FAA Aviation Mechanic Powerplant (AMP)Fuel, Induction and LubricationHard
A mechanic is comparing the oil pressure and scavenge pumps in a turbine engine lubrication system. The scavenge pump has a significantly greater pumping capacity than the pressure pump. What is the primary reason for this design?
- AThe scavenge pump also removes fuel that has leaked into the oil system
- BThe scavenge pump must overcome higher discharge pressure than the pressure pump
- CA larger scavenge pump reduces the need for an oil cooler in the system
- DOil returning from the bearings is aerated (mixed with air), increasing its volume and requiring greater capacity to return it to the tank
Show answer & explanationAnswer & explanation
Correct answer: D. Oil returning from the bearings is aerated (mixed with air), increasing its volume and requiring greater capacity to return it to the tank
As oil passes through bearing compartments, it becomes aerated with air, significantly increasing its volume. The scavenge pump must therefore have greater capacity than the pressure pump to ensure all the aerated oil is returned to the tank and bearing sumps do not flood, even though the mass of oil is the same.
Why the other options are wrong
- A. Fuel leakage into oil is an abnormal fault condition, not the design reason for scavenge pump sizing.
- B. Scavenge pumps actually operate against lower discharge pressure, not higher, since they return oil to the low-pressure tank.
- C. Scavenge pump capacity is unrelated to eliminating the need for an oil cooler; deaerated oil is still cooled before recirculation.
Turbine Scavenge Pump Sizing
Turbine engine oil scavenge pumps are built with greater capacity than the pressure pump to handle the increased volume of aerated oil returning from bearing sumps.
- Oil becomes aerated (foamy) after lubricating bearings
- Aeration increases oil volume without increasing mass
- Scavenge pump capacity must exceed pressure pump capacity to prevent sump flooding
Memory trick: Foamy oil takes up more room, so scavenge needs a bigger scoop.