Attitude Indicator Operation and Errors

Purpose and Types

The attitude indicator's main role is to show the aircraft's orientation—pitch (nose up or down) and bank (left or right)—relative to the horizon. There are two main types: the basic attitude indicator and the Attitude and Director Indicator (ADI), which combines the attitude display with flight director command bars for guidance.

Principle of Operation

A gyroscope spinning about a vertical axis is stabilized by gravity. The instrument uses two gimbals to allow movement in pitch and roll. As the aircraft maneuvers, the gyroscope remains aligned with the Earth's horizon, and mechanical linkages translate this into the instrument display. Attitude indicators can be powered pneumatically (using engine bleed air) or electrically, with electric types often providing greater reliability and freedom of movement.

Display and Markings

The typical display shows a symbolic aircraft against a background with a horizon bar, pitch scale (usually marked in degrees), and bank angle marks (often at 10°, 20°, 30°, 60°, and 90°). In the basic T arrangement, the attitude indicator is centrally located for optimal pilot scan.

Attitude Indicator Errors and Limitations

Despite their reliability, attitude indicators have limitations:

• Precession and Drift: Over time, gyroscopic precession can cause small errors, especially after prolonged turns or acceleration.
• Gyroscopic Topple: Excessive pitch or bank angles (beyond instrument limits, typically ±85° pitch and ±110° bank) can cause the gyro to tumble, resulting in significant errors.
• Acceleration and Deceleration Errors: Rapid changes in speed or turns can cause the instrument to momentarily indicate incorrect pitch or bank.
• Power Source Failures: Pneumatic or electrical failures can cause the instrument to become unreliable or inoperative.

Recognizing and Managing Errors

Pilots must cross-check the attitude indicator with other instruments, especially if unusual indications appear after abrupt maneuvers or system failures. Fast erection features on some electric indicators allow rapid realignment, but should only be used on the ground or in straight and level flight to avoid introducing further errors.