An attitude indicator is a device used in aircraft to alert the pilot of the aircraft's orientation relative to Earth’s horizon. It works on the principle of gyroscopic rigidity in space. The fast-whirling rotor disk resists any attempt to interrupt its orientation. Hence, the gyro offers a steady horizontal reference against which the pitch and roll attitudes of the aircraft can be determined.
Many airplanes are mounted with an air-driven attitude indicator powered by the aircraft’s vacuum or pressure system. At the core of the attitude indicator is a hygroscope – a huge metal rotor disk that whirls at about 10,000 revolutions per minute in the horizontal axis about a vertical axle. The spinning earth gyro inside the attitude indicator is gimbaled with pivots in both the pitch axes and the roll. The rotor of the air-driven gyro is mounted in a sealed housing. The passages in the rear pivot, the side pivots and the gimbal ring allow filtered air into the gyro housing.
The attitude indicator (also known as gyro horizon, attitude director indicator or artificial horizon) is the only device on the panel that offers a perfect picture of the flight attitude of the aircraft. Without this instrument, the pilot would be forced to determine flight attitude by incorporating information from many other devices such as turn-and-bank or turn coordinator, vertical-speed indicator, airspeed indicator or altimeter that offer only limited, and often untimely, information about flight attitude.