Stability Augmentation

Stability augmentation is the artificial improvement, generally by electromechani­cal feedback systems, of airplane stability and control while the airplane remains under the control of the human pilot. Stability augmentation generally changes the airplane’s stability derivatives and modes of motion.

We make the important distinction between stability augmentation, artificial feel systems, and airplane automatic pilots. While artificial feel systems, discussed in Chapter 5, may alter stick-free stability for the better, their main function is providing manageable control forces. Automatic pilots replace the human pilot when they are in use.

20.1 The Essence of Stability Augmentation

To be a true stability augmenter, the device must change the airplane’s flight characteristics without the pilot’s perception. This means that augmenter outputs must add to those of the pilot in a series fashion. Augmenter outputs put into the primary control circuit between the cockpit and the control surfaces must move only the control surfaces, and not the cockpit controls. The requirement to not move the pilot’s controls is sidestepped if the augmenter is not inserted into the primary control circuit but moves a separate, or dedicated, control surface. Still another way around the need for augmenters not to move the pilot’s controls is the integrated control surface actuator (Chapter 5), used in fly-by­wire control systems. Integrated servo actuators accept and add electrical signals from both cockpit controls and stability augmenters.

In fly-by-cable control systems, isolation of primary-control-circuit stability augmenter outputs from the cockpit controls is a surprisingly difficult mechanical design problem. Control valve friction in control surface actuators acts to hold the surfaces fixed for small stability augmenter signals. When this happens, the augmenter in effect backs up and moves the cockpit controls instead. The result is an unaugmented airplane for small disturbances and limit cycle oscillations, such as yaw snaking. One cure for excessive valve friction can be as bad as the small signal backup problem. This is to center the cockpit controls with husky spring detents, which have to be overcome by the pilot in normal control use.

The degree of authority of stability augmentation systems is another important design consideration. Since augmenters operate ideally without moving the pilot’s controls, the pilot will be unaware of abrupt failures to the limit of augmenter authority until the airplane reacts. Then, there should be enough pilot control authority left to add to and cancel the failed augmenter inputs, with something to spare. This was the design philosophy until the advent of redundant, self-correcting augmentation systems, which make feasible augmentation at full authority or control surface travel.

Automatic pilots, which replace the human pilot when they are in use, are expected to move the cockpit controls. Abrupt full autopilot failures are instantly apparent to an attentive flight crew. Larger control authority than for stability augmenters is feasible, even for systems without the redundant, self-correcting feature.