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Instrument Flying Handbook
Flight Instruments
Vacuum Pump Systems

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Instrument Flying
Handbook

Preface

Table of Contents

Chapter 1. Human Factors
Chapter 2. Aerodynamic Factors
Chapter 3. Flight Instruments
Chapter 4. Section I
Airplane Attitude Instrument
Flying
Using Analog Instrumentation
Chapter 4. Section II
Airplane Attitude Instrument
Flying
Using an Electronic Flight
Display

Chapter 5. Section I
Airplane Basic
Flight Maneuvers
Using Analog Instrumentation
Chapter 5. Section II
Airplane Basic
Flight Maneuvers
Using an Electronic Flight
Display

Chapter 6. Helicopter
Attitude Instrument Flying

Chapter 7. Navigation Systems
Chapter 8. The National
Airspace System

Chapter 9. The Air Traffic
Control System

Chapter 10. IFR Flight
Chapter 11. Emergency
Operations

venturi tube system
Figure 3-27. A venturi tube system that provides necessary vacuum
to operate key instruments.

Vacuum Pump Systems

Wet-Type Vacuum Pump
Steel-vane air pumps have been used for many years to
evacuate the instrument cases. The vanes in these pumps
are lubricated by a small amount of engine oil metered into
the pump and discharged with the air. In some aircraft the
discharge air is used to inflate rubber deicer boots on the
wing and empennage leading edges. To keep the oil from
deteriorating the rubber hoots, it must be removed with an
oil separator like the one in Figure 3-28.

The vacuum pump moves a greater volume of air than is
needed to supply the instruments with the suction needed,
so a suction-relief valve is installed in the inlet side of the
pump. This spring-loaded valve draws in just enough air to
maintain the required low pressure inside the instruments,
as is shown on the suction gauge in the instrument panel.
Filtered air enters the instrument cases from a central air
filter. As long as aircraft fly at relatively low altitudes,
enough air is drawn into the instrument cases to spin the
gyros at a sufficiently high speed.

Dry Air Vacuum Pump
As flight altitudes increase, the air is less dense and more air
must he forced through the instruments. Air pumps that do not
mix oil with the discharge air are used in high flying aircraft.

Single-engine instrument vacuum system using a steel-vane wet type vacuum pump.
Figure 3-28. Single-engine instrument vacuum system using a steel-vane wet type vacuum pump.

 

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