Does this sound familiar? You head to the airport for a checkout in a new-to-you complex airplane, a high-performance single perhaps. You're mentally prepared to master the airplane, and you have already studied a copy of the aircraft flight manual that you bought from the FBO. The CFI gives what seems to be a thorough overview of the airplane, and the two of you proceed to the ramp for the preflight. As you strap yourself into the left seat, an unexpected wrinkle catches you off guard. Staring you in the face is an unfamiliar autopilot control panel that looks nothing like the model referred to in the generic flight manual you studied. You've never really used any autopilot before, and a bead of sweat appears on your brow. The astute CFI notices your apprehension and mutters something about it being recently installed, adding, "We won't be using that today." You breathe a silent sigh of relief and finish the checkout but go home inwardly worried that you still know little about how to operate this or any other autopilot.
Understanding autopilots means starting with the basics, and the first rule is easy. An autopilot is a useful, but not very bright, assistant (see " Out of the Pattern: Applied Avionics," April Pilot). It exists to help you manage the flight, period. Depending on the model, it can fly approaches, track an airway or GPS direct-to course, hold an altitude, or do any number of other useful tasks that you might ask of it.
With an autopilot you'll arrive less fatigued after long flights or bouts in the weather. Sometimes it even makes you look like a better pilot than you might feel on a given day. But despite what nonpilots assume ("you just let the autopilot fly that thing, don't you?") George can't think for you. In fact, his appalling ignorance can bite the unwary. While your aircraft is "on autopilot," someone needs to be minding the store.
Knowing how to use an autopilot begins with understanding all its possible operational modes as well as its limitations. During a coupled ILS approach, at what point must you disengage the autopilot? How soon after takeoff can you engage it? Can it be used to shoot a localizer back-course approach? Will it couple to that fancy new panel-mount GPS unit? If you lose an engine at cruise in a twin, can you safely continue to use the autopilot? Unless you don't intend to turn it on, no autopilot-equipped aircraft checkout is complete until you understand the autopilot's operation as completely as you do the aircraft itself.
The training scenario described above might have had a better ending if the CFI had had experience with this autopilot (he didn't), or if the FBO had advised you beforehand that an after-market autopilot had recently been installed. You then could have previewed the FBO's copy of the FAA-approved supplement to the airplane flight manual describing the autopilot's operation as installed in that airplane. Or you might have contacted the autopilot manufacturer directly for a copy of the pilot's operating handbook. In preparing for a recent checkout in a Beechcraft Baron, I found a free downloadable copy of the POH supplement for the autopilot installed in the rental aircraft. This generic manual, available on S-Tec Corporation's Web site ( www.s-tec.com), didn't contain aircraft limitations specific to the Baron, since the same autopilot is used in a variety of other aircraft. But it did give me a head start in understanding the system prior to my checkout.
Airplanes move around three axes of flight, but the simplest autopilots may manage just a single axis, as in the case of basic wing levelers. Other light-aircraft autopilots control pitch as well, and some also incorporate yaw dampers. Yaw dampers smooth out or "dampen" oscillations that result from an aircraft's particular degree of instability along the yaw axis. Their primary job is to help provide a smoother ride, especially in turbulence. Only the most sophisticated autopilots found on certain corporate- and airline-type aircraft take yaw control beyond mere yaw dampening. These can supply the proper rudder correction during an engine failure and assist in runway alignment during autolandings.
Depending on their complexity, autopilots have pitch, roll, and yaw computers that process signals from various sources to control aircraft movement. For example, S-Tec Corporation autopilots use the aircraft's turn coordinator as the signal source for the roll computer. A solid-state absolute pressure transducer and accelerometer supply the pitch computer inputs. According to the company, these signal sources eliminate the precession and acceleration/deceleration errors that are possible when depending upon the artificial horizon as the signal source. S-Tec autopilots are therefore rate-based rather than attitude-based systems. Failure of either the attitude indicator or the vacuum system will not affect a rate-based autopilot design. Another advantage is that a turn coordinator-style rate gyro will not tumble because of unusual attitudes.
The pilot issues commands through a control panel, part of every autopilot. These come in many shapes and sizes, but they all allow a pilot to select various modes of operation. During descent, for example, selecting the "vertical speed" pitch mode on the control panel tells the autopilot that its priority is to maintain a particular vertical speed. Once that vertical speed is established, however, it's still up to the pilot to set the appropriate power setting to maintain aircraft speed. Otherwise, George just might allow the aircraft to exceed V NE.
The pilot's commands are communicated through the computers to control servos, the muscles of the system. The servos, which are often electrically powered in light aircraft, are what actually move the aircraft's control surfaces to carry out the commanded action.
Autopilots are tested during certification to ensure that they do the job expected of them, and also to be sure pilots can safely recover the aircraft in the event the autopilot malfunctions. For instance, an electric-pitch trim fault that causes an autopilot to disconnect in an out-of-trim condition might result in altitude loss. This could be a real handful close to the ground. Based on the results of certification flight tests, autopilot usage may be limited below certain altitudes, in certain aircraft configurations, or above certain speeds. For instance, the Cessna 400B Nav-O-Matic autopilot installed in some Cessna 402 aircraft cannot be used with flaps selected more than 15 degrees down or at speeds greater than 225 KIAS. The flight manual advises that the possible altitude loss because of autopilot malfunction could be as great as 600 feet in cruise or 200 feet in the approach configuration.
Pilots also must be able to quickly disengage the autopilot should it malfunction. Many systems utilize a disconnect switch installed in the pilot control yoke or stick, as well as an alternate disconnect switch on the autopilot control panel. But switches can fail, so a good operating practice is to know ahead of time which circuit breakers can be pulled to quickly disable the autopilot.
As said before, autopilots can bite the unwary in various ways. The following are some actual events culled from the NTSB accident database, the first involving a Beechcraft King Air A100 at Manchester, New Hampshire, a few years ago.
While executing a coupled ILS approach to Runway 35, at about 100 feet above decision height the localizer bar went hard right, the glideslope flag came out, and the aircraft turned hard right. The pilot countered with opposite yoke and felt for the autopilot disconnect button on the yoke. The pilot managed to disconnect the autopilot and go around, but not before striking a light pole and damaging the aircraft. Investigation discovered that the glideslope signal was out of service.
Poor instrument competency and improper operation of the autopilot played a role in the fatal crash in California of a Beechcraft Baron, showing perhaps that an autopilot can't make up for a basic lack of proficiency. While cleared to intercept the localizer, the aircraft passed through the approach course, eventually striking rising terrain. As noted in the accident summary, the pilot's former wife recalled instances when he had improperly set the autopilot OBS; on those occasions, he had similarly deviated from the localizer at intercept and resisted her suggestion to take over and manually fly the approach, instead persisting in trying to make the autopilot fly it. A flight instructor who administered a flight review one month earlier found the pilot's instrument skills weak and refused to certify his instrument competency.
The ATP-rated pilot of a Beechcraft Bonanza wrote the book on how not to operate autopilots. On one occasion he attempted takeoff with the autopilot engaged — something expressly forbidden — and discovered the Bonanza "wanting to take off" at 30 knots. On another flight he found the pitch trim stuck in an 18-degree nose-up position following a 360-degree autopilot turn at 65 kt. The manufacturer prohibited autopilot operation below 85 kt because of just this possibility. After yet another flight he complained to the manufacturer that the autopilot had failed to return automatically to a previous altitude — something it wasn't designed to do in the first place. His luck nearly ran out on another flight when he became confused about whether the autopilot was engaged. According to the pilot, the airplane "kept wanting to climb" and he was having difficulty holding a heading. After he turned the autopilot on and off several times, the aircraft went out of control. The wings were severely bent and buckled in the subsequent recovery, but fortunately they remained attached.
Accidents have occurred when pilots became confused about which mode the autopilot was operating in. In a number of cases, pilots executing ILS approaches believed the autopilot was engaged in Approach or Glideslope modes (where the aircraft tracks the electronic glideslope and localizer) when in fact they were in Vertical Speed and Localizer modes, tracking the localizer but maintaining a set vertical speed instead of following the glideslope. For whatever reasons, these pilots failed to notice the aircraft descend below the glideslope before the aircraft crashed short of the runways.
There are many easy-to-imagine scenarios in which the autopilot is doing one thing while the pilot believes it is doing another. In the final analysis, George is much like the computer on your desktop: garbage in, garbage out.
Vincent Czaplyski holds ATP and CFI certificates. He flies as a Boeing 757/767 captain for a major U.S. airline.
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