The heading indicator (HI), called a directional gyro (DG) or direction indicator (DI), is a primary flight instrument that shows, at-a-glance, the direction the aircraft's nose is pointing. Mounted underneath the artificial horizon in the middle of the instrument panel, it displays heading on a 360 degree azimuth with the final zero omitted. HI works by using a gyroscope; being north-seeking, it is unaffected by banks, turbulence, and other errors, yet it must be manually set to the magnetic compass heading unless it is a slaved type that continuously receives magnetic slaved information from a flux gate.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
What is a heading indicator on an aircraft?
The heading indicator, known as a directional gyro or direction indicator, is a flight instrument used in an aircraft to inform the pilot of the aircraft's heading, and it allows the pilot to see the direction the aircraft's nose is pointing. The heading indicator is arranged such that the gyro axis is used to drive the display, and reading the instrument will tell you your magnetic heading.
The heading indicator on an aircraft is an instrument that measures how much the aircraft has turned around the vertical axis, and it displays the heading - the direction the aircraft's nose is pointed relative to magnetic north. The instrument presents this heading on a circular compass card calibrated in degrees, with the nose of a symbolic airplane serving as the lubber line.
The display consists of a drum-like card marked in the same 360° azimuth style as a magnetic compass, but the final zero is omitted: 6° means 060°, 21° means 210°. A gyroscope spins horizontally inside the case, remaining rigid in space so the card stays fixed while the aircraft turns around it. The pilot views the card from the back and sees the nose of the symbolic aircraft point to the aircraft's current heading.
What is the purpose of the heading indicator?
The heading indicator gives pilots a simple, reliable direction reference. It tells the pilot which way the nose is pointing with reference to magnetic North. Because the card inside the instrument rotates to show the current heading, the pilot can read the number at the lubber line and know the exact heading without trying to steady a wobbling magnetic compass.
In visual flight the indicator is not legally required, yet pilots still use it. During gentle or moderate turns the magnetic compass swings and dips, so the heading indicator provides steadier information and helps the pilot roll out on the desired course. It also assists cross-country navigation by letting the pilot set and maintain a selected track, refining situational awareness and making it easier to set up clean, co-ordinated turns.
What is the difference between a magnetic compass and a heading indicator?
The magnetic compass is the primary indicator of direction in most airplanes and it uses a magnetic needle that aligns with the magnetic north pole. Because the magnetic north pole is not the same as the geographic north pole, pilots must apply magnetic variation - adding westerly or subtracting easterly variation - to convert true course to magnetic course. Isogonic lines on sectional charts show where these corrections apply.
A magnetic compass is liquid-filled and relies on earth's magnetic field, so any turning or acceleration introduces error. During a turn from a northerly heading the compass briefly indicates the opposite direction and during a turn from a southerly heading it shows the correct direction but at an exaggerated rate. These errors are remembered with the acronym ANDS: Accelerate North, Decelerate South. Additional error, called deviation, arises when nearby aircraft instruments interfere with the magnetic field. A compass card attached to the instrument lists the few degrees of correction needed for each heading.
The heading indicator, sometimes called a direction indicator, is gyroscopic, not magnetic. It presents a 360-degree azimuth similar to the magnetic compass yet remains steady during turns and accelerations. Because it does not use magnetic north as a datum, it must be realigned in flight to correct for apparent drift caused by gyroscopic precession. Both VFR and IFR pilots must understand how the two instruments practically relate to each other: the compass provides the steady magnetic reference, while the heading indicator offers stable, instantaneous heading information free from the dynamic errors present in the magnetic compass.
How does a heading indicator work?
Heading indicators work on the principle of rigidity in space imparted by a gyroscope. A small wheel inside the instrument spins horizontally at 24,000 rpm, held in two or three gimbals so that no moment turns the axis once the rotor has been set. The resulting gyroscopic rigidity creates an inertial plane that remains fixed relative to distant space. Whenever the aircraft yaws, the aeroplane turns about the vertical axis while the gyro-spin axis keeps its original orientation. The case, fixed to the airframe, rotates around the stable gyro, and this relative movement is taken off mechanically to drive a circular compass card that is calibrated in degrees. By watching the numbers pass the lubber line, the pilot reads the instantaneous magnetic heading although the instrument itself senses only inertial yaw.
Because the Earth continues to rotate under the platform at 15° per hour, uncorrected precession slowly turns the gyro away from the selected datum and introduces reading drift. A flux gate - situated remotely in the wingtip or tail - continuously senses the Earth's magnetic field. Every time the slaving meter detects deviation between flux-gate heading and gyro heading, the system applies a gentle torque to precess the gyro back into agreement with the magnetic meridian. Thus, the heading indicator combines the short-term steadiness of a gyroscope with the long-term truth of terrestrial magnetism. When the knob is pushed in, the gimbals are employed for normal slaved operation and when it is pulled out, the gimbals are released so that the pilot can set the card to the magnetic compass before engaging the sync again.
What is a heading indicator powered by?
The heading indicator is powered by suction. It is powered by a vacuum pump mounted on the engine. Most training aircraft power the heading indicator with a vacuum system. The instrument can also be powered by an electrical system. In some planes, it is powered by a direct-current electrical system for redundancy. A modern HSI, which merges heading information with navigation sources, is electrically driven by an electric gyro.
What are the components of a heading indicator?
A heading indicator consists of three mechanical gimbals that allow movement around the vertical and the horizontal axes so that the gyroscope can remain rigid in space while the aircraft yaws, pitches, or rolls. Inside the instrument the gyroscope, spinning at up to 24,000 rpm on its horizontal axis, is connected by a gear to the circular compass card. The card itself is divided into 360 equal segments called degrees and displays cardinal points or letters denoting the eight cardinal points. A fixed lubber line extending from the nose of the little airplane shape painted on the face of the gauge intersects the card and serves as the zero point for reading the instantaneous heading.
Complementing the basic gyro assembly, the system contains a slaving control and compensator unit that has two manual heading-drive buttons and a pushbutton providing a means of selecting either the slaved gyro or the free gyro mode, and a slaving meter that indicates the difference between the displayed heading and the magnetic heading supplied by the remote magnetic slaving transmitter. The transmitter contains a flux valve, the direction-sensing device that eliminates the possibility of magnetic interference inside the cockpit, and it continuously compares the aircraft yawing plane - defined by the longitudinal and horizontal axes - with the local Earth horizontal to keep the gyro oriented. Thus, the heading indicator unites a circular compass card, the freedom of three gimbals, and the correcting intelligence of the slaving electronics into a single instrument ready for panel mounting within the basic aviation six-pack.
How is the heading indicator gyro mounted?
The gyro in a heading indicator is mounted with its spin axis horizontal, supported inside a double gimbal suspension that leaves it free to stay rigid in space while the aircraft yaws around it. This arrangement copies the attitude indicator's double gimbal, but with the difference that the spin axis is laid flat, aligned fore-and-aft with the aircraft, so only the horizontal axis is used to drive the display card. A control knob on the front of the instrument can be pushed in to engage the gimbals, allowing the pilot to set the card against the magnetic compass. When the same knob is pulled out it disengages the gimbals and the gyro remains rigid in its frame. Vacuum or electric power spins the gyro at a very high rpm, and friction - inevitable where the gimbals rotate - gradually slows the rotor. As it loses rigidity, the instrument drifts, making periodic resetting against the magnetic compass necessary.
The gyroscope inside the heading dial is held within a closed case, and the whole unit is fixed onto a set of gimbals. These gimbals permit the gyro to stay level and to keep its spatial attitude, allowing it to orient in an unmoving heading. During a classroom demonstration, the instructor manually turned a cockpit mock-up while the device was powered up on land. The gyroscope inside the navigation device rejected the motion, keeping the indicated bearing steady.
What are the types of heading indicators in aircraft?
The two types of heading indicators are the simple directional gyro (DG) heading indicator and the slaved heading indicator. The DG type contains a gyro that remains rigid when its erection knob is disengaged. The aircraft is then free to turn around the fixed card while the pilot steers toward the bug heading. Because the gyro is not automatically corrected, the instrument drifts and must be manually realigned to the magnetic compass. The tool suffers inherent errors like magnetic dip.
A slaved heading indicator solves drift by coupling the gyro to a magnetic slaving transmitter. This transmitter, called a flux-gate magnetometer, continuously senses earth’s magnetism and drives a torque motor that keeps the gyro card aligned with magnetic north. The result is an indicator that behaves like a stable, error-free compass yet retains the gyro's freedom from the magnetic dip and other errors that plague the stand-by magnetic compass. An advanced form of slaved instrument is the horizontal situation indicator, which combines the slaved gyro with course-deviation cues to present the complete horizontal situation on one display.
How to read a directional gyro?
To read a directional gyro, first look at the face: the directional gyro looks like a compass and is connected to a compass card. The compass card is divided into 360 equal segments called degrees and displays a compass-rose in 5-degree increments. A letter at each cardinal point denotes the direction the aircraft is facing. The instrument translates resistance into a reading on the compass card, so you simply read the number that lines up with the lubber line: that number, in degrees, is the aircraft's heading. Zero is north, 090 is east, 180 is south, 270 is west. The heading shown is magnetic, not true, because the directional gyro must be aligned with the magnetic compass at regular intervals approximately every 10 to 15 minutes. When the directional gyro is slaved to the magnetic flux detector it automatically keeps this alignment. The gyroscope inside the directional gyro resists change to its position, so the DG is holding its position while the plane turns around the directional gyro. The card therefore stays steady and gives an immediate, lag-free indication.
What are the errors with the heading indicator?
Heading indicator error sources include friction, precession, human error, vacuum pump failure and suction errors. Friction inside the gyroscope causes mechanical drift and worn bearings increase friction-created drift that reaches 15 degrees per hour. Apparent drift is precession caused by Earth’s rotation at 15 degrees per hour. During a turn or turbulence the gyroscope experiences precession, giving a temporary error in the displayed heading, if the limits are exceeded the gyro tumbles past 180 degrees then corrects slowly or continues to tumble. Suction errors occur when airflow over the gyro wheel is blocked. Suction system failure causes the instrument to display erroneous data. Human error occurs if the pilot forgets to check gyro power and reset the heading indicator before take-off, or if caging or alignment is mishandled. The instrument suffers gimbal error or parallax created by reading the instrument from different angles. Tolerance is normally 5 degrees after reset, but drift beyond that must be corrected by periodic realignment with the magnetic compass.
What are the limitations of the heading indicator?
The heading indicator is limited by the need for periodic calibration because apparent drift accumulates as Earth rotates 15 degrees per hour. Manual alignment every fifteen minutes in straight level flight keeps correction within the 3 degree tolerance allowed by the FAA handbook. Failure of the vacuum pump or blockage of the airflow over the gyro wheel produces suction errors that render the instrument inoperative, and mechanical drift caused by gimbal friction adds a slow but steady departure from truth.
Instrument limits are physical: excessive bank or pitch during aerobatics exceeds gyroscopic limits and tumbles the card, while any high-rate turn pegs the indication. Once disturbed, the gyro offers misleading information until it is again caged and recaged. Parallax impedes accuracy; because the pilot views the rotating card from the back and from varying angles, a slight shift in eye position changes the perceived heading, compounding the other errors.
The heading indicator offers only an indirect display of heading change and is loosely combined with other navigational systems. Crosswind makes course over ground differ from displayed heading, and the instrument does not sense magnetic dip or magnetic variation. Safe flight therefore depends on regular scanning, disciplined comparison with the magnetic compass, and an understanding that reliability is maintained only through continuous resets and adherence to the procedures outlined in the FAA handbook.
How to reset a heading indicator?
To reset a heading indicator, depress the heading-drive button on the slaving control unit if the compass card is adjustable. Caging and uncaging the knob also realigns a free-gyro unit after tumble. Make the reset at the same moment you compare the directional reading to the compass card and its correction values. Once reset, the directional gyro will again match the magnetic compass reading for the next 15-minute segment of flight.
