What does engine oil do besides lubricate?

It cools, by carrying heat from pistons, bearings, and cylinder walls to the sump, tank, or oil cooler. It cleans, by picking up carbon and metal particles and delivering them to the filter or screen. It seals the gap between piston rings and cylinder walls, cushions shock loads on gears and bearings, and leaves a film that protects internal surfaces from corrosion when the engine sits.

Where is the oil stored in a dry-sump engine?

In a separate oil tank mounted outside the engine, not in the crankcase. A pressure pump draws from that tank and feeds the engine, and one or more scavenge pumps pull oil out of the sump and return it to the tank. The tank includes expansion space above the oil for foaming and thermal expansion, so it is never filled to the very top.

Why is the scavenge pump larger than the pressure pump?

Because the oil coming back is not the same oil that went out. After being thrown around by the crankshaft, gears, and bearings, it is hot and full of entrained air, so the same weight of oil occupies noticeably more volume. Extra scavenge capacity keeps that frothy return moving out of the sump instead of accumulating where it would be churned further.

What does the oil cooler bypass valve do?

It routes oil around the cooler core instead of through it. When the oil is cold and thick at start-up, or when the core is congealed or obstructed, sending oil through the cooler would slow warm-up and could burst the core. The valve senses that condition and opens a path around the cooler, then closes it as the oil reaches normal operating temperature.

What causes high oil temperature during cruise?

Treat it as a symptom rather than an answer. Common causes are low oil quantity, so less oil absorbs the same heat, restricted cooling airflow or a blocked cooler core, a bypass valve stuck open, the wrong oil grade, or an internal problem such as a failing bearing adding friction. Check quantity and the cooling path first, then look inside.

Engine Lubrication Systems — FAA A&P Test Questions (ACS AM.III.G)

Engine Lubrication Systems asks you to trace oil from the tank or sump, through the pump, past every valve, and back again, and to say what each component does when something goes wrong. AM.III.G questions cover what oil is actually for beyond lubrication, how wet-sump and dry-sump layouts differ, which pump type the engine drives, and what the relief, bypass, and thermostatic valves each protect. Follow one drop of oil around the whole circuit and most of this area becomes a memory of the path rather than a list.

What ACS AM.III.G covers

This area covers reciprocating and turbine lubrication systems, their components, and the indications that reveal a problem, following the Powerplant Handbook, FAA-H-8083-32. The FAA begins with purpose. Oil primarily reduces friction between moving parts, but the test also expects the secondary jobs: carrying heat away from pistons, bearings, and cylinder walls, cleaning by carrying carbon and metal particles to the filter, sealing the small gaps between piston rings and cylinder walls, cushioning shock loads on gear teeth and bearings, and protecting internal surfaces from corrosion. Layout comes next. In a wet-sump engine the supply lives in the crankcase sump beneath the engine, while a dry-sump engine keeps the supply in a separate external tank and uses scavenge pumps to return oil there, which is the arrangement used where oil quantity is large or attitude changes are severe. The engine-driven pressure pump is normally a constant-displacement gear pump, and because it delivers more than the engine needs, an oil pressure relief valve limits maximum system pressure by returning excess oil to the pump inlet or sump. Two further valves are easy to confuse and are tested separately: the filter bypass valve routes oil around a clogged filter so the engine still gets oil, while the oil cooler bypass, or thermostatic, valve routes oil around the cooler core when the oil is cold or the core is blocked. Scavenge capacity is deliberately greater than pressure capacity, because returning oil is hot, foamy, and mixed with air, so it occupies more volume than it did on the way out. Indication questions close the area, treating a high cruise oil temperature as a symptom to be traced to oil quantity, cooling airflow, the cooler and its bypass, or internal engine condition.

Where this sits on the test

ACS AM.III.G is tested on the FAA Powerplant written test, one of 1,355 ACS-tagged questions in the Powerplant bank. Every question tagged to this area carries a worked rationale and its FAA handbook reference, so you can drill the code itself rather than the whole test.

FAA handbook references

FAA-H-8083-32

Three traps candidates fall into

Candidates size the scavenge pump smaller than the pressure pump, reasoning that the pressure side feeds the whole engine. Scavenged oil is aerated and foamy after being churned by the crankshaft and gears, so it occupies more volume, and the scavenge pump must be larger to keep the sump from filling up.
The three bypass paths get answered with one component. The pressure relief valve limits system pressure, the filter bypass sends oil around a clogged filter element, and the oil cooler bypass routes oil around the cooler when it is cold or obstructed. Each protects a different failure, and the test asks about them separately.
A dry-sump engine gets described as having no oil reservoir at all. The supply is simply not kept in the crankcase; it lives in a separate external tank, and scavenge pumps continually return oil there. Questions about where oil is stored, and about tank expansion space, hinge on knowing which layout is installed.

7 free sample questions from ACS AM.III.G

AM.III.GTap an answer
What is the primary purpose of the oil system in a reciprocating aircraft engine?
AM.III.GTap an answer
Besides lubrication, which additional function does engine oil perform in a reciprocating engine?
AM.III.GTap an answer
In a dry-sump oil system, where is the main supply of engine oil stored?
AM.III.GTap an answer
Which type of pump is most commonly used as the engine-driven oil pump in a reciprocating engine?
AM.III.GTap an answer
What is the function of the oil pressure relief valve in a lubrication system?
AM.III.GTap an answer
What is the purpose of an oil cooler bypass (thermostatic) valve?
AM.III.GTap an answer
In a dry-sump engine, why is the scavenge pump usually built with greater capacity than the pressure pump?

What is the primary purpose of the oil system in a reciprocating aircraft engine?

ACS code: AM.III.G

Correct answer: To reduce friction between moving parts by lubricating bearing surfaces

Rationale: The handbook lists several functions of engine oil, but the principal one is lubrication—forming an oil film between moving parts to reduce friction and wear. Oil also cools, cleans, seals, and cushions, but lubrication is the primary purpose. Oil does not raise combustion temperature and is not the source of landing-gear hydraulic power.

Besides lubrication, which additional function does engine oil perform in a reciprocating engine?

ACS code: AM.III.G

Correct answer: It carries heat away from internal engine parts

Rationale: In addition to reducing friction, oil cools the engine by absorbing heat from pistons, bearings, and other internal parts and transferring it to the oil cooler. Oil also seals, cleans, and cushions, but it has no role in fuel delivery to the carburetor or in producing ignition spark.

In a dry-sump oil system, where is the main supply of engine oil stored?

ACS code: AM.III.G

Correct answer: In a separate external oil tank

Rationale: A dry-sump system stores the bulk of its oil in a separate tank mounted away from the engine, and a scavenge pump returns oil from the engine sump to that tank. A wet-sump system, by contrast, keeps the oil supply in the crankcase sump. The oil cooler is a heat exchanger, not a storage reservoir.

Which type of pump is most commonly used as the engine-driven oil pump in a reciprocating engine?

ACS code: AM.III.G

Correct answer: A gear-type positive-displacement pump

Rationale: The handbook identifies the gear-type pump as the most widely used oil pump in reciprocating engines. It is a positive-displacement pump in which two meshing gears carry oil around the housing and force it out under pressure. Centrifugal pumps lack positive displacement, and diaphragm pumps are typically used for fuel, not engine oil.

What is the function of the oil pressure relief valve in a lubrication system?

ACS code: AM.III.G

Correct answer: It limits oil pressure to a safe maximum value

Rationale: The oil pressure relief valve regulates system pressure by opening to bypass excess oil back to the inlet or sump once pressure reaches a set value, protecting the system from excessive pressure. It does not boost pressure on cold starts, and metal-particle removal is the job of the oil filter, not the relief valve.

What is the purpose of an oil cooler bypass (thermostatic) valve?

ACS code: AM.III.G

Correct answer: It routes oil around the cooler until the oil reaches operating temperature

Rationale: The thermostatic bypass valve senses oil temperature and directs cold oil around the cooler core, sending oil through the cooler only after it warms up. This helps the oil reach operating temperature quickly and keeps it within the proper range. It does not divert cold oil into the cooler, nor does it gate flow to the bearings.

In a dry-sump engine, why is the scavenge pump usually built with greater capacity than the pressure pump?

ACS code: AM.III.G

Correct answer: Because the returning oil is aerated and occupies more volume

Rationale: Scavenged oil returning from the engine is mixed with air and foams, so it occupies a greater volume than the solid oil delivered by the pressure pump. To keep the sump clear and prevent the oil from backing up, the scavenge pump is given greater capacity. Tank position and oil viscosity are not the reasons for the larger scavenge capacity.

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Frequently asked questions

What does engine oil do besides lubricate?: It cools, by carrying heat from pistons, bearings, and cylinder walls to the sump, tank, or oil cooler. It cleans, by picking up carbon and metal particles and delivering them to the filter or screen. It seals the gap between piston rings and cylinder walls, cushions shock loads on gears and bearings, and leaves a film that protects internal surfaces from corrosion when the engine sits.
Where is the oil stored in a dry-sump engine?: In a separate oil tank mounted outside the engine, not in the crankcase. A pressure pump draws from that tank and feeds the engine, and one or more scavenge pumps pull oil out of the sump and return it to the tank. The tank includes expansion space above the oil for foaming and thermal expansion, so it is never filled to the very top.
Why is the scavenge pump larger than the pressure pump?: Because the oil coming back is not the same oil that went out. After being thrown around by the crankshaft, gears, and bearings, it is hot and full of entrained air, so the same weight of oil occupies noticeably more volume. Extra scavenge capacity keeps that frothy return moving out of the sump instead of accumulating where it would be churned further.
What does the oil cooler bypass valve do?: It routes oil around the cooler core instead of through it. When the oil is cold and thick at start-up, or when the core is congealed or obstructed, sending oil through the cooler would slow warm-up and could burst the core. The valve senses that condition and opens a path around the cooler, then closes it as the oil reaches normal operating temperature.
What causes high oil temperature during cruise?: Treat it as a symptom rather than an answer. Common causes are low oil quantity, so less oil absorbs the same heat, restricted cooling airflow or a blocked cooler core, a bypass valve stuck open, the wrong oil grade, or an internal problem such as a failing bearing adding friction. Check quantity and the cooling path first, then look inside.

All 40 ACS areas

General

Airframe

Powerplant

Primary sources

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