automatic pilot

An autopilot is a mechanical, electrical, or hydraulic system used to guide a vehicle without assistance from a human being. Most people understand an autopilot to refer specifically to aircraft, but self-steering gear for ships and boats is sometimes also called by this term.

First autopilots

In the early days of aviation, airplanes required the continuous attention of a pilot in order to fly safely. As airplane range increased, allowing flights of many hours, the constant attention led to serious fatigue. An autopilot is designed to perform some of the tasks of the pilot.

The first aircraft autopilot was developed by Sperry Corporation in 1912. Lawrence Sperry (Son of famous inventor Elmer Sperry) demonstrated it two years later in 1914, and proved the credibility of the invention by flying the plane with his hands up.

The autopilot connected a gyroscopic attitude indicator and magnetic compass to hydraulically operated rudder, elevator, and ailerons. It permitted the aircraft to fly straight and level on a compass course without a pilot's attention, greatly reducing the pilot's workload. This straight-and-level autopilot is still the most common and least expensive type of autopilot.

In the early 1920s, the Standard Oil tanker J.A Moffet became the first ship to use an autopilot.

Modern autopilots

Modern autopilots generally divide a flight into taxi, take-off, ascent, level, descent, approach and landing phases. Autopilots exist that automate all of these flight phases except the taxiing. Landing on runway and controlling the aircraft on rollout i.e keeping it on the centre of the runway is CAT 3b landing, available on the majority of major runways today. Landing, rollout and taxi control to stand is CAT 3c. This is not usually used to date but may be used in the future. Some incorporate automated collision-avoidance; the most popular collision avoidance for aircraft is called TCAS (Traffic alert and Collision Avoidance System). An autopilot is often an integral component of a Flight Management System.

Modern autopilots use computer software to control the aircraft. The software reads the aircraft's current position, and controls a flight control system to guide the aircraft. In such a system, besides classic flight controls, many autopilots incorporate thrust control capabilities that can control throttles to optimize the air-speed, and move fuel to different tanks to balance the aircraft in an optimal attitude in the air.

Although autopilots handle new or dangerous situations inflexibly, they generally fly an aircraft with a lower fuel-consumption than a human pilot.

The autopilot reads its position and the aircraft's attitude from an inertial guidance system. Inertial guidance systems accumulate errors over time. They will incorporate error reduction systems such as the carousel system that rotates once a minute so that any errors are dissipated in different directions and have an overall nulling effect. Error in gyroscopes is known as drift. This is due to physical properties within the system be it mechanical or laser guided that corrupt positional data. The disagreements between the two are resolved with digital signal processing, most often a six-dimensional Kalman filter. The six dimensions are usually roll, pitch, yaw, altitude, latitude and longitude. Aircraft may fly routes that have a required performance factor, therefore the amount of error or actual performance factor must be monitored in order to fly those particular routes. The longer the flight the more error accumulates within the system. Radio aids such as DME, DME updates and GPS may be used to correct the aircraft position. Inertial reference units, i.e. gyroscopes, are the basis of aircraft on board position determining, as GPS and other radio update systems depend on a third party to supply information. IRU's are completely self-contained and use gravity and earth rotation to determine their initial position (earth rate). They then measure acceleration to calculate where they are in relation to where they were to start with. From acceleration one can get speed and from speed one can get distance. As long as one knows the direction (from accelerometers) the IRU's can determine where they are (software dependent).

Computer system details

The hardware of a typical autopilot is a set of five 80386 CPUs, each on its own printed circuit board. The 80386 is an inexpensive, well-tested design that can implement a true virtual computer. New versions are being implemented that are radiation-resistant, and hardened for aerospace use. The very old computer design is intentionally favored, because it is inexpensive, and its reliability and software behavior are well-characterized.

The custom operating system provides a virtual machine for each process. This means that the autopilot software never controls the computer's electronics directly. Instead it acts on a software simulation of the electronics. Most invalid software operations on the electronics occur during gross failures. They tend to be obviously incorrect, detected and discarded. In operation, the process is stopped, and restarted from a fresh copy of the software. In testing, such extreme failures are logged by the virtualization, and the engineers use them to correct the software.

Usually, one of the processes on each computer is a low priority process that continually tests the computer.

Generally, every process of the autopilot runs more than two copies, distributed across different computers. The system then votes on the results of those processes. For triple autoland, this is called camout, and uses median values of autopilot commands versus mechanical centre and feel mechanism positioning as a possible computation. Extreme values are discarded before they can be used to control the aircraft.

Some autopilots also use design diversity. In this safety feature, critical software processes will not only run on separate computers, but each computer will run software created by different engineering teams. It is unlikely that different engineering teams will make the same mistakes. As the software becomes more expensive and complex, design diversity is becoming less common because fewer engineering companies can afford it.

Aviation Autopilot Categories of Landing

Instrument aided landings are defined in categories by the ICAO. These are dependent upon the required visibility level and the degree to which the landing can be conducted automatically without input by the pilot.

CAT I - This category permits pilots to land with a decision height (where the pilot takes over from the autopilot) of 200 ft (≈ 60 m) and a forward visibility of 2400 ft (≈ 730 m). Simplex autopilots are sufficient.

CAT II - This category permits pilots to land with a decision height between 200 ft and 100 ft (≈ 30 m) and a forward visibility (RVR = Runway Visual Range) of 1000 ft (300 m). Autopilots have a fail passive requirement.

CAT IIIa -This category permits pilots to land with a decision height as low as 50 ft (≈ 15 m) and a forward visibility (RVR) of 700 ft (200 m). It needs a fail-passive autopilot. The probability of landing within the prescribed area must be better than 1 - 10^-6.

CAT IIIb - As IIIa but with the addition of automatic roll out after touchdown incorporated with the pilot taking control some distance along the runway. This category permits pilots to land with a decision height less than 50 feet or no decision height and a forward visibility of 250 ft (75 m, compare this value to the aircraft size...) or 300 ft (100m) in the US. For a landing without decision aid, a fail-operational autopilot is needed. Obviously for this category some form of runway guidance system is needed : at least fail passive but it needs to be fail-operational for landing without decision height or for RVR below 375 feet (125 m).

CAT IIIc - As IIIb but without decision height or visibility minima, also known as "zero-zero". No aircraft is approved for this category. It would necessitate a reliable way for the aircraft and ground vehicle to maneuver on the ground without any visual reference.

Fail-passive autopilot: in case of failure, the aircraft stays in a controllable position and the pilot can take control of it to go around or finish landing. It is usually a dual-channel system.

Fail-operational autopilot: in case of a failure below alert height, the approach, flare and landing can still be completed automatically. It is usually a triple-channel system or dual-dual system.

↑ - Patent Storm <http://www.patentstorm.us/patents/5945943-description.html>

See also

• Gyrocompass
• Driverless car

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