VACUUM SYSTEM

Vacuum System Lesson by wifiCFI

Vacuum System

How it Works

The aircraft’s vacuum system is typically run via vacuum pump connected to the aircraft’s engine.

The vacuum pump sucks air through the vacuum system to power the vacuum associated instrument gauges.

The typical instrument on an small aircraft’s vacuum system are the:

Attitude Indicator

Heading Indicator

Below is a typical vacuum system schematic found on light general aviation aircraft.

Gyroscope Operation

The air passing through the Attitude and Heading Indicators spin up gyroscopes located within the instruments.

Gyroscopes work based on 2 principles:

Rigidity in Space

Precession

Note:

On light general aviation aircraft there is typically a 3rd instrument that is gyroscopically operated, it is the Turn Coordinator. However, the Turn Coordinator is typically driven electrically instead of via vacuum pump. This is to provide a back up to pilot’s in case the aircrafts vacuum pump system becomes inoperative in flight.

Rigidity In Space

Rigidity in Space means a gyroscope (when spinning) tends to resist the motion of the aircraft in flight.

You can think of it as the airplane rotating and pitching “around” the instrument gyroscopes.

This helps a pilot to know his/her orientation in space (ie pitching, banking, etc.)

A gyroscope is able to maintain its position in space due to its rotational velocity.

This is why a bicycle can ride straight up when the tires begin to rotate fast enough.

This is because more “G-Forces” are pulling out on the gyroscope than are pulling down on the gyroscope.

Precession

Precession deals with how a gyroscope will react when a force is applied to it.

Because of the rotational velocity of the gyroscope, precession states that the resulting for will take place 90 degrees ahead of the applied force in the direction of rotation.

This is depicted below.

While gyroscopes are efficient at remaining “Rigid in Space,” they can “tumble” when excessive forces are applied.

Attitude Indicator

May tumble with pitch attitude of 60-70 degrees (up or down) or

With a banking attitude of 100-110 degrees.

Heading Indicator

Excessive bank angles.

Turn Coordinator

Excessive bank angles.

FAA Sources Used for This Lesson

Instrument Flying Handbook