Helicopter DAFCS Problems?: Ask the right questions first

What to ask the flight crew when problems arise.

Were you in ATT or SAS?

Up to now we've looked at flight director issues when coupled to the autopilots, but what about when the flight director isn't being used? If you're not using the FD, you're down to the two basic modes of autopilot operation, ATT and SAS. In addition to knowing the difference between autopilots and flight directors, you need to know the difference between these two modes.

The ATT mode maintains the pitch and roll attitude of the helicopter, always returning the aircraft to the pilot-set attitude after a disturbance. It is typically used for "hands-off" flight and coupling to the flight director. The mode relies primarily on one sensor, the vertical gyro or AHRS (attitude/heading reference system), to provide real-time pitch and roll attitude. Regardless of gyro or AHRS, the attitude source is the most important sensor in any automatic flight control system.

The SAS mode is basically a rate damper in the pitch and roll axes and is used when the pilot prefers to be flying the aircraft "hands-on," which may be during takeoff and landing or where a lot of maneuvering is needed. Stability augmentation systems, in general, make the aircraft easier to fly by controlling pitch and roll attitude rates while in turbulence and by controlling the pitch and roll rate response to the pilot's input on the cyclic stick. Again, the attitude source is the primary sensor which the autopilot uses to control the aircraft rate response. However, to enhance the aircraft's handling qualities (or lack thereof) position sensors are installed to detect pitch and roll stick inputs. The SAS mode uses the stick inputs to provide the desired aircraft response. One of the big differences between SAS and ATT is that ATT almost never uses stick position.

Now that you know the difference, so what?

Well, at some time, you may hear a pilot comment that the helicopter doesn't "feel" right. Vague? You bet. So you ask for more details and find out that it's only during takeoff and landing and that the SAS mode was being used. Now you ask, "How does the ATT mode perform?" The pilot says "no problem, just fine."

Since the SAS and ATT modes use the same attitude source, it should make sense that if the source was faulty, both modes would be affected. However, since only the SAS mode was squawked, it's not the attitude source, it's somewhere in the stick position sensor or signal. Now you just need to know whether the problem was in the No. 1 or No. 2 autopilot and whether it was in the pitch axis or roll axis, which can be learned by flying with only one AP engaged. If you're lucky, the pilot already did this.

Once you get the symptoms isolated to one mode, one autopilot, one axis, and one sensor you should be able to find and fix the problem.

Other questions

Depending on the symptoms, there are dozens of other questions that might have to be asked. Were any flags in view? What did the actuator position indicator show? Can you reproduce the symptom consistently? What was your airspeed and altitude? Is the problem associated with any other system?

Coming up with the right questions isn't always easy, but the more you know about any system, and how to properly use it, the easier it will be and the more natural it will become. AMT

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