Aircraft Flight Control

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Aircraft Flight Control

Aircraft in flight require adherence to a specific course for long periods of time, which can be tiring for the pilot. Likewise, steering ships on the water for hours is tedious and leads to crew fatigue. For more than a century, ships have had auto-pilots or devices that maintain a heading without human operators. Controlling an aircraft is similar but more complex because an aircraft operates in three dimensions and travels at considerably higher speeds. In the air, small deviations can become large errors more quickly.

The automatic control of aircraft was not considered until aircraft were capable of practical long distance flight. In the 1920s when radio navigation became available for aircraft, air travel became practical, eventually leading to the development of auto-pilot systems for aircraft. The first aircraft autopilots were based on gyros and were similar to those found in a ship. Such simple auto-pilots were capable of maintaining a set heading, but did not ensure the aircraft would follow a specific course. Heading is just what the word implies, the direction the aircraft is heading, which is not always the same as the path the aircraft actually flies. That path is called its "track." Its "course" is the desired path for the aircraft to fly. Because the medium in which aircraft fly, air, is not still, winds will blow the aircraft around in a somewhat unpredictable manner and the track and course may not agree. Flight crews needed to determine how much to change the heading in order keep the aircraft on course. The crew also had to adjust the power, ailerons , and elevators to prevent the aircraft from gaining or losing altitude as well as keep it flying level.

When radio navigation and its accompanying electronics came along, auto-pilots became more sophisticated, allowing the auto-pilot to follow a radio beam. Auto-pilots could keep the aircraft at a constant altitude and keep the wings level as well.

Auto-Pilot Operation

An aircraft is said to have six degrees of freedomup-down, left-right, forward-aftand can rotate around three axesroll, pitch, and yaw. Roll is rotation around the forward-to-aft line of the aircraft. Pitch is around the axis of the wing, and yaw is around the vertical. Auto-pilots used for en route travel can completely control an aircraft by changing only three of the six degrees of freedom: roll, pitch, and yaw. The most capable autopilots can actually land an aircraft, control engine power, and manipulate all six degrees of freedom.

An auto-pilot requires input information to control the motion of an aircraft. Gyros are used to sense motion while an altimeter senses altitude. Radio receivers provide information relative to the desired course of an aircraft. When the more sophisticated long distance navigation information is available, an aircraft may be controlled on a course to a specific point called a waypoint. Essentially, an aircraft could be programmed with a destination and automatically flown to that destination. When an auto-pilot reaches this level of sophistication it is called a "flight management system" (FMS). At the heart of an FMS is a "flight management computer" (FMC).

Microprocessors in Auto-Pilots

Although many auto-pilots existed before the introduction of the microprocessor, they were difficult to implement and required each aircraft type to have its own model auto-pilot. This is because each aircraft has its own characteristics and the auto-pilot reacts differently for each aircraft type. A programmable auto-pilot was the ideal solution and a perfect application of the microprocessor.

With the introduction of the microprocessor, an auto-pilot or FMS could be found even in small, private aircraft. However, the microprocessor also brought new challenges. The microprocessor must perform a series of operations, some of which jump from one address location to another and then return while keeping track of the entire process in what is called a stack. On some occasions, because of a software or hardware problem, the computer loses track and executes a bogus program. This situation can have catastrophic results. Just as personal computers sometimes "hang up," requiring a reboot, aircraft computers can similarly hang up, potentially leading to a disaster.

Therefore, every piece of equipment that is installed in an aircraft registered in the United States must be certified by the Federal Aviation Administration(FAA) and similar agencies in other countries. All automatic flight control equipment must demonstrate that it is "fail-safe." This means that if the computer has a failure, the auto-pilot does not place the aircraft in a dangerous situation. It was difficult for the early microprocessor-controlled autopilots to demonstrate this characteristic.

Aircraft certification, until the introduction of the microprocessor, meant hardware. However, when microprocessor-based equipment appeared in aircraft, it became clear that software, like hardware, could also fail with serious consequences. Something needed to be done to insure the integrity of the software. The FAA developed certification procedures for software, one of the first organizations to recognize this need.

Until then, while the microprocessor was revolutionizing the world, analog auto-pilots were the mainstay in aircraft. Eventually, the new procedures adopted by the FAA to certify software were implemented and digital auto-pilots using microprocessors became common. The first microprocessor auto-pilots introduced in the late 1970s were programmed using assembly language . This was necessary because of the small size of read-only memories of the time. It was not uncommon to find memories as small as 4K bits (not bytes). It was not long before 64K-bit ROMs were available, but by modern standards that is very small. Also, many of the functions performed by the microprocessor involved switching the state of one bit of a port . Assembly language provided the closest connection to the actual inner workings of the hardware.

Flight Management Systems

In the late 1970s, the long range navigation system LORAN-C became fully operational. This navigation system provided aircraft position in latitude and longitude. The desired course was set by entering waypoint information, and deviation from the selected course was provided to the auto-pilot. This development led to the integration of the auto-pilot and the long distance navigation computer into one unit, and later, the integration of other navigation systems, such as short distance navigation and landing systems, into the auto-pilot as well. This is the concept behind a flight management system or FMS.

The FMS controls the aircraft and the navigation equipment. It contains databases for airways, airports, radio frequencies, and radio navigation aids. With the FMS, a desired course is entered and the FMS will automatically select the navigation equipment and radio frequencies. With so much stored data, changes occur regularly. Therefore, it is necessary to update the databases periodically to reflect the changes using a CD (compact disc), floppy disk, or a notebook computer.

see also Airline Reservations; Navigation; Satellite Technology; Telecommunications.

Albert D. Helfrick


Billings, C. E. Aviation Automation: The Search for a Human-Centered Approach. Mahwah, NJ: Lawrence Erlbaum Associates Publishers, 1996.

Garrison, Paul. Autopilots, Flight Directors, and Flight-Control Systems. Blue Ridge Summit, PA: TAB Books, 1985.

Tischler, Mark B., ed. Advances in Aircraft Flight Control. London: Taylor and Francis, 1996.