Gyroscopic flight instruments incorporate a mechanical gyroscope to display aircraft attitude. Electrically or vacuum-driven, the artificial horizon, heading indicator, and turn coordinator apply basic gyroscopic principles. These instruments - named the Attitude Indicator, Heading Indicator, and Turn Coordinator - feature an internal gyro powered by vacuum, pressure, or electricity. Found on most general aviation and older commercial aircraft, they form three of the six primary cockpit instruments.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
What are gyro instruments in an aircraft?
Gyroscopic flight instruments are instruments which have a mechanical gyroscope incorporated into their design. The three main gyroscopic instruments are the artificial horizon, the heading indicator, and the turn coordinator and together they constitute what pilots term a ‘full panel’. Each instrument contains a spinning wheel or disc mounted in a low-friction gimbal set-up that allows movement in orthogonal planes. The wheel spins at 10,000 to 15,000 revolutions per minute and exhibits rigidity in space, providing a datum from which the aircraft's orientation can be measured. Because gyros detect changes that the human vestibular system cannot, these instruments react to short-term movements of the airplane and help the pilot sense pitch, bank, and yaw.
The attitude indicator, or artificial horizon, is a pictorial display whose disc is split into two segments: blue represents the sky and brown the earth. It uses the principle of rigidity in space to show aircraft attitude relative to the horizon, and it contains a set of weights intended to drive the instrument toward level flight by sensing gravity. The heading indicator is fundamentally a mechanical instrument designed to facilitate the use of the magnetic compass. Its card provides clear heading information about the aircraft's yaw axis. The turn coordinator operates similarly to the older turn indicator, but an angled, or canted, gyro allows it to respond to roll as well as yaw forces. It displays rate of turn and includes a standard-rate turn index so the pilot can establish and maintain a precise 3 per second turn.
All gyro instruments are secured to the panel with brass instrument screws, are shock and contaminant sensitive, and are normally driven by a vacuum system that sucks air through the gyro. Current practice additionally installs electric turn/bank indicators. Together with the pitot-static instruments - airspeed indicator, altimeter, and vertical speed indicator - the gyro instruments complete the ‘six pack’ of primary flight instruments.
How does gyroscope work in aircraft?
The gyroscope works by resisting tilting. At high speed the wheel becomes rigid and resists tilting, so any change in aircraft orientation is seen as the aircraft moving around the gyroscope. When the box is tipped over the gyro stays in place while gimbals pivot around the gyroscope. Gyros are useful to pilots because they can detect changes in orientation, and deviation from datum is measured and displayed to the pilot. The aircraft symbol flies around the fixed axis of the artificial horizon gyroscope, and this provides pitch information on one instrument. Rate indicators operate on the principle of precession, and Gyro precession results 90° from applied force. The horizontal axis of the heading indicator is aligned with the aircraft centerline and the angular velocity helps determine the direction that the aircraft is headed.
A rotating gyroscope resists any alteration in the aircraft’s attitude keeping it stable. This is useful in cases when visibility is reduced due to fog or other weather elements. The turn coordinator relies on the gyroscope's precession, in which the gyro reacts to applied torque, to show the speed of rotation. The heading index relies on the gyrocompass's precession to show the course of movement. Together, these measuring devices give assurance particularly during IFR circumstances, allowing pilots to comprehend the aircraft's movement.
What are the two sources of power used to operate gyroscopic instruments in an aircraft?
Gyroscopic instruments are powered in one of two ways: via a vacuum system or an electrical system. In the vacuum system, an engine-driven pump creates suction that spins the gyro rotor. This source typically drives the attitude indicator and the heading indicator found in most training aircraft. The same suction is also produced by a venturi tube mounted on the fuselage, providing the necessary vacuum when engine pumps are not used. In the electrical system, the gyro is spun by an electric motor that receives its energy from the aircraft battery and alternator. This arrangement powers the turn coordinator, the horizontal situation indicator, and some attitude gyros and autopilots that require alternating current. Direct-current instruments are available in 14- or 28-volt models, and when only direct current is on board, an inverter supplies the required alternating current.
What are the errors associated with gyroscopes in aviation?
Gyroscope instruments show errors and become inaccurate when vacuum pressure drops below the normal operating range. Vacuum failure makes the attitude indicator erratic, while a failing directional gyro drifts as much as 15° every hour of flight. This slow wander, often called drift error, is chiefly caused by friction in the gimbal bearings and is unrelated to acceleration or magnetic influences, since the heading indicator is unaffected by dip or deviation.
Earth rotation likewise imposes an apparent wander that makes the heading indicator stray. The resulting precession appears without any physical movement of the instrument and must be reset by aligning it with the magnetic compass before take-off and periodically in flight. Blocked or loose fittings in the vacuum lines, oil settling out of unused bearings, lack of use, and bearing failure also add friction-induced precession, producing wander, tilt or tumble errors.
Dynamic manoeuvres add further disturbances. Acceleration and deceleration introduce precession errors, appearing momentarily as a slight pitch-up or pitch-down on a pendulously corrected attitude indicator, while centrifugal force in normal coordinated or aerobatic turns causes temporary pitch and bank errors that usually self-correct when the aircraft returns to level, balanced flight. Harsh manoeuvres exceed erection limits so that a vacuum-driven attitude indicator tumbles past 180° if pitch exceeds 60-70° or bank exceeds 100-110°. When the mechanical failure persists, the gyro stays stuck or continues tumbling, leaving the pilot with meaningless references.
Because attitude indicator errors lead to loss of control if uncorrected, pilots must cross-check the AI against the heading indicator, turn coordinator and magnetic compass, and reset the system as soon as a discrepancy appears. Glass cockpits help by automatically detecting and annunciating gyro failures, but in any aeroplane the remedy is the same: re-establish attitude with primary flight instruments, eliminate the cause of vacuum or power loss, and if a directional gyro wanders continuously, treat it as an early sign of impending mechanical failure.
What are the types of gyroscopes in aviation?
Three types of gyros are common in aviation: mechanical, light-based and MEMS. A mechanical gyroscope is the traditional type in which a large spinning rotor is mounted into two or three gimbals, and rigidity in space is the primary operating principle.
Light-based gyros include laser-ring and fiber-optic types and both operate on the Sagnac effect. Laser-ring gyro is the next-generation technology, and fiber-optic gyroscope is a cheaper version of the same principle.
MEMS gyro is a type of gyro that uses a vibrating element and works based on the Coriolis force. MEMS gyroscopes are compact in size and give adequately good performance.