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Attitude Indicator Explained:
Why is My Attitude Indicator Precessing?

airplane instrument panel with attitude indicator in the center

July 25, 2019

Attitude indicator errors can be problematic because AIs are pivotal to keeping airplanes level in flight. There are a number of factors that can cause unwanted precession, from bearing issues to unusual aircraft maneuvers. Understanding how AIs work and why they may precess unexpectedly will allow you to be proactive in case you need a repair or overhaul.


How Does an Attitude Indicator Work?

AIs, sometimes referred to as gyros or artificial horizons, operate with a gyroscope that indicates how level the aircraft is oriented to the earth. Rotating the gyroscope around the longitudinal axis indicates the degree of bank or roll, whereas the lateral axis indicates pitch — nose up, nose down or level. The gyroscope is gimbaled at a lateral axis for pitch attitude and a longitudinal axis for roll.

The erecting mechanism of a vacuum-driven attitude indicator

Source: Aviation Safety Bureau

The gyroscope works by creating a rigidity in space while spinning and is placed to take into account the tilting of that instrument. An AI functions properly when its gyroscope remains vertically upright while the aircraft rolls and pitches around it.

The bearings in AIs must have minimum friction. Even a small amount places a strain on the gyroscope, causing it to tilt. To minimize this tilting, a mechanism inside the instrument applies a force when the gyroscope tilts from its vertical position. This force returns the gyroscope to its upright position.

All mechanical gyroscopes have inner and outer bearings in a gimbal assembly or rotor assembly. In the rotor assembly, bearings contain bearing oil for anti-friction and are made to operate at high RPMs.


What Is Precession?

In the closed system of an AI, the gyroscope stands erect and banking of the plane will tilt it off the axis. When an outside force tries to tilt a spinning gyroscope, it responds as if the force had been applied at a point 90 degrees further around in the direction of rotation.

Precession occurs when the vacuum or electric motor of an AI operates outside the design limits, or when any excess frictional force disturbs the free rotation of the gyroscope at design speed. This causes a slow falling out or sluggish movement of the indicator. Precession will keep the instrument from giving a correct reading.


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What Causes an Attitude Indicator to Precess?

Bearing Issues

Contamination from the vacuum pump, deteriorating air lines and moisture can all contribute to bearing issues. Contamination in the bearing can cause the bearing oil to break down over time and cause the rotor to slow down. When the rotor is no longer operating at the higher RPM, it will cause the attitude to precess.

Low pressure or vacuum from the pump can also cause an attitude gyroscope to precess. The proper vacuum level should be 4.5 inches to 5.5 inches. Pressure should be 2 pounds to 2.5 pounds. Because of various factors, vacuum or pressure gauges may not give accurate information, so pilots are encouraged to double-check their readings.


Wear and Tear

Unwanted precession can be caused by friction in the gimbals and bearings. Worn-out bearings would indicate overhaul or replacement. Loose fittings or blockages in the lines to the gyroscope could also contribute to precession.

Lack of use can also affect the bearings. If the attitude indicator is not periodically exercised, then the oil will settle to the bottom of the bearings, which leaves the bearing exposed to possible corrosion. This will quickly wear out the bearings, causing them to not be as effective or even fail.

Bad bearings are unable to keep up with the required RPM to keep an attitude indicator true.


Other Causes

Normal coordinated turns can also introduce temporary errors in both pitch and bank indications. The errors are caused by centrifugal force, and will correct in a normally operating AI. Acceleration and deceleration also cause precession errors.

Harsh maneuvers and aerobatics will cause precession errors also. The gyroscope in the AI will tumble past 180 degrees, then will correct slowly. Over time, this self-correction may be slower and readings may be less accurate. However, most aerobatics planes include AI with a cage function to aid with 180 inversion of AI.

Finally, electrical gyros may also experience motor electrical system failures that could result in precession.


Mid-Continent Instruments and Avionics Repair and Overhaul Services

If your attitude indicator needs repair or replacement, Mid-Continent Instruments and Avionics has the expertise you need. We have been an industry leader in the overhaul, exchange, repair, design and manufacturing of aircraft instruments, avionics and advanced power solutions for more than half of a century.

We operate one of the largest instrument overhaul and exchange programs in the world, supporting a customer base that includes original equipment manufacturers, fixed base operators, instrument and avionics shops, flight schools and government agencies.

Our well-earned reputation for reliability, quality and exceptional service makes us a favorite among pilots. Ask your local avionics dealer about our products and overhaul services. Locate an avionics dealer near you.


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