The NTSB calls your attention to Advisory Circular AC 91-75, a Safer Skies initiative recommendation authored by the FAA and the industry that highlighted vacuum system failures as a significant cause or contributor to fatal accidents in IMC.

The organizations conducted research to further analyze the potential hazards of partial panel operations in IMC. As part of the study, the FAA’s Civil Aeromedical Institute conducted simulations studying pilot responses to vacuum system failures, while the AOPA’s Air Safety Foundation conducted parallel flight studies in a Piper Archer and a Beech Bonanza. These studies included pilots with varying experience levels in both high- and low-performance aircraft.

The studies showed that a vacuum system or gyro failure is insidious because the gyro fails slowly, making the failure difficult to recognize. Data indicated that pilots might not have the proficiency to safely recover and land the airplane, particularly in high-performance airplanes, even though instrument-rated pilots receive partial panel training.

It also identified that, because 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.

Since 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.