Chapter 6 - Oil & Cooling Systems

Oil System Malfunction - Fluctuating Oil Pressure (e.g: Normal reading to low or zero and back again).

Pressure pump is drawing air either from the sump or the storage tank. Likely, due to low quantity of oil available to the pump. Or, for a dry sump, the scavenger pump has failed and is unable to lift oil into the oil (storage) tank.

Oil System Malfunction - Low Oil Pressure

1. Low oil quantity - a lack of proper replenishment, a leak (tank or supply line) or scavenger pump failure;

2. Loss of oil pressure (sudden and complete) - Failure of the pressure pump or the pressure line between it and the engine.

3. Loss of oil pressure (likely a gradual loss) - Failure of the pressure system elsewhere in the engine.

4. Faulty oil pressure gauge - Indication falls, but not accompanied by a rise in oil temperature. Likely to be a faulty gauge, but make sure to manage both.

Oil System Malfunction - High Oil Pressure

1. Faulty pressure relief valve - Excessively high pressure can damage the system (e.g: Damage an oil seal, cause loss of oil then eventual failure).

Oil System Malfunction - High Oil Temperature (at best)

At best, general overheating of the engine, when the pressure indications are normal and when doing things conducive to high engine temperature (e.g: climbing)

Oil System Malfunction - High Oil Temperature (at worst)

At worst, an increasing or high oil temperature, particularly when coupled with low/reducing oil pressure. This is an indication of a potentially serious issue with lubrication system.

Low or fluctuating oil pressure, accompanied by a rise in oil temperature.

This is an indication of a serious issue with lubrication system. Land ASAP.

Gradual loss of oil pressure, accompanied by a rise in oil temperature.

Gradual loss of oil - due to lack of replenishment, leak, etc. Land sooner rather than later.

Indication of oil pressure gauge falls, but not accompanied by a rise in oil temperature

Likely a faulty oil pressure gauge, but monitor both temperature and pressure.

In regard to the cooling system, what are the main conditions that lead to engine overheating?

The engine is most likely to overheat when operated at a high power setting and slow airspeed. Because heat production is at its maximum, while cooling airflow is low. i.e: Engine is producing lots of heat, not enough cooling.

What operating conditions can lead to engine overcooling?

Low power and high airspeed. Engine is producing little heat, but there' is lots of cooling.

Where are cooling fins on the engine found?

Around the cylinder heads and cylinder barrels.

What are the purpose of the cooling cowl flaps on equipped aircraft?

Cooling cowl flaps are fitted on the cowl of some aircraft. They are electrically or manually opened and closed. Opening the cowl flaps allow more air to escape from the engine compartment, which increases flow and the cooling. They also increase parasite drag (aka cooling drag).

Why is an engine more likely to overheat at a high power setting?

Because at high power, the engine generates the maximum amount of heat, so the need for cooling is greatest.

Why does slow airspeed increase the chance of engine overheating?

Slow airspeed reduces the effectiveness of the cooling system, because less air flows over the cooling fins.

When is the effectiveness of cooling fins greatest?

The cooling fins are most effective at high airspeed.

Why can an engine overheat during ground operation?

During ground operation, there is little or no airspeed, so less air flows over the cooling fins and cooling is less effective.

If unavoidably halted on the ground for a lengthy period, what should you do to assist engine cooling?

Face the aircraft into wind where possible and run the engine at idling RPM.

In general, what three operating practices should be avoided?

Running the engine on the ground for long periods with the throttle closed, extended climbs at high power and low airspeed, and extended descents with the throttle closed.

What is the purpose of periodically opening the throttle during a long descent?

To maintain cylinder temperatures, provide carburettor heat, reduce the risk of throttle ice, and prevent lead fouling of the spark plugs.

What is a CHT gauge? And what does it indicate?

A Cylinder Head Temperature Gauge gives a direct indication of how well the engine cooling system is functioning.

What can cause excessive engine temperature besides high power and low airspeed?

A too-lean mixture or low oil level can also cause excessive engine temperature.

When should cowl flaps generally be open?

They should generally be open during taxiing once the CHT has risen into the recommended range, during takeoff, and on final approach in readiness for a go-around.

What are the main advantages of air cooling systems?

Air cooling systems are simpler, more reliable, potentially cheaper to maintain, and lighter than liquid cooling systems.

What is the main disadvantage of air cooling?

It is difficult to design an air cooling system that cools all cylinders evenly.