Accurate Flight Instruments: How Directional Gyros Work

As a pilot, making good time is always great, but knowing where you are is more important. That’s why you need accurate flight instruments to operate from point A to B safely, which is where the directional gyro comes in. Directional gyros indicate the direction that the aircraft is heading.

Directional gyros, also called heading indicators or direction indicators, are the fastest moving component in a piston-powered aircraft. They can spin at up to 24,000 rpm, and are among a plane’s most critical systems. At a glance, the directional gyro looks like a compass. In fact, the shortcomings of the compass are remedied by the directional gyro.

Directional gyros are unaffected by the conditions that compromise a compass. The directional gyro uses a gyroscope that resists change to its position. It’s connected to a compass card, which moves with changes to the aircraft heading and displays the compass rose direction in 5-degree increments. Mechanical friction eventually catches up and will cause the directional gyro to precess.

Normal procedure is to realign the direction indicator once every 10-to-15 minutes during routine in-flight checks. Failure to do this is a common source of navigation errors among new pilots.

The directional gyro is one of six critical instruments installed in an aircraft and adopted by both commercial and general aviation.

Read more in The Six Pack: Aircraft Instruments Explained.

Like all instruments, directional gyros can fail. Gyros often, but not always, give clues of imminent failure. Heading drift in the directional gyro is a pre-indicator of failure that is often only apparent in flight. Abnormal sound or vibration from the instrument can also indicate failure.

The most common cause of directional gyro problems is bearing failure. It can be caused by any of the following factors:

• Normal wear due to time in service or not using the instrument for long periods of time.
• Adverse wear due to the instrument ingesting dirty air. This is caused by a missing or defective filter in a vacuum system.
• Contamination by debris from a failed vacuum pump in a pressure system where the filter was inadequate, or the system was not purged correctly following pump failure.
• Impact damage due to a hard landing or rough handling of the gyro rotor and gimbal bearings.

To maintain directional gyro accuracy, the instruments require regular and delicate maintenance. Dropping the gyro, even less than a quarter of an inch, will damage most modern gyros, as the instrument is very sensitive and a small drop is equivalent to applying 1 unit of G-force, or more, to it. A heavy landing can also cause damage, as can rough handling during installation, storage or shipping.

Here are eight key maintenance points for directional gyros:

1. When you order a gyro, check it as soon as you receive it.
2. Do not unplug or plug your gyro in with aircraft power on. (Not affected on air driven gyros)
3. Allow the aircraft to come to a complete stop and sit for at least 15 minutes before attempting to remove the gyro. This gives it time to completely spool down and stop spinning. The newer and more efficient the unit, the longer the gyro will spin.
4. For vacuum gyros, always keep system and gyro filters clean. Don't plumb gyros in series, as this tends to reduce vacuum pressure to the units.
5. Do not lubricate fittings on vacuum gyros. This will contaminate the gyro. Teflon tape is recommended.
6. If gyros are in storage or are not used for periods up to six months, they need to be powered up and operated for 30 minutes and allow the gyro to rotate on all axes and the bearings to be exercised and lubricated.
7. Anytime a gyro is removed from the aircraft, it should be immediately put into an adequately padded shipping container.
8. Store the gyro flat on a shelf, resting on padding, in a temperature-controlled environment.