EAA Sport Aviation - December 2009

nuts & bolts
maintenance & restoration
Aircraft Vacuum Systems
Why they fail and tips for installation
JEFF SIMON, EAA 478233

Aircraft vacuum pumps are one of the most critical yet controversial components in any general- aviation aircraft. Vacuum power drives the gyros, supports the autopilot, and produces the air pressure for de-icing boots; all are critical to safe instrument flight rules (IFR) navigation. The source of vacuum power on most general aviation aircraft is dry, rotary-vane-style pumps that are lightweight and self-lubricating. The controversy and challenge lie in the fact that these pumps are also one of the least predictable devices on the aircraft, in terms of failure. And when they fail, it is usually catastrophic, with no residual vacuum power at all.

Predictability is the holy grail of aircraft design and maintenance. It’s not critical that an aircraft component last forever, just that we know when it’s likely to fail so that we can repair or replace the component proactively. That’s the concept behind TBOs (time between overhauls) and life-limited parts.

In theory, vacuum pump life should be extremely predictable. Pump manufacturers have studied pump life closely enough to be able to accurately predict vane wear over time. It would be logical to conclude that if we know exactly how a component will wear over time, we should be able to replace the component well before that wear becomes a safety issue.

Unfortunately, excessive wear is only one of many ways a vacuum pump can fail. The remaining failures are caused by issues outside of the pump itself. Therefore, if we want to increase the reliability of the vacuum pump, we need to address the health and maintenance of the entire vacuum system.

Vacuum System Basics

As soon as gyros were implemented for aircraft navigation, there was a need to power them. Vacuum power was an excellent solution that could be used to power multiple gyros from a single power source. By pulling air over a rotor vane inside the gyro, the mechanical gyro could be kept spinning at a constant rate. Using a vacuum, rather than a positive pressure flow, made it easy to regulate the pressure and provide for clean intake air through a filter inside the aircraft. It was an eloquent solution in a time

Unfortunately, excessive wear is only
one of many ways a vacuum pump can
fail. The remaining failures are caused
by issues outside of the pump itself.

before today’s modern electric motor technology.

A typical vacuum system consists of an intake filter, followed by a pressure regulator and a vacuum gauge. The air then flows through the gyroscopic instruments and, finally, to the vacuum pump. On most single-engine aircraft, the pump outlet simply exhausts the pressure side into the engine cowl. However, on twin aircraft with de-icing boots, two vacuum pumps are combined to provide a redundant vacuum source, while one of the pumps includes an additional regulator and a series of valves to direct the exhaust air for use to inflate the boots.