Helicopter DAFCS Problems?: Ask the right questions first

What to ask the flight crew when problems arise.


The autopilot provides or enhances helicopter stability through its two basic modes: ATT (attitude retention) and SAS (stability augmentation system). With the exception of an "autopilot heading hold" feature in the SPZ systems, the autopilot knows nothing about holding such things as altitude, airspeed, or course.

On the other hand, the flight director is essentially clueless about stabilizing the aircraft, but does know how to compute pitch and roll attitude corrections to keep the aircraft on the desired lateral and/or vertical path when navigating between points A and B. These corrections from the FD, called "computed steering commands," are displayed by command bars (cues) on the Attitude Director Indicator (ADI) to "direct" the human pilot to fly the aircraft up, down, left, right, or some combination thereof.

Compared to the AP's two basic modes, flight directors have a number of modes such as HDG (heading select), ALT (altitude hold), ALTPRE (altitude preselect), IAS (indicated airspeed hold), NAV (navigation) and many more for use during en route, approach, SAR, or hovering.

What were you "coupled" to?

Many years ago autopilots and flight directors had no interaction, however, today they do, which simplifies the flying task tremendously. This is accomplished by taking the steering commands from the flight director and inputting them to both the ADI and the autopilot, which is called "coupling." On the SPZ systems coupling (which is only possible during AP ATT mode) is indicated by a CPL "ON" annunciation on the AP controller. If desired, the pilot can "uncouple" the FD commands from the AP and manually steer according to the command bars, while the AP returns to maintaining pitch and roll attitude. Uncoupling the FD commands from the AP is sometimes useful to compare the basic performance of the autopilot to the behavior of the command bars. For example, if the aircraft is stable and the command bars are wandering, the basic AP is most likely OK but something isn't right with one or more parts of the FD system, such as sensors or wiring.

If the flight crew squawks problems with one or more FD modes, you'll need to know which ones were being used to isolate the possible causes. For example, if the pilot writes up "porpoising when coupled to ALT" you would investigate the pitot-static and air data systems and other potential contributors to porpoising, but before you start tearing things apart you might have to ask the next question.

Did you try the other FD?

This is the first question that should be asked when a pilot reports FD mode problems in a helicopter that has a dual SPZ system, which has two full-time autopilots and two flight directors, FD1 and FD2. While modes can be selected on both FDs, only the "active" flight director, as annunciated on the FD1/2 or FDSEL button, can be coupled to both autopilots at one time.

Dual systems have a great advantage of dual AP servos and, in most cases, fully dual sensors (gyros/AHRS, air data, radios, etc.) which facilitate "box swapping." For example, if a problem is reported while coupled to a specific mode (or modes) using FD2, the first thing you want to know is how the mode behaves using FD1. If FD1 operation is satisfactory you would then swap the appropriate sensors for that mode from side to side. If the problem follows a suspected sensor from FD2 over to FD1, it's a no-brainer, replace the sensor. Don't forget, the problem might be in FD2 itself, so you may need to swap the flight control computers (each which contains an AP and a FD). If the problem doesn't follow a box you'll need to look elsewhere, usually in the wiring.

In some installations a single sensor, such as an encoding altimeter, is wired to both computers. Be careful! This has several possibilities. If the mode performance is equally bad on both sides, your first inclination would be to suspect the single sensor that supports the mode, and rightfully so. However, if the wiring from that sensor is routed to a splice block, which then divides and routes the signals to each computer, you could be dealing with wiring problems between the sensor and the block or the block itself. If the mode performance is bad on only one side, and the sensor has only one set of outputs, don't blame the sensor. The problem may be in the splice blocks, the wiring from the blocks to the computer, or the computer itself.

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