Attitude Indicator Explained: My Attitude Indicator Isn't Working

Depending on your flight plan, failure of the attitude indicator (AI) may not keep you from flying. Often, common attitude indicator errors will give you a hint that it's time to have the instrument examined by a professional. It may only need an overhaul, or it could be time to replace the unit.

One of the most common attitude indicator errors occurs when the AI doesn't erect properly. When this happens, it's usually due to the gyro failing to spin as fast as it should. A properly functioning AI should erect or stabilize within three minutes of powering up. A malfunctioning AI will oscillate or precess when the aircraft is level and not moving. You may notice errors during preflight or at the run-up check.

We'll look at several reasons why the AI may fail to erect.

Bad Bearings

Worn out bearings on the gyro means the gyro won't be able to spin as fast as it should to maintain the required RPMs to display a true attitude. The slow falling out or sluggish movement of the indicator is known as precessing. In the case of bearing failure, there is too much friction. The attitude indicator will not be able to erect or get into the correct starting position. Environmental factors such as temperature changes can affect bearing performance, and hard landings may damage the bearings. Carbon contamination from the pump vanes can also cause problems. Add or replace the filter in the system line to help alleviate this issue.

System Leak/Obstruction

The vacuum system that powers the gyro could develop a leak or become crimped, reducing airflow through the instrument. Typical attitude indicator errors include being slow to erect or displaying incorrect indications inflight. Depending on the size of the leak, the error can be challenging to detect.

Vacuum or Pressure Pump Failure

The vacuum or pressure pump can fail slowly or all at once. Like a leak, intermittent pump problems can present as failure to erect or other errors. The pump may also fail instantly, putting the AI out of commission.

Flight Envelope

Typical vacuum-driven attitude indicators have a limit of 100–110 degrees of bank and 60–70 degrees of pitch. For older AIs, the limits are closer to 60 degrees in pitch and 100 degrees in roll. If these limits are exceeded, the attitude indicator can tumble or oscillate until the aircraft returns to the limits and the AI erects again. It can take a few seconds to several minutes for the gyro to stabilize and the instrument to center properly.