Par 8-14 Page 8-10
(d) Install an adapter in the spark plug bushing and connect the compression tester to the cylinder.
(e) Adjust the pressure regulator to obtain a reading of 20 psi on the regulator pressure gauge. At this time, the cylinder pressure gauge should also register 20 psi.
(f) Turn the crankshaft, by hand, in the direction of rotation until the piston (in the cylinder being checked) is coming up on its compression stroke. Slowly open the air valve and pressurize the cylinder to 80 psi.
CAUTION: Care must be exercised in opening the air valve since sufficient air pressure will have built up in the cylinder to cause it to rapidly rotate the propeller if the piston is not at TDC.
(g) Continue rotating the engine against this pressure until the piston reaches TDC.
Reaching TDC is indicated by a flat spot or sudden decrease in force required to turn the crankshaft. If the crankshaft is rotated too far, back up at least one-half revolution and start over again to eliminate the effect of backlash in the valve operating mechanism and to keep piston rings seated on the lower ring lands.
(h) Open the air valve completely. Check the regulated pressure and readjust, if necessary, to read 80 psi.
(i) Observe the pressure indication of the cylinder pressure gauge. The difference between this pressure and the pressure shown by the regulator pressure gauge is the amount of leakage through the cylinder. A loss in excess of 25 percent of the input air pressure is cause to suspect the cylinder of being defective; however, recheck the readings after operating the engine for at least 3 minutes to allow for sealing of the rings with oil.
(j) If leakage is still occurring after a recheck, it may be possible to correct a low reading. This is accomplished by placing a fiber drift on the rocker arm directly over the valve stem and tapping the drift several times with a hammer to dislodge any foreign material between the valve face and seat.
NOTE: When correcting a low reading in this manner, rotate the propeller so the piston will not be at TDC. This is necessary to prevent the valve from striking the top of the piston in some engines. Rotate the engine before rechecking compression to reseat the valves in the normal manner.
8-15. SPARK PLUGS. The spark plug provides the high-voltage electrical spark to ignite the fuel/air mixture in the cylinder. The types of spark plugs used in different engines will vary with regard to heat range, reach, thread size, and other characteristics required by the particular installation.
a. Heat Range. The heat range of a spark plug is the principal factor governing aircraft performance under various service conditions. The term “heat range” refers to the classification of spark plugs according to their ability to transfer heat from the firing end of the spark plug to the cylinder head.
Par 8-15 Page 8-11
(1) Spark plugs have been classified as “hot,” “normal,” and “cold.” However, these terms may be misleading because the heat range varies through many degrees of temperature from extremely hot to extremely cold. Thus, the words “hot,” “cold,” and “normal” do not necessarily tell the whole story.
(2) Since the insulator is designed to be the hottest part of the spark plug, its temperature can be related to the pre-ignition and fouling regions as shown in Figure 8-2.
Pre-ignition is likely to occur if surface areas in the combustion chamber exceed critical limits or if the spark plug core nose temperature exceeds 1,630 ºF (888 ºC). However, fouling or short circuiting of the plug due to carbon deposits is likely to occur if the insulator tip temperature drops below approximately 800 ºF (427 ºC). Since spark plugs must operate between fairly well-defined temperature limits, they must be supplied in various heat ranges to meet the requirements of different engines under a variety of operating conditions.
FIGURE 8-2. CHART OF SPARK PLUG TEMPERATURE RANGES (
3) From the engineering standpoint, each individual plug must be designed to offer the widest possible operating range. This means that a given type of spark plug should operate as hot as possible at low speeds and light loads and as cool as possible under cruising and takeoff power. Plug performance, therefore, depends on the operating temperature of the insulator nose with the most desirable temperature range falling between 1,000 ºF and 1,250 ºF (538 ºC and 677 ºC).
(4) Fundamentally, an engine which runs hot requires a relatively cold spark plug, whereas an engine which runs cool requires a relatively hot spark plug. If a hot spark plug is installed in an engine which runs hot, the spark plug tip will be overheated and cause pre-ignition. If a cold spark plug is installed in an engine which runs cool, the tip of the spark plug will collect unburned carbon, causing fouling of the plug. The principal factors governing the heat range of aircraft spark plugs are:
Par 8-15 Page 8-12
(a) The distance between the copper sleeve around the insulator and the insulator tip;
(b) The thermal conductivity of the insulating material;
(c) The thermal conductivity of the electrode;
(d) The rate of heat transfer between the electrode and the insulator;
(e) The shape of the insulator tip;
(f) The distance between the insulator tip and the shell; and
(g) The type of outside gasket used.
(5) “Hot” plugs have a long insulator nose; thereby, creating a long heat transfer path, whereas “cold” plugs have a relatively short insulator to provide a rapid transfer of heat to the cylinder head (see Figure 8-3).
FIGURE 8-3. HOT AND COLD SPARK PLUGS
b. R each. The spark plug reach (see Figure 8-4) is the threaded portion which is inserted into the spark plug bushing of the cylinder. A plug with the proper reach will ensure that the electrode end inside the cylinder is in the best position to achieve ignition. Spark plug seizure or improper combustion within the cylinder will probably occur if a plug with the wrong reach is used. Shell threads of spark plugs are classified as 14- or 18-mm spark plug diameter, long reach or short reach, thus:
Par 8-15 Page 8-13 FIGURE 8-4. SPARK PLUG REACH Diameter Long Reach Short Reach 14 mm 1/2 in (12.7 mm) 3/8 in (9.53 mm) 18 mm 13/16 in (20.64 mm) 1/2 in (12.7 mm)
c. Insta llation Procedures. When installing spark plugs, observe the following procedure.
(1) Visually inspect the plug for cleanliness and the condition of the threads, ceramic, and electrodes.
NOTE: Never install a spark plug which has been dropped and always use new gaskets every time you install a spark plug.
(2) Check the plug for the proper gap setting using a round wire feeler gauge as shown in Figure 8-5. In the case of used plugs, procedures for cleaning and regapping are usually contained in the various manufacturers’ manuals.
FIGURE 8-5. METHOD OF CHECKING SPARK PLUG GAP
Par 8-15 Page 8-14
(3) Check the plug and cylinder bushing to ascertain that only one gasket is used per spark plug. When a thermocouple-type gasket is used, no other gasket is required.
(4) Apply anti-seize compound sparingly to the shell threads but do not allow the compound to contact the electrodes since the material is conductive and will short out the plug. If desired, the use of anti-seize compound may be eliminated on engines equipped with stainless steel spark plug bushings or inserts.
(5) Screw the plug into the cylinder head as far as possible by hand. If the plug will not turn easily to within two or three threads of the gasket, it may be necessary to clean the threads.
NOTE: Cleaning inserts with a tap is not recommended as permanent damage to the insert may result.
(6) Seat the proper socket securely on the spark plug and tighten to the torque limit specified by the engine manufacturer before proceeding to the next plug.
CAUTION: A loose spark plug will not transfer heat properly and, during engine operation, may overheat to the point in which the nose ceramic will become a “hot spot” and cause pre-ignition. However, avoid over-tightening as damage to the plug and bushing may result.
(7) Connect the ignition lead after wiping clean with a dry, lint-free cloth. Insert the terminal assembly into the spark plug in a straight line. (Care should be taken as improper techniques can damage the terminal sleeves.) Screw the connector nut into place until finger tight, then tighten an additional one-quarter turn while holding the elbow in the proper position.
(8) Perform an engine runup after installing a new set of spark plugs. When the engine has reached normal operating temperatures, check the magnetos and spark plugs in accordance with the manufacturer’s instructions.
8-16. OPERATIONAL PROBLEMS. Whenever problems develop during engine operation and appear to be caused by the ignition system, it is recommended that the spark plugs and ignition harnesses be checked first before working on the magnetos. The following are the more common spark plug malfunctions and are relatively easy to identify.
a. Fouling.
(1) Carbon fouling (see Figure 8-6) is identified by the dull black, sooty deposits on the electrode end of the plug. Although the primary causes are excessive ground idling and rich idle mixtures, a cold heat range may also be a contributing factor.
Par 8-16 Page 8-15 FIGURE 8-6. TYPICAL CARBON-FOULED SPARK PLUG (
2) Lead fouling is characterized by hard, dark, cinder-like globules which gradually fill up the electrode cavity and short out the plug (see Figure 8-6a). The primary cause for this condition is poor fuel vaporization combined with a high tetraethyllead content fuel. A cold heat range may also contribute to this condition.
FIGURE 8-6a. TYPICAL LEAD-FOULED SPARK PLUG
Par 8-16 Page 8-16
(3) Oil fouling is identified by a wet, black carbon deposit over the entire firing end of the plug as shown in Figure 8-6b. This condition is fairly common on the lower plugs in horizontally opposed engines and both plugs in the lower cylinders of radial engines. Oil fouling is normally caused by oil drainage past the piston rings after shutdown. However, when both spark plugs removed from the same cylinder are badly fouled with oil and carbon, some form of engine damage should be suspected and the cylinder more closely inspected. Mild forms of oil fouling can usually be cleared up by slowly increasing power while running the engine until the deposits are burned off and the misfiring stops.
FIGURE 8-6b. TYPICAL OIL-FOULED SPARK PLUG
b. F used Electrodes. There are many different types of malfunctions which result in fused spark plug electrodes; however, most are associated with pre-ignition as either the cause or the effect. For this reason, any time a spark plug is found with the following defects, further investigation of the cylinder and piston should be conducted.
(1) Occasionally, the ceramic nose core will crack, break away, and remain trapped behind the ground electrode. This piece of insulation material will then build up heat to the point it will ignite the fuel/air mixture prematurely. The high temperatures and pressures encountered during this condition can cause damage to the cylinder and piston and ultimately lead to fusing and shorting out of the plug (see Figure 8-6c).
Par 8-16 Page 8-17 FIGURE 8-6c. TYPICAL SPARK PLUG WITH CRACKED CORE NOSE (
2) Corrosive gases formed by combustion and the high voltage spark have eroded the electrodes. Spark plugs in this condition require more voltage to fire—often more than the ignition system can produce (see Figure 8-6d).
FIGURE 8-6d. TYPICAL WORN OUT SPARK PLUG
Par 8-16 Page 8-18
c. Bridged Electrodes. Occasionally, free combustion chamber particles will settle on the electrodes of a spark plug and gradually bridge the electrode gap, resulting in a shorted plug.
Small particles may be dislodged by slowly cycling the engine as described for the oil-fouled condition; however, the only remedy for more advanced cases is removal and replacement of the spark plug. This condition is shown in Figure 8-6e.
FIGURE 8-6e. TYPICAL SPARK PLUG WITH BRIDGED ELECTRODES
d. Met al Deposits. Whenever metal spray is found on the electrodes of a spark plug, it is an indication that a failure of some part of the engine is in progress. The location of the cylinder in which the spray is found is important in diagnosing the problem, as various types of failures will cause the metal spray to appear differently. For example, if the metal spray is located evenly in every cylinder, the problem will be in the induction system, such as an impeller failure. If the metal spray is found only on the spark plugs in one cylinder, the problem is isolated to that cylinder and will generally be a piston failure. In view of the secondary damage which occurs whenever an engine part fails, any preliminary indication, such as metal spray, should be thoroughly investigated to establish and correct the cause.
e. Flashover. It is important that spark plug terminal contact springs and moisture seals be checked regularly for condition and cleanliness to prevent “flashover” in the connector well.
Foreign matter or moisture in the terminal connector well can reduce the insulation value of the connector to the point the ignition system voltages at higher power settings may flash over the connector well surface to ground and cause the plug to misfire. If moisture is the cause, hard starting can also result. The cutaway spark plug shown in Figure 8-7 illustrates this malfunction.
Any spark plug found with a dirty connector well may have this condition and should be reconditioned before reuse.
Par 8-16 Page 8-19 FIGURE 8-7. SPARK PLUG WELL FLASHOVER 8-
17. SPARK PLUG PRE-RECONDITIONING INSPECTION. All spark plugs should be inspected visually before reconditioning to eliminate any plug with obvious defects. A partial checklist of common defects includes:
a. Chipped or cracked ceramic either at the nose core or in the connector well.
b. Damaged or badly worn electrodes.
c. Badly nicked, damaged, or corroded threads on shell or shielding barrel.
d. Dented, bent, or cracked shielding barrel.
e. Connector seat at the top of the shielding barrel badly nicked or corroded.
8-18. IGNITION HARNESSES INSPECTION. Aircraft-quality ignition harness is usually made of either medium or high-temperature wire. The type used will depend upon the manufacturing specification for the particular engine. In addition to the applicable manufacturer’s maintenance and repair procedures, the following is a quick reference checklist for isolating some of the malfunctions inherent to ignition harnesses.
a. Carefully inspect the lead conduit or shielding. A few broken strands will not affect serviceability, but if the insulation in general looks worn, replace the lead.
b. When replacing a lead, if the dressing procedure is not accomplished properly, strands of shielding may be forced through the conductor insulation. If this occurs, a short will exist in the conductor; therefore, it is essential this task be performed properly.
c. The high-temperature coating used on some lightweight harnesses is provided for vibration abrasion resistance and moisture protection. Slight flaking or peeling of this coating is not serious, and a harness assembly need not be removed from service because of this condition.
d. Check the spark plug contact springs for breaks, corrosion, or deformation. If possible, check the lead continuity from the distributor block to the contact spring.
e. Check the insulators at the spark plug end of the lead for cracks, breaks, or evidence of old age. Make sure they are clean.
Par 8-18 Page 8-20
f. Check to see that the leads are positioned as far away from the exhaust manifold as possible and are supported to prevent any whipping action.
g. When lightweight harnesses are used and the conduit enters the spark plug at a severe angle, use clamps, as shown in Figure 8-8, to prevent overstressing the lead.
FIGURE 8-8. TYPICAL METHOD OF CLAMPING LEADS 8-
19. MAGNETO INSPECTION. Whenever ignition problems develop and it is determined that the magneto is the cause of the difficulty, the following are a few simple inspection procedures which may locate the malfunction quickly. However, conduct any internal inspection or repair of a magneto in accordance with the manufacturer’s maintenance and overhaul manuals.
a. Inspect the distributor block contact springs. If broken or corroded, replace.
b. Inspect the felt oil washer, if applicable. It should be saturated with oil. If it is dry, check for a worn bushing.
c. Inspect the distributor block for cracks or a burned area. The wax coating on the block should not be removed. Do not use any solvents for cleaning.
d. Look for excess oil in the breaker compartment. If oil is present, it may indicate a bad oil seal or bushing at the drive end. This condition could require complete overhaul, as too much oil may foul and cause excessive burning of the contact points.
e. Look for frayed insulation on the leads in the breaker compartment of the magneto.
See that all terminals are secure. Be sure that wires are properly positioned.
f. Inspect the capacitor visually for general condition and check the mounting bracket for cracks or looseness. If possible, check the capacitor for leakage, capacity, and series resistance.
g. Examine the points for excessive wear or burning. Discard points which have deep pits or excessively burned areas. Desired contact surfaces have a dull gray, sandblasted (almost rough) or frosted appearance over the area where electrical contact is made. Figure 8-9 shows
how the normal contact point will look when surfaces are separated for inspection. Minor irregularities or roughness of point surfaces are not harmful (see Figure 8-10), neither are small pits or mounds, if not too pronounced. If there is a possibility of the pit becoming deep enough to penetrate the pad (see Figure 8-11), reject the contact assembly.
FIGURE 8-9. NORMAL CONTACT POINT
NORMAL POINT IS SMOOTH AND FLAT. SURFACE HAS DULL GRAY "SANDBLASTED" APPEARANCE.
FIGURE 8-10. POINT WITH MINOR IRREGULARITIES
MINOR IRREGULARITIES - SMOOTH ROLLING
HILLS AND DALES WITHOUT ANY DEEP PITS OR HIGH PEAKS. THIS IS A NORMAL CONDITION OR POINT OF WEAR.
FIGURE 8-11. POINT WITH WELL-DEFINED MOUND 9/8/98
WEEL-DEFINED MOUND EXTENDING NOTICEABLY ABOVE SURROUNDING SURFACE. THE MATING
POINT HAS A PIT OR HOLE CORRESPONDING TO THE MOUND SEEN HERE.
h. Generally, no attempt should be made to dress or stone contact point assemblies;
however, if provided, procedures and limits contained in the manufacturer's manuals may be followed.
Par 8-19 Page 8-21