Engine Lubrication Systems

Every aircraft engine carries its own engine oil system. An engine-driven pump sends oil to the bearings, gears and other moving parts, and the oil returns to be filtered, cooled and used again. Oil starvation destroys an engine within minutes, so the oil pressure, temperature and quantity are among the first indications a pilot learns to watch.

Piston and gas turbine engines share the principles but differ in emphasis. A piston engine uses its oil on sliding surfaces, cylinder walls and piston rings as well as bearings, and loses some of it in combustion. A gas turbine uses oil mainly on the shaft bearings and gearboxes, at high temperature but in small quantities: the CFM56-5B of the A320 consumes an average of about 0.5 quarts an hour. The accessory gearbox, which the oil system also lubricates and which drives the pumps, is covered at the end of this article.

Purpose of engine lubrication

Engine oil does several jobs at once:

• it lubricates, keeping a film between moving surfaces to reduce friction and wear;
• it cools, carrying heat from parts the cooling air cannot reach, such as pistons and bearings, to the oil cooler;
• it seals, between piston rings and cylinder walls;
• it cleans, holding combustion residues and wear particles in suspension until the filter traps them;
• it cushions shock loads between parts and protects against corrosion.

Oil is also a hydraulic fluid. In a constant-speed propeller the governor uses engine oil to change the blade angle, and in many turboprops the engine oil is the propeller's pitch-control fluid and works the oil-pressure torquemeter (see propellers and propeller control). In a typical turbocharged piston engine, oil pressure closes the wastegate against a spring (see supercharging and turbocharging).

Wet and dry sump systems

In a wet sump oil system, used on most light training aeroplanes, the sump at the bottom of the crankcase is the oil reservoir. The engine-driven pump draws oil from it and sends it through the filter and cooler to the engine, and the oil drains back to the sump by gravity. The quantity is read on a dipstick with the engine stopped and kept within the range the flight manual gives, rather than simply topped up to the maximum.

In a dry sump system the oil is kept in a separate oil tank. A pressure pump feeds the engine, and scavenge pumps return the oil from the sumps, bearing chambers and gearboxes to the tank. Some larger and aerobatic piston engines use dry sumps, and so do airliner turbofans: on the Boeing 737 NG, engine-driven scavenge pumps return the oil to the tank. The 737's EASA type-certificate data sheet lists a usable oil capacity of 10.3 litres for the NG, with CFM56-7B engines, and 19.25 litres for the 737 MAX, with LEAP-1B engines. On the A320 the crew check the oil level on the ECAM ENG page and on the walk-round, and the minimum before flight is 9.5 quarts plus the estimated consumption.

Inverted and radial piston engines have a particular hazard: oil drains past the rings into the lowest cylinders while the engine stands, and a cylinder full of incompressible oil can be damaged on its compression stroke. These engines are pulled through by hand before start to clear this hydraulic lock (see piston engine principles).

Oil types

Piston engines use mineral-based oils. Straight mineral oil is often specified for the first hours after an overhaul, to help the new piston rings seat. After that the usual choice is ashless dispersant oil, a detergent oil that holds contaminants in suspension so that they are carried to the filter instead of settling in the engine. Oil viscosity falls as it warms: cold oil is thick, which is why oil pressure reads high just after a cold start and should fall back into range as the oil warms.

Gas turbines use synthetic oils that tolerate the high temperatures of the bearing chambers. The A320's type-certificate data sheet, for example, cites the MIL-L-23699 specification among the approved oils for the V2500 engine. Turbine oil must also stay out of the air: oil leaking past a degraded bearing seal into the compressor can contaminate the bleed air and cause a fume event in the cabin (see in-flight fire, smoke and fumes).

For weight and balance, the FAA's standard weight for oil is 7.5 lb per US gallon, and the basic empty weight of most modern light aeroplanes includes full engine oil.

Pressure and relief valves

The pressure pump is engine-driven, so its output rises with engine speed while the bearings' needs do not. An oil pressure relief valve holds the feed pressure at the set value; at high rpm a large part of the pump's output is spilled back through it to the tank. A relief valve that sticks produces small fluctuations in the oil pressure indication. On the Boeing 737, the oil temperature, oil pressure and LOW OIL PRESSURE sensors sit downstream of the pump, before the oil reaches the engine.

Oil pressure has a regulatory status of its own. CS 25.1305, which lists the required powerplant instruments, requires an oil pressure warning or caution. On the A320's CFM56-5B, a dedicated pressure switch triggers the ENG OIL LO PR warning at 13 psi; on the 737 the amber LOW OIL PRESSURE alert comes on at or below the red line, and the amber caution band that begins at the red line varies with N2 and is shown only above 65% N2.

A gas turbine that loses its oil pump must be shut down at once, because nothing else will feed its bearings. The indications read together:

In a light single, falling pressure with rising temperature means reducing power and landing as soon as possible at the nearest suitable aerodrome, while the engine still runs.

Scavenge system

The oil scavenge system returns oil from the bearing chambers, sumps and gearboxes to the tank. Scavenging in this sense has nothing to do with the exhaust scavenging of a piston engine's valve overlap. The scavenge pumps are engine-driven like the pressure pump and are normally given a larger capacity than it, so that they keep the sumps and bearing chambers clear. On the 737 the scavenged oil passes through the scavenge oil filter and the main fuel-cooled oil cooler before returning to the tank.

Because oil is spread through the engine while it runs, the tank quantity varies. On the 737, the indicated oil quantity may fall markedly during start, take-off and climb and recover in level flight; at low N2 during an in-flight start the scavenge pumps may not return enough oil to the tank, giving a low quantity indication; and a reading as low as zero is normal when a windmilling engine turns below about 8% N2.

Oil filters and chip detectors

Filters trap the wear particles and residues the oil collects. As a filter clogs, the pressure difference across it rises, and at a set value an oil filter bypass valve opens and lets the oil flow round it: dirty oil is better than no oil. The crew are warned first. On the 737 the amber OIL FILTER BYPASS alert shows an impending bypass of the scavenge filter; on the A320 the ECAM alerts the crew to a filter blockage; on the E190-E2 an impending oil filter bypass gives only an advisory, whereas the fuel filter equivalent is a caution.

Chip detectors are magnetic plugs, typically fitted in the scavenge lines, that collect ferrous particles from the returning oil. Metal found on one is an early warning of bearing or gear wear (see engine indications and condition monitoring).

Oil cooling and fuel-oil heat exchangers

Piston engines usually cool their oil in an air-cooled radiator. A thermostatic bypass valve in the cooler circuit sends cold, viscous oil straight back to the engine, because forcing it through the narrow passages of the cooler would waste pressure and could burst the matrix, and because oil needs to reach its working temperature to lubricate properly and boil off absorbed moisture. As it warms, the valve progressively sends it through the cooler; this is why the oil temperature must be in limits before high power is used. In very cold weather, oil coring can occur: the cold, viscous oil does not flow properly through the system, heat is not carried away evenly, and the oil temperature at the sensor rises rapidly while the bulk of the oil stays cold. The ATPL texts' remedy is to close the oil cooler flaps and warm the oil first. After high power, a turbocharged engine is idled for a few minutes before shutdown so that oil keeps flowing through the hot turbocharger bearing; stagnant oil would bake into carbon, a process called coking.

Gas turbines use the fuel as their heat sink. In a fuel-cooled oil cooler (FCOC), or fuel/oil heat exchanger, the oil gives up its heat to the fuel on its way to the engine, which also warms the fuel and keeps ice crystals from blocking the fuel filter (see gas turbine engine fuel system). The 737 NG uses two: one for the engine oil and one for the IDG oil. If the matrix is blocked or the oil very viscous, the FCOC's bypass valve opens at a set inlet oil pressure and routes the oil round it. In the design the ATPL texts describe, a pressure-maintaining valve keeps the oil pressure above the fuel pressure, so that a leak in the matrix lets oil into the fuel rather than fuel into the oil.

Oil temperature is limited. On the A320's CFM56-5B the maximum is 140 °C continuous and 155 °C transient, for up to 15 minutes; the ENG OIL HI TEMP caution comes on above 140 °C for more than 15 minutes or at 155 °C, and the minimum is −40 °C for starting and −10 °C for take-off. The FADEC can raise the idle N1 automatically when the oil is warmer than normal. On the A320neo the limits are engine-specific. As on the ceo, an exceedance turns the digits amber rather than red, an arrangement EASA accepted for the neo because it needs no immediate crew action; the ECAM procedure is to reduce thrust and shut the engine down if the temperature cannot be kept within the limit.

Accessory gearbox and shear necks

The accessory gearbox (AGB), or external gearbox, drives the engine's accessories: fuel and oil pumps, hydraulic pumps, the generator or IDG, the dedicated alternator that powers the FADEC, and usually the starter. The accessory drive is taken from the HP compressor shaft, at the cool end of the engine, because a drive from the hot end would suffer high losses and need exotic shaft materials. On the CFM56-5B the gearbox sits at the bottom of the fan case, driven by the HP rotor through a horizontal drive shaft. On engines with two external gearboxes, the oil and fuel pumps are grouped on the high-speed gearbox driven by the HP shaft, because the starter turns that shaft first and these pumps must run before light-up. The gear teeth are sized to the load each accessory takes, and every extra accessory load, such as a further hydraulic pump, takes power from the engine and raises its specific fuel consumption.

Accessory drives are protected by a shear neck, a deliberately weakened section of shaft. If an accessory seizes, the neck breaks and the accessory stops, while the gearbox and the engine keep running; without it, one failed accessory could destroy the gearbox and stop the engine. The exception is the oil pump drive, which has no shear neck, because the pump must keep turning as long as it physically can. A generator drive has its own protection, the disconnect of a CSD or IDG (see AC generators, CSD and IDG).

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.