OPERATIONAL CONSIDERATIONS

Indications of oil pressure and temperature give the pilot a good idea of the mechanical integrity of the engine. Of course the pilot must then interpret these indications correctly.

On Radial and Inverted engines the pilots knowledge of the lubrication system is required even before starting the engines. These engine can suffer from a problem called ‘ Hydraulicing’, where oil accumulates in the lower cylinders between piston and cylinder head. As oil is incompressible damage to the engine could occur as the piston moves on the compression stroke.

Prior to starting, these engines should be pulled through the cycle by use of the propeller, to ensure no hydraulic lock has occurred, ( Confirm Magneto’s are OFF before turning engine ).

On starting positive engine oil pressure should be indicated within a specified time. ( Piper Warrior 30 seconds ). If the engine is started from cold the oil pressure could be excessively high. This would be ‘Normal’ as long as it drops to within its normal range as the engine warms up. Correct engine operating pressure and temperatures are dependant on each other. High oil temperature could give low pressure. The oil pressure should be within its operating range at the correct operating temperature.

Fluctuations in pressure could be the result of low oil levels, or system faults. Low pressure at normal temperature would indicate imminent engine failure, and a landing should be made as soon as possible.

A problem that can occur during starting in very cold weather is ‘Coring’. It is caused by the fact the cold viscous oil does not flow correctly through the engine. It should be remembered that an important task of the oil is to cool. The reduction in flow rate will not dissipate the heat being generated in the engine. The result is that the ‘Oil temperature rapidly rises’, but this is only locally at the point of sensing. The problem is that the majority of the oil is ‘Cold’. To overcome coring oil cooler flaps should be ‘Closed’, this will initially increase temperature but should improve flow particularly through the cooler, and then bring temperatures down.

It should be appreciated that as the oil is used to lubricate the moving parts of the engine, the oil will come in contact with the combustion gases. Sealing of the valves and pistons is not 100%

and as a result some oil will be burnt and the engine will therefore have an oil consumption rate. Ignoring external leakage, oil consumption varies between engines. A light aircraft would use around 1 pint per hour. A consumption rate greater than this would indicate wear in the engine.

The oil contents should always be checked prior to flight. If the engine has a ‘Dry Sump’ system, the contents should be checked ‘Immediately’ after the engine has stopped, ( realistically within a few minutes of shut down). This ensures that the tank contents are recorded accurately before the oil migrates under gravity down into the engine sump. Large piston engines have oil tanks fitted with a ‘Check Valve’ which is underneath the oil tank and closes under spring pressure or by an electrically operated actuator on engine shut-down. The closing of the check valve prevents oil migration into the sump. The ‘wet sump’ system is the opposite. A period of a least 15-20 minutes should have elapsed before the contents are checked in a similar fashion to motor cars. In any event, the oil-level is checked ‘after a period of time’.

QUESTIONS

1. From the following list select the correct combination of statements.

The primary task of the lubrication is to:

1. Reduce friction

2. In a piston engine dry sump oil system, the oil temperature and pressure are sensed:

a. When the oil is leaving the sump.

b. For the temperature when the oil is leaving the tank, and for the pressure when the oil is leaving the pressure pump.

c. For the oil temperature when the oil is entering the tank and for the pressure when it is entering the pressure pump.

d. At the same point.

3. Oil returning to the oil tank is filtered by:

a. The oil pressure filter.

b. The oil tank filter.

c. A micron size multi-bore filters assembly.

d. The scavenge filter.

4. Engine oil pressure is:

a. Low at idle R.P.M. and high at high R.P.M.

b. Controlled by the oil cooler.

c. Substantially decreased when the oil pressure relief valve opens.

d. Relatively unaffected by engine speed.

5. The purpose of the crankcase breather is to:

a. Maintain the pressure in the oil tank at atmospheric pressure.

b. Ease the task of the oil scraper ring.

c. Prevent pressure building up inside the crankcase.

d. Prevent distortion of the crankcase.

6. The most probably cause of small fluctuations in the oil pressure would be:

a. Lack of oil.

b. The pressure relief valve sticking.

c. Air in the oil tank.

7. The extra space in the oil tank is to cater for:

a. Frothing and aeration of the oil as it passes through the engine.

b. Fire protection.

c. The accommodation of extra oil contents on long duration flights.

d. Anti-surge action.

8. The scavenge pump system in a lubrication system has:

a. A by-pass in case of blockage.

b. A smaller capacity than the pressure pump.

c. A bifurcated tertiary drive system.

d. A larger capacity than the pressure pump.

9. In a “wet sump” oil system, the oil is contained in the:

a. Engine and tank.

b. Tank and oil cooler.

c. Sump and tank.

d. Engine and sump.

10. The oil contents of a piston engine (wet sump) are checked:

a. When the engine is running at idle power.

b. As soon as possible after the engine is stopped because the oil will drain away from the sump.

c. After approximately 15 minutes once the engine has stopped.

d. When the oil has reached a specific temperature.

ANSWERS

1 C

2 B

3 D

4 D

5 C

6 B

7 A

8 D

9 D

10 C

CHAPTER FOUR