Lubrication Systems
Systems and flow
Reciprocating engines use either a wet-sump system (oil stored in the engine sump) or a dry-sump system (oil in an external tank). Both have a pressure pump feeding oil to the bearings and one or more scavenge pumps returning it. Know the path: tank, pressure pump, relief valve, oil cooler (with bypass valve), filter or screen (with bypass valve), galleries, bearings, scavenge pump, tank.
Turbine engines also use a dry-sump arrangement but require a different oil - never interchange turbine and reciprocating oils. Ashless dispersant oil is standard for opposed engines; mineral oil may be specified for break-in.
Traps
The oil pressure relief valve sets maximum pressure; the cooler bypass valve protects against cold, thick oil blocking flow; an open filter bypass means unfiltered oil is circulating. A sudden rise or drop in pressure, or a sudden change in consumption, is a red flag. Do not confuse a pressure gauge reading with oil quantity.
Worked example
For most light reciprocating engines, steady consumption of roughly one quart per 4-8 hours is normal; large radials can use more. If an engine goes from one quart per 8 hours to one quart per hour, look for a broken ring, worn valve guides, or a leaking seal before further flight.
Rule of thumb
After starting, oil pressure should appear within about 30 seconds; if it does not, shut down. For turbines, follow the AFM - typically no oil pressure indication within about 30 seconds means abort the start.