Lubrication
Lubrication tests the oil system's job: supply clean, cool oil at the right pressure to bearings and gears, then scavenge it and detect distress. In helicopter engines and transmissions, oil condition is a primary health indicator.
Core rules
- A dry-sump system uses a pressure pump, filter, cooler, jets and scavenge pumps returning oil to the tank.
- Oil jets meter flow; pressure is set by the pump and relief valve, not by the jets alone.
- Filters have a bypass and differential-pressure indicator; bypass means unfiltered oil is circulating.
- Chip detectors and magnetic plugs detect ferrous debris; fine fuzz is normal, flakes are not.
- Synthetic oils resist coking but still have temperature limits; overheating leaves varnish and carbon.
- Oil analysis tracks viscosity, water, metals and additive depletion.
Worked example
Oil flows at 30 L/min with density 0.90 kg/L and specific heat 1.9 kJ/kg·K. The temperature rise across the engine is 30°C. Mass flow = 30 × 0.90 = 27 kg/min. Heat rejected = 27 × 1.9 × 30 = 1539 kJ/min = 25.65 kW. That heat must be removed by the cooler; if the cooler is blocked, oil temperature rises and viscosity falls, reducing film strength.
Traps
- Overfilling a tank and causing aeration or overflow.
- Mixing oil brands or grades without approval.
- Ignoring a bypass indicator because pressure looks normal.
- Treating a chip detector warning as a nuisance.
- Checking oil level immediately after shutdown without allowing drain-down.