Autopilot
From Free net encyclopedia
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 autopilots for boats and ships are called by the same name and serve the same purpose.
In the early days of transport aircraft, aircraft required the continuous attention of a pilot in order to fly in a safe manner. This created very high demands on crew attention and high fatigue. The autopilot is designed to perform some of the tasks of the pilot.
The first aircraft autopilot was developed by Sperry Corporation. It 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, thus covering more than 80% of the pilot's total workload on a typical flight. This straight-and-level autopilot is still the most common, least expensive and most trusted type of autopilot. It also has the lowest pilot error, because it has the simplest controls.
Modern autopilots generally divide a flight into taxi, take-off, ascent, level, descent, approach, landing, and taxi 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, used on the majority of major runways today. Landing, rollout and taxi control to stand is cat3c. This is not usually used to date but may be used in the future- and some incorporate automated collision-avoidance, The most popular collision avoidance for aircraft is called TCAS (Trafffic 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 all but a few of the best pilots.
The autopilot reads its position and the aircraft's attitude from an inertial guidance system. Inertial guidance systems accumulate errors over time. These errors are corrected by using satellite navigation systems and altimeters. They also may 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 positonal 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 positon. 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).
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.
See also
de:Autopilot it:Pilota automatico nl:Automatische piloot no:Autopilot pl:Autopilot uk:Автопілот