Attitude indicator
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Image:VMS Artificial Horizon.jpg Image:Attitude indicator drawing.png An attitude indicator (AI), aka gyro horizon or artificial horizon, is an instrument used in an aircraft to inform the pilot of the orientation of the airplane relative to the ground. It indicates pitch (fore and aft tilt) and roll (side to side tilt), and is considered the most important instrument for flight in instrument meteorological conditions. Attitude indicators also have significant application under visual flight rules.
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.
Attitude indicators in aircraft work using a gyroscope to establish an inertial platform. In Western aircraft the gyroscope is geared to a display that has two dimensions of freedom, simultaneously displaying pitch and roll. The display is coloured to indicate the horizon as the division between the two coloured segments (typically blue for sky and brown for ground), 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 5° or 10° of pitch. Most Russian-built aircraft have a somewhat different design. The background display is coloured as in a Western instrument, but moves up and down only to indicate pitch. A symbol representing the aircraft (which is fixed in a Western instrument) rolls left or right to indicate roll angle.
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 and vertical speed indicator.
Most attitude indicators can only tolerate a specific range of roll angles. If the aircraft banks too steeply—while performing aerobatics, for example—the attitude indicator will "tumble" and become unusable. For this reason, some attitude indicators are fitted with a "cage," a device to restore the gyroscope to an erect position.
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 three-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 less than $15,000, making them practical for general aviation aircraft.cs:Umělý horizont de:Künstlicher Horizont nl:Kunstmatige horizon ru:Авиагоризонт
