FAA Aviation Mechanic Powerplant (AMP)Reciprocating EnginesMedium
An engine equipped with a full-flow oil filter experiences a partially clogged filter element during operation. What is the purpose of the bypass (relief) valve built into the oil filter assembly?
- ATo allow unfiltered oil to continue flowing to the engine if the filter becomes restricted, preventing oil starvation
- BTo divert oil to the propeller governor when filter flow is reduced
- CTo route excess oil overboard through the breather to relieve crankcase pressure
- DTo increase oil pressure above the normal relief valve setting during cold starts
Show answer & explanationAnswer & explanation
Correct answer: A. To allow unfiltered oil to continue flowing to the engine if the filter becomes restricted, preventing oil starvation
A clogged filter element would otherwise starve the engine of lubricating oil. The bypass valve opens when a pressure differential across the filter exceeds a set value, allowing oil to bypass the clogged element and continue circulating to protect the engine, even though the oil is temporarily unfiltered.
Why the other options are wrong
- B. Governor oil supply is a separate system unrelated to the filter bypass valve.
- C. This describes a crankcase breather function, not the filter bypass valve.
- D. The bypass valve does not raise system pressure; it relates only to differential pressure across the filter.
Oil Filter Bypass Valve
A spring-loaded valve in a full-flow oil filter that opens when the filter element becomes clogged, allowing unfiltered oil to continue lubricating the engine.
- Opens based on differential pressure across the filter element
- Prevents oil starvation from a clogged filter
- Better to circulate unfiltered oil than none at all
Memory trick: When the filter clogs, the bypass door swings open — flow must go on!