Learn how an aircraft heading indicator works, how to align it with the magnetic compass, set the heading bug, and fix wrong readings.
In aviation, a heading indicator uses a gyroscope or electronic heading system to show the aircraft’s magnetic heading on a stable compass card. To set a conventional unit, fly straight and level, compare it with the magnetic compass, then turn its adjustment knob until both agree. Set the heading bug separately.
How does a heading indicator work?
A traditional heading indicator, also called a directional gyro, DG or DI, uses a rapidly spinning gyroscope that resists changes to its orientation. As the aircraft turns around the gyro, the instrument mechanism moves the compass card to show the new heading beneath the fixed index or lubber line.
The display is read in degrees clockwise from magnetic north: 090° is east, 180° is south and 270° is west. On compact cards, labels may omit the final zero, so 3 means 030° and 27 means 270°. Our explanation of Cessna 172 controls and traditional flight instruments shows how the directional gyro fits into a typical light-aircraft panel.
Unlike a magnetic compass, a free directional gyro does not detect north. It must first be aligned with the compass and then checked periodically because friction, precession and manoeuvring cause it to drift. Depending on the aircraft, the gyro may be powered by the vacuum system or electrically.
How do you set a conventional heading indicator?
Set a conventional heading indicator by aligning its compass card with the magnetic compass while the aircraft is steady and the compass has settled.
- Check the instrument is powered. Allow the gyro to stabilise and confirm that no failure flag or abnormal vacuum indication is present.
- Stabilise the aircraft. On the ground, remain stationary where practical. In flight, maintain straight, level and unaccelerated flight; do not align the indicator during a turn.
- Read the magnetic compass. Wait for its swinging to settle, then note the indicated magnetic heading.
- Align the compass card. Operate the heading-indicator adjustment knob as specified for that instrument, rotating the card until the same heading sits beneath the lubber line. Some older units require the knob to be pushed or pulled before it turns.
- Release and cross-check. Make sure the card remains on the selected heading and compare it with the compass again.
Realign it at the interval specified by the aircraft checklist and after manoeuvres that may have increased gyro error. Roughly every 15 minutes is a common training cross-check, but the aircraft’s operating instructions take precedence.
A magnetic compass is least reliable while turning, accelerating or decelerating, so copying its reading at that moment transfers compass error into the heading indicator. This is one of the most common causes of an apparently inaccurate DG.
Is the heading bug the same as the heading setting?
No. Aligning the compass card corrects the heading being displayed; moving the heading bug selects a target heading without changing the indicated aircraft heading.
Turn the heading-bug selector until the small marker sits on the heading you want. When an autopilot is operating in HDG mode, it uses that marker as its steering command. The autopilot must be engaged with HDG mode active; merely positioning the bug will not turn the aircraft. For a practical example, see our KAP 140 heading-mode and heading-bug instructions.
On an HSI, the course pointer is another separate control. It selects a VOR, localiser or GPS course rather than the aircraft’s heading. Wind can also make heading differ from ground track; our FSX explanation of heading, course and bearing illustrates that distinction.
Do all heading indicators need manual alignment?
No. Manual alignment is mainly required by free-gyro instruments; slaved and modern electronic systems normally obtain heading from a magnetic sensor.
| Instrument type | Heading source | Pilot setting |
|---|---|---|
| Traditional DG or DI | Vacuum- or electrically driven free gyro | Align the compass card manually and correct periodic drift |
| Slaved HSI or compass system | Gyro system corrected by a magnetic sensor | Normally automatic in slave mode; manual synchronisation may be needed in free mode |
| Glass cockpit or AHRS | Magnetometer and inertial sensors | Heading alignment is normally automatic, but the heading bug is still selected manually |
Some electronic displays can use magnetic or true heading. If two instruments disagree by roughly the local magnetic variation, check that both are using the same reference before treating either as faulty.
How do you set the heading indicator in a flight simulator?
In a flight simulator, first identify whether the aircraft models a free directional gyro, a slaved system or an AHRS display.
- Check the aircraft’s magnetic compass. Let it settle while stationary or in steady flight.
- Synchronise a traditional DG. Use the virtual cockpit adjustment knob or the simulator command for setting or synchronising the heading indicator.
- Set the heading bug separately. Turn its selector to the desired heading for visual reference or autopilot HDG mode.
- Verify the active mode. If the autopilot ignores the bug, confirm that HDG rather than NAV, LOC or another lateral mode is active.
Control-assignment wording varies between simulators and add-on aircraft. Look for separate commands covering the heading bug and gyro synchronisation; assigning the bug command will not correct gyro drift. For the wider cockpit scan, see our guide to reading the main flight instruments in Microsoft Flight Simulator.
Why is the heading indicator showing the wrong heading?
A wrong heading usually comes from normal gyro drift, an unreliable comparison made during a turn, loss of gyro power or confusion between magnetic heading, true heading and ground track.
- Slow disagreement with the compass: realign a powered free gyro during straight and level flight.
- Rapid drift, a frozen card or a warning flag: check the relevant vacuum or electrical system. In a real aircraft, follow the approved failure checklist rather than repeatedly resetting the instrument.
- Disagreement only while turning or accelerating: wait until the magnetic compass settles before making an adjustment.
- Autopilot does not follow the bug: verify that the autopilot and HDG mode are engaged and that the simulator control is mapped to the heading bug rather than the compass card.
- Runway number does not exactly match the indication: runway designators are rounded magnetic headings, not calibration references.
If a real aircraft’s heading indicator continues to drift rapidly after correct alignment, or disagrees substantially with the compass alongside abnormal power indications, treat it as an instrument or system fault rather than a setting error.