In an earlier blog, I - well, actually our Test Pilot, Chad Howard - answered a few questions on autopilot operations that came up at AERO Friedrichshafen. While operational questions were the majority, we did get a few pertaining to troubleshooting a particular issue called "pitch porposing."
It's not an easy question to answer. Because they are so integrated into the aircraft's airframe and avionics, autopilot troubleshooting can be very challenging. And it's something that's best left to the experts. So I asked our Field Service Engineer and resident expert, Al Gialousis, to share a few tips on tracking down this particular problem.
One of the more difficult autopilot system issues to its isolate to root-cause is a pitch oscillation, also known as "pitch porpoising." The reason is that there are a number of variables at work, and because differing pitch performance issues often result in the same situation. Since each component has a unique and subtle effect on system performance, getting an accurate description of the problem is a must.
In isolating potential sources of a problem, a good first step is to determine whether the issue is related to system "signal" or system "slop." For example, is the autopilot driving the oscillation or is the root cause is that the autopilot simply can't fly the aircraft correctly due to external forces?
If the aircraft is equipped with a flight director indicator it is one of the best tools used for determining the cause. When in FD mode, you have a visual reference of how the autopilot system would be flying the aircraft. Only doing so while the servo solenoids are disconnected allows the aircraft to be maintained in straight and level flight. Hence, oscillating command bars would indicate a system signal issue while stable command bars would indicate slop in the flight control system or pitch servo.
You will also have to identify exactly what type of pitch oscillation you are dealing with. Two of the most common are divergent oscillations and trim-induced oscillations.
Divergent oscillations, which get progressively worse in magnitude with each cycle, are generally an indication of a loose elevator cable, loose pitch servo bridle cable or high pitch servo start-up voltage.
Trim-induced oscillations are generally caused by pitch servo sensor switch torque settings. The easiest way to identify this problem is by simply turning off the Trim Master switch
Additionally, when dealing with an S-TEC System-55X, always confirm the unit is fully seated into the tray. It is not uncommon for incorrect hardware to be used, which inhibits unit seating. Also, ensure the connectors are securely fastened to the tray's back-plate and will not push back when mating with the unit.
Troubleshooting an oscillation requires gathering quantifiable data, determining a most probable cause, and isolating suspect components through testing & adjustment. Since many reported autopilot discrepancies are often related to the condition of the aircraft, confirming the aircraft's flight control system is rigged in accordance with the manufacturer's specifications for cable tensions and system friction is a great starting point. At the same time, the servo's bridle cable and start-up voltage can be checked. These simple steps will resolve many performance issues. When these steps do not resolve a pitch issue, additional troubleshooting guidance is available by contacting the Genesys Aerosystems Tech Support department.
Excellent insights Al, thank you. If you have a particular question about autopilot operations or troubleshooting, please send it along and I'll have one of the Genesys Aerosystems autopilot experts answer it.
Until next time, fly safely,