Lubrication and cooling systems
Lubrication and cooling systems
This section covers oil storage, pressure and scavenge, filtration and temperature control, plus air and liquid cooling. The exam tests system flow, why oil pressure and temperature behave as they do, and cooling limits.
Core rules and concepts
- Most aircraft piston engines use a dry-sump system: a pressure pump sends oil from the tank to bearings and rocker areas; a scavenge pump returns it. A relief valve limits pressure.
- The oil cooler and vernatherm (or bypass valve) regulate temperature. A filter bypass ensures lubrication if the element blocks. Low pressure can mean low level, pump wear, relief-valve trouble or a leaking gallery.
- Air cooling depends on fin area, baffling and cowl flaps. Cylinder-head temperature is the limiting measurement; uneven cooling causes warped cylinders and stuck valves.
- Liquid cooling uses coolant, a radiator and a thermostat; it needs correct coolant mix and pressure cap operation.
- Oil does more than lubricate: it cools, cleans, seals and protects against corrosion.
Worked example
After a run-up, oil pressure is below the green arc and falls as temperature rises, while the tank level is correct. The engine is shut down. The mechanic checks the relief valve, oil cooler bypass and pressure gauge before flight. A worn pump or stuck relief valve can cause low pressure; continuing to run risks bearing damage.
Traps
Do not confuse high oil temperature with high oil pressure; do not ignore a missing baffle seal; and do not use automotive oil unless approved for the engine.