Lubrication and Sealing Systems
What this domain tests
You must know the oil system path, oil types and limits, how seals keep oil out of the gas path, and what indications warn of seal or pump failure.
Core rules
- Typical turbine oils are MIL-PRF-23699 (standard), MIL-PRF-23899 (high thermal stability) and MIL-PRF-7808 (low temperature). They are synthetic esters; never mix grades unless the manual permits.
- The system has a tank with expansion space and breather, pressure pump, filter, fuel/oil or air/oil cooler, scavenge pumps, chip detector and pressure and temperature transmitters.
- Seals: labyrinth seals meter air rather than touch; carbon seals run on a mating face; brush seals reduce leakage; hydraulic seals use oil pressure. Bearing compartments are pressurised with compressor bleed so oil leaks outward, not into the gas path.
- Low oil pressure at idle can be normal when hot; low pressure at high power is not. Rising oil temperature often points to a cooler or scavenge problem.
Worked example
A 20-hour flight starts with 12.0 qt and ends with 10.5 qt. Consumption = 1.5 qt ÷ 20 h = 0.075 qt/h. If the limit is 0.1 qt/h, the engine is within limits, but trend it: a sudden rise from 0.02 to 0.075 qt/h deserves a defect report because it may indicate a seal or breather problem.
Traps
- Overfilling the tank; foam and breather loss follow.
- Using automotive or piston-engine oil.
- Ignoring a chip detector warning or resetting it without inspection.
- Assuming oil pressure gauges are direct-reading; many are electrical and can fail.
Key points
- MIL-PRF-23699, 23899 and 7808 are synthetic turbine oils; do not mix grades.
- Pressure pump, filter, cooler, scavenge pumps and chip detector form the loop.
- Labyrinth, carbon and brush seals control oil leakage; bearing compartments are pressurised.
- Low oil pressure at idle can be normal; low pressure at high power is not.
- Oil consumption trend matters more than a single reading.