Oil system: design, function, and gauges
Oil system functions:
- Lubricates moving parts
- Cools: reduces friction, pulls heat from cylinders
- Seals cylinder walls against pistons
- Carries away contaminants
Wet-sump vs dry-sump:
| Wet-sump | Dry-sump | |
|---|---|---|
| Oil held in | Sump built into engine | Separate external tank |
| Supply | Oil pump (main component) draws from sump, routes oil to engine (pressure lubrication) | Oil pump still supplies engine |
| Return | Oil returns to sump after circulation | Scavenge pumps return oil from engine to tank |
| Extra | Some engines: rotating crankshaft also splashes oil onto parts (PHAK ch.7) | More oil carried: very large reciprocating engines |
Oil type and quantity:
- AFM/POH or placards near access panel: correct oil type and weight, minimum and maximum quantity
- Dipstick measures quantity only (PHAK ch.7)
Gauges:
| Gauge | Reads | Notes |
|---|---|---|
| Oil pressure | Pressure of oil supplied to engine, usually psi | Green = normal, red = min/max limits. Actual limits and recommended pressures by phase of flight: Aircraft Flight Manual |
| Oil temperature | Lubricating oil temperature | Slower than pressure gauge: cold-start rise takes several minutes or longer (PHAK ch.7) |
Abnormal oil temperature:
- High: plugged oil line, low oil quantity, blocked oil cooler, or defective gauge. Also: oil pressure system malfunction, inadequate engine cooling (TP 1102).
- Low in cold weather: may signal improper oil viscosity (PHAK ch.7).
- Running hotter than designed: power loss, more oil burned, risk of scored cylinder walls, damaged pistons and rings, burned or warped valves.
Oil grade:
| Oil used | Engine | Allowed | Ref |
|---|---|---|---|
| Detergent | Non-detergent-oil | Never | - |
| Non-detergent | Detergent-oil | Emergency only | TP 1102 |
- Use proper grade for seasonal conditions (FAA-H-8083-3 AFH, Engine Starting).
Cold oil: aircraft engine oil more viscous than most engine oils, very thick when cold (TP 1102).
Trap: high oil-temperature reading is not automatically a bad oil pump; blocked cooler or defective gauge reads the same.
References
- TP 1102 Aeroplane Flight Training Manual, Ex.2 Aircraft Familiarization, Engine Instruments
- TP 1102 Aeroplane Flight Training Manual, Ch.3 Engine Handling, Inspection (items 5-6)
- FAA-H-8083-25 PHAK ch.7