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Air Fronts: Aircraft Manuals - RAF Pilot's Notes: A.P 2095, Pilot's Notes - General, 1943 - Part 2: Engines and Propellers

A.P 2095, PILOT'S NOTES GENERAL, PROMULGATED BY ORDER OF THE AIR COUNCIL , 2ND Edition , April 1943. FOR OFFICIAL USE ONLY

PART II - ENGINES AND PROPELLERS

NOTE A. ENGINE LIMITATIONS

1. Introductory.

(i) The Pilot's Notes for each type of aircraft lay down certain engine limitations which the pilot should observe in the handling of the engine. The principal figures are quoted on the Cockpit Data Plate for convenient reference in the air.

(ii) These limitations are based in part upon calculations and in part upon type tests on the bench. They may subsequently be modified in the light of Service experience and operational requirements. While normally the. same for any one type of engine in all aircraft, they may vary from one type of aircraft to another.

(iii) The limitations are designed to secure an adequate margin of safety against immediate breakdown and to give the engine a reasonable life between overhauls. Proper handling throughout the life of an engine will improve reliability towards the end of the periods between overhauls, and will also improve the chance of the engine standing up to operational overloads.

2. R.P.M. and Boost.

(i) The engine is stressed, and wear is correspondingly caused both by high r.p.m. (inertia stress) and by high boost (gas pressure stress). Consequently, the two main power ratings of the engine involve changes both in the r.p.m. and in boost, e.g.

Rated Power 2,400 r.p.m. +4 lb./sq. in boost.

Combat Power 2,800 ,, +8 1/4 "I

(ii) It should be noted, however, that a reduction of r.p.m. - say from 2,800 to 2,400 - at take-off or combat boost increases the strain on the engine, because :-

(a) It reduces the velocity of gas flow and consequent throttling at the valves and gives a higher pressure in the cylinder at given boost, in extreme cases leading to detonation.

(b) Inertia stresses partly balance gas stresses.

(iii) On the other hand, the engine gives more power for the fuel used if power is reduced by reducing r.p.m. rather than boost, and it is correct handling to reduce power by reducing r.p.m. as indicated in the following sub-paragraphs (iv) and (v).

(iv) Up to a certain boost the mixture strength can be weak (or economical) and the fuel is fully burnt. Higher boosts can only be used with a rich mixture in which surplus fuel acts as a coolant to prevent detonation. This maximum weak mixture boost defines a third (maximum weak) power rating of the engine. It is generally permissible to obtain the required power for economical cruising at the maximum weak mixture boost (or the highest attainable at the altitude) with any r.p.m. from the specified maximum down to the lowest obtainable; if not, Pilot's Notes will quote a lower limit.

(v) When it is necessary to use more power than can be obtained on a weak mixture, the same practice may be followed, using the full boost and any r.p.m. down to the figure that will give the power that can be obtained on weak mixture, when of course, the boost should be reduced and the r.p.m. raised to the weak limitations.

3. Duration and Flight Condition Limits of Use.

(i) The general operation of the aircraft is planned on the basis that the higher engine limitations will only be reached during certain periods of time at a stretch and over certain total fractions of the whole working life of the engine. The pilot is accordingly instructed that specific limitations are to be reached only :

(a) for certain purposes, e.g.:

take-off and climb, if necessary to 1,000 ft;

climbing to the operational height;

combat bursts of power.

(b) for certain periods at a time, e.g.:-

for 5 minutes maximum in take-off;

for 5 minute periods' in combat ;

for 1 hour of climbing.

(ii) These figures provide a general guide to the reasonable use of the engine. In combat and emergency other considerations may justify the pilot in disregarding these
restrictions.

4. Automatic Safeguards.

(i) The engine is normally safeguarded against exceeding the maximum r.p.m. and boost by the constant-speed propeller and the automatic boost control.

(ii) It is possible however, to overspeed the engine by incorrect handling (see Note C), and to overboost American engines with no boost control (Note H).

(iii) Overspeeding in dives occurs with certain propellers and is permissible within the diving limitations (see Note D).

5. Temperatures and Pressures.

(i) Serious damage may occur quickly from over-heating. The limitations include permissible cylinder head or coolant temperatures and oil temperatures.

(ii) The installation should be so designed that these limitations will not be exceeded with proper handling of the engine; but :

(a) Cowling systems are not. always quite adequate for all conditions.

(b) The engine may at times be worked abnormally hard.

(d) The cooling is partly under the pilot's control and unnecessary opening of gills or radiator shutters reduces performance.

The pilot should, therefore, regularly watch the temperature gauges and adjust gills or shutter, or airspeed, or power accordingly (see Note C).

(iii) The oil pressure gauge and the specified normal and emergency pressures provide a further check on satisfactory lubrication. If the pressure falls to the emergency minimum the engine has been seriously overworked or some defect has developed.

6. Running Up and Stopping.

(i) The engine limitations include :

(a) Minimum oil temperature

(b) Minimum cylinder or coolant temperature to be attained before opening up the engine. This is to ensure proper functioning of the lubrication system, proper distribution of the charge, and to prevent damage to the engine by uneven heating.

(ii) Engines should also be run gently before stopping to drain away surplus oil, to leave an adequate oil film, and to prevent damage by rapid and uneven cooling.

APPENDIX

The limitations for supercharged engines with variable pitch propellers were defined by A.M.O. A415/38. (See A.P. 129, Chap. II, 66) in relation to flight operation as under:

(a) Maximum for take- off conditions (3 min. or to 1,000 ft.)

(b) climbing (30 min. limit)

(d) cruising (maximum with no time limit)

(e) economical cruising (maximum with weak conditions mixture)

(f) diving conditions

The condition of level flight (c) covers short periods when necessary in combat or emergency, irrespective of whether the aircraft is in level flight, or climbing or descending, and is now called combat. The condition of climbing (b) similarly covers other conditions in which it is necessary to use this power, e.g. in a single-engine return of a twin-engine aircraft. Cruising (d) denotes continuous operation for an indefinite period and is called continuous cruising in A.P.129. Both (d) and (e) have been quoted as cruising rich and cruising weak in some Pilot's Notes and continuous rich and continuous weak in other Notes. The time limits have been increased to 5 minutes for take-off and one hour for climb.

PART II - NOTE B - STARTING, RUNNING. UP AND TESTING

1. Preliminary.

(i) The aircraft should be faced into wind to ensure the best cooling during running up.

(ii) Fuel cocks and engine controls should be set for starting with the exception of the ignition, which should be off.

Throttle . . slightly more open than for normal idling

Mixture . . NORMAL or RICH (if fitted)' ¹

Propeller .. for maximum r.p.m.²

Supercharger M ratio (if two speed)

Air intake .. COLD

(iii) The engine should be turned over, if possible, by hand two revolutions of the propeller to break down the sticky oil films in cold weather. This turning is always essential with radial or inverted cylinder engines to prevent damage by hydraulic shock if oil or fuel should have drained into the cylinder heads. If resistance to turning indicates leakage of oil or fuel into the lower cylinders, turning should be done with the plugs removed from these cylinders.

(iv) The ignition should then be switched ON.

2. Priming and Starting with Electric Starter ³ (Cold Engine).

(i) The priming pipe line must first be filled. The line may be assumed to be full if an increase is felt in the resistance to pumping. Alternatively, the strokes needed to fill the line may be estimated from their length and the pump capacity; the K40 pump (T handle 40 c.c.) will fill 7 feet of 1/4 inch O.D. pipe per stroke, and the A.M. type B (round handle 10 c.c.) 7 feet per 4 strokes.

(ii) Fill the priming pump by withdrawing the plunger, switch on the starter and booster-coil and prime vigorously, pushing the plunger home forcefully and at once with-drawing steadily, giving the pump time to fill and then

^{------------------
1} For Bendix-Stromberg injection carburettor; see Note H.
² Except for certain propellers; see Note D, paras. 2 and 3. Including Inertia and Direct Cranking Starter (Note H).
^{------------------}

at once delivering the next stroke. (It is, however, advantageous to prime when the engine fires even if the pump must be withdrawn too quickly to fill fully.) The engine should start as soon as enough priming has been injected to give a suitable mixture, and this should occur with the number of strokes stated in the Pilot's Notes.

NOTE. - With the American inertia and direct cranking starter the flywheel must first be energized, see Note H.

(iii) Release the starter switch when the engine starts, but keep the booster-coil on and continue priming, though less vigorously, until the engine has picked up speed. The booster coil should then be switched off. The priming pump should be kept full to give further priming should the engine falter.

(iv) When the engine is running satisfactorily on the carburettor and it is clear that further priming will not he needed, push the plunger gently home and screw it down.

NOTE 1. - Priming while turning gets the fuel properly atomised and evenly distributed to the cylinders.

NOTE 2. - The amount of priming required varies with the atmospheric temperature and the normal number of strokes needed is tabulated in the Pilot's Notes. If the engine fails to start with approximately this number of strokes a second attempt may be made without further priming, or with a little more. Much more priming should not be given; some other cause than insufficient priming should be sought.

NOTE 3. - To avoid overheating the starter motor, turning periods must be limited to the period (commonly 20 sec.) stated in the Pilot's Notes, and if the engine fails to start, the prescribed interval (commonly 30 sec.) for cooling between starts must be observed.

NOTE 4.-If the engine fails to start and overpriming is suspected, it should be turned forwards several revolutions, with ignition off and throttle well open, to clear it.

3. Starting with Cartridge or Inertia Starter (Cold Engine).

(i) Prime the engine with the number of strokes specified in the Pilot's Notes, according to the atmospheric temperature, switching on the booster coil and firing the cartridge (or engaging the starter) while the last stroke is being delivered. The pump must be operated vigorously as described in para. 2 (ii).

NOTE.-With the American inertia starter the flywheel must be energized first (see Note H).

(ii) Keep the booster coil on and continue priming, though less vigorously, until the engine has picked up speed. Then switch off the booster coil and stop priming, but keep the pump full to give further priming should the engine falter.

NOTE.-The booster-coil switch is often combined with the cartridge firing switch, or the inertia starter engaging switch ; the instruction still holds good.

(iii) When the engine is running satisfactorily on the carburettor and it is clear that further priming will not be required, push the plunger gently home and screw it down.

NOTE.-If the engine fails to start and overpriming is suspected, it should he turned forwards several revolutions with ignition off and throttle well open to clear it.

4. Cold Weather Starting.

(i) In very cold weather engines which have been stopped sufficiently long for the oil temperature to fall below 0° C, are difficult to start because :

(a) the oil thickens and makes the engine hard to turn ;

(b) the fuel does not so readily vaporize and provide a combustible mixture.

(ii) The Worth system of diluting the oil with petrol has been introduced to make the engine easier to turn. It also safeguards the engine by ensuring an immediate flow of oil to all parts. Operating instructions will be found in Note G.

(iii) Fuel of high volatility may he used in cold weather to provide a combustible mixture without an excess of liquid fuel which might damage the engine and create a risk of fire. This fuel must be used for starting at atmospheric temperatures below 0° C on all installations adapted for its use. The adaptation consists of the insertion of a three-position cock in the line supplying fuel to the priming pumps. The special fuel is drawn through this cock from a can, the cock being turned to the off position after use.

(iv) Engines must in addition be protected from the weather as much as possible.

(v) Fuel and oil system vents must be inspected and any ice or moisture removed.

(vi) The ignition system must be inspected for ice or moisture.

(vii) After starting, the oil pressure should be watched during the first minute. If it does not build up to normal or higher in the first few seconds the engine should be shut down. It may be necessary to drain and refill with warm oil, or to use a mobile defroster to warm the tanks and pipelines.

5. Warming-up.

The throttle should be opened gradually until the engine is running at a fast tick-over, 1,000 to 1,200 r.p.m. (or at the r.p.m. specified in the Pilot's Notes). Warm up at this speed until the prescribed temperatures (oil, coolant, cylinder head) are reached.

6. Testing and Exercising.

(i) The engine can now be run up for various tests and to exercise various parts. Check that :

Mixture Control (if fitted) is AUTO RICH (or NORMAL)

Propeller Control is set for take-off r.p.m.

Supercharger gear ratio is M.

Air intake (if variable) is COLD.

The engine should be opened up sufficiently to move the aircraft forward against the chocks, and the brakes should then be applied.

IMPORTANT NOTE.-To avoid overheating the engine, the tests must be made as quickly as possible, especially with air-cooled engines. Gills must be fully open.

(ii) Open up to the maximum boost permitted for operation in weak mixture (in RICH setting).

(a) Change the supercharger gear to S ratio, moving the lever over smartly. The boost should read the same as before the change, being under automatic control; but the r.p.m. should drop slightly because the supercharger takes more power from the engine in the higher ratio. The r.p.m. drop provides a check that the change has been effected. Return to M. (But see Note 2 to this sub-para., and any special instructions in Pilot's Notes for the type.)

(b) Move the propeller control lever well back, watching the r.p.m. There should be a marked fall in the r.p.m. which shows that the propeller is controlling. Move the lever fully forward again.

NOTE 1.-These tests should be made daily; but not necessarily by the pilot before take-off. In addition to testing the mechanism, changing the supercharger gear removes any sludge from the clutch and exercising an hydraulic propeller fills it with warmer oil.

NOTE 2.-With Bristol sleeve valve engines (Hercules) the supercharger gear test is to be made at 1,500 r.p.m. only, to safeguard the sleeves against damage by momentary high boost on changing to S ratio. A momentary drop in the oil pressure provides an indication that the change has been effected.

(iii) Now move the throttle to the maximum take-off position and check for satisfactory running, noting the boost and r.p.m.

NOTE 1.-This take-off power test provides a general test of the ignition. A single ignition test should not be made, as it is not a reliable test of the ignition system on account of the very rich mixture used at this boost and may indicate defects which are not present. The single ignition test should follow as laid down at (iv) below.

NOTE 2.-Take-off boost is quoted for some engines as so many lb./sq. in. " or Full Throttle". This is because on an average day the throttle valve needs to be fully open to give the prescribed boost and with a low barometer and/or in warm weather the nominal boost may not be attainable.

NOTE 3.-(a) During the running up the propeller is virtually in fixed pitch (the blades set at their finest pitch against the fine pitch stop) until the boost rises to 2 to 4 lb/sq. in. below the maximum take-off figure, when the propeller starts " constant speeding". The attainment of take-off r.p.m. at take-off boost provides no proper check on performance; for any small deficiency in power is masked by the propeller adjusting its pitch to absorb less power at the same r.p.m. An accurate check can only be made at the lower boost at which the blades are against the stops, when any deficiency of power will be shown by low r.p.m. because the pitch is fixed.

(b) With propellers of small pitch range " constant speeding " is usually attained only after the aircraft has gathered some speed on the take-off and the full take-off r.p.m. are not reached on the run-up. The r.p.m. actually obtained on running up then provide a precise check of power.

(iv) After the take-off power test, reduce to the maximum boost for continuous rich operation and test the ignition by switching off each side in turn. The drop in r.p.m. on single ignition should not exceed 5 per cent.

PART II-NOTE C HANDLING THE POWER UNIT

1. Taxying.

(i) Care must be taken to avoid overheating engines while taxying.

(ii) The aircraft should always be taxied with propeller set for maximum r.p.m., as this will give the greatest tractive effort and the best cooling for the power used.

(iii) Gills or radiator shutters should be fully open.

(iv) The aircraft must not be taxied fast, but sufficient speed should be maintained to keep it in continuous motion and avoid bursts of power to restart movement.

(v) Temperatures should be watched. Oil inlet temperatures, cylinder head or coolant temperatures should not exceed the cruising limitations for the engine. If engines are unavoidably overheated, they should be allowed to cool before take-off.

(vi) When possible, tractors should be used for moving heavy aircraft uphill to or from dispersal points.

2. Take-off.

(i) Engines should not be kept idling longer than can be avoided. Any necessary idling should be done at about 1,000 r.p.m. to minimise fouling of plugs.

(ii) If the take-off is delayed for more than two or three minutes after taxying out, the plugs may have become fouled and the engine(s) must be cleared (one at a time) by opening up to zero boost or as much as possible against the brakes, and the ignition should be tested. Clearing must be repeated if there is further similar delay.

(iii) Take-off boost and r.p.m. are permissible until the aircraft has climbed 1,000 feet or for five minutes, whichever is less. It may not be necessary to use the full permitted boost, or to continue the use of take-off r.p.m. and whatever boost is used up to the height and time allowed. The life of the engine will be prolonged by not doing so. But the power used in take-off should always be continued until the under-carriage is up, safety speed has been reached, and the aircraft is well away from the ground on a gentle climb.

3. The Climb.

(i) The r.p.m. and the boost are automatically maintained at the figures selected by the pilot until the aircraft reaches full throttle height, after which the boost falls progressively. The supercharger is capable of giving an excessive boost at low heights and the throttle valve is partly closed by the boost control to limit the boost to the required amount. The valve opens as the aircraft ascends, and so the boost is maintained until the valve is fully open.

(ii) The boost can be restored by changing the supercharger to S ratio; but the change should not be made until there has been a drop of 2 to 5 lb. per sq. in. in the boost pressure, the amount varying according to the type of engine. For the change should be made at the height at which the engine gives the same power in either ratio and, at the same boost and r.p.m., the engine gives less power in S ratio for two reasons: firstly, more power is consumed in driving the supercharger in S ratio; secondly, the supercharger heats the charge more in S ratio and less weight of charge is drawn in at each stroke.

(iii) Boost will be maintained in S ratio to a second full throttle height and then begin to fall. The diagram below shows how the power available varies with altitude in the two supercharger gear ratios.

VARIATION OF POWER WITH HEIGHT AT GIVEN R.P M.

NOTE 1.—Bristol Engines.-As the boost falls (above the full throttle height) the throttle lever should be moved progressively back, " following the boost back with the throttle ", so that the lever is never further forward than is necessary to get the maximum obtainable boost. Unless this is done, the engine will run on too rich a mixture and will give neither its best power nor the best fuel economy. This rule does not, however, apply in the economical mixture range of boost; or to Merlin engines.

NOTE 2.-Merlin Engines.-The throttle valve cannot open fully unless the throttle lever is advanced to a certain point on the quadrant. It is therefore necessary, when climbing, to advance the throttle lever when the boost begins to fall.

NOTE 3.-The height above which S ratio must be used for maximum power varies with the boost, the r.p.m. and the airspeed.

(a) The variation of full throttle height with boost follows from the consideration that, when an aircraft is climbing and the boost begins to fall, every height is a full throttle height and each succeeding height is the full throttle height for a lower boost than the last.

Graphicd

(b) The variation with r.p.m. can be seen from the consideration that raising the r.p.m. speeds up the super-charger and makes it capable of producing any required boost to a greater height.

(c) The variation with airspeed arises from the ram effect of the air pressure on the forward facing intakes to the carburettor, which considerably helps the supercharger at high speeds of flight. The following figures show roughly how ram raises full throttle height :-

4. Economical Flying.

(i) The mixture control (if fitted) must be set WEAK for flying at any boost up to the maximum WEAK limit. The setting should only be RICH at the higher boosts, or when the engine is temporarily throttled back, or for taxying. If there is no manual mixture control, weak mixture will be obtained automatically by setting the boost at or below the weak limit.

(ii) The pilot has a choice, in reducing power for cruising at an economical I.A.S., of throttling back or dropping the r.p.m.; the correct choice has a considerable effect on fuel consumption. The throttle lever should be pulled back to the position for maximum weak boost and the propeller lever should then be pulled back until the desired I.A.S. is obtained, or the minimum practicable r.p.m. reached, after which further reduction of power must be got by throttling back.

NOTE.-In Merlin engines the throttle valve cannot open fully unless the throttle lever is moved forward of the setting which gives maximum weak boost at the lowest altitudes. Therefore, at the higher altitudes the lever must be set forward-up to the climbing position if necessary - in order to get maximum permissible, or obtainable, boost for economical cruising.

(iii) S ratio should only be used if the required I.A.S. cannot be obtained in M ratio, or if the r.p.m. required to obtain the I.A:S. in M ratio are near the maximum permissible (100 to 150 r.p.m. less).

5. General Handling.

(i) (a) The throttle lever should be moved slowly and evenly. Sudden throttle changes throw an undesirable strain on the engine. Moreover if the throttle is opened too quickly serious overspeeding may result owing to the time required for the propeller to readjust its pitch.

(b) The propeller speed control lever should also be moved slowly because, owing to lag in response of the mechanism, the r.p.m. are likely to overshoot the desired figure if the lever is moved too quickly.

(ii) Serious damage may be done through detonation if the engine is subjected to high boost at low r.p.m., and any reduction of r.p.m. at high power actually increases the strain on the engine. Therefore:

(a) For a big increase of power raise the r.p.m. first and, conversely, for a big reduction reduce the boost first.

(b) Aerobatics and manoeuvres should be performed with a high r.p.m. setting in order that high power can be obtained by simply advancing the throttle lever without risk of damaging the engine.

(iii) The later propellers (of 35° or greater pitch range) will keep the r.p.m. within the normal range during a dive. The r.p.m. setting should be fairly high:

(a) in order that the propeller lever will not require attention on pulling out to level flight or climb;

(b) because maximum braking effect on throttling hack in the dive will be obtained by a high setting.

If the propeller should fail to control in the dive and the all-out r.p.m. be exceeded, the pilot must endeavour to limit the rise of r.p.m. and the duration of excessive r.p.m. by pulling out, and he should have the throttle at least one-third open. (See note D on 20° propellers.)

(iv) If an engine cuts-through a tank running dry, or through the action of negative g on the carburettor during a push-down or a roll - the throttle should at once be closed and slowly re-opened as power returns. If this is not done, the propeller may fine its pitch - in the attempt to maintain high r.p.m. with no power behind it - and then fail on return of power to coarsen pitch quickly enough to prevent serious overspeeding.

NOTE.-If the engine does not pick up quickly after a cut when pushing down into a dive, overspeeding can some-times occur owing to the power of the constant-speed unit being reduced when the propeller is wind-milling at high speed. If this should occur, the aircraft should be pulled out to reduce the I.A.S. as quickly as possible until the C.S.U. regains control.

(v) It is common practice to run an engine on one tank until it cuts, in order to keep a check on fuel consumption. When the fuel is getting low, the pilot should watch for the first signs of cutting (fall of fuel pressure or irregular running) and close the throttle. Turn off the tank and then at once turn on a new tank. Pause for a second or two and slowly reopen the throttle. This procedure will avoid the risk of damage to intakes by backfires.

(vi) On a flight of several hours' duration the supercharger gear should be changed once or twice to remove any sludge that may have accumulated.

6. Engine and Oil Temperatures.

(i) The importance of watching cylinder (or coolant) and oil temperatures and keeping them within the limitations has been emphasized in Note A.

(ii) The cooling of air-cooled cylinders is controlled by the setting of the gills, which in general will be:

(a) Fully open for all ground running.

(b) Closed or part open for the take-off.

(d) More or less closed in level flight.

They must be set according to the conditions of flight to maintain a suitable cylinder-head temperature. They should not be opened more than is necessary because they add considerably to drag as they are opened.

NOTE.-Closing gills for take-off is a precautionary measure against engine failure. If this practice leads to overheating the gills must he partly opened.

(iii) The cooling of liquid-cooled engines is in part regulated automatically, the circulation of the coolant being thermostatically controlled to provide quick warming up and to keep the engine warm on a glide. But the pilot generally has control over the flow of air through the radiator and the setting of this control affects both the cooling of the engine and the performance of the aircraft. Radiator shutters should be open only as much as is necessary for adequate cooling.

(iv) If difficulty arises in keeping the temperature within the limitations on climb, the cooling may be improved by climbing at a higher I.A.S.; the loss in rate of climb by climbing at 10 to 15 m.p.h. above the I.A.S. for best rate of climb will not be large.

(v) Weak mixture climbing must not be practised if it leads to excessive temperatures.

(vi) The oil cooling is automatically regulated (by means of a viscosity or a thermostatic by-pass), but the pilot has some-times control of the flow of air through the oil-cooler. Excessively high temperature or low pressure indicates that the engine has been overworked, that the oil cooler is not working properly, or that some defect has developed.

NOTE.-Coring occurs in some oil-coolers at low atmospheric temperatures, the oil congealing and choking the flow so that the cooling is reduced. It may be possible to thaw a cooler by closing the shutters (if fitted) or by increasing the flow by increase of r.p.m.

(vii) The engine must not be allowed to get cold, or it may not respond when required. The engine will be kept warmer by diving moderately with throttle well open than by gliding with throttle closed. In a long glide gills must be closed and the engine should be opened up at intervals.

7. Landing and Stopping the Engine.

(i) After lowering the undercarriage the pilot should

(a) See that the mixture control (if fitted) is RICH, air intake (if variable) COLD and the supercharger in M ratio.

(b) Set the propeller control for take-off r.p.m. (or as recommended in the Pilot's Notes).

He is then ready for getting full power in any emergency by simply advancing the throttle lever.

NOTE.-To minimise over-speeding on quick opening up to take-off boost, the control may he set for not more than 5% below take-off r.p.m. On high-powered aircraft, if climbing power is considered ample for a baulked landing, the control may be set for climbing r.p.m.; but if climbing r.p.m. are more than 5% below take-off r.p.m., the engine should only be opened up to climbing boost, or the propeller control should also he advanced.

(ii) After landing, open the gills or radiator flaps for taxying.

(iii) Idle the engine (with the aircraft headed into wind if possible) at about 800 r.p.m. to cool it before stopping. (Air-cooled cylinders should be reduced to between 150 and 200°C.)

(iv) Throttle back and stop the engine by pulling and holding the slow-running cut-out, releasing smartly when the engine has stopped.

(v) Switch off the ignition and turn off the fuel.

(vi) Close the cowling gills or radiator shutters.

NOTE.-Engines fitted with a slow running cut-out (not operated by the master fuel cock) must not be stopped by turning off the fuel.

APPENDIX

1. Boost Control Cut-outs and Over-rides.

(i) Some earlier Merlin engines have a cut-out which renders the automatic boost control inoperative to permit an " emergency " power to be obtained.

(ii) Later Merlin cut-outs retain the automatic control of boost but reset the control so that each position of the throttle lever gives a higher controlled boost.

(iii) A similar device called a high boost control or over-ride is used with Mercury engines in Blenheim IV and V. Owing to the regulation of mixture strength by means of the throttle lever on this engine the control should only be used in flight with the lever fully forward, and when it is used for take-off the throttle must be opened steadily and fairly quickly to the take-off position after selecting high boost.

(iv) High boost for take-off is obtained in some Merlin installations by moving the throttle lever through a gate and thereby over-riding the boost control and setting the throttle valve to a definite position to give a high take-off boost. A cut-out may be provided in addition to give high boost under automatic control in flight.

2. Automatic Supercharger Gear Change.

(i) An automatic gear change in some installations safeguards the engine against the inadvertent use of S ratio at low altitudes which would quickly wreck the engine.

(ii) The change is effected automatically at a certain altitude; but the gear can be returned from S to M above this height by an over-ride switch.

(iii) The gear may also be returned automatically from S to M ratio if the charge temperature becomes excessive; but it can be reset subsequently by the pilot if and when the excessive temperature no longer occurs.

(iv) A testing switch, with warning light, allows engagement of S ratio on the ground for testing purposes.

PART II - NOTE D - THE VARIOUS TYPES OF PROPELLER

1. Introductory: -

(i) There are four main classes of variable-pitch propellers.

(a) Two Pitch-pitch directly controlled by pilot with two settings only (nearly obsolete).

(b) Constant speed - pitch controlled by a governor (" constant speed unit "), pilot selecting r.p.m. required.

(c) Constant Speed with Alternative Manual Control - pitch controlled by governor or directly by pilot (electrical propellers).

(d) Classes (b) or (c) with Feathering control also.

(ii) Power to turn the blades in their sockets to change the pitch is derived :

(a) From engine-driven hydraulic pumps; or

(b) From batteries and engine-driven generators (electrical propellers and feathering of both electrical and hydraulic propellers).

It follows that only the electrical propellers can have the pitch changed when the engine is not running, except for feathering and unfeathering.

2. Two-pitch Propellers.

The pitch is directly controlled by the pilot and has two settings, FINE and COARSE. The correct use is :

(1) Start and warm up COARSE.

(2) Change to FINE before running up the engine.

(3) After reaching safety speed reduce the boost to the climb figure and then change to COARSE.

(4) Change to FINE preparatory to landing.

(5) Before stopping the engine change to COARSE; after moving the lever open up the engine sufficiently to change the pitch.

NOTE.-Leaving the propeller in COARSE pitch facilitates inspection and maintenance. The propeller is emptied of oil and, if the pitch is only returned to FINE after the engine oil has warmed somewhat, initial sluggishness is avoided.

3. De Havilland and Hamilton 14 and 20 degree Constant-speed Propellers.

(i) These constant-speed propellers were developed from the two-pitch propellers by introducing the constant-speed governor under the pilot's control of r.p.m. setting. The pilot retains one direct setting of pitch-when the lever is full back (low r.p.m.) the propeller locks in POSITIVE (i.e., fixed) COARSE PITCH.

The same stopping and starting procedure should be followed as for the two-pitch propellers :—

(a) Start and warm up POSITIVE COARSE.

(b) Change to take-off r.p.m. setting before running up the engine.

(c) Set the r.p.m. as required. in flight, using POSITIVE COARSE PITCH for cruising if it is found possible to cruise at lower r.p.m. by so doing.

(d) Change to coarse before stopping the engine, opening up sufficiently to ensure change of pitch.

(ii) These propellers tend to sluggish operation at very low temperatures and they may fail to function after a time below - 35° C. Cruising in POSITIVE COARSE PITCH minimises the risk of sluggishness. To prevent or to overcome sluggishness the propeller should be exercised as follows :

(a) Move the control to maximum r.p.m.

(b) Slowly return to POSITIVE COARSE.

At extreme temperatures exercising should be done two or three times every quarter of an hour. It should seldom be necessary on Merlin fighters below 25,000 ft. in winter or 30,000 ft. in summer; but sluggish operation may occur at lower heights with air-cooled engines.

NOTE.-The development of sluggishness has been over-come in some installations (Spitfire V) by introducing a bleed to ensure continual circulation of oil.

4. Rotol 20 deg. Constant-speed Propellers.

These propellers are also designed for POSITIVE COARSE setting, but the system of control is different and the special stopping and starting procedure, and the need for exercising at low temperatures, do not apply.

NOTE.-On Fulmar the control is so arranged that the POSITIVE COARSE setting is not available.

5. Diving with Two-pitch and 14 and 20 deg. Constant-speed Propellers.

(i) These propellers, unlike the 35 deg. propellers, have not sufficient range of pitch. to keep the r.p.m. within the normal limits in a dive to very high speed. The permissible diving airspeed may therefore be limited by the r.p.m. rising beyond the normal maximum to the maximum permissible revolutions for diving laid down in the engine limitations in the Pilot's Notes.

In order to balance inertia stresses by explosion stresses, the throttle must always be at least one third open at these high r.p.m. The r.p.m. should not be allowed to exceed the maximum for level flight for more than 20 seconds.

(ii) De Havilland and Hamilton 14 and 20 deg. Constant-Speed Propellers:

The pilot will normally dive with the throttle well open and the propeller under constant-speed control. But if he wishes to throttle back before diving, the control should be moved to POSITIVE COARSE PITCH when the throttle is closed. If this is not done the blades may move to fine pitch and stay in fine pitch while windmilling, the air forces on the backs of the blades overpowering the centrifugal forces and springs upon which this design relies to coarsen the pitch; serious overspeeding will then occur however the propeller lever may be set.

6. Electrical Propellers (Curtiss and Rotol).

(i) The electrically operated propellers can either be set to function under automatic, constant-speed control, or the pitch can be locked and changed by manual control. When the selector switch is set to AUTO the propeller is under the control of the governor, "constant speeding". The r.p.m. are generally adjusted by the usual lever, but sometimes by an electric motor controlled by a switch. When the selector switch is central, the pitch is fixed but can be changed by deflecting the switch to IN Crease R.P.M. or DECrease R.P.M., the switch returning to central on release.

(ii) There is also a safety switch which is tripped by any electrical overload; it must be reset ON after a 30 seconds pause before the propeller can be controlled in any way.

(iii) The propeller should normally be operated AUTO, but it can be set to manual control to relieve the load on the battery if it is run down. Manual control may also be used for an ignition test in flight, for mixture adjustment with non-automatic carburettors, when flying in icing conditions to detect icing by drop in r.p.m., or if the governor should break down.

(iv) Electrical propellers change pitch considerably more slowly than the hydraulic types and greater care is needed to avoid overspeeding by too quick throttle opening. When diving, the propeller should be set for 200 to 400 r.p.m. below the maximum level flight r.p.m.

7. Feathering of Propellers.

(i) Feathering the propeller stops the engine and may prevent further damage after a breakdown. It reduces propeller drag and so improves performance and control. Feathering turns the blades into a more or less fore and aft plane, and the big change of pitch required to do this is made considerably faster than the normal change of pitch.

(ii) "Hydromatic" Propellers (Three Blade):

(a) These propellers are feathered by pressing the feathering button, which is then held in electrically until the process is complete. The button may not stay in at the first pressure and it must be held until it is locked in electrically; this may take two or three seconds. If the throttle has not been closed, it should be closed at once after pressing the button. The button should spring out when feathering is complete ; if it does not, it should be pulled out.

(b) If the wrong button has been pressed and the error is detected at once, pulling out the button will stop feathering and return the propeller quickly to constant speed control. But if this action is not taken within two or three seconds the propeller must be feathered fully and unfeathered, closing the throttle at once and re-opening when the correct r.p.m. for return to constant speed control are reached.

(c) The propeller is unfeathered - after setting the engine controls for starting - by setting the propeller control lever for minimum r.p.m. and pressing the same button, but now holding it in by hand until the correct r.p.m. are reached. The button must be released when the engine is turning at 1,000 to 1,300 r.p.m., except on the following aircraft for which the correct range is 1,500 to 1,800 r.p.m.

Flamingo"

Mosquito, all marks

Lancaster I

Wellington II

(d) If the propeller does not return of itself into the range of constant speed control, the throttle should be opened slightly and momentarily to give the required rise of r.p.m.

(iii) Rotol Hydraulic Propellers:

(a) This propeller is feathered by moving the propeller control lever fully back through a gate and then pressing the feathering button, which must be held in throughout the process. The throttle, if not already closed, should be closed at once after pulling back the lever, which starts the feathering process. (Pressing the button accelerates the feathering. The propeller will feather slowly, at a decreasing rate, and not quite to the full, on the lever alone.)

(b) The propeller is unfeathered - after setting the engine controls for starting - by returning the propeller lever through the gate and then holding in the button until the corresponding r.p.m. are attained. (The propeller will not unfeather without the electrical assistance introduced by pressing the button.) If the propeller does not start to unfeather on pressing the button, advance the lever slightly to take up any " backlash " in the control.

(iv) Electric Propellers:

(a) These propellers are feathered by setting the feathering switch to FEATHER; the blades will then feather however the selector switch may be set. If not closed already, close the throttle at once after operating the feathering switch. After feathering fully the switch may be left at FEATHER, or it may be returned to NORMAL, but only if the selector switch is central (or the propeller may unfeather).

(b) The propeller can also be feathered slowly (45 to 60 seconds) by holding the selector switch to DECrease R.P.M. until the engine stops. This method avoids strain on the battery and mechanism.

(c) To unfeather, set the engine controls for starting and set or check selector switch central and feathering switch NORMAL; propeller control back. Deflect the selector switch to INCrease R.P.M. Until the engine is running at about 1,000 r.p.m.; then set to AUTO.

(v) Warming up after Unfeathering:

(a) Engines may be warmed up in flight at as much as 2,000 r.p.m. at a small throttle opening, provided that the oil inlet temperature has not fallen very low (below 5°C.), and it will seldom be necessary to warm up at the r.p.m. used on the ground.

(b) If the oil has become very cold in an idle installation after a long flight, the engine can be warmed up at low r.p.m. as described below; but the advisability of warming up at all before attempting to use the engine for landing will depend upon the reason for shutting down. If the reason was loss of coolant or oil, the engine should be used for the minimum time and it should not be warmed up.

(c) Hydromatic propellers can be unfeathered to run at 600 to 800 r.p.m. by releasing the button at this speed. If the button is pressed again the propeller will start feathering; so, if the r.p.m. rise too high, they can be checked by pressing the button for a second or two and pulling it out again. To unfeathcr fully, the propeller must first be fully feathered and then it can be fully unfeathered.

(d) Rotol hydraulic propellers can be run at low r.p.m. by releasing the button during unfeathering. The r.p.m. will then rise slowly, but they can be checked and made to fall slowly by moving the propeller lever to the feathering position; so the r.p.m. can be kept within suitable limits by alternate movements of the lever.

(e) Rotol electric propellers can be run at low r.p.m. by returning the selector switch central when the required r.p.m. are reached.

(f) All American propellers govern down to 1,000 r.p.m. and no special action is necessary.

(vi) Practice and Test Feathering:

(a) Feathering imposes a severe drain on the battery. It is, therefore, important that the battery should be fully charged at the start of a flight on which feathering is to be practised; and not more than three featherings and three unfeatherings should be made on the aircraft in one flight.

(b) Feathering checks on the ground should be done with a ground trolley battery.

(c) If two or more propellers have been feathered, the propeller of the engine which drives the generator should be unfeathered first so that the generator can supply current for the second unfeathering. A pause of some minutes between opening up the first engine and unfeathering the second propeller will allow the battery to be partly recharged.

8. Further Information.

Reference may be made to the Propeller Manuals, A.P. 1538 and suffix letters.

PART II - NOTE E - THE EFFECTS OF LOW AND NEGATIVE G ON THE ENGINE

1. Introduction.

When pushing down into a dive the pilot imposes a reduced. g on himself and on everything in the aircraft. A sufficiently sharp push-down leads to g becoming negative and the pilot finds his weight transferred from the seat to the harness. At the same time fuel and oil move to the top of the tanks and the float in the carburettor float chamber floats downward. Reduced g occurs also in other manoeuvres, such as loops and rolls, and in a true slow roll and in badly executed aerobatics g may become negative.

2. The Effects on the Carburettor.

(i) In float chamber carburettors reduction of g upsets the balance between float forces and fuel pressure and - usually around 1/2 g - causes flooding of the carburettor and a rich cut of the engine. The engine will cut less readily and recover more quickly with the throttle well open; but, for the reason explained below, the throttle must be closed when the engine cuts.

(ii) To reduce the tendency to rich cutting at reduced g a restrictor has been introduced in the fuel supply pipe to Merlin engines. This device limits the rate of fuel feed in order to prevent flooding. Its effectiveness depends on the relation of supply to demand and it fails at low power. It, therefore, remains beneficial to have the throttle well open when g is reduced.

(iii) When g becomes negative quickly the engine cuts weak from the uncovering of jets at the base of the float chamber. The duration of the weak cut is brief and it is followed at once by the rich cut due to flooding of the carburettor.

(iv) Recovery from a cut takes normally about 10 seconds from the removal of the cause. The restrictor reduces the recovery period to 3 to 5 seconds.

(v) Diaphragm and Injection carburettors are independent of g, and reduced and negative g will not cause cutting.

(vi) The " Anti-g " S.U. float carburettor is immune from cutting under negative g, except at low power when its behaviour is similar to that of the standard carburettor fitted with a restrictor.

3. Danger of Overspeeding with a Constant-Speed Propeller when the Engine Cuts.

Although the tendency to cut is less and recovery is quicker with the throttle well open, the pilot must close the throttle before power returns to avoid the risk of serious overspeeding of the engine; for as soon as power fades the propeller starts to fine its pitch and it is unable to readjust the pitch quickly enough to cope with a rapid return of high power. It is, therefore, advisable to close the throttle when a cut has occurred from any cause and to reopen it carefully when power returns.

4. Oil Starvation under Negative G.

(i) Negative g uncovers the oil tank outlet and the oil pressure falls almost immediately. The engine will stand only a momentary running at less than the emergency minimum oil pressure, and repeated short periods of low oil pressure will ultimately cause failure. It is, therefore, important :

(a) to avoid unnecessary applications of negative g;

(b) to limit the duration of negative g to a very few seconds ;

(d) if practicable, to watch the oil gauge and not to continue under negative g after the oil pressure falls below the emergency minimum.

(ii) Oil starvation can be avoided by specially designed negative g oil tanks.

PART II - NOTE F ENGINE ICING

1. Engine Icing in Aircraft Icing Conditions.

(i) The weather conditions that cause ice to form on the leading edges of wings, etc., will also cause ice to form in a forward-facing external air intake which is not fitted with an ice-guard. Ice may also form on any obstruction to the flow within the induction system before the introduction of the fuel. After the fuel has been injected the reduction of temperature due to vaporisation will put the stream below the icing range of temperature and ice will not form there in these atmospheric conditions. (See para. 2.)

(ii) Gradual blocking of the intake produces irregular running and/or flame from the. exhaust; total blocking will stop the engine and may cause fire.

(iii) Merlin engines are, therefore, fitted with gapped ice guards. Ice forms on the grid-screen carried a little distance ahead of the intake mouth and when the screen is blocked by ice, air is drawn through the gap. The only effect on the engine is that the ram effect is lost and the full throttle height (and the power at greater heights) is lowered.

(iv) Gapless ice guards are fitted to some Bristol engines; when the guard screen is blocked, air is admitted by the automatic opening of an intake from the engine bay.

(v) If no ice guard is fitted, the alternative manually operated WARM intake must be used in aircraft icing conditions.

(vi) Ice guards also provide protection against blocking of the intake by snow or sleet and WARM intake must be used in these conditions if no ice guard is fitted.

2. Carburation Icing Conditions.

(i) Ice may form in the induction system in warm weather and under a clear sky; the atmospheric temperature may be as high as + 18°C. This is because the evaporation of the fuel causes a big drop of temperature. The drop in temperature causes condensation of the water vapour in the air and the resulting low temperature causes the droplets to be super-cooled and ready to freeze on any cold object such as an unheated butterfly valve.

(ii) This refrigeration icing is partly avoided in the American injection carburettors by introducing the fuel after the air has passed the butterfly; but ice may form in the vicinity of the injection nozzle and in the blower entry trunk.

(iii) Merlin and the later Hercules engines have the vulnerable parts of the carburettor heated with oil or coolant and should not experience refrigeration icing so long as oil or coolant is kept above 60° C.

(iv) Another means of protection is provided by the WARM intake, which draws heated air from inside the engine bay.

(a) In Bristol engines and some American engines the degree of heating is small and there are two settings - COLD and WARM.

(b) In other American engines more heat is available and the degree of heating can be controlled by the pilot with the aid of a thermometer in the intake.

(c) In Merlin engines the WARM intake is for use only in aircraft icing conditions in the absence of an ice guard or in the event of failure of the ice guard to protect the induction system.

(v) In Bristol engines without oil-heated throttles the WARM intake is inadequate to prevent icing in all conditions and a remedy has been found in the use of alcohol, which mixes readily with water and ice, lowers the freezing point of water, and makes ice soft and non-adhering. The alcohol can be mixed with the fuel or injected when necessary - into the induction system. The use of alcohol injection is discussed later.

(vi) A drop in temperature is also caused by the reduction of pressure at the butterfly, particularly at small throttle openings. This may cause throttle icing in. a Bendix-Stromberg carburettor, especially when appoaching to land, owing to the carburettor being unheated; the amount will be small, but it may prevent the throttle closing. In damp or wet weather with air temperature between 0° and 5° C, it is advisable to use warm intake for landing.

(vii) Engine icing may be detected by falling boost; but with boost control the throttle butterfly will open to counteract throttling by ice and considerable icing may occur before any indication is given if the aircraft is flying well below the full throttle height. With Merlin engines, however, icing will cause an immediate fall in boost if the butterfly is as far open as the boost control can set it without the throttle lever being further advanced. (See Note C under "Economical Flying".)

3. Effects of WARM Intake on Engine Performance.

(i) The use of WARM intake causes a loss of power at given r.p.m. and boost by reducing the weight of charge drawn in; so COLD intake should be used for maximum power.

(ii) WARM intake may also lead to detonation at high boost in a warm atmosphere.

(iii) When running on WARM intake - or when the ice guard is blocked by ice or snow - the ram effect is lost; the full throttle height is lowered and the power obtainable above full throttle height is reduced.

(iv) If the carburettor is, like the Bendix-Stromberg, fully compensated for varying charge temperature at given altitude, the use of warm intake may have no adverse effect on range; it may even increase range by improving the distribution at low power, especially in cold weather. On the other hand, unless the warm intake is correctly designed, disturbance of the flow may adversely affect the mixture strength.

(v) If the carburettor is not temperature compensated, WARM intake will decrease range. The adverse effect may not be serious; but with some installations it has been found to be very serious, largely owing to disturbance of the flow.

(vi) In general, with Bristol engines WARM intake will give a smoother running engine at some sacrifice of economy of fuel. With some Bristol engines - e.g. Mercury - WARM intake must be used when cruising at low power in other than very warm conditions, or the engine is liable to "fade".

4. Use of Cold Intake for Starting.

(i) Cold intake must always be used for starting because if WARM is used:

(a) the shutters may be damaged by a backfire;

(b) with an up-draught carburettor, a backfire may ignite priming fuel which may have reached the engine bay.

(ii) WARM may be used to accelerate warming up in very cold weather, but the engine must be running evenly before the change to WARM is made.

5. Use of WARM Intake to prevent Icing.

(i) In the absence of an ice guard, use WARM intake in aircraft icing conditions - i.e. in rain or cloud at temperatures between -1° and -7° C and in sleet or snow.

(ii) On earlier Bristol engines it is generally advisable to use WARM intake when cruising at low power at temperatures below 15° C.

(iii) On later Bristol engines - Hercules III, VI and XI-WARM intake need only be used if icing is indicated by falling boost.

(iv) When using WARM intake against carburettor icing, the cylinders should be kept up to 180° to 200° C. The use of high boost and low r.p.m. and WEAK mixture will help to keep the temperature high.

6. Use of Alcohol Injection.

(i) The system should be primed by giving 12 strokes of the pump just before starting the engine. (Increased resistance to pumping indicates when the system has been primed.)

(ii) On the first sign of falling boost, operate the spray for a few seconds; this will clear the ice for the moment.

(iii) Then change the carburettor heat conditions - from COLD . to WARM, or from WARM to COLD; or climb to a considerably greater height COLD; or descend WARM.

PART II-NOTE G - OIL DILUTION FOR COLD WEATHER STARTING

1. Purpose and Principle.

Oil dilution has been introduced to make the starting of engines in cold weather easier by reducing the effort needed to turn them. In addition to giving easier starting, oil dilution ensures an immediate flow of lubricant to all parts of the engine at the normal working pressure. The underlying principle is that cold oil thinned with petrol gives satisfactory lubrication so long as the diluted oil has the correct viscosity.

2. The System.

A pipe is run from the delivery side of the fuel pump (or reducing valve) to the suction side of the oil pump, and fuel is admitted to the oil on pressing a push button in the cockpit, the rate of flow being determined by a metering jet suited to the particular installation. To confine the dilution as far as possible to the oil in circulation, a "hot pot" is required in the main tank, from the bottom of which the oil is drawn and into the top of which it is returned by the scavenge pump. The level in the hot pot is maintained by the entry of oil from the main tank through holes at the base of the hot pot. The correct period for dilution varies from 1 to 4 minutes, according to the installation and the lowest atmospheric temperature to be anticipated. When the engine is run up subsequently, the diluent evaporates, the greater part in the first ten minutes.

3. When to use Dilution.

Oil should be diluted whenever there is likelihood of difficulty in starting, which of course depends upon the weather and the time during which the engine is likely to be idle. Whatever the day temperature, if there is risk of sharp frosts at night, dilution should be used to prevent the bursting of flexible pipes, couplings and oil coolers.

4. Procedure for Dilution.

(i) (a) When possible the oil tank should be topped up before dilution. (The air space in a filled tank is ample to contain the increased bulk due to the diluent.)

(b) But if this is not practicable, topping up should be left until just before or after starting. If the tank is topped up earlier, cold undiluted oil will have time to work its way from the full tank into the partly empty hot well; it will enter it at its base and, being denser, lie at the bottom and be drawn into circulation when the engine is started.

(ii) (a) The engine and the oil should be cool before the oil is diluted; otherwise, some of the diluent will evaporate. The recommended oil temperature for dilution is from 10° to 45° C.

(b) Although the full benefit will only be obtained by diluting at these temperatures, considerable assistance in starting will still be obtained if the dilution is done at higher temperatures.

(iii) The engine should be run at 1,000-1,200 r.p.m. or at the r.p.m. specified in the Pilot's Notes, and the dilution switch should be pressed for the recommended period, which will be quoted in the Pilot's Notes. The engine should he stopped before releasing the switch.

(iv) The operator should check that the switch returns on release.

5. Benefit of a Subsequent Short Run.

The benefit of dilution can be increased by allowing the engine to cool thoroughly and then restarting and running at 1,000 to 1,200 r.p.m. for 30 seconds with the dilution switch pressed. This second short run distributes the diluted oil to the cylinder walls and other parts which are normally so hot that they evaporate the fuel at the time of dilution. The additional run is particularly beneficial if the first dilution has been made at a high temperature.

6. Period of Effectiveness.

(i) After dilution the engine can be left for two or three days during normally cold weather, without the usual frequent running up, or re-dilution.

(ii) If, however, the engine is run up on the ground after dilution and is then to remain idle for some time, oil dilution should be repeated before closing down, but only for a period necessary to replace fuel evaporated from the oil during this run, normally a quarter of the standard period.

7. Starting on Diluted Oil.

The engine is started in the normal manner and it may be opened up as soon as the oil pressure is normal and the oil and coolant temperatures are the normal minima quoted in the Pilot's Notes.

8. Overdilution.

If the instructions are followed, overdilution should only occur if the fuel valve leaks or if the push button sticks (so that dilution occurs at other than the intended times). Over-dilution will result in low oil pressure. So long as the pressure is above the minimum quoted in the Pilot's Notes, over-dilution will have no ill effects, and the excess fuel will quickly evaporate. The only precaution necessary is to warm up thoroughly till normal oil pressure is obtained.

PART II-NOTE H - SPECIAL FEATURES OF AMERICAN ENGINES

1. Boost Control.

(i) American engines have (with some recent exceptions) been built without automatic boost control and the pilot must advance the throttle continually on the climb to maintain constant boost pressure up to full throttle height. Similarly, in diving below full throttle height he must watch the boost gauge and close the throttle as necessary to avoid over-boosting. In changing from M to S supercharger ratio, he may need to throttle back slightly to avoid over-boosting.

(ii) In the absence of any boost control a spring catch has been fitted to all American aircraft supplied to the R.A.F. in order that the pilot may be able to feel the take-off setting without having to watch the boost gauge during take-off.

(iii) A form of automatic boost control is, however, added to certain American aircraft in use by the R.A.F. as a better safeguard against inadvertent over-boosting. When this control is fitted :

(a) Take-off boost will be obtained by moving the lever fully forward.

(b) Climbing boost will be obtained at low altitudes by pulling back the lever to the climbing gate; but in general the lever must be progressively advanced to maintain the boost as height increases.

(c) When cruising it will be necessary to move the throttle lever if it is desired to maintain the same boost at a different height.

(d) The lever must be moved fully forward to obtain maximum power at any height.

It will be seen that this control does not automatically - maintain the boost constant up to full throttle height. What it does is to set a limit to the boost, a limit which is governed by the position of the throttle lever. For example, if the aircraft is dived to a low level with the lever at the climbing gate, the boost will not rise above the climb limit; if the lever is further back, it will not rise above some lower limit; and it can never exceed the take-off and all-out limit with the lever fully forward.

(iv) American boost gauges read the manifold pressure in inches of mercury absolute, whereas the British gauges read the boost in lb. per sq. inch above the standard atmospheric pressure, 14'7 lb./sq. in., boost being negative when the manifold pressure is, below this figure; so 30 ins. Hg. is the equivalent of zero boost and every inch of Hg. is approximately 1/2 lb./sq. in. boost.

2. Mixture Control.

(i) Bendix-Stromberg Injection Carburettor :

The control is marked :

FULL RICH

AUTO RICH

AUTO LEAN

IDLE CUT-OFF

FULL RICH is for use only in emergency-if the automatic control should fail, or if the mixture is weakened by ice forming in the induction system. The range from AUTO RICH to AUTO LEAN gives a progressive weakening of the mixture strength; but normally the AUTO RICH and AUTO LEAN positions only should be used. On some aircraft, however, the mixture may become too rich with large throttle openings at high altitudes and the use of an intermediate setting will then give smoother running. IDLE CUT-OFF is used both for stopping and for starting and cuts off the supply of fuel at all positions of the throttle lever. The control, which must be at IDLE CUT-OFF while priming, is moved to AUTO RICH as soon as the engine fires regularly. (See also 2 (iv) and Appendix to this note.)

(ii) Other Automatic Carburettors (later Holley Injection type):

The markings are :

RICH

LEAN

IDLE (or FUEL) CUT-OFF

The mixture is automatically compensated for altitude and the control should normally be operated as for a two position carburettor. But the range from RICH to LEAN gives a progressive change of mixture strength which may be used to give a smoother running engine with large throttle openings at high altitudes.

(iii) Non-automatic Carburettors (Chandler-Evans, Chandler-Groves, Holley)

The markings are :