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.
The AI 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).
Under some circumstances, some types of AI may "topple", 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.
