Advanced Pilot Training: Flight Operation of Engines

Operating Limits Depend Upon Grade of Fuel Used

Military aircraft engines designed to use 100/130 grade of fuel will be operated in combat and on certain missions with the grade of fuel for which they were designed; but much of your flying within the domestic area will be on a lower grade fuel than that for which the engines were designed.

You must be absolutely certain of the grade of fuel you are using and accommodate the operating range of your engines to it.

To conserve 100 octane aircraft fuel, 91 octane is now specified for most military aircraft operating in continental United States on training or routine flights.

All aircraft which require and are serviced with 100 octane fuel will be operated at all times using the most economical power and cruising conditions.

Mixed Grades of Fuel

On any mission named above, never mix 100/130 octane with a lower grade fuel. If your tanks contain any 91 octane, have them fully drained before servicing with the higher grade fuel.

It is permissible when occasions demand to mix fuels of grade 91 and grade 100/130. But when these grades are mixed, you must observe operating limits for grade 91.

Reduction of Full Rated Power

When fuel grade of Spec. No. AN-VV-781, amendment 5, AN-F-27, or AN-F-28 is used, full rated military power is allowed. When fuel grade of Spec. No. AN-VV-781, amendment 4, is used, you will reduce maximum boost by 10% below full rated power.

Watch Your Operating Limits

In using the lower grade fuel, pilots must use special care in flight operations of engines to avoid any condition that may lead to detonation, which is always a danger in engines operating on a grade of fuel lower than that for which they were designed.

All aircraft serviced with the lower grade fuel will have attached to the control column or ignition switch it red warning tag bearing the following notation:

Warning To Pilot Tag

This airplane serviced with fuel, grade 91, Specification No. AN-F-26. Operate engine within the maximum limits prescribed in T. 0, No. 02-1-38.

Detonation in Aircraft Engines

Detonation may be indicated by unusual engine roughness, cylinder temperature increase, erratic fuel-air ratio, and by the exhaust flame.

Engine roughness does not necessarily indicate that there is detonation, but when unusual roughness is present it may be due to detonation.

Cylinder temperature increase cannot be relied on as a definite indication of detonation but since detonation liberates an unusual amount of heat to the cylinder walls an increase in cylinder temperatures may be due to detonation.

An erratic fuel-air ratio reading may indicate detonation. If, as the mixture is leaned out, the needle does not show a leaner mixture or backs up the scale towards the rich side, detonation has probably occurred and the mixture should be richened.

Exhaust flame is the most reliable indication of detonation when exhaust stacks are visible. Intermittent puffs of dense black smoke, often accompanied by sparks or glowing carbon, indicate detonation. Black puffs occurring with great regularity indicate severe -detonation. A rich mixture is indicated by full red flames with steady black smoke. Black smoke and red flame may also be caused by high oil consumption, ice in the carburetor, or other causes of poor fuel distribution. Light blue flames, almost invisible, indicate a mixture setting which produces maximum power.

Detonation may be caused by use of fuels of low octane number, low fuel-air ratio, and engine operating conditions.

Use of fuels of low octane number will cause detonation since fuels are rated in terms of their resistance to detonation. The greater their resistance to detonation the higher the octane number of the fuel. A higher octane number fuel than that specified may safely be used but a lower octane number must be used only when specified in technical orders or in an emergency. In this case the engine instructions covering the use of a lower grade fuel must be strictly followed.

The greatest tendency for fuel to detonate occurs near the fuel-air ratio for best power. Leaning the mixture increases the detonation, while rich mixture increases anti-knock value and cools the cylinders.

A change in engine operating conditions which increases the pressure or temperature in the cylinder increases the chance of detonation. Therefore, detonation may be caused by the following:

1. Increasing the manifold pressure.

2. Advancing the ignition timing or by operating on one spark plug.

3. Increasing intake air temperature by use of the carburetor air heater.

4. Increasing fuel-air mixture temperature, as by using high blower gear ratio instead of low blower to obtain a given manifold pressure.

5. An increase of cylinder temperature.

6. Building up of engine deposits which tend to decrease the rate at which heat is conducted away from the combustion chambers.

7. Operating engine for an excessive period on the ground.

8. Leaning fuel-air mixture.

Results of detonation are loss of power, overheating, preignition, and damage to the engine.

To obtain detonation-free operation immediately reduce the manifold pressure, richen the mixture, and reduce carburetor air heat to the minimum at which icing may be prevented.

Changing Power Condition

To avoid excessive pressures within cylinders with resultant detonation and possibility of failure, use the following procedure when changing condition of power:

Increasing Engine Power

1. Adjust mixture control to obtain the fuel-air ratio specified for power condition desired.

2. Adjust propeller control to obtain desired rpm.

3. Adjust throttle control to obtain desired manifold pressure.

4. Readjust mixture control, if necessary.

Decreasing Engine Power

1. Adjust throttle control to obtain desired manifold pressure.

2. Adjust propeller control to obtain desired rpm.

3. Readjust throttle controls, if necessary.

4. Adjust mixture controls to obtain desired fuelair ratio.

Fuel-Air Ratios

The fuel-air ratio values are the absolute minimum with which dependable engine operation can be expected. Use fuel-air ratios somewhat richer than the minimums specified except when extreme ranges are essential. This increase will help prevent sticking and burning of piston rings.

Air Speeds During Climbs

To obtain proper cooling, maintain a calibrated IAS of at least the best climbing speed specified for the particular airplane, with at least a 10 per cent greater IAS desirable unless the tactical situation prevents, or unless maximum altitudes are desired.

NOTE: Under no circumstances will airplanes be climbed for protracted periods at calibrated IAS less than the best climbing speed when using rated engine power.

The true climbing speeds for all later model airplanes appear in Flight Characteristics Section of the Handbook of Flight Operating Instructions for each type of airplane.

Overspeeding of Aircraft Engines

When manifold pressures and engine speeds have exceeded those specified, follow this procedure, without exception.

Report the following information to the Squadron, Station, or Sub-Depot Engineering Officer, as the case may be, immediately upon landing.

1. The maximum rpm and manifold pressure which was obtained during flight.

2. Duration in minutes of the overspeed and overpower operation.

3. Reason for overspeed and overpower, if known.

4. Total engine time and time since last overhauled when overspeed or overpower operations occur.

REFERENCE: Technical Orders 02-1-29, dated November 23,1942, and 02-1-69, dated May 22, 1943.

Side Note: Fuel Grades

Starting with an octane rating of 50, or even less, fuel development finally reached a high of 150 PN (performance number). However, the most common high performance aviation fuel used at the end of the large displacement piston engine era was 1151145 PN. A brief dissertation on the meaning of performance number and its relationship to octane number is in order. Sam Heron  came up with the performance number label. The PN scale was introduced in 1943, some years after the British had a crude version of it based on a 100 octane number commercial gasoline (Grade 100/125). The PN scale was introduced after the army and navy specified fuel with a lean rating of 100 octane number and a rich mixture rating equal to isooctanc plus one cubic centimeter (I cc) of tetraethyl lead (Grade 1001125). In simple terms this means pure isooctane I's 100 octane and adding one cubic centimeter of tetraethyl lead boosted the performance of the fuel to 125 PN when running on the rich side of stoichiometric (Ref. 1.4 and 1.5). Army and Navy personnel were confronted with the problem of explaining to military brass that it was not possible to assign an octane number to isooctane plus one cubic centimeter of lead. Operating personnel in the supply and maintenance divisions of the services arbitrarily started to assign octane numbers to the new fuel and consequently mark ftiel-servicing trucks. Of course, no specs existed at the time so fuel might have been described as 104 octane at one location and 108 octane at another location. One can imagine the confusion caused by this mislabeling. A pilot landing at a field with fuel designated as 104 octane and needing " 109" would, in all likelihood, refuse the 104 fuel as being unsafe. This state of confusion and, at times, chaos needed a fix. The army, navy and British Air Commission representatives decided that fuels of 100 octane number and higher should be described by performance number (PN). The PN scale was based on 100 octane number, i.e., pure isooctane, as permitting 100 percent power in a supercharged engine, and 13 0 PN fuel permitting 1310 percent of the power for the same engine operating on 100 octane fuel. The relation between permissible power (i.e., maximum power before the onset of detonation) and various concentrations of lead in isooctane (which, of course, had been the rating scale previously in use) was derived by averaging a large number of single-cylinder and full-scale engine and laboratory single-cylinder engine data.

Source: Graham White, R-2800. Pratt and Whitney's Dependable Masterpiece, London 2001, p. xxi