Principles of Gyroscopic Instruments

Three of the six primary flight instruments use gyroscopes to provide pilots with critical flight information about the airplane's attitude, heading, and rate of turn.

Rigidity in Space and Precession

Gyroscopes work like spinning tops. They have two properties—rigidity in space and precession—that make them useful in flight instruments.The attitude indicator and heading indicator are based on a gyro's rigidity in space. Because a gyro resists being tipped over, it can provide a stable reference to the real horizon or to a specific direction.

The turn coordinator uses precession to display information about the direction and rate of turn.

Gyro Power

In most light airplanes, an engine-driven vacuum pump spins the gyros in the attitude indicator and the heading indicator. To provide a backup if the vacuum fails, the turn coordinator usually has a gyro spun by an electric motor.

Attitude Indicator

Click on the attitude indicator to watch the movie.

Sometimes called the "artificial horizon," the attitude indicator is the only instrument that simultaneously displays both pitch and bank information.

How It Works

The gyro mounted in the attitude indicator rotates in the horizontal plane and maintains its orientation relative to the real horizon as the airplane banks, climbs, and descends.

Note, however, that the attitude indicator alone can't tell you whether the airplane is maintaining level flight, climbing, or descending. It simply shows the aircraft's attitude relative to the horizon. To determine your flight path, you must crosscheck the airspeed indicator, altimeter, heading indicator, and other instruments.

The pointer at the top of the attitude indicator moves along a scale with marks at 10, 20, 30, 60, and 90 degrees of bank. The horizontal lines show the aircraft's pitch attitude in degrees above or below the horizon. The converging white lines in the bottom section of the indicator can also help you establish specific bank angles.

Limitations

The gyros in the attitude indicators used in most small aircraft tumble if the pitch attitude exceeds +/-70 degrees or if the angle of bank exceeds 100 degrees. When the gyro tumbles, it gives unreliable indications until it realigns itself, a process that usually requires several minutes of straight and level flight. Aerobatic and large aircraft are often equipped with gyros that are reliable through 360 degrees of pitch and bank.

Many modern attitude indicators have a blue "sky" and brown "earth," which is the origin of the phrase "keep the blue side up."

Heading Indicator

The heading indicator, sometimes called the "directional gyro" or "DG," is one of the three gyro instruments. When aligned with the compass, it provides an accurate, stable indication of the aircraft's magnetic heading.

The heading indicator is an important aid because the compass is subject to errors caused by acceleration, deceleration, and the curvature of the earth's magnetic field, especially at high latitudes. The compass often oscillates or leads or lags a turn and it is especially hard to read in turbulence or during maneuvers. (To see how difficult it is to fly with only a compass, you can display a compass in a separate window. On the Views menu, click Instrument Panel and then click Compass.)

How It Works

The gyro in the heading indicator rotates in the vertical plane. A card marked with headings maintains its orientation as the airplane turns. The apparent movement of the card gives the pilot an immediate, precise indication of the airplane's heading and the direction in which the airplane is turning.

The card is marked off in five-degree increments, with numbers every 30 degrees and the cardinal directions indicated by N, S, E, W.

Aligning the Heading Indicator

On small aircraft like the Cessna 182RG, the pilot sets the heading indicator to coincide with the compass before takeoff and resets it periodically during flight to make sure that it remains in sync with the compass. The heading indicator drifts because it's based on a gyro, which precesses with time. As a rule, the heading should drift no more than three degrees every 15 minutes.Larger aircraft usually have "slaved" heading indicators that automatically keep the instrument properly aligned with the compass.

Turn Coordinator

Click on the turn coordinator to watch the movie.

The turn coordinator is really two instruments. The gyro portion shows the aircraft's rate of turn—how fast it's changing direction. A ball in a tube called the "inclinometer" or "slip/skid indicator" shows the quality of the turn—whether the turn is "coordinated."

How It Works

The gyro in the turn coordinator is usually mounted at a 30-degree angle. When the airplane turns, forces cause the gyro to precess. The rate of precession makes a miniature airplane on the face of the instrument bank left or right. The faster the turn, the greater the precession, and the steeper the bank of the miniature airplane.

Standard Rate Turn

When the wings of the miniature airplane align with the small lines next to the "L" and "R," the aircraft is making a standard rate turn, which in the Cessna 182RG is three degrees per second. This means the aircraft completes a 360-degree turn in two minutes.

Balancing Act

The black ball in the slip/skid indicator stays between the two vertical reference lines when the forces in a turn are balanced and the airplane is in coordinated flight. If the ball drops toward the inside of the turn, the airplane is slipping. If the ball moves toward the outside of the turn, the airplane is skidding.

To correct a skid, reduce rudder pressure being held in the direction of the turn and/or increase the bank angle.

To correct a slip, add rudder pressure in the direction of the turn and/or decrease the bank angle.The auto-coordination feature in Flight Simulator automatically moves the rudder to maintain coordinated flight.

Useful Backup

The turn coordinator is usually electrically powered so that it's available if the vacuum pump fails and disables the attitude indicator and heading indicator.

Needle and Ball

The turn coordinator is common in modern light aircraft such as the Cessna 182RG. Older airplanes often have a similar instrument called the "turn and slip indicator" or the "needle and ball," which uses a different presentation to display the same information.