FAA Aviation Mechanic Airframe (AMA)Hydraulic and Pneumatic SystemsHard
A technician performing troubleshooting on a variable-displacement engine-driven hydraulic pump measures the case drain flow rate and finds it significantly higher than the value specified in the maintenance manual for a pump at normal operating pressure. What does this excessive case drain flow most likely indicate?
- AA clogged pump inlet filter restricting suction flow
- BAn overcharged accumulator upstream of the pump
- CExcessive internal wear or leakage within the pump
- DNormal pump operation at high system demand
Show answer & explanationAnswer & explanation
Correct answer: C. Excessive internal wear or leakage within the pump
The case drain line carries internal pump leakage (used for lubrication and cooling) back to the reservoir. A higher-than-specified case drain flow rate indicates increased internal leakage past the pump's internal seals and pistons, a sign of excessive wear, and is a standard diagnostic method for assessing variable-displacement pump condition.
Why the other options are wrong
- A. A clogged inlet filter would starve the pump and reduce output pressure/flow, not increase case drain flow.
- B. An overcharged accumulator affects surge damping downstream, not internal pump case leakage.
- D. Normal operation produces a consistent, specified case drain flow; higher-than-normal flow is not simply due to demand.
Case Drain Line
A line that routes internal leakage fluid from within a variable-displacement pump housing back to the reservoir, used to cool and lubricate the pump and serving as a wear-monitoring point.
- Internal leakage normally cools/lubricates pump components
- Case drain flow rate is compared to manufacturer specifications
- Excessive flow indicates worn pistons, cylinder bores, or seals
Memory trick: Case drain = the pump's 'blood test' — too much flow means internal wear.