Attitude Indicator

The attitude indicator (also known as an artificial horizon) shows the aircraft's attitude relative to the horizon. From this the pilot can tell whether the wings are level and if the aircraft nose is pointing above or below the horizon. This is a primary instrument for instrument flight and is also useful in conditions of poor visibility. Pilots are trained to use other instruments in combination should this instrument or its power fail.

An attitude indicator (AI), also known as gyro horizon or artificial horizon, is an instrument used in an aircraft to inform the pilot of the orientation of the aircraft relative to earth. It indicates pitch (fore and aft tilt) and bank or roll (side to side tilt) and is a primary instrument for flight in instrument meteorological conditions. Attitude indicators also have significant application under visual flight rules, though some light aircraft do not have them installed.

How an Attitude Indicator Works

Attitude indicators use a gyroscope (powered via vacuum pump or electrical motor) to establish an inertial platform. The gyroscope is geared to a display that has two dimensions of freedom, simultaneously displaying pitch and bank. The display may be colored to indicate the horizon as the division between the two colored segments (typically blue for sky and brown for ground), and is intended to be intuitive to use. The actual bank angle is 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 depending on design.

Some attitude indicators can only tolerate a specific range of bank angles. If the aircraft rolls too steeply — while performing aerobatics, for example — the attitude indicator can "tumble" and become temporarily unusable. For this reason, some attitude indicators are fitted with a "cage" (a device to restore the gyroscope to an erect position). Most modern attitude indicators slowly re-erect back to level after a tumble. Others do not tumble at all.

Individual mechanical gyros are slowly being replaced by Attitude and Heading Reference Systems (AHRS), which use solid-state or miniature gyroscopes (MEMS gyroscope) 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.

With most AHRS systems, if an aircraft's AIs have failed there will be a standby AI located in the center of the instrument panel, where other standby basic instruments such as the airspeed indicator and the attitude indicator are also available. These mostly mechanical standby instruments may be available even if the electronic flight instruments fail, though the standby attitude indicator is electrically driven and will, after a short time, fail if electrical power to it fails.