Aircraft Gyroscopic Instruments and the Vacuum System

Posted on November 11, 2019 Jerold Perkins Aviation

Gyroscopic instruments are an essential part of flight navigation. They include the gyro compass, attitude, turn and bank, and turn coordinator instruments within the electronic flight instrument systems, or EFIS, that pilots rely on. Two of the popular gyroscopic instruments that are run using the vacuum gyro system are the pressure pump system, as well as older models that use a venturi. Newer models, however, have begun to use conventional gyro systems as a backup while the main system is electrically driven. With vacuum systems still remaining a very popular and widely used style in aircraft, how do the types function?

Most gyroscopic vacuum systems consist of a few main parts to monitor the vacuum in the system: namely the pressure relief valve, attitude and directional gyro, suction gauge, in-cockpit air filter, and the engine driven vacuum pump. The system functions as air is brought into the vacuum and pulled through the systems, causing the gyroscopes to spin which enables the instruments to function.

Venturi tubes are the vacuum gyro system that are primarily used in older aircraft models, and they function by supplying vacuum during aircraft flight. They are placed directly in the airflow, thus produce some drag at the benefit of not needing engine power. They do, however, also have the drawback of potential ice pickup which makes them unable to be used for instrument flight rules (IFR) flights due to it being unsafe to reference outside visuals.

The pump vacuum gyro system avoids the problem of drag at the cost of some engine power, and is a dry type and utilizes carbon vanes, holding a lifespan of around 500-1000 hours. They are more useful in IFR flights due to not sitting in the airflow, and their lifespan can be monitored by the slow drop in suction and when instrument tumble begins.

Electrical driven gyros are also a form of gyroscopic system that is becoming more popular for use in newer aircraft moving away from traditional vacuum gyro systems, as well as for electronic driven glass EFIS. While they are more expensive, they have the added benefit of being completely sealed to dust, more stable indication, have a greater lifespan, and run on higher RPM. Nevertheless, these models of aircraft often have traditional vacuum gyro systems as a backup.

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