The dictionary defines an autopilot as “a mechanical, electrical, or hydraulic system used to guide an aircraft without assistance from a human being.” The words, without assistance from a human being, convey to the naïve that when the autopilot is on, nothing is required of the pilot. A pilot displaying this behavior reveals his ignorance of the operational characteristics and limitations of a two-axis autopilot, and it is up to the instructor when first introducing autopilot concepts to ensure that the student understands this. The word “autopilot” is somewhat of a misnomer because a two-axis autopilot does not automatically control all aspects of flying. With the autopilot on, the pilot still is required to perform several flying tasks.
Think of it like this: a two-axis autopilot, which controls roll and pitch, shares aircraft control with the pilot. Even with the best two-axis GA autopilot operating, the pilot must make appropriate power and rudder control inputs. Consider a Nav III-equipped Cessna 172S in level cruise flight at 1,000 feet, at 110 KIAS with the power set at 2,300 rpm. An expedited climb to 5,000 feet is desired. The pilot inputs the new altitude into the autopilot and then activates the vertical speed mode to climb at a rate of 800 feet per minute. The autopilot pitches the airplane up to an attitude to maintain this rate of climb without considering the effect on airspeed. If the pilot does not increase power and apply appropriate right rudder pressure to keep the ball centered, and the autopilot pitches the airplane up too steeply to maintain the rate of climb, airspeed may erode precipitously and the critical angle of attack may be exceeded. This could result in a power-on stall with the nose to the left (ball to the right), and a spin to the left.
To avert this scenario, “workload sharing” dictates that while the autopilot pitches the airplane’s nose up and climbs at a preselected rate, the pilot must add power to control airspeed appropriately, and apply sufficient right rudder pressure to keep the ball centered. The pilot operating handbook for this airplane explicitly warns that when operating the Bendix/King KAP 140 two-axis autopilot “in the vertical speed mode with power set for climb and airspeed at or near best rate of climb, continued operation in VS mode can result in a stall.” Best rate-of-climb speed for this airplane is about 75 KIAS. While climbing in the VS mode following takeoff, a safer speed to consider may be about 85 KIAS.
As an alternative to using the VS mode, the flight level charge (FLC) mode may be set with the Garmin GFC 700 automatic flight control system to maintain a constant-airspeed climb. Suppose an autopilot climb to 3,000 feet is desired following takeoff in a Cessna 172 NAV III airplane. On departure the pilot pitches the airplane up to maintain airspeed at 85 KIAS and applies sufficient right rudder pressure to keep the ball centered. At about 1,000 feet agl the autopilot is switched On and the heading and FLC buttons are pushed, activating these modes. Pushing the FLC button can be thought of as issuing an airspeed hold command to the GFC 700, which pitches the airplane as needed to maintain 85 KIAS during the climb. Airspeed is maintained fairly constant, while the actual rate of climb is variable (the opposite occurs using the VS mode). At 3,000 feet, the GFC 700 captures this altitude, gradually pitches the nose to level flight attitude, and airspeed increases. Here, workload sharing means the pilot adjusts the throttle and mixture controls as needed, and applies appropriate rudder control pressure following the level-off.
Initial autopilot applications
An autopilot can be an invaluable aid to flight safety. When a pilot’s reference to the horizon is impaired—when flying in instrument meteorological conditions (IMC) or at night—the autopilot can be engaged to maintain proper attitude, heading, and altitude. Because an autopilot relieves the pilot from the high level of concentration required for manual instrument flying, instrument scanning becomes easier. An autopilot also allows the pilot to perform other tasks related to the safety of the flight like reviewing charts, clearances, instrument approach procedures, and weather information.
Noninstrument-rated pilots, as well as student pilots preparing to fly solo cross-country flights, should be taught how to use the autopilot to extricate themselves from inadvertent flight into IMC. Flight under these conditions predisposes to spatial disorientation, and eventual loss of aircraft control. Turning the autopilot on, activating the altitude hold and HDG functions, and then setting the heading bug to turn about 180 degrees from the current heading, is a method to safely fly out of IMC and back into visual meteorological conditions. This is a practical and potentially life-saving use of an autopilot.
In some situations it may be necessary to temporarily disable the aforementioned horizontal and vertical tracking modes with a function called control wheel steering (CWS). For example, suppose following takeoff during autoclimb to a designated altitude, several large birds appear suddenly in front of the airplane, requiring immediate evasive action. Pressing and holding the CWS button on the yoke temporarily disengages control outputs from the autopilot to the aileron, elevator, and elevator trim servomotors, permitting the pilot to control the airplane for evasive maneuvers.
An autopilot is designed to reduce, not totally eliminate, a pilot’s workload. To fully exploit all capabilities of an autopilot while operating safely, instructors should ensure that their students thoroughly understand the autopilot’s operational characteristics. With the autopilot on, the pilot must always be aware that the flying workload is shared with the autopilot, and have no doubts about what flying tasks are being performed by the autopilot and those that are the responsibility of the pilot.
Michael J. Banner is a professor at the University of Florida and an instructor pilot in the Civil Air Patrol. A CFII, he has 3,000 hours of flight time.
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