FAA Aviation Mechanic Powerplant (AMP)Fuel, Induction and LubricationHard
During overhaul of a wet-sump reciprocating engine's oil pump, a mechanic measures excessive gear-to-housing clearance beyond the manufacturer's limits. What is the most likely operational effect of this condition?
- AIncreased oil pressure due to reduced pump displacement
- BComplete loss of oil quantity through external seals
- CImproved lubrication due to increased oil churning within the pump
- DIncreased internal oil leakage past the gears, reducing pump output and oil pressure, especially at low RPM
Show answer & explanationAnswer & explanation
Correct answer: D. Increased internal oil leakage past the gears, reducing pump output and oil pressure, especially at low RPM
The gear-type oil pump depends on close clearances between the gear teeth and housing to create a positive-displacement seal. Excessive clearance allows oil to leak backward internally (slippage), reducing volumetric efficiency and oil pressure, which becomes most noticeable at low engine RPM when pump speed and output are already reduced.
Why the other options are wrong
- A. Increased clearance reduces, not increases, effective displacement and pressure.
- B. Internal gear clearance does not directly cause external seal leakage.
- C. Excessive clearance harms, rather than improves, pump efficiency.
Oil Pump Gear Clearance
The precise gap between pump gears and housing in a positive-displacement gear-type oil pump; excessive clearance allows internal oil slippage, reducing pump output and system oil pressure.
- Wet-sump engines commonly use gear-type positive-displacement oil pumps
- Clearance must be checked against manufacturer limits during overhaul
- Excessive clearance is most noticeable as low oil pressure at idle/low RPM
Memory trick: Loose gears leak oil backward like a leaky faucet.