Lubrication and Cooling Systems
Domain 6 - Lubrication and Cooling Systems
The exam tests oil system layout, oil condition monitoring and engine heat management.
Core rules
- A dry-sump system has a tank, pressure pump, filter, oil/air or fuel/oil cooler, main bearing jets, scavenge pumps, chip detector and breather. Scavenge capacity is larger than pressure capacity so the sump never floods.
- Oil lubricates, cools, cleans and protects against corrosion. MIL-PRF-23699 oils are standard for turbine engines; mixing grades is normally prohibited unless the manual allows it.
- Key indications are pressure, temperature and chip detector. Low pressure with normal temperature suggests a leak, pump wear or a filter bypass opening. High temperature with normal pressure suggests cooler fouling, low quantity or bearing distress.
- Oil analysis by spectrometry, water content and particle count finds wear metals and can detect a pending failure before a chip light.
- Cooling air serves turbine blades and nozzles, active clearance control, IDG and generator cooling, and cabin bleed. Fuel/oil heat exchangers use fuel as the ultimate heat sink, so fuel temperature limits matter.
Worked example
At idle, oil pressure is 25 psi against a 30 psi minimum; at takeoff it reaches 45 psi. Pump output rises with N2, so the takeoff figure masks the defect. A shortfall at idle points to a worn pump or a leaking relief valve, not a gauge error. The correct action is a check with a calibrated gauge, then pump and relief valve inspection.
Traps
- Topping up with a different oil grade as a temporary measure.
- Ignoring a chip detector warning because the oil looks clean.
- Diagnosing high oil temperature as a sensor fault before checking the cooler and fuel flow.