GROUND OPERATIONS

Conducting an engine runup is a ground operation that can present a safety hazard to personnel and can damage an aircraft and surrounding equipment.

Therefore, certain precautions must be taken. For example, all engine runups should be conducted in an area specifically designated for that purpose.

Furthermore, the aircraft should be positioned on a clean level surface and aimed so the blast from the propeller does not blow dirt into any hangar or onto another aircraft. Rather than relying solely on the aircraft's brakes, chock the wheels securely, or tie the aircraft down to prevent movement during engine power checks.

Ground service equipment such as auxiliary power carts or hydraulic service units should be positioned well away from the propeller arc with their wheels chocked and brakes set. In addition, adequate fire protection should be stationed nearby, being certain that personnel and fire extinguishers are well clear of the propeller area. Furthermore, a reliable means of communication between the engine operator and ground personnel should be established.

HYDRAULIC LOCK

Hydraulic lock is a condition that can develop in a radial engine after shutdown, where oil or liquid fuel accumulates and pools in the lower cylinders, or lower intake pipes. Since fluids are not com-pressible, any attempt to start an engine with hydraulic lock can cause severe damage to the pis-ton, connecting rod, valves, or cylinder. Therefore, before attempting to start any radial engine that has been shut down for more than 30 minutes, you should check for hydraulic lock. To do this, make sure the ignition switches are "off," then pull the propeller through in the direction of rotation a min-imum of two complete revolutions. Any liquid pre-sent in a cylinder will be indicated by an abnormal amount of effort required to rotate the propeller.

To eliminate a hydraulic lock, remove either the front or rear spark plug and pull the propeller through in the direction of rotation, allowing the piston to expel any liquid that is present. Never attempt to clear the hydraulic lock by pulling the propeller through in the opposite direction of

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Figure 2-21. Oil or fuel can seep past the piston rings on the lower cylinders of a radial engine, causing hydraulic lock.

The presence of hydraulic lock is identified by an abnormal amount of effort required to rotate the propeller. If a start is attempted under these conditions, severe engine damage can result.

rotation, since that could inject the liquid from the cylinder into the intake pipe, increasing the possi-bility of a complete or partial lock on a subsequent start. [Figure 2-21]

ENGINE STARTING

Starting an aircraft engine is a specialized procedure and varies with an individual engine and aircraft.

Therefore, before starting any aircraft engine, be sure to study the procedures in the appropriate air-plane flight manual and get instruction from an experienced operator. However, certain general guidelines apply to all reciprocating engine pow-ered aircraft. Before attempting an engine start, check engine fluids and verify that all cockpit engine controls are intact and fully operational. In addition, station extra personnel with fire extin-guishing equipment nearby in safe areas.

There is always a possibility of fire when starting an engine. Because of this, you should always have a carbon dioxide fire extinguisher of ade-quate capacity available. For starting large aircraft where it is not possible to see the engine when it

is being started, a fire guard should be stationed near the engine.

Induction system fires are the most common type of fire and occur most frequently in reciprocating engines. The reason for this is if an engine is over-primed and then fires back through the carburetor, the gasoline in the induction system can ignite. If an induction fire occurs, continue cranking the engine to draw the fire back into the cylinders.

If this fails, signal the fire guard to extinguish the fire.

To start an engine with a typical float-type carbure-tor, place the mixture control in the full rich posi-tion. Almost all reciprocating engines are equipped with either a carburetor-heat or an alternate-air posi-tion on the carburetor air inlet system. For starting and ground operation these controls should be in the cold or closed position. Prime the engine as required, and open the throttle about one-half inch.

Turn the master switch on, and turn the engine over with the starter switch. When the engine starts, check for positive oil pressure and adjust the throt-tle to produce about 1,000 rpm.

If an engine becomes flooded during a start attempt, place the mixture control in the idle cutoff position.

This shuts off all fuel flow to the cylinders. With the mixture in the cutoff position, place the ignition switch in the off position, open the throttle all the way, and crank the engine with the starter until the fuel charge in the cylinders has been cleared. Once this is done, repeat the normal start procedure.

Fuel-injected engines have several different starting requirements. For example, once the mixture is placed in the full rich position and the throttle is opened about one-half inch, both the master switch and the fuel pump are turned on until adequate fuel flow is observed. This procedure is required to prime the engine. Once primed, the fuel pump is turned off, the magneto switches are turned on, and the starter is engaged. When the engine starts, check for positive oil pressure. ,

ENGINE WARM-UP

Proper engine warm-up is important, particularly when the condition of the engine is unknown. An improperly adjusted idle mixture, intermittently fir-ing spark plugs, or improperly adjusted engine valves all have an overlapping effect on engine sta-bility. Therefore, the warm-up should be made at a speed that results in maximum engine stability. This typically results when the engine speed is between 800 and 1,000 rpm.

Reciprocating Engine Operation, Maintenance, Inspection, and Overhaul 2-15

During warm-up, monitor the engine instruments to ensure normal operation. For example, the oil pres-sure gauge should indicate prespres-sure within 30 sec-onds after a start. If that does not happen, the engine should be shut down immediately.

At warm-up rpm, the mixture control's effectiveness in changing the fuel/air ratio is minimal between the rich and lean positions. Control of the fuel/air ratio in this rpm range is predominantly governed by throttle position. Therefore, you should set the mix-ture control at the recommended position and refrain from attempts to try and hasten the warm-up by leaning the mixture.

Carburetor heat can be used as required when icing conditions are present. For engines equipped with a float-type carburetor, it may be desirable to raise the carburetor air temperature during warm-up to pre-vent ice formation and to ensure smooth operation.

However, the warm air provided by the carburetor heat system is not filtered and, therefore, should not be used if dust and dirt may be ingested.

A magneto safety check can be performed during warm-up. The purpose of this check is to ensure that all ignition connections are secure and that the igni-tion system will permit operaigni-tion at the higher power settings used during later phases of the ground check. This test is accomplished at idle rpm with the propeller in the high rpm, low pitch posi-tion. To conduct the check, move the ignition switch from the "both" position to "right" then return to

"both," from "both" to "left" and return to "both,"

from "both" to "off" momentarily, and return to

"both." While switching from the "both" position to a single magneto, a slight but noticeable drop in rpm should occur. This indicates that the opposite mag-neto is properly grounded out. Complete cutting out of the engine when switching from "both" to "off"

indicates that both magnetos are properly grounded.

Failure to obtain any drop in rpm while in a single magneto position, or failure of the engine to cut out while switching to "off" indicates that one or both ground connections are incomplete.

GROUND CHECK

After engine warm-up, a ground check is performed to verify the operation of the powerplant and acces-sory equipment. This check typically requires you to properly interpret instrument readings based on established performance criteria. Generally, the ground check items are included in the preflight runup checklist.

During the ground check, head the aircraft into the wind, if possible, to take advantage of the cooling airflow. A ground check may be per-formed as follows:

1. Open the cowl flaps.

2. Set the mixture in the full rich position.

3. Verify that the propeller control is in the low pitch, high rpm position.

4. Place the carburetor heat control in the cold position.

5. Open the throttle to the specified rpm and lean the mixture as required.

6. If the engine is carbureted, apply carburetor heat and observe a slight drop in rpm. Once rpm stabilizes, return the carburetor heat to the cold position.

7. Move the magneto switch from

"both" to

"right" and back to "both." Then, switch from

"both" to "left" and back to "both." You should observe a slight rpm drop while operating on the right and left magnetos. The drop on either magneto should be approximately the same, and the maximum drop should not exceed that specified by the engine manufacturer.

8. If the aircraft is equipped with a constant-speed propeller, check the propeller operation accord ing to propeller manufacturer's instructions.

9. Check the fuel pressure and oil pressure. They must be within the established tolerances.

10. Check the ammeter and suction gauge for

proper system operation.

11. Retard the throttle to the idle position.

In addition to the operations outlined above, check the aircraft generator and hydraulic systems. The test procedures for these systems are generally detailed on the aircraft checklist for runup.

IGNITION OPERATION

By comparing the rpm drop encountered when checking the magnetos to a known standard, you can determine if a magneto is properly timed and if all the ignition leads are properly grounded. For example, a rapid rpm drop which occurs when you switch to one magneto may indicate that the spark plugs or ignition harness is faulty because these defects take effect immediately. Faulty spark plugs or a defective ignition harness is often manifested by dead cylinders or intermittent firing at the instant the magneto switch is moved. On the other hand, a slow rpm drop is usually caused by incorrect igni-tion timing or faulty valve adjustment. These condi-tions result in a loss of power, but do not occur as rapidly as a dead spark plug.

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An engine that quits firing completely when switched to one magneto is definite evidence that the selected magneto ignition system is malfunc-tioning. On the other hand, an absence of an rpm drop could indicate a defective ground connection on one side of the ignition system. Another indica-tion of a defective igniindica-tion system is an excessive rpm difference between the left and right switch positions which may indicate a difference in timing between the left and right magnetos.

POWER CHECK

When conducting a ground check, most aircraft manufacturers also require a power check. The pur-pose of a power check is to measure an engine's per-formance against an established standard. The standard is determined by the manufacturer and represents the amount of power an engine can develop at a given rpm and manifold pressure. With a constant air density, a given propeller and blade angle always requires the same rpm to absorb the same horsepower from the engine.

To conduct a power check, place the propeller in the low pitch, high rpm position and advance the throt-tle to obtain the target rpm established by the manu-facturer. Under these conditions, the manifold pressure gauge should indicate the pressure speci-fied by the manufacturer if all of the cylinders are operating properly. However, if the engine is weak, or if one or more cylinders are dead or intermittently firing, the operating cylinders must provide more power for a given rpm. Consequently, the throttle must be opened further, resulting in higher mani-fold pressure. Therefore, a higher than normal man-ifold pressure for a given rpm usually indicates a dead cylinder or late ignition timing. On the other hand, an excessively low manifold pressure indi-cates that the ignition timing is early. In addition to causing a low manifold pressure, early ignition tim-ing can cause detonation and loss of power at high power settings.

IDLE SPEED AND MIXTURE

When an engine is operated at idle for long periods of time, many pilots tend to use an excessively rich fuel/air mixture to aid in cylinder cooling. However, after prolonged operation, the excess fuel has a ten-dency to build up and foul out the spark plugs. With a properly adjusted idle mixture setting, it is possi-ble to run the engine at idle rpm for long periods.

Such a setting results in minimal plug fouling and exhaust smoking.

In addition to properly adjusting the mixture

con-trol in the cockpit, you should conduct a mixture check during the ground check to verify that the car-buretor mixture screw is properly adjusted. To per-form a mixture check, close the throttle and move the mixture control to the "idle cutoff" position.

Observe the change in rpm just before it drops off dramatically and return the mixture control to the

"rich" position.

If the mixture is adjusted properly, engine rpm should increase by an amount recommended by the manufacturer (usually 20 rpm). If the increase in rpm is less than that recommended or if no increase occurs, the idle mixture is too lean. However, if the rpm increases above the recommended value, the mixture is too rich.

ACCELERATION AND DECELERATION

Aircraft engines must be capable of accelerating and decelerating rapidly. Therefore, when conducting a ground check, you should conduct an acceleration and deceleration check. To perform an acceleration test, move the throttle from idle to full power smoothly and rapidly. The engine should accelerate without hesitation and with no evidence of engine backfiring. The deceleration check is made by retarding the throttle from full power back to idle.

The rpm should decrease smoothly and evenly, with little or no tendency for the engine to afterfire.

An acceleration and deceleration check often reveals borderline conditions that are not apparent during other checks. This is true because the high cylinder pressures developed during this check put added strain on both the ignition system and the fuel metering system. This added strain is typically sufficient to point out certain defects that otherwise would go unnoticed.

ENGINE STOPPING

The procedure used to shut down an engine varies from engine to engine based on the type of carbure-tor or fuel injection system installed. Therefore, the shutdown instructions provided by the manufac-turer should be followed exactly. As a rule, most engines are shut down by placing the mixture con-trol in the "idle cut off" position. This procedure helps ensure that all of the fuel in the cylinders and induction system is burned. If all the fuel is burned, the chances of an accidental start caused by pro-peller movement is minimized. Once the engine quits, the ignition switch is turned off and the key is taken out of the ignition.

If an engine becomes excessively warm during taxi,

Reciprocating Engine Operation, Maintenance, Inspection, and Overhaul 2-17

you should allow the engine to idle for a short time before you shut it down. This allows both the cylin-der head temperature and oil temperature to cool to reasonable levels prior to engine shutdown.