Saturday, June 1, 2013
Automatic Flight Control Systems
Helicopter drivers usually start out their lives in very basic machines limited in power, and with no help when it comes to commanding the aircraft’s petulant tendencies. The green pilot must sense every rotation, translation, and acceleration and react accordingly to produce the desired flight condition. This can make for a very tired pilot at the end of the day.
As we gain skill and experience and move on to larger, more capable aircraft, we are rewarded with the addition of automatic flight control systems (AFCS) designed to lower workload by producing a more well-behaved flying machine. The term “AFCS” covers a list of devices used in various ways to alter or improve aircraft stability and handling qualities, or to permit certain parts of a mission to be flown automatically. It can be considered the portion of the flight controls that are moved automatically by a device other than the pilot.
There is a usual hierarchy to any AFCS (see figure); and although the acronyms describing the different levels can vary, the hierarchy can be divided into an inner and outer loop. The inner loop is primarily governed by sensors monitoring internal conditions directly related to the helicopter, such as roll, pitch, and yaw attitudes, rates, and accelerations. The outer loop deals with conditions external to the helicopter such as airspeed, altitude, and navigational information.
Beginning at the innermost level of an AFCS, you will usually find a stability augmentation system, or SAS. The job of the SAS is to provide short-term “rate damping.” Disturbances such as wind gusts are detected as roll, pitch, and yaw rates through the use of gyros, and the AFCS quickly commands an actuator to move a control surface to counter it. Just like a child on a swing who is pumping their legs out of sync with the pendulum motion, after a few moments the oscillation is brought to a stop. SAS actuators are usually in series with the pilot’s controls, and typically don’t move them, so their motion is transparent. These types of actuators tend to be fast-moving, and have limited authority. SAS ultimately makes the helicopter less “twitchy.”
In its basic form a SAS cannot tell the difference between a pilot-induced disturbance through use of the controls, or a wind gust. This can actually end up making the helicopter handle sluggishly. The stability and control augmentation system, or SCAS, is the next level in the AFCS hierarchy, and addresses the problem of a pilot’s control inputs being damped by the SAS. In this case, when a control movement is sensed, it is fed forward into the AFCS, causing the initial movement of the SAS actuator to be in the pilot’s desired direction. This “rate quickening,” as its called, enhances aircraft feel to the pilot.
The last level in the inner loop employs attitude stabilization equipment, or ASE. At this level, through the use of rate or attitude gyros, the system will produce control inputs to restore the actual roll, pitch, or yaw attitude that had just been disturbed. This is especially helpful to the pilot that finds themselves in the clouds, as a SAS or SCAS will not really help keep them right side up.
Moving to the outer loop of AFCS hierarchy, we finally get to what is considered an “autopilot.” At this level, the system sensors work to maintain desired airspeed, altitude, and sideslip. This is commonly done with the use of actuators placed in parallel to the pilot’s controls. They are generally slower moving, but can have up to full authority to move the control surfaces as necessary to maintain the desired flight condition. When you see the flight controls moving eerily by themselves, it is due to the AFCS commanding the parallel actuators.
The highest level and outermost loop of an AFCS is the “operational autopilot.” It is at this level that the system can command higher functions such as point-to-point navigation or automatic tracking of an instrument landing system (ILS).
This basic description of an AFCS would not be complete without mentioning the force trim or artificial feel system, which aids in holding the controls in a desired trim condition, and is a necessary part of most automatic flight control systems. It “anchors” the cockpit side of the system and lets the SAS do its job. With the trim system turned off, the pilot becomes the anchor. The best AFCS have excellent “fly-through” characteristics, in other words, the pilot can get on the controls and make changes to the aircraft’s flight path/condition without having to fight or disengage the AFCS. That said, having an understanding of your automatic flight control system, all its capabilities, and all the possible failure modes still remains an essential part of any pilot’s knowledge base.