circuits to other wires and short circuits to the airframe, and find nothing wrong. Now what?

Powers and Grounds

Something that gets overlooked many times, are the various (and numerous) powers and grounds that avionics require to make everything work properly. This is where GIGO still applies because if the power is garbage going into a device, the output of the device (regardless of what it is) will most likely be garbage coming out. Some of the most familiar power problems include AP drop-offs, sloppy (loose) AP and/or FD performance, nuisance error codes, and more.

Regardless of vintage, avionics require voltage, current, and frequency (for AC systems) that is clean (noise-free) and stable (though it tends to be even more important for today’s digital avionics).

Good grounds are essential to proper performance so don’t forget about them. Many times if you’re having strange problems that you can’t seem to fix, it usually ends up being a bad ground. Many times when I’m providing troubleshooting assistance I’ll ask the mechanic to find and clean the appropriate ground blocks. It doesn’t always solve the problem but it certainly couldn’t hurt.

Aircraft Power Comes First

Power on the aircraft comes from sources such as batteries, generators, or alternators. Depending on the equipment installed in the aircraft, there may be additional devices such as inverters, transformers, and rectifiers to provide suitable power to the avionics and other systems. When a number of different aircraft systems are having problems, it is likely that something is wrong with the main aircraft power which, if you’re lucky, may be visible in the cockpit on the voltmeters, ammeters, or caution/advisory panel. If you suspect power problems check the aircraft sources first to make sure they are working properly. This includes communicating with the flight crews to find out if they’ve seen anything unusual. If you’re satisfied that the aircraft power system is good then go further downstream.