How does a heading indicator work, and how do you set it?
How does a heading indicator work? Learn to set a directional gyro, use the heading bug, correct drift and troubleshoot simulator errors.
A heading indicator shows the aircraft’s heading on a stable compass card. A traditional unit uses a gyroscope, but it does not sense magnetic north: the pilot sets it to the magnetic compass with the aircraft steady, then periodically corrects drift. The heading bug is a separate target-heading control.
In Aviation & Real-World Flying, the key distinction is between synchronising the indication and selecting a heading. The same distinction applies in a simulator: one control corrects the displayed heading, while another moves the heading bug for reference or autopilot use.
How does a heading indicator work?
A traditional heading indicator, also called a directional gyro, DG or DI, uses a rapidly spinning gyroscope whose rigidity provides a stable directional reference. As the aircraft and instrument case turn around that reference, the mechanism rotates the compass card beneath the fixed index, or lubber line.
The card is graduated clockwise from north. A heading of 090° is east, 180° is south and 270° is west. Compact cards usually omit the final zero, so 3 means 030°, 21 means 210° and 33 means 330°.
A free gyro does not remain perfectly accurate. Bearing friction, mechanical imperfections, Earth rotation and manoeuvre-induced precession gradually move its reference, producing gyro drift. The instrument therefore needs periodic comparison with an independent heading source.
Is a heading indicator magnetic?
A conventional directional gyro is not itself a magnetic instrument. It displays an approximation of magnetic heading only because the pilot aligns it with the aircraft’s magnetic compass.
The term “magnetic heading indicator” can also refer loosely to a slaved or electronic display receiving data from a remote magnetic sensor. Those systems can correct themselves automatically. Some electronic displays can be switched between magnetic and true heading, so a disagreement close to the local magnetic variation may indicate different reference settings rather than a failed instrument.
What is used to set the heading indicator in an aircraft panel?
The heading indicator is set with its card-setting, adjustment or synchronisation knob, normally located on the instrument bezel. This is not the heading-bug selector, even when both controls sit beside the same display.
In many classic Cessna 172 six-pack panels, the heading indicator occupies the centre of the lower row and has one control for aligning the card and another for the bug, where a bug is fitted. Cessna 172 panels vary widely, however: an aircraft may have a vacuum-driven DG, an electronic replacement, an HSI or an integrated glass display. Our guide to the main Cessna 172 controls and six-pack instruments shows where it fits within the wider panel.
How do you align a conventional directional gyro?
Align a conventional heading indicator by copying the settled magnetic-compass indication while the aircraft is stationary or flying straight, level and unaccelerated.
- Confirm that the gyro is powered. Allow it to run up and check for a failure flag or an abnormal vacuum or electrical indication. The required power source depends on the installation.
- Stabilise the aircraft. On the ground, remain stationary where practical. In flight, hold straight-and-level flight at a constant speed so the magnetic compass can settle.
- Read the magnetic compass. Do not take the comparison while turning, accelerating or decelerating; its indication can swing or lag during those manoeuvres.
- Operate the correct setting control. Turn the heading indicator’s adjustment knob until the compass heading appears beneath the lubber line. Some older controls must be pushed or pulled before they engage, so follow the instrument markings rather than forcing the knob.
- Release and cross-check. Confirm that the card remains in position and compare it with the compass once more.
Normally, the compass indication is copied directly. Do not add or subtract magnetic variation unless the aircraft’s approved procedure explicitly tells you to do so.
How often should a heading indicator be reset?
A free directional gyro should be checked before flight and periodically in flight at the interval specified by the aircraft checklist. A comparison about every 15 minutes is common in light-aircraft training, with additional checks after prolonged manoeuvring, but the aircraft’s operating instructions take precedence.
Is the heading bug the same as the heading setting?
No. Synchronising the compass card corrects the heading being displayed; moving the heading bug selects a desired heading without changing the aircraft’s indicated heading.
The bug can serve as a visual reminder even without an autopilot. With an autopilot operating in HDG mode, it becomes the commanded heading. Merely moving the bug will not turn the aircraft: the autopilot must be engaged and its heading mode must be active. Our separate explanation covers setting the heading bug and using HDG mode in a simulator.
Do all heading indicators need manual alignment?
No. Manual alignment is required mainly by free-gyro instruments; slaved and modern electronic heading systems normally correct their heading from a magnetic sensor.
| Display type | Heading source | What the pilot sets |
|---|---|---|
| Traditional DG or DI | Vacuum- or electrically driven free gyro | Manually align the card and correct periodic drift |
| Slaved HSI or remote compass display | Gyro reference corrected by a remote magnetic sensor | Usually automatic in slave mode; manual synchronisation may be available in free mode |
| AHRS or glass cockpit | Magnetometer and inertial sensor data | Heading alignment is normally automatic; the heading bug remains pilot-selectable |
A slaved or AHRS system may still require time to initialise after start-up, and some installations require the aircraft to remain stationary during that process. Do not apply the free-gyro setting procedure to an electronic display unless its operating instructions provide a manual alignment or free mode.
How do you set the heading indicator in a flight simulator?
In a flight simulator, identify the modelled heading system first, then use the virtual card-setting control only if the aircraft has a free directional gyro.
- Identify the instrument. A traditional round DG may need synchronisation; a slaved HSI or glass display will usually align automatically.
- Power and initialise the aircraft. A vacuum-driven instrument may require the engine to be running, while an electric or AHRS display needs the appropriate electrical supply.
- Establish a trustworthy comparison. Keep the aircraft stationary or fly straight and level until the magnetic compass settles.
- Synchronise a free gyro. Use the cockpit adjustment knob or the simulator assignment for directional-gyro or heading-indicator synchronisation. Control names vary between simulators and aircraft packages.
- Set the bug separately. Use the heading-bug selector if the purpose is to mark a desired heading or command the autopilot.
A mistake we see constantly is assigning the heading-bug increase and decrease controls, then expecting them to correct gyro drift. If the small marker moves but the compass card does not, the bug is being adjusted. Look for a separate directional-gyro synchronisation control.
Some simulator aircraft simplify the system or disable gyro drift, even when the panel artwork resembles a conventional DG. If a slaved display already agrees with the compass, there is nothing to synchronise. If it does not, check its magnetic-versus-true reference, electrical state, initialisation and simulated failures before trying to force the card into position.
Does a heading indicator show course or ground track?
No. Heading is the direction the aircraft’s nose points; ground track is its actual path over the ground, course is an intended path, and bearing is the direction from one position to another.
Wind commonly makes heading and track different. A GPS map can therefore show a track of 085° while the heading indicator reads 090° without either being wrong. On an HSI, the course pointer is also separate from the heading card and heading bug. Our simulator explanation of heading, course and bearing covers these distinctions in practical use.
Why is the heading indicator showing the wrong heading?
An incorrect indication usually comes from normal gyro drift, an unreliable compass comparison, loss of instrument power or confusion between heading, track and true or magnetic references.
- Slowly increasing compass disagreement: this is typical free-gyro drift. Realign the powered instrument while the aircraft is steady.
- Rapid drift, a frozen card, tumbling or a warning flag: suspect a vacuum, electrical, gyro or sensor failure rather than ordinary drift.
- Disagreement only during turns or speed changes: wait for straight, level and unaccelerated flight before using the magnetic compass as the reference.
- A nearly constant offset resembling local magnetic variation: check whether one display is using true heading and the other magnetic heading.
- Disagreement with a GPS or moving map: confirm that the map is showing ground track rather than aircraft heading.
- The bug moves but the card does not: the wrong cockpit control or simulator assignment is being used.
- The autopilot ignores the bug: verify that the autopilot is engaged and HDG mode, rather than NAV, LOC or another lateral mode, is active.
- The runway number differs slightly: runway designators are rounded magnetic directions with the final zero omitted; they are not precise calibration references.
In a real aircraft, repeated or rapid disagreement after correct alignment should be treated as an instrument or system fault. Stop trying to reset it continuously, cross-check independent references and follow the approved checklist. We cover the practical indications and priorities in our guide to recognising and handling a heading-indicator failure.