Continuing on our review of the ‘six pack’ of flight instruments from the instruments that are powered by the pitot-static system, below we review those that are gyroscopes.
A gyroscope is a rotor or spinning wheel rotating and high speed, and exhibits two fundamental characteristics upon which all practical applications are based. These are:
- Gyroscopic intertia – or rigidity in space. This is the tendency of the rotating body to maintain it’s plane of rotation if undisturbed.
- Precession: This is the tendency of the rotating body, when a force is applied to it at a point perpendicular to the plane of rotation to react as if the force had been applied 90 degrees in the direction of rotation
The three gryroscopic instruments are:
- The heading indicator. The main instrument we use to detect heading of the aircraft. Only operates when the engine is running. It runs off a vacuum system so we have to adjust it to the magnetic compass every time we fly. Frictional forces in the gyro bearings cause it to precess, resulting in a creep or drift in reading approximately 3 degrees every 15 minutes.
- Turn and bank coordinator, sometimes called the needle and ball. The needle shows the direction and approximate rate of turn. The ball shows the amount of bank in the turn and whether there is any slipping or skidding. The ball is controlled by gravity and centrifugal force. In a coordinated turn, the ball will be in the center as the centrifugal force offsets the pull of gravity. The instrument reacts to yaw but can be used for roll control since the aircraft yaws when banked. It can show a rate one turn which gives us 3 degrees per second or a two-minute turn.
- The attitude indicator. Modern attitude indicators have virtually no limits of pitch and roll and will be accurate indicate pitch up to 85 degrees, and will not ‘tumble’ in 360 degree rolls.
The instruments are typically powered by the vacuum system and an electrical system for redundancy in case one of the power sources fails. Often the heading indicator and attitude indicator operate on the vacuum system while the turn and bank coordinator is electrically operated.