The Attitude Indicator

The attitude indicator, or artificial horizon, is a basic flight instrument found in every aircraft. Its operation is based on gyroscopic principles. In light aircraft, a vacuum pump typically drives its gyros, though some can be electrically driven. This instrument measures an aircraft's attitude relative to the horizon in terms of pitch and bank.

The bank angle is the inclination of the wings relative to the horizon, indicated on the instrument's banking scale. The pitch angle is the inclination of the aircraft's nose relative to the horizon. The instrument displays these inclinations intuitively, showing a miniature aircraft against an artificial horizon line that separates sky from ground.

Key components include a miniature aircraft, an artificial horizon line, and various markings. The banking scale, located at the top, uses a yellow bug to indicate the current bank angle, with markings for 10, 20, 30, 45, 60, and 90 degrees. The pitch scale uses a central point on the miniature aircraft to indicate pitch, with long lines for 10-degree increments and short lines for 5-degree increments. It's important to remember this instrument shows attitude, not whether the aircraft is turning, climbing, or descending.

Examples demonstrate how to interpret combined pitch and bank readings. Some attitude indicators feature a sky pointer, which remains fixed while the banking scale moves. This can be confusing as the bank indicator moves opposite to the direction of banking. The key is to use the bank indicator for the angle and the miniature aircraft for the direction.

Some instruments, especially US-manufactured ones, include an adjustment knob to align the miniature aircraft with the horizon from the pilot's perspective. Improper adjustment can lead to incorrect readings, particularly in IMC or at night. Therefore, adjustments should only be made on the ground or during straight and level flight in VMC. A red flag indicates when the gyroscope is not working, signaling the instrument should not be used.

The attitude indicator utilizes the gyroscopic effect of rigidity in space. A vertical gyroscope, with its plane of rotation parallel to the real horizon, remains rigid regardless of aircraft movements. This allows it to accurately measure pitch and bank, as demonstrated by how the instrument and aircraft move around the stable gyro.

Although theoretically having three degrees of freedom, the gyro has mechanical stops. Exceeding certain pitch or bank angles can cause the instrument to 'topple' and become inoperative until it realigns. Older instruments have more restrictive limits than modern ones. Another limitation is gyroscopic apparent wander, caused by Earth's rotation, which can lead to the gyro's plane of rotation progressively drifting from the true horizon. To counteract this, a control system (pendulous veins for air-driven, torque motor for electric) constantly realigns the gyro with gravity.

Some electrically driven attitude indicators have a caging knob. This allows decoupling the gyro from the instrument face, preventing it from toppling during aggressive maneuvers by acting as a free gyro. It can also quickly realign a toppled gyro, but this procedure should only be done in straight, level flight or on the ground.