Attitude indicator

The attitude indicator (AI), also often called the artificial horizon, is an instrument used in an aircraft to inform the pilot of the attitude of the plane - it indicates attitude in both pitch (fore and aft tilt) and roll (side to side tilt). The instrument is considered the most important for flight under the instrument flight rules (IFR), but has minimal application under the visual flight rules (VFR), except in emergencies, when the pilot may lose visual reference to the ground.

Related Topics:
Instrument - Aircraft - Instrument flight rules - Visual flight rules

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The first artificial horizons were used in maritime navigation where latitude is measured through the observation of celestial bodies in relation to the horizon. The horizon in this context is a fixed number of degrees below the actual tangent at the observation point. Various devices were developed (many relying on a liquid such as mercury) to indicate the level plane. Obviously fog and rain whilst obscuring the horizon might still allow the Sun's position to be clearly measured.

Related Topics:
Navigation - Latitude - Mercury

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AIs in aircraft works using a gyroscope to establish an inertial platform, geared to a display that has two degrees of freedom, simultaneously displaying pitch and roll. The display is coloured to indicate the horizon as the division between the two coloured segments, and as such is intended to be intuitive to use. The actual roll angle is also calibrated around the circumference of the instrument. The pitch angle is indicated by a series of calibration lines, each representing 10° of pitch. The pitch angle is relative to the ground, which is not as helpful as knowing the angle of attack of the aircraft, a much more critical measure of performance. The pilot must infer the total performance by using other instruments such as the airspeed indicator (ASI).

Related Topics:
Gyroscope - Inertial platform - Angle of attack - Airspeed indicator

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Under some circumstances, some types of AI may "tumble", which is, they lose the ability to display anything sensible, particularly if the aircraft is in an extreme or unusual attitude, or performing aerobatics. Some types are fitted with a "cage", which locks the gyroscope to prevent damage under these conditions.

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Individual mechanical gyros are slowly being replaced by Attitude and Heading Reference Systems (AHRS), which use solid-state or miniature gyroscopes (MEMS) to supply aircraft orientation information, supplemented by magnetometers to supply heading information. Historically, heading information was supplied by a separate gyroscopic instrument known as a directional gyro (DG, or heading indicator). AHRS are able to provide 3-axis information that can be shared with multiple devices in the aircraft, such as "glass cockpit" primary flight displays (PFDs). AHRS have been proven to be highly reliable and are in wide use in commercial and business aircraft. Recent advances in MEMS manufacturing have brought the price of FAA certified AHRS down to below $15,000, making them practical for general aviation aircraft.

Related Topics:
Attitude and Heading Reference Systems - MEMS - Magnetometers - Directional gyro - Glass cockpit - PFDs - FAA - General aviation

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