Aircraft Reciprocating Engine Operation

The operation of the powerplant is controlled from the cockpit or flight deck. Some installations have numerous control handles and levers connected to the engine by rods, cables, bellcranks, pulleys, etc. In most cases, the control handles are conveniently mounted on quadrants in the flight deck. Placards, or markings, are placed on the quadrant to indicate the functions and positions of the levers. In some installations, friction clutches are installed to hold the controls in place.

Engine Instruments

The term engine instruments usually includes all instruments required to measure and indicate the functioning of the powerplant. The engine instruments are generally installed on the instrument panel so that all of them can easily be observed at one time. Manifold pressure, rpm, engine temperature, oil temperature, CAT, and the fuel-air ratio can be controlled by manipulating the flight deck controls. Coordinating the movement of the controls with the instrument readings protects against exceeding operating limits.

Engine operation is usually limited by specified operating ranges of the following:
  1. Crankshaft speed (rpm)
  2. Manifold pressure
  3. Cylinder head temperature
  4. CAT
  5. Oil temperature
  6. Oil pressure
  7. Fuel pressure
  8. Fuel flow meter
  9. Fuel/air mixture setting

The procedures, pressures, temperatures, and rpm used throughout this section are solely for the purpose of illustration and do not have general application. The operating procedures and limits used on individual makes and models of aircraft engines vary considerably from the values shown here. For exact information regarding a specific engine model, consult the applicable instructions.

Engine Starting

Before starting the engine, observe the manifold pressure gauge that should read approximate atmospheric (barometric) pressure when the engine is not running. At sea level, this is approximately 30 "Hg, and at fields above sea level, the atmospheric pressure is less, depending on the height above sea level. Also, observe all engine gauges for the correct reading for engine off settings.

Correct starting technique is an important part of engine operation. Improper procedures often are used, because some of the basic principles involved in engine operation are misunderstood.


Engines that have undergone overhaul or major maintenance can have air trapped in some of the oil passages that must be removed before the first start. This is done by pre-oiling the engine by cranking, with the spark plugs removed, the engine with the starter or by hand (turning) until oil pressure is indicated. A second method is to pump oil under pressure through the oil system using an external pump until oil comes out of the oil outlet of the engine.

Hydraulic Lock

Whenever a radial engine remains shut down for any length of time beyond a few minutes, oil or fuel may drain into the combustion chambers of the lower cylinders or accumulate in the lower intake pipes ready to be drawn into the cylinders when the engine starts. [Figure] As the piston approaches top center of the compression stroke (both valves closed), this liquid being incompressible, stops piston movement. If the crankshaft continues to rotate, something must give. Therefore, starting or attempting to start an engine with a hydraulic lock of this nature may cause the affected cylinder to blow out or, more likely, may result in a bent or broken connecting rod.

Aircraft Reciprocating Engine Operation
Initial step in developing a hydraulic lock

To eliminate a lock, remove either the front or rear spark plug of the lower cylinders and pull the propeller through in the direction of rotation. The piston expels any liquid that may be present. If the hydraulic lock occurs as a result of overpriming prior to initial engine start, eliminate the lock in the same manner (i.e., remove one of the spark plugs from the cylinder and rotate the crankshaft through two turns). Never attempt to clear the hydraulic lock by pulling the propeller through in the direction opposite to normal rotation. This tends to inject the liquid from the cylinder into the intake pipe with the possibility of a complete or partial lock occurring on the subsequent start.

Engine Warm-Up

Proper engine warm-up is important, particularly when the condition of the engine is unknown. Improperly adjusted idle mixture, intermittently firing spark plugs, and improperly adjusted engine valves all have an overlapping effect on engine stability. Therefore, the warm-up should be made at the engine speed where maximum engine stability is obtained. Experience has shown that the optimum warm-up speed is from 1,000 to 1,600 rpm. The actual speed selected should be the speed at which engine operation is the smoothest, since the smoothest operation is an indication that all phases of engine operation are the most stable.

Some engines incorporate temperature-compensated oil pressure relief valves. This type of relief valve results in high engine oil pressures immediately after the engine starts, if oil temperatures are very low. Consequently, start the warmup of these engines at approximately 1,000 rpm and then move to the higher, more stable engine speed as soon as oil temperature reaches a warmer level.

During warm-up, watch the instruments associated with engine operation. This aids in making sure that all phases of engine operation are normal. For example, engine oil pressure should be indicated within 30 seconds after the start. Furthermore, if the oil pressure is not up to or above normal within 1 minute after the engine starts, the engine should be shut down. Cylinder head or coolant temperatures should be observed continually to see that they do not exceed the maximum allowable limit.

A lean mixture should not be used to hasten the warm-up. Actually, at the warm-up rpm, there is very little difference in the mixture supplied to the engine, whether the mixture is in a rich or lean position, since metering in this power range is governed by throttle position.

Carburetor heat can be used as required under conditions leading to ice formation. For engines equipped with a float-type carburetor, it is desirable to raise the CAT during warm-up to prevent ice formation and to ensure smooth operation.

The magneto safety check can be performed during warmup. Its purpose is to ensure that all ignition connections are secure and that the ignition system permits operation at the higher power settings used during later phases of the ground check. The time required for proper warm-up gives ample opportunity to perform this simple check, which may disclose a condition that would make it inadvisable to continue operation until after corrections have been made.

The magneto safety check is conducted with the propeller in the high rpm (low pitch) position, at approximately 1,000 rpm. Move the ignition switch from “both” to “right” and return to “both;” from “both” to “left” and return to “both;” from “both” to “off” momentarily and return to “both.”

While switching from “both” to a single magneto position, a slight but noticeable drop in rpm should occur. This indicates that the opposite magneto has been properly grounded out. Complete cutting out of the engine when switching from “both” to “off” indicates that both magnetos are grounded properly. While in the single magneto position, failure to obtain any rpm drop, or failure of the engine to cut out while switching to off, indicates that one or both ground connections are faulty. This indicates a safety problem; the magnetos are not secured at shut down and may fire if the propeller is turned.

Ground Check

The ground check is performed to evaluate the functioning of the engine by comparing power input, as measured by manifold pressure, with power output, as measured by rpm or torque.

The engine may be capable of producing a prescribed power, even rated takeoff, and not be functioning properly. Only by comparing the manifold pressure required during the check against a known standard is an unsuitable condition disclosed. The magneto check can also fail to show shortcomings, since the allowable rpm dropoff is only a measure of an improperly functioning ignition system and is not necessarily affected by other factors. Conversely, it is possible for the magneto check to prove satisfactory when an unsatisfactory condition is present elsewhere in the engine.

The ground check is made after the engine is thoroughly warm. It consists of checking the operation of the powerplant and accessory equipment by ear, by visual inspection, and by proper interpretation of instrument readings, control movements, and switch reactions. During the ground check, the aircraft should be headed into the wind, if possible, to take advantage of the cooling airflow. A ground check procedure is outlined below:
  1. Control position check
  2. Cowl flaps (if equipped)—open
  3. Mixture—rich
  4. Propeller—high rpm
  5. Carburetor heat—cold
  6. Check propeller according to propeller manufacturer’s instruction.
  7. Open throttle to the run-up rpm setting as per manufacturer’s instructions (specified RPM and manifold pressure).
  8. Ignition system operational check.

In performing the ignition system operational check (magneto check), the power-absorbing characteristics of the propeller in the low fixed-pitch position are utilized. In switching to individual magnetos, cutting out the opposite plugs results in a slower rate of combustion, which gives the same effect as retarding the spark advance. The drop in engine speed is a measure of the power loss at this slower combustion rate.

When the magneto check is performed, a drop in torquemeter pressure indication is a good supplement to the variation in rpm. In cases where the tachometer scale is graduated coarsely, the torquemeter variation may give more positive evidence of the power change when switching to the individual magneto condition. A loss in torquemeter pressure not to exceed 10 percent can be expected when operating on a single magneto. By comparing the rpm drop with a known standard, the following are determined:
  1. Proper timing of each magneto.
  2. General engine performance as evidenced by smooth operation.
  3. Additional check of the proper connection of the ignition leads.

Any unusual roughness on either magneto is an indication of faulty ignition caused by plug fouling or by malfunctioning of the ignition system. The operator should be very sensitive to engine roughness during this check. Lack of dropoff in rpm may be an indication of faulty grounding of one side of the ignition system. Complete cutting out when switching to one magneto is definite evidence that its side of the ignition system is not functioning. Excessive difference in rpm drop off between the left and right switch positions can indicate a difference in time between the left and right magnetos.

Sufficient time should be given to the check on each single switch position to permit complete stabilization of engine speed and manifold pressure. There is a tendency to perform this check too rapidly with resultant wrong indications. Operation as long as 1 minute on a single ignition system is not excessive.

Another point that must be emphasized is the danger of sticking tachometer. The tachometer should he tapped lightly to make sure the indicator needle moves freely. In some cases using older mechanical tachometers, sticking has caused errors in indication to the extent of 100 rpm. Under such conditions, the ignition system could have had as much as a 200 rpm drop with only a 100 rpm drop indicated on the instrument. In most cases, tapping the instrument eliminates the sticking and results in accurate readings.

In recording the results of time ignition system check, record the amount of the total rpm drop that occurs rapidly and the amount that occurs slowly. This breakdown in rpm drop provides a means of pinpointing certain troubles in the ignition system. This can reduce unnecessary work by confining maintenance to the specific part of the ignition system that is responsible for the trouble.

Fast rpm drop is usually the result of either faulty spark plugs or faulty ignition harness. This is true because faulty plugs or leads, take effect at once. The cylinder goes dead or starts firing intermittently the instant the switch is moved from “both” to the “right” or “left” position.

Slow rpm drop usually is caused by incorrect ignition timing or faulty valve adjustment. With late ignition timing, the charge is fired too late (in relation to piston travel) for the combustion pressures to build up to the maximum at the proper time. The result is a power loss greater than normal for single ignition because of the lower peak pressures obtained in the cylinder. However, this power loss does not occur as rapidly as that which accompanies a dead spark plug. This explains the slow rpm drop as compared to the instantaneous drop with a dead plug or defective lead. Incorrect valve clearances, through their effect on valve overlap, can cause the mixture to be too rich or too lean. The too rich or too lean mixture may affect one plug more than another, because of the plug location and show up as a slow rpm drop on the ignition check. Switch from “both” to “right” and return to “both.” Switch from “both” to “left” and return to “both.” Observe the rpm drop while operating on the right and left positions. The maximum drop should not exceed that specified by the engine manufacturer.

Fuel Pressure and Oil Pressure Check

Fuel pressure and oil pressure must be within the established tolerance (green arc) for the engine.

Propeller Pitch Check

The propeller is checked to ensure proper operation of the pitch control and the pitch-change mechanism. The operation of a controllable pitch propeller is checked by the indications of the tachometer and manifold pressure gauge when the propeller governor control is moved from one position to another. Because each type of propeller requires a different procedure, the applicable manufacturer’s instructions should be followed.

Power Check

Specific rpm and manifold pressure relationship should be checked during each ground check. This can be done at the time the engine is run-up to make the magneto check. The purpose of this check is to measure the performance of the engine against an established standard. Calibration tests have determined that the engine is capable of delivering a given power at a given rpm and manifold pressure. The original calibration, or measurement of power, is made by means of a dynamometer in a test cell. During the ground check, power is measured with the propeller. With constant conditions of air density, the propeller, at any fixed-pitch position, always requires the same rpm to absorb the same horsepower from the engine. This characteristic is used in determining the condition of the engine.

With the governor control set for full low pitch, the propeller operates as a fixed-pitch propeller, because the engine is static. Under these conditions, the manifold pressure for any specific engine, with the mixture control in rich, indicates whether all the cylinders are operating properly. With one or more dead or intermittently firing cylinders, the operating cylinders must provide more power for a given rpm. Consequently, the carburetor throttle must be opened further, resulting in higher manifold pressure. Different engines of the same model using the same propeller installation, and at the same barometer and temperature readings, should require the same manifold pressure to within 1 "Hg. A higher than normal manifold pressure usually indicates a dead cylinder or late ignition timing. An excessively low manifold pressure for a particular rpm usually indicates that the ignition timing is early. Early ignition can cause detonation and loss of power at takeoff power settings.

The accuracy of the power check may be affected by the following variables:
  1. Wind—any appreciable air movement (5 mph or more) changes the air load on the propeller blade when it is in the fixed-pitch position. A head wind increases the rpm obtainable with a given manifold pressure. A tail wind decreases the rpm.
  2. Atmospheric temperatures—the effects of variations in atmospheric temperature tend to cancel each other. Higher carburetor intake and cylinder temperatures tend to lower the rpm, but the propeller load is lightened because of the less dense air.
  3. Engine and induction system temperature—if the cylinder and carburetor temperatures are high because of factors other than atmospheric temperature, a low rpm results since the power is lowered without a compensating lowering of the propeller load.
  4. Oil temperature—cold oil tends to hold down the rpm, since the higher viscosity results in increased friction horsepower losses.

Idle Speed and Idle Mixture Checks

Plug fouling difficulty is the inevitable result of failure to provide a proper idle mixture setting. The tendency seems to be to adjust the idle mixture on the extremely rich side and to compensate for this by adjusting the throttle stop to a relatively high rpm for minimum idling. With a properly adjusted idle mixture setting, it is possible to run the engine at idle rpm for long periods. Such a setting results in a minimum of plug fouling and exhaust smoking, and it pays dividends from the savings on the aircraft brakes after landing and while taxiing.

If the wind is not too strong, the idle mixture setting can be checked easily during the ground check as follows:
  1. Close throttle.
  2. Move the mixture control to the idle cutoff position and observe the change in rpm. Return the mixture control back to the rich position before engine cutoff.

As the mixture control lever is moved into idle cutoff, and before normal dropoff, one of two things may occur momentarily:
  1. The engine speed may increase. An increase in rpm, but less than that recommended by the manufacturer (usually 20 rpm), indicates proper mixture strength. A greater increase indicates that the mixture is too rich.
  2. The engine speed may not increase or may drop immediately. This indicates that the idle mixture is too lean. The idle mixture should be set to give a mixture slightly richer than best power, resulting in a 10- to 20-rpm rise after idle cutoff.

Engine Stopping

With each type of engine installation, specific procedures are used in stopping the engine. The general procedure, outlined in the following paragraphs, reduces the time required for stopping, minimizes backfiring tendencies, and prevents overheating of tightly baffled air-cooled engine during operation on the ground.

In stopping any aircraft engine, the controls are set as follows, irrespective of the type or fuel system installation.
  1. Cowl flaps and any other shutters or doors are always placed in the full open position to avoid overheating the engine and are left in that position after the engine is stopped to prevent engine residual heat from deteriorating the ignition system.
  2. Carburetor air-heater control is left in the cold position to prevent damage that may occur from backfire.
  3. Constant speed propeller is usually stopped with the control set in the high pitch (decrease rpm) position.

No mention is made of the throttle, mixture control, fuel selector valve, and ignition switches in the preceding set of directions because the operation of these controls varies with the type of carburetor used with the engine. An engine equipped with a carburetor incorporating an idle cutoff mixture control is stopped as follows:
  1. Idle the engine by setting the throttle for 800 to 1,000 rpm.
  2. Move the mixture control to the idle cutoff position. In a float-type carburetor, it equalizes the pressure in the float chamber and at the discharge nozzle.
  3. After the propeller has stopped rotating, place the ignition switch in the off position.

In addition to the operations outlined previously, check the functioning of various items of aircraft equipment, such as generator systems, hydraulic systems, etc.

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