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Aviation Learning Center Document AC 91-75 - Attitude Indicator
Author: Federal Aviation Administration Date: June 2003
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4. Discussion
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a. The attitude indicator is centrally located in the pilot's primary field of view. It provides easily interpreted pitch and bank information in one instrument. For this reason, most pilots tend to rely heavily on the attitude indicator in IMC to maintain aircraft control. Most small general aviation airplanes typically have an attitude and heading indicator powered by a single vacuum source. These airplanes normally do not have redundant vacuum systems or a second attitude indicator. Therefore, to recognize that a failure condition exists and isolate which instrument has failed, the pilot must cross-check other instruments that indicate pitch or bank information.

NOTE: Standby vacuum pumps and attitude indicators with failure warning flags are available in newer model aircraft and are slowly making their way into the general aviation fleet as retrofits. Many of the newly manufactured aircraft intended for use in IMC are being delivered with these systems. Additionally, some new aircraft only have electric gyro instruments and are equipped with redundant, independent electrical power sources. The systems described above offer mitigation to the risk of pilots being misled by a failed attitude indicator.

b. The rate-of-turn indicator (either a turn coordinator or turn and bank indicator) is used to cross-check the attitude indicator and directional gyro for bank information. Turn coordinators also include a failure warning flag that provides an indication if the gyro power source is lost. The rate-of-turn indicator typically has an independent power source to provide redundancy for the attitude indicator and directional gyro. However, the rate-of-turn indicator only provides information that the aircraft is in a turn and whether the turn is at the standard rate (a standard rate turn is 3 degrees per second, or 2 minutes for a full 360-degree circle). It does not provide information on the bank angle.

c. A disagreement between the vacuum-powered gyro instruments and the rate-of-turn indicator is typically the first indication of a failure condition. To confirm which gyro has failed, the pilot must cross-check other flight instruments.

d. Loss of aircraft control can occur:

  • (1) When the pilot does not recognize that a failure condition exists and continues following erroneous instrument indications.

  • (2) During the time it takes the pilot to determine which system has failed.

  • (3) During subsequent partial panel instrument flying, which can be confusing if the pilot cannot cover the failed instrument(s).

e. Loss of control is most likely to happen during a high workload environment such as instrument departure or instrument approach. The risk increases in aircraft with higher complexity and speed.

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