Text-only reference. Published from the recorded official FAA Chapter 4 PDF. Diagrams, photographs, and figure artwork are not reproduced here; use the official FAA PDF for those materials.
4-26 Reciprocating Engine Ignition System Maintenance & Inspection An aircraft’s ignition system is the result of careful design and thorough testing. The ignition system usually provides good, dependable service, provided it is maintained and inspected properly. However, difficulties can occur with normal wear, which affects ignition system performance, especially with magneto systems. Breakdown and deterioration of insulating materials, breaker point wear, corrosion, bearing and oil seal wear, and electrical connection problems are all possible defects that can be associated with magneto-ignition systems. The ignition timing requires precise adjustment and painstaking care so that the following four conditions occur at the same instant: 1. The piston in the No. 1 cylinder must be in a position a prescribed number of degrees before top dead center on the compression stroke.
2. The rotating magnet of the magneto must be in the E-gap position. 3. The breaker points must be just opening on the No. 1 cam lobe. 4. The distributor finger must be aligned with the electrode serving the No. 1 cylinder. If one of these conditions is out of synchronization with any of the others, the ignition system is out of time. If the spark is out of time, it is not delivered to the cylinder at the correct time and engine performance decreases. When ignition in the cylinder occurs before the optimum crankshaft position is reached, the timing is said to be early. If ignition occurs too early, the piston rising in the cylinder is opposed by the full force of combustion. This condition results in a loss of engine power, overheating, and possible detonation and pre-ignition.
If ignition occurs at a time after the optimum crankshaft position is reached, the ignition timing is said to be late. If it occurs too late, not enough time is allowed to consume the air-fuel charge, and combustion is incomplete. As a result, the engine loses power and requires a greater throttle opening to carry a given propeller load. Moisture forming on different parts of the ignition system causes more common irregularities. Moisture can enter ignition system units through cracks or loose covers, or it can result from condensation. Breathing, a situation that occurs during the readjustment of the system from low to high atmospheric pressure, can result in drawing in moisture- laden air. Ordinarily, the heat of the engine is sufficient to evaporate this moisture, but occasionally the moist air condenses as the engine cools. The result is an appreciable moisture accumulation which causes the insulation materials to lose electrical resistance. A slight amount of moisture contamination may cause reduction in magneto output by short-circuiting to ground a part of the high-voltage current intended for the spark plug. If the moisture accumulation is appreciable, the entire magneto output may be dissipated to ground by way of flashover and carbon tracking. Moisture accumulation during flight is extremely rare because the high operating temperature of the system is effective in preventing condensation. Difficulties from moisture accumulation are probably more evident during starting and ground operation.
Spark plugs are often diagnosed as being faulty when the real malfunction exists in a different system. Malfunctioning of the carburetor, poor fuel distribution, too much valve overlap, leaking primer system, or poor idle speed and mixture settings show symptoms that are the same as those for faulty ignition. Unfortunately, many of these conditions can be temporarily improved by a spark plug change, but the trouble recurs in a short time because the real cause of the malfunction has not been eliminated. A thorough understanding of the various engine systems, along with meticulous inspection and good maintenance methods, can substantially reduce such errors.
Magneto-Ignition Timing Devices
Built-In Engine Timing Reference Marks
Most reciprocating engines have timing reference marks built into the engine. The timing reference marks vary by manufacturer. [Figure 4-43] When the starter gear hub is installed correctly, the timing marks are marked on it that line up with the mark on the starter. On an engine that has no starter gear hub, the timing mark is normally on the propeller flange edge. [Figure 4-44] The top center (TC) mark stamped on the edge aligns with the crankcase split line below the crankshaft when the No. 1 piston is at top dead center. Other flange marks indicate degrees before top center. Some engines have degree markings on the propeller reduction drive gear. To time these engines, the plug provided on the exterior of the reduction gear housing must be removed to view the timing marks. On other engines, the timing marks are on a crankshaft flange and can be viewed by removing a plug from the crankcase. In every case, the engine manufacturer’s instructions give the location of built- in timing reference marks.
In using built-in timing marks to position the crankshaft, be sure to sight straight across the stationary pointer or mark on the nose section, the propeller shaft, crankshaft flange, or bell gear. [Figure 4-45] Sighting at an angle results in an error in positioning the crankshaft. Normally, the No. 1 cylinder is used to time or check the timing of the magnetos. When installing magnetos, the timing marks must be lined up and the No. 1 cylinder must be on the compression stroke. 4-27 Index mark is a dot on the starter Number of degrees of crank- shaft rotation is marked on the propeller hub T C 24 32 T C 24 32 Timing marks Split line 20 Indexing groove gear.
The amount of gear backlash in any system of gears varies between installations because there is clearance between the gear teeth. Always take timing when reading, or stop movement of the engine for timing set up, in the direction of rotation. Another unfavorable aspect in the use of timing marks on the reduction gear is the small error that exists when sighting down the reference mark to the timing mark inside the housing on the reduction gear. This can occur because there is depth between the two reference marks.
Timing Discs
Most timing disc devices are mounted to the crankshaft flange and use a timing plate. [Figure 4-46] The markings vary according to the specifications of the engine. This plate is temporarily installed on the crankshaft flange with a scale numbered in crankshaft degrees and the pointer attached to the timing disc.
Piston Position Indicators
Any given piston position, whether it is to be used for ignition, valve, or injection pump timing, is referenced to a piston position called top dead center. This piston position is not to be confused with a piston position called top center. A piston in top center has little value from a timing standpoint because the corresponding crankshaft position may vary from 1° to 5° for this piston position. This is illustrated in zone of the piston. Notice that the piston does not move while the crankshaft describes the small arc from position A to position B. This no-travel zone occurs between the time the crankshaft and connecting rod stop pushing the piston upward, and continues until the crankshaft has swung the lower end of the connecting rod into a position where the crankshaft can start pulling the piston downward. Top dead center is a piston and crankshaft position from which all other piston and crankshaft locations are referenced. When a piston is in the top dead center position of the crankshaft, it is also in the center of the no-travel zone. The piston is in a position where a straight line can be drawn through the center of the crankshaft journal, the crankpin, and the piston pin. This is 4-28 A B Crankshaft moves about 8° without moving piston Top center Top dead center T C 25° shown on the right of Figure 4-47. With such an alignment, a force applied to the piston could not move the crankshaft.
Timing Lights
Timing lights are used to help determine the exact instant that the magneto points open. There are two general types of timing lights in common use. Both have two lights and three external wire connections. Although both have internal circuits that are somewhat different, their function is very much the same. [Figures 4-48 and 4-49] Three wires plug into the light box. [Figure 4-49] There are two lights on the front face of the unit, one green and one red, and a switch to turn the unit on and off. To use the timing light, the center lead, which is black, marked “ground lead” is connected to the case of the magneto being tested. The other leads are connected to the primary leads of the breaker point assembly of the magnetos being timed. The color of the lead corresponds to the color of the light on the timing light.
With the leads connected in this manner, it can be easily determined whether the points are open or closed by turning on the switch and observing the two lights. If the points are closed, most of the current flows through the breaker points and not through the transformers, and the lights do not come on. If the points are open, the current flows through the transformer and the lights glow. Some models of timing lights operate in the reverse manner (i.e., the light goes out when the points open). Each of the two lights is operated separately by the set of breaker points to which it is connected. This makes it possible to observe the time, or point in reference to magneto rotor rotation, that each set of points opens.
Most timing lights use batteries that must be replaced after long use. Attempts to use a timing light with weak batteries may result in erroneous readings because of low current flow in the circuits. 4-29 Timing mark Chamfered tooth of a magneto. Red marks visible through vent hole must align Timing pin
Checking the Internal Timing of a Magneto
When replacing or preparing a magneto for installation, the first concern is with the internal timing of the magneto. For each magneto model, the manufacturer determines how many degrees beyond the neutral position a pole of the rotor magnet should be to obtain the strongest spark at the instant of breaker point separation. This angular displacement from the neutral position, known as the E-gap angle, varies with different magneto models. On one model, a step is cut on the end of the breaker cam to check internal timing of the magneto. When a straightedge is laid along this step and it coincides with the timing marks on the rim of the breaker housing, the magneto rotor is then in the E-gap position, and the breaker contact points should just begin to open.
Another method for checking E-gap is to align a timing mark with a pointed chamfered tooth. [Figure 4-50] The breaker points should be just starting to open when these marks line up. In a third method, the E-gap is correct when a timing pin is in place and red marks visible through a vent hole in the side of the magneto case are aligned. [Figure 4-51] The contact points should be just opening when the rotor is in the position just described. Bench timing the magneto, or setting the E-gap, involves positioning the magneto rotor at the E-gap position and setting the breaker points to open when the timing lines or marks provided for that purpose are perfectly aligned.
High-Tension Magneto E-Gap Setting (Bench Timing) The following steps are taken to check and adjust the timing of the breaker points for the S-200 magneto, which does not have timing marks in the breaker compartment: 1. Remove the timing inspection plug from the top of the magneto. Turn the rotating magnet in its normal direction of rotation until the painted, chamfered tooth on the distributor gear is approximately in the center of the inspection window. Then, turn the magnet back a few degrees until it is in its neutral position. Because of its magnetism, the rotating magnet holds itself in the neutral position.
2. Install the timing kit and place the pointer in the zero position. [Figure 4-52] 3. Connect a suitable timing light across the main breaker points and turn the magnet in its normal direction of rotation 10° as indicated by the pointer. This is the E-gap position. The main breaker points should be adjusted to open at this point. 4. Turn the rotating magnet until the cam follower is at the highpoint on the cam lobe, and measure the clearance between the breaker points. This clearance must be 0.018 inch ± 0.006 inch [0.46 millimeter (mm) ± 0.15 mm]. If the breaker point clearance is not within these limits, the points must be adjusted for correct setting. It is then necessary to recheck and readjust the timing for breaker opening. If the breaker points cannot be adjusted to open at the correct time, they should be replaced.
4-30
Timing the High-tension Magneto to the Engine
When replacing magnetos on aircraft engines, two factors must be considered: 1. The internal timing of the magneto, including breaker point adjustment, which must be correct to obtain maximum potential voltage from the magneto. 2. The engine crankshaft position where the spark occurs. The engine is usually timed by using the No. 1 cylinder on the compression stroke. The magneto must be timed by first adjusting or checking the internal timing with the magneto off the engine. This is done by checking and adjusting the ignition points to open at the E-gap position. The chamfered tooth should line up (reference timing mark for the magneto) in the middle of the timing window. The magneto is set to fire the No. 1 cylinder. Remove the most accessible spark plug from the No. 1 cylinder. Pull the propeller through in the direction of rotation until the No.
1 piston is coming up on the compression stroke. This can be determined by holding a thumb over the spark plug hole until the compression air is felt. Set the engine crankshaft at the prescribed number of degrees ahead of true top dead center as specified in the applicable manufacturer’s instruction, usually using the timing marks on the engine. With the engine set at a prescribed number of degrees ahead of true top dead center on the compression stroke and with final movement of the engine stopped in the direction of normal rotation, the magneto can be installed on the engine. [Figure 4-53] While holding the magneto drive in the firing position for the No. 1 cylinder as indicated by the alignment of the reference marks for the magneto, install the magneto drive into the engine drive. It should be installed in the middle of its slotted flange to allow for fine timing of the magneto to the engine.
Attach a timing light to both magnetos. With the engine still in the firing position, the magnetos should be timed by moving them in the flange slots until the breaker points in the magneto just open. If the slots in the mounting flange of the magneto do not permit sufficient movement to effect breaker point opening for the No. 1 cylinder, move the magneto out of position far enough to permit turning the magneto drive shaft. Then, install the magneto in position again and repeat the previous check for point opening. Install the magneto attaching nuts on the studs and tighten slightly. The nuts must not be tight enough to prevent the movement of the magneto assembly when the magneto mounting flange is tapped with a mallet. Reconnect the timing light to the magneto and breaker points. With the light and ignition switch turned on, rotate the magneto assembly first in the direction of rotation and then in the opposite direction.
This is done to determine that the points just opened. After completing this adjustment, tighten the mounting nuts. Move the propeller one blade opposite the direction of rotation and then, while observing the timing light, move the propeller in the direction of rotation until the prescribed number of degrees ahead of top dead center is reached. Be sure that the lights for both sets of points come on points open, within the prescribed timing position. Both right and left sets of breaker points should open at the same instant, proper magneto-to-engine timing exists, and all phases of magneto operation are synchronized. Some early engines had what was referred to as staggered timing where one magneto would fire at a different number of degrees before top dead center on the compression stroke. In this case, each magneto had to be timed separately. If staggered ignition timing is used, the spark plug nearest the exhaust valve will fire first.
4-31 In the following example, a timing light is used for timing the magneto to the engine. The timing light is designed in such a way that one of two lights come on when the points open. The timing light incorporates two lights. When connecting the timing light to the magneto, the leads should be connected so that the light on the right side of the box represents the breaker points on the right magneto, and the light on the left side represents the left magneto breaker points. The black lead or ground lead must be attached to the engine or an effective ground. When using the timing light to check a magneto in a complete ignition system installed on the aircraft, the ignition switch for the engine must be turned to both. Otherwise, the lights do not indicate breaker point opening.
Performing Ignition System Checks
The ignition system has checks performed on it during the aircraft engine run-up, which is the engine check before each flight. The magneto check, as it is usually referred to, is performed during the engine run-up check list. One other check is accomplished prior to engine shutdown. The ignition system check is used to check the individual magnetos, harnesses, and spark plugs. After reaching the engine rpm specified for the ignition system check, allow the rpm to stabilize. Place the ignition switch in the right position and note the rpm drop on the tachometer. Return the switch to the both position. Allow the switch to remain in the both position for a few seconds so that the rpm stabilizes again.
Place the ignition switch in the left position and again note the rpm drop. Return the ignition switch to the both position. Note the amount of total rpm drop that occurs for each magneto position. The magneto drop should be even for both magnetos and is generally in the area of a 25–75 rpm drop for each magneto. Always refer to the aircraft operating manual for specific information. This rpm drop is because operating on one magneto combustion is not as efficient as it is with two magnetos providing sparks in the cylinder. Remember, this tests not only the magnetos but also the ignition leads and spark plugs. If either magneto has excessive rpm drop while operating by itself, the ignition system needs to be checked for problems. If only one magneto has a high magneto drop, the problem can be isolated and corrected by operating on that magneto. This ignition system check is usually performed at the beginning of the engine run-up because rpm drops not within the prescribed limits affect later checks.
Ignition Switch Check
The ignition switch check is performed to see that all magneto ground leads are electrically grounded. The ignition switch check is usually made at 700 rpm. On those aircraft engine installations that do not idle at this low rpm, set the engine speed to the lowest possible to perform this check. When the speed to perform this check is obtained, momentarily turn the ignition switch to the off position. The engine should completely quit firing. After a drop of 200–300 rpm is observed, return the switch to the both position as rapidly as possible. Do this quickly to eliminate the possibility of afterfire and backfire when the ignition switch is returned to both.
If the ignition switch is not returned quickly enough, the engine rpm drops off completely and the engine stops. In this case, leave the ignition switch in the off position and place the mixture control in the idle-cutoff position to avoid overloading the cylinders and exhaust system with raw fuel. When the engine has completely stopped, allow it to remain inoperative for a short time before restarting. If the engine does not cease firing in the off position, the magneto ground lead, more commonly referred to as the P lead, is open, and the trouble must be corrected. This means that one or more of the magnetos are not being shut off even when the ignition switch is in the off position. Turning the propeller of this engine can result in personnel injury or death. If the propeller is turned in this condition, the engine can start with personnel in the propeller arch.
Maintenance & Inspection of Ignition Leads
Inspection of ignition leads should include both a visual and an electrical test. During the visual test, the lead cover should be inspected for cracks or other damage, abrasions, mutilated braid, or other physical damage. Inspect leads for overheating if routed close to exhaust stacks. Disconnect the harness coupling nuts from the top of the spark plugs and remove the leads from the spark plug lead well. Inspect the contact springs and compression springs for any damage or distortion and the sleeves for cracks or carbon tracking. The coupling nut that connects to the spark plug should be inspected for damaged threads or other defects.
Each lead should be checked for continuity using a high- tension lead tester by connecting the black lead to the contact spring and the red lead to the eyelet of the same lead in the cover. The continuity lamp on the tester should illuminate when tested. The insulation resistance test of each lead is accomplished using the high-tension lead tester by attaching the red, or high-voltage, lead to the spring of the harness lead. Then, attach the black lead to the ferrule of the same lead. Depress the press-to-test push button switch on the lead tester. Observe that the indictor lamp flashes and gap fires simultaneously as long as the press-to-test switch is held in the depressed position.
4-32 Elbow Seal Terminal sleeve assembly Protector cap 5mm ferrule 7mm sleeve 5mm ignition cable5mm ferrule 5mm ignition cable For distributor blocks with 5mm wells For distributor blocks with 7mm wells If the indicator lamp flashes and the gap fails to fire, the lead under test is defective and must be replaced. The indicator lamp flashes to show that a high-voltage impulse was sent out. If it fails to pass through the tester, then the electrical pulse leaked through the wire showing it to be defective. When defective leads are revealed by an ignition harness test, continue the test to determine whether the leads or distributor block are defective. If the difficulty is in an individual ignition lead, determine whether the electrical leak is at the spark plug elbow or elsewhere. Remove the elbow, pull the ignition lead out of the manifold a slight amount, and repeat the harness test on the defective lead. If this stops the leakage, cut away the defective portion of the lead and reinstall the elbow assembly, integral seal, and terminal (sometimes referred to as cigarette). [Figure 4-54] When installing the leads, you should avoid bends because weak points may develop in the insulation through which high-tension current can leak.
If the lead is too short to repair in the manner described, or the electrical leak is inside the harness, replace the defective lead. Single ignition lead replacement procedures are as follows: 1. Disassemble the magneto or distributor so that the distributor block is accessible. 2. Loosen the piercing screw in the distributor block for the lead to be replaced, and remove the lead from the distributor block. 3. Remove approximately 1 inch of insulation from the distributor block end of the defective lead and approximately 1 inch of insulation from the end of the replacement cable. Splice this end to the end of the lead to be replaced and solder the splice.
4. Remove the elbow adapter from the spark plug end of the defective lead, then pull the old lead out and put the new lead into the harness. While pulling the leads through the harness, have someone push the replacement lead into the ignition manifold at the distributor end to reduce the force required to pull the lead through the ignition manifold. 5. When the replacement lead has been pulled completely through the manifold, force the ignition lead up into the manifold from the distributor block end to provide extra length for future repairs, which may be necessary because of chafing at the spark plug elbow.
6. Remove approximately 3⁄8 inch of insulation from the distributor block end. Bend the ends of the wire back and prepare the ends of the cable for installation into the distributor block well. Insert the lead in the distributor and tighten the piercing screw. 7. Remove approximately ¼ inch of insulation from the spark plug end of the lead and install the elbow, integral seal, and cigarette. [Figure 4-54] 8. Install a marker on the distributor end of the cable to identify its cylinder number. If a new marker is not available, use the marker removed from the defective cable. 4-33
Replacement of Ignition Harness
Replace a complete ignition harness only when the shielding of the manifold is damaged or when the number of defective leads makes it more practical to replace the harness than to replace the individual leads. Replace a cast- filled harness only when leakage in the cast-filled portion is indicated. Before replacing any harness to correct engine malfunctioning, make extensive ignition harness tests. Typical procedures for installing an ignition harness are detailed in the following paragraphs. Install the ignition harness on the engine. Tighten and safety the hold down nuts and bolts and install and tighten the individual lead brackets according to instructions. The ignition harness is then ready for connection of the individual leads to the distributor block. A band is attached to each lead at the distributor end of the harness to identify the cylinder for the lead. However, each lead should be checked individually with a continuity or timing light prior to connecting it.
Check for continuity by grounding the lead at the cylinder and then checking at the distributor block end to establish that the lead grounded is as designated on the band for the lead. After checking all leads for proper identification, cut them to the proper length for installation into the distributor block. Before cutting the leads, however, force them back into the manifold as far as possible to provide surplus wire in the ignition manifold. This extra wire may be needed at a later date in the event that chafing of a lead at the spark plug elbow necessitates cutting a short section of wire from the spark plug end of the harness. After cutting each lead to length, remove approximately 3⁄8 inch of insulation from the end and prepare the lead for insertion into the distributor block. Before installing the lead, back out the set screw in the distributor block far enough to permit slipping the end of the wire into the hole without force. Insert the lead into the block and tighten the set screw. Connect the wires in firing order (the first cylinder to fire No. 1 location on the block, the second in the firing order to No. 2 location, etc).
After connecting each lead, check continuity between the lead and its distributor block electrode with continuity light or timing light. To perform one test lead, touch the other test lead to the proper distributor block electrode. If the light does not indicate a complete circuit, the set screw is not making contact with the ignition wire or the lead is connected to the wrong block location. Correct any faulty connections before installing the distributor block. Checking Ignition Induction Vibrator Systems To check the induction vibrator, ensure that the manual mixture control is in idle cutoff, the fuel shutoff valve and booster pump for that engine are in the off position, and the battery switch is on. Since the induction vibrator buzzes whether the ignition switch is on or off, leave the switch off during the check. If the engine is equipped with an inertia or combination starter, make the check by closing the engage mesh switch; if the engine is equipped with a direct-cranking starter, see that the propeller is clear and close the start switch.
An assistant stationed close to the induction vibrator should listen for an audible buzzing sound. If the unit buzzes when the starter is engaged or cranked, the induction vibrator is operating properly.
Spark Plug Inspection & Maintenance
Spark plug operation can often be a major source of engine malfunctions because of lead, oil, graphite, carbon fouling, and spark plug gap erosion. Most of these failures, which usually accompany normal spark plug operation, can be minimized by good operational and maintenance practices. A spark plug is considered fouled if it has stopped allowing the spark to bridge the gap either completely or intermittently. Carbon Fouling of Spark Plugs Carbon fouling from fuel is associated with mixtures that are too rich to burn or mixtures that are so lean they cause intermittent firing. [Figure 4-55] Each time a spark plug does not fire, raw fuel and oil collect on the nonfiring electrodes and nose insulator. These difficulties are almost invariably associated with an improper idle mixture adjustment, a leaking primer, or carburetor malfunctions that cause too rich a mixture in the idle range. A rich air-fuel mixture is detected by soot or black smoke coming from the exhaust and by an increase in rpm when the idling air-fuel mixture is leaned to best power. The soot that forms as a result of overly rich idle air-fuel mixtures settles on the inside of the combustion chamber because the heat of the engine and the turbulence 4-34 in the combustion chamber are slight. At higher engine speeds and powers, however, the soot is swept out and does not condense out of the charge in the combustion chamber.
Oil Fouling of Spark Plugs Even though the idling air-fuel mixture is correct, there is a tendency for oil to be drawn into the cylinder past the piston rings, valve guides, and impeller shaft oil seal rings. At low engine speeds, the oil combines with the soot in the cylinder to form a solid that is capable of shorting out the spark plug. Spark plugs that are wet or covered with lubricating oil are usually grounded out during the engine start. In some cases, these plugs may clear up and operate properly after a short period of engine operation. Engine oil that has been in service for any length of time holds in suspension minute carbon particles that are capable of conducting an electric current. Thus, a spark plug will not arc the gap between the electrodes when the plug is full of oil. Instead, the high-voltage impulse flows through the oil from one electrode to the other without a spark as though a wire conductor were placed between the two electrodes.
Combustion in the affected cylinder does not occur until, at a higher rpm, increased airflow has carried away the excess oil. Then, when intermittent firing starts, combustion assists in emitting the remaining oil. In a few seconds, the engine is running clean with white fumes of evaporating and burning oil coming from the exhaust. Lead Fouling of Spark Plugs Lead fouling of aviation spark plugs is a condition likely to occur in any engine using leaded fuels. Lead is added to aviation fuel to improve its anti-knock qualities. The lead, however, has the undesirable effect of forming lead oxide during combustion. This lead oxide forms as a solid with varying degrees of hardness and consistency. Lead deposits on combustion chamber surfaces are good electrical conductors at high temperatures and cause misfiring. At low temperatures, the same deposits may be good insulators. In either case, lead formations on aircraft spark plugs prevent their normal operation. [Figure 4-56] To minimize the formation of lead deposits, ethylene dibromide is added to the fuel as a scavenging agent that combines with the lead during combustion.
Lead fouling may occur at any power setting, but perhaps the power setting most conducive to lead fouling is cruising with lean mixtures. At this power, the cylinder head temperature is relatively low and there is more oxygen than needed to consume all the fuel in the air-fuel mixture. Oxygen, when hot, is very active and aggressive. When all the fuel has been consumed, some of the excess oxygen unites with some of the lead and some of the scavenger agent to form oxygen compounds of lead or bromine or both. Some of these undesirable lead compounds solidify and build up in layers as they contact the relatively cool cylinder walls and spark plugs. Although lead fouling may occur at any power setting, experience indicates that the lead buildup is generally confined to a specific combustion temperature range. Combustion temperatures outside this specific range minimize the lead fouling tendency.
If lead fouling is detected before the spark plugs become completely fouled, the lead can usually be eliminated or reduced by either a sharp rise or a sharp decrease in combustion temperature. This imposes a thermal shock on cylinder parts, causing them to expand or contract. Since there is a different rate of expansion between deposits and metal parts on which they form, the deposits chip off or are loosened and then scavenged from the combustion chamber by the exhaust or are burned in the combustion process. Several methods of producing thermal shock to cylinder parts are used. The method used depends on the accessory equipment installed on the engine. A sharp rise in combustion temperatures can be obtained on all engines by operating them at full takeoff power for approximately 1 minute.
When using this method to eliminate fouling, the propeller control must be placed in low pitch, or high rpm, and the throttle advanced slowly to produce takeoff rpm and manifold pressure. Slow movement of the throttle control provides reasonable freedom from backfiring in the affected cylinders during the application of power. 4-35 Another method of producing thermal shock is the use of excessively rich air-fuel mixtures. This method suddenly cools the combustion chamber because the excess fuel does not contribute to combustion; instead, it absorbs heat from the combustion area. Some carburetor installations use two- position manual mixture controls that provide a lean mixture setting for cruising economy and a richer mixture setting for all powers above cruising. Neither manual mixture control setting in this type of configuration is capable of producing an excessively rich air-fuel mixture. Even when the engine is operated in auto-rich at powers where an auto-lean setting would be entirely satisfactory, the mixture is not rich enough.
Graphite Fouling of Spark Plugs As a result of careless and excessive application of thread lubricant, called antiseize compound, to the spark plug, the lubricant flows over the electrodes and causes shorting. Shorting occurs because graphite is a good electrical conductor. The elimination of service difficulties caused by graphite is up to the aircraft technician. Use care when applying the lubricant to make certain that smeared fingers, shop towels, or brushes do not contact the electrodes or any part of the ignition system except the spark plug threads. Never apply to the first set of threads.
Gap Erosion of Spark Plugs Erosion of the electrodes takes place in all aircraft spark plugs as the spark jumps the air gap between the electrodes. [Figure 4-57] The spark carries with it a portion of the electrode, part of which is deposited on the other electrode. The remainder is blown off in the combustion chamber. As the airgap is enlarged by erosion, the resistance that the spark must overcome in jumping the air gap also increases. This means that the magneto must produce a higher voltage to overcome the higher resistance. With higher voltages in the ignition system, a greater tendency exists for the spark to discharge at some weak insulation point in the ignition system. Since the resistance of an air gap also increases as the pressure in the engine cylinder increases, a double danger exists at takeoff and during sudden acceleration with enlarged airgaps.
Insulation breakdown, premature flashover, and carbon tracking result in misfiring of the spark plug and go hand in hand with excessive spark plug gap. Wide gap settings also raise the coming in speed of a magneto and therefore cause hard starting. Spark plug manufacturers have partially overcome the problem of gap erosion by using a hermetically sealed resistor in the center electrode of spark plugs. This added resistance in the high-tension circuit reduces the peak current at the instant of firing. This reduced current flow helps prevent metal disintegration in the electrodes. Also, due to the high erosion rate of steel or any of its known alloys, spark plug manufacturers are using tungsten or an alloy of nickel for their massive electrode plugs and iridium/platinum plating for their fine wire electrode plugs.
Spark Plug Removal Spark plugs should be removed for inspection and servicing at the intervals recommended by the manufacturer. Since the rate of gap erosion varies with different operating conditions, engine models, and type of spark plug, engine malfunction traceable to faulty spark plugs may occur before the regular servicing interval is reached. Normally, in such cases, only the faulty plugs are replaced. Since spark plugs can be easily damaged, careful handling of the used and replacement plugs during installation and removal of spark plugs from an engine cannot be overemphasized. To prevent damage, spark plugs should always be handled individually and new and reconditioned plugs should be stored in separate cartons. A common method of storage is illustrated in Figure 4-58. This is a drilled tray, which prevents the plugs from bumping against one another and damaging the fragile insulators and threads. Additionally, the tray helps identify where the spark plug came from and its proper rotation. If a plug is dropped on the floor or other hard surface, it should not be installed in an engine, since the shock of impact usually causes small, invisible cracks in the insulators. A dropped spark plug should be discarded.
Before a spark plug can be removed, the ignition harness lead must be disconnected. Using the special spark plug coupling elbow wrench, loosen and remove the spark plug to elbow coupling nut from the spark plug. Take care to pull the lead 4-36 Damage to shielding barrel insulator at this point straight out and in line with the centerline of the plug barrel. If a side load is applied, damage to the barrel insulator and the ceramic lead terminal may result. [Figure 4-59] If the lead cannot be removed easily in this manner, the neoprene collar may be stuck to the shielding barrel. Break loose the neoprene collar by twisting the collar as though it were a nut being unscrewed from a bolt.
After the lead has been disconnected, select the proper size deep socket for spark plug removal. Apply steady pressure with one hand on the hinge handle, holding the socket in alignment with the other hand. Failure to hold the socket in correct alignment causes the socket to tilt to one side and damage the spark plug. [Figure 4-60] In the course of engine operation, carbon and other products of combustion are deposited across the spark plug and cylinder, and some carbon may even penetrate the lower threads of the shell. As a result, a high torque is generally required to break the spark plug loose. This factor imposes a shearing load on the shell section of the plug. After removing the plugs, they should be placed in a spark plug tray. [Figure 4-58] Spark Plug Reconditioning Service A visual inspection should be the first step in servicing spark plugs. The threads on the shielding barrel and on the shell that screws into the cylinder should be inspected for damaged or nicked threads. Inspect the lead shielding barrel for corrosion, nicks, and cracks. The firing end should be checked for insulator cracks, chips, and excessive electrode wear. The shell hex or wrench hex should be checked to see if it is rounded off or mutilated. If the spark plug passes the visual check, then it should be degreased using petroleum solvent. Take care to keep solvent out of the shielding barrel.
Never soak the plugs in solvent. After drying the firing end of the plugs, remove the lead compound deposits using a vibrator cleaner. [Figure 4-61] The firing end can now be cleaned by using an abrasive blaster. This is usually done using a spark plug cleaner tester. [Figure 4-62] As the firing end is subjected to the abrasive blast, the plug should be rotated so all the area of the firing end is cleaned. After the abrasive blast, the firing end gets a thorough air blast to remove the abrasive material. The shielding barrel insulators may be cleaned with a cotton cloth or felt swab saturated with solvent, wood alcohol, or other approved cleaner. The firing end should be inspected using a light and a magnifying glass.
If the plug passes the firing end visual and cleaning checks, then the spark gap should be set using a round thickness 4-37 gauge. The spark plug should be tested by using a tester as shown in Figure 4-62, which passes a high-voltage through the spark plug and fire the gap. As this test takes place, the firing end of the plug is subjected to air pressure to simulate the pressure in the engine’s cylinder. If the firing pattern is good, the plug should be returned to its holder ready for installation in the engine. Inspection Prior to Installation Before installing new or reconditioned spark plugs in the engine cylinders, clean the spark plug bushings or Heli- Coil inserts.
Brass or stainless steel spark plug bushings are usually cleaned with a spark plug bushing cleanout tap. Before inserting the cleanout tap in the spark plug hole, fill the flutes of the tap, or channels between threads, with clean grease to prevent hard carbon or other material removed by the tap from dropping into the inside of the cylinder. Align the tap with the bushing threads by sight where possible, and start the tap by hand until there is no possibility of it being cross-threaded in the bushing. To start the tap on installations where the spark plug hole is located deeper than can be reached by a clenched hand, it may be necessary to use a short length of hose slipped over the square end of the tap to act as an extension. When screwing the tap into the bushing, be sure that the full tap cutting thread reaches the bottom thread of the bushing. This removes carbon deposits from the bushing threads without removing bushing metal, unless the pitch diameter of the threads has contracted as the result of shrinkage or some other unusual condition. Replace the cylinder if, during the thread-cleaning process, the bushing is found to be loose, loosened in the cylinder, or the threads are cross-threaded or otherwise seriously damaged.
Spark plug Heli-Coil inserts are cleaned with a round wire brush, preferably one having a diameter slightly larger than the diameter of the spark plug hole. A brush considerably larger than the hole may cause removal of material from the Heli-Coil proper or from the cylinder head surrounding the insert. Also, the brush should not disintegrate with use, allowing wire bristles to fall into the cylinder. Clean the insert by carefully rotating the wire brush with a power tool. When using the power brush, be careful that no material is removed from the spark plug gasket seating surface, since this may cause a change in the spark plug’s heat range, combustion leakage, and eventual cylinder damage. Never clean the Heli- Coil inserts with a cleaning tap, since permanent damage to the insert results. If a Heli-Coil insert is damaged as a result of normal operation or while cleaning it, replace it according to the applicable manufacturer’s instructions.
Using a lint-free rag and cleaning solvent, wipe the spark plug gasket seating surface of the cylinder to eliminate the possibility of dirt or grease being accidentally deposited on the spark plug electrodes at the time of installation. Before the new or reconditioned plugs are installed, they must be inspected for each of the following conditions: 1. Ensure that the plug is of the approved type, as indicated by the applicable manufacturer’s instructions. 2. Check for evidence of rust-preventive compound on the spark plug exterior and core insulator and on the inside of the shielding barrel. Rust-preventive compound accumulations are removed by washing the plug with a brush and cleaning solvent. It must then be dried with a dry air blast.
3. Check both ends of the plug for nicked or cracked threads and any indication of cracks in the nose insulator. 4-38 Installation with Normal Torque Installation with Excess Torque Stretching occurs over this length Seal for combustion gas intact Seal is opened at this point 4. Inspect the inside of the shielding barrel for cracks in the barrel insulator, and the center electrode contact for rust and foreign material that might cause poor electrical contact. 5. Install a new spark plug gasket. When the thermocouple gasket is used, do not use an additional gasket. The gap setting should be checked with a round wire- thickness gauge. [Figure 4-63] A flat-type gauge gives an incorrect clearance indication because the massive ground electrodes are contoured to the shape of the round center electrode. When using the wire thickness gauge, insert the gauge in each gap parallel to the centerline of the center electrode. If the gauge is tilted slightly, the indication is incorrect. Do not install a plug that does not have an air gap within the specified clearance range.
Spark Plug Installation Prior to spark plug installation, carefully coat the first two or three threads from the electrode end of the shell with a graphite base antiseize compound. Prior to application, stir the antiseize compound to ensure thorough mixing. When applying the antiseize compound to the threads, be extremely careful that none of the compound gets on the ground, center electrodes, or on the nose of the plug, where it can spread to the ground or center electrode during installation. This precaution is mentioned because the graphite in the compound is an excellent electrical conductor and could cause permanent fouling.
To install a spark plug, start it into the cylinder without using a wrench of any kind, and turn it until the spark plug is seated on the gasket. If you can screw the plug into the cylinder with comparative ease using your fingers, this indicates good, clean threads. In this case, only a small amount of additional tightening torque is needed to compress the gasket to form a gastight seal. If a high torque is needed to install the plug, dirty or damaged threads on either the plug or plug bushing are indicated. The use of excessive torque might compress the gasket out of shape and distort and stretch the plug shell to a point where breakage would result during the next removal or installation. Shell stretching occurs as excessive torque continues to screw the lower end of the shell into the cylinder after the upper end has been stopped by the gasket shoulder. As the shell stretches, the seal between the shell and core insulator is opened, creating a loss of gas tightness or damage to the core insulator. After a spark plug has been seated with the fingers, use a torque wrench and tighten to the specified torque. [Figure 4-64] Spark Plug Lead Installation Before installing the spark plug lead, carefully wipe the terminal sleeve and the integral seal with a cloth moistened with acetone or an approved solvent. After the plug lead is cleaned, inspect it for cracks and scratches. If the terminal sleeve is damaged or heavily stained, replace it.
Application of a light coating of an insulating material to the outer surface of the terminal sleeve, as well as filling the space occupied by the contact spring, is sometimes recommended. By occupying the space in the electrical contact area of the shielding barrel, the insulating material prevents moisture from entering the contact area and shorting the spark plug. 4-39 One drop here And one drop here Some manufacturers recommend the use of such insulating compounds only when moisture in the ignition system becomes a problem, and others have discontinued the use of such materials. After inspection of the spark plug lead, slip the lead into the shielding barrel of the plug with care. Then, tighten the spark plug coupling elbow nut with the proper tool. Most manufacturers’ instructions specify the use of a tool designed to help prevent an overtorque condition. After the coupling nut is tightened, avoid checking for tightness by twisting the body of the elbow.
After all plugs have been installed, torqued, and the leads properly installed, start the engine and perform a complete ignition system operational check. Breaker Point Inspection Inspection of the magneto consists essentially of a periodic breaker point and dielectric inspection. After the magneto has been inspected for security of mounting, remove the magneto cover, or breaker cover, and check the cam for proper lubrication. Under normal conditions, there is usually ample oil in the felt oiler pad of the cam follower to keep the cam lubricated between overhaul periods. However, during the regular routine inspection, examine the felt pad on the cam follower to be sure it contains sufficient oil for cam lubrication. Make this check by pressing the thumbnail against the oiler pad. If oil appears on the thumbnail, the pad contains sufficient oil for cam lubrication. If there is no evidence of oil on the fingernail, apply one drop of a light aircraft engine oil to the bottom felt pad and one drop to the upper felt pad of the follower assembly. [Figure 4-65] After application, allow at least 15 minutes for the felt to absorb the oil. At the end of 15 minutes, blot off any excess oil with a clean, lint-free cloth. During this operation, or any time the magneto cover is off, use extreme care to keep the breaker compartment free of oil, grease, or engine cleaning solvents, since each of these have an adhesiveness that collects dirt and grime that could foul an otherwise good set of breaker contact points.
After the felt oiler pad has been inspected, serviced, and found to be satisfactory, visually inspect the breaker contacts for any condition that may interfere with proper operation of the magneto. If the inspection reveals an oily or gummy substance on the sides of the contacts, swab the contacts with a flexible wiper, such as a pipe cleaner dipped in acetone or other approved solvent. By forming a hook on the end of the wiper, ready access can be gained to the back side of the contacts. To clean the contact mating surfaces, force open the breaker points enough to admit a small swab. Whether spreading the points for purposes of cleaning or checking the surfaces for condition, always apply the opening force at the outer end of the mainspring and never spread the contacts more than 1⁄16 inch. If the contacts are spread wider than recommended, the mainspring, the spring carrying the movable contact point, is likely to take a permanent set. If the mainspring takes a permanent set, the movable contact point loses some of its closing tension and the points then either bounce or float, preventing the normal induction buildup of the magneto.
A swab can be made by wrapping a piece of linen tape or a small piece of lint-free cloth over one of the leaves of a clearance gauge and dipping the swab in an approved solvent. Pass the swab between the carefully separated contact surfaces until the surfaces are clean. During this entire operation, take care that drops of solvent do not fall on lubricated parts, such as the cam, follower block, or felt oiler pad. To inspect the breaker contact surfaces, it is necessary to know what a normal operating set of contacts looks like, what surface condition is considered as permissible wear, and what surface condition is cause for dressing or replacement.
The probable cause of an abnormal surface condition can be determined from the contact appearance. The normal contact surface has a dull gray, sandblasted, almost rough appearance over the area where electrical contact is made. [Figure 4-66] This gray, sandblasted appearance indicates that the points have worn in and have mated to each other and are providing the best possible electrical contact. This does not imply that this is the only acceptable contact surface condition. Slight, smooth-surfaced irregularities, without deep pits or high peaks, such as shown in Figure 4-67, are considered normal wear and are not cause for replacement.
However, when wear advances to a point where the slight, smooth irregularities develop into well-defined peaks extending noticeably above the surrounding surface, the breaker contacts must be replaced. [Figure 4-68] Unfortunately, when a peak forms on one contact, the mating 4-40 contact has a corresponding pit or hole. This pit is more troublesome than the peak because it penetrates the platinum pad of the contact surface. It is sometimes difficult to judge whether a contact surface is pitted deeply enough to require replacement because this depends on how much of the original platinum is left on the contact surface. The danger arises from the possibility that the platinum pad may already be thin as a result of long service life and previous dressings.
At overhaul facilities, a gauge is used to measure the remaining thickness of the pad, and no difficulty in determining the condition of the pad exists. But at line maintenance activities, this gauge is generally unavailable. Therefore, if the peak is quite high or the pit quite deep, remove and replace them with a new assembly. A comparison between Figures 4-67 and 4-68 will help to draw the line between minor irregularities and well-defined peaks. Some examples of possible breaker contact surface conditions are illustrated in Figure 4-69. Item A illustrates an example of erosion or wear called frosting. This condition results from an open-circuited condenser and is easily recognized by the coarse, crystalline surface and the black “sooty” appearance of the sides of the points. The lack of effective condenser action results in an arc of intense heat being formed each time the points open. This, together with the oxygen in the air, rapidly oxidizes and erodes the platinum surface of the points, producing the coarse, crystalline, or frosted appearance.
Properly operating points have a fine-grained, frosted, or silvery appearance and should not be confused with the coarse- grained and sooty point caused by faulty condenser action. stage, these points are identified by a fairly even contact edge and minute pits or pocks in or near the center of the contact surface with an overall smoky appearance. In more advanced stages, the pit may develop into a large, jagged crater, and eventually the entire contact surface takes on a burned, black, and crumpled appearance. Pitted points, as a general rule, are caused by dirt and impurities on the contact surfaces. If points are excessively pitted, a new breaker assembly must be installed.
recognized by the mound of metal that has been transferred from one point to another. Buildup, like the other conditions mentioned, results primarily from the transfer of contact material by means of the arc as the points separate. But, unlike the others, there is no burning or oxidation in the process because of the closeness of the pit of one point and the buildup of the other. This condition may result from excessive breaker point spring tension that retards the opening of the points or causes a slow, lazy break. It can also be caused by a poor primary condenser or a loose connection at the primary coil.
If excessive buildup has occurred, a new breaker assembly must be installed. by their smoked and smudged appearance and by the lack of any of the previously mentioned irregularities. This condition may be the result of excessive cam lubrication or of oil vapors that may come from within or outside the magneto. A smoking or fuming engine, for example, could produce the oil vapors. These vapors then enter the magneto through the magneto ventilator and pass between and around the points. These conductive vapors produce arcing and burning on the 4-41 A B C D E F contact surfaces. The vapors also adhere to the other surfaces of the breaker assembly and form the sooty deposit. If so, install new breaker assembly.
Dielectric Inspection
Another phase of magneto inspection is the dielectric inspection. This inspection is a visual check for cleanliness and cracks. If inspection reveals that the coil cases, condensers, distributor rotor, or blocks are oily or dirty or have any trace of carbon tracking, they require cleaning and possibly waxing to restore their dielectric qualities. Clean all accessible condensers and coil cases that contain condensers by wiping them with a lint-free cloth moistened with acetone. Many parts of this type have a protective coating. This protective coating is not affected by acetone, but it may be damaged by scraping or by the use of other cleaning fluids. Never use unapproved cleaning solvents or improper cleaning methods. Also, when cleaning condensers or parts that contain condensers, do not dip, submerge, or saturate the parts in any solution because the solution used may seep inside the condenser and short out the plates.
Coil cases, distributor blocks, distributor rotors, and other dielectric parts of the ignition system are treated with a wax coating when they are new and again at overhaul. The waxing of dielectrics aids their resistance to moisture absorption, carbon tracking, and acid deposits. When these parts become dirty or oily, some of the original protection is lost, and carbon tracking may result. If any hairline carbon tracks or acid deposits are present on the surface of the dielectric, immerse the part in approved cleaning solvent and scrub it vigorously with a stiff bristle brush. When the carbon track or acid deposits have been removed, wipe the part with a clean, dry cloth to remove all traces of the solvent used for cleaning. Then, coat the part with a special ignition- treating wax. After wax treating the part, remove excess wax deposits and reinstall the part in the magneto.
Ignition Harness Maintenance
Although the ignition harness is simple, it is a vital link between the magneto and spark plug. Because the harness is mounted on the engine and exposed to the atmosphere, it is vulnerable to heat, moisture, and the effects of changing altitude. These factors, plus aging insulation and normal gap erosion, work against efficient engine operation. The insulation may break down on a wire inside the harness and allow the high-voltage to leak through the insulation to the harness shielding instead of going to the spark plug. Open circuits may result from broken wires or poor connections. A bare wire may be in physical contact with the shielding, or two wires may be shorted together.
Any serious defect in an individual lead prevents the high- tension impulse from reaching the spark plug to which the lead is connected. As a result, this plug will not fire. When only one spark plug is firing in a cylinder, the charge is not consumed as quickly as it would be if both plugs were firing. This factor causes the peak pressure of combustion to occur later on in the power stroke. If the peak pressure in the cylinder occurs later, a loss of power in that cylinder results. However, the power loss from a single cylinder becomes a minor factor when the effects of a longer burning time is considered. A longer burning time overheats the affected cylinder, causing detonation, possible pre-ignition, and perhaps permanent damage to the cylinder.
High-Tension Ignition Harness Faults Perhaps the most common and most difficult high-tension ignition system faults to detect are high-voltage leaks. This is leakage from the core conductor through insulation to the ground of the shielded manifold. A certain small amount of leakage exists even in brand new ignition cable during normal operation. Various factors combine to produce first a high rate of leakage and then complete breakdown. Of these factors, moisture in any form is probably the worst. Under high-voltage stress, an arc forms and burns a path across the insulator where the moisture exists. If there is gasoline, oil, or grease present, it breaks down and forms carbon. The burned path is called a carbon track, since it is actually a path of carbon particles. With some types of insulation, it may be possible to remove the carbon track and restore the insulator to its former useful condition. This is generally true of porcelain, ceramics, and some of the plastics because these materials are not hydrocarbons and any carbon track forming on them is the result of a dirt film that can be wiped away.
4-42 A B C D CABLE TESTER MODEL E5 PUSH TO TEST 12 Volt 24 Volt Differences in location and amount of leakage produce different indications of malfunction during engine operation. Indications are generally misfiring or crossfiring. The indication may be intermittent, changing with manifold pressure or with climate conditions. An increase in manifold pressure increases the compression pressure and the resistance of the air across the air gap of the spark plugs. An increase in the resistance at the air gap opposes the spark discharge and produces a tendency for the spark to discharge at some weak point in the insulation. A weak spot in the harness may be aggravated by moisture collecting in the harness manifold.
With moisture present, continued engine operation causes the intermittent faults to become permanent carbon tracks. Thus, the first indication of ignition harness unserviceability may be engine misfiring or roughness caused by partial leakage of the ignition voltage. shows a short from one cable conductor to another. This fault usually causes misfiring, since the spark is short circuited to a plug in a cylinder where the cylinder pressure is low. Fault B illustrates a cable with a portion of its insulation scuffed away. Although the insulation is not completely broken down, more than normal leakage exists, and the spark plug to which this cable is connected may be lost during takeoff when the manifold pressure is quite high. Fault C is the result of condensation collecting in the lowest portion of the ignition manifold. This condensation may completely evaporate during engine operation, but the carbon track that is formed by the initial flashover remains to allow continued flashover whenever high manifold pressure exists. Fault D may be caused by a flaw in the insulation or the result of a weak spot in the insulation that is aggravated by the presence of moisture. However, since the carbon track is in direct contact with the metal shielding, it probably results in flashover under all operating conditions.
Harness Testing
The electrical test of the ignition harness checks the condition or effectiveness of the insulation around each cable in the harness. [Figure 4-71] This test involves application of a definite voltage to each lead, and then measurement with a very sensitive meter of the amount of current leakage between the lead and the grounded harness manifold. This reading, when compared with known specifications, becomes a guide to the condition or serviceability of the cable. As mentioned earlier, there is a gradual deterioration of flexible insulating material. When new, the insulation has a low rate of conductivity; so low that, under several thousand volts of electrical pressure, the current leakage is only a very few millionths of an ampere. Natural aging causes an extremely slow, but certain, change in the resistance of insulating material, allowing an ever-increasing rate of current leakage.
The procedures for testing ignition harness and leads were discussed earlier in this chapter.