If the vacuum pump air filter becomes blocked, how will this affect the instruments?
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A
Air driven instruments will respond erratically, or slowly due to the low airflow
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B
Air driven instruments will respond correctly until the system becomes totally blocked
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C
None - as the vacuum relief valve is designed to prevent this being a problem
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D
It will cause the air driven instruments to be overly sensitive due to gyro overspeed
Refer to figure.
The figure shows the vacuum system for a typical light training aircraft. Ambient air at atmospheric pressure enters through an air filter from outside, and the engine drives a vacuum pump to create low pressure. This pulls air through the system, driving air driven gyroscopic instruments such as the Attitude Indicator, Direction/Heading Indicator, Turn Co-ordinator and the suction gauge to monitor the vacuum pressure.
If the inlet filter becomes blocked, then the flow of air through the system will become impeded. The vacuum pressure will become quite low, as atmospheric air is not flowing in. This means that the airflow powering gyroscopic instruments is severely reduced. If the gyroscopes are not spinning fast enough, these air driven instruments will respond erratically, or slowly due to the low airflow.
The option "None - as the vacuum relief valve is designed to prevent this being a problem" seems like a sensible choice, however surprisingly there tends not to be any alternative air source for vacuum/air powered instruments.
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