Magnetos (i.e. the Ignition System)

Magnetos (i.e. the Ignition System) B-17 Technical Session for Flight Engineers 6/10/2017 (with post meeting revisions 6/11/2017) The B-17G (specifically our Texas Raiders, TR) has an electrical system AND an ignition system. The Electrical System is the batteries, generators, motors, starters, lights, solenoids, circuit breakers, inverters and the like. The Ignition System generates a timed pulse of electricity and directs it to the appropriate spark plug - and that is all it does. The two systems are completely independent except briefly during engine start when the Electrical System assists the magnetos in generating an initial spark. The significance of this is that anything, or everything, can go wrong with the Electrical System and yet there will be no effect upon the Ignition System. Dead batteries, popped circuit breakers, burnt up wires, all of that has no effect upon the ignition systems. The Ignition System consists of a magneto, an ignition wiring harness and the spark plugs (there are two spark plugs on each cylinder on the engine). Each engine has TWO ignition systems: one for the front spark plugs, the second for the back spark plugs. With four engines on TR, that amounts to EIGHT ignition systems. The ignition systems are mounted on the back of the engines (attached to the supercharger section). Each ignition system is independent. Again the significance is failure of one has no effect upon any other ignition systems, meaning one ignition system on an engine can fail completely and that engine will still continue to run with the other system. The reason each engine has TWO ignition systems is first for redundancy and, secondly, for engine performance efficiency. The magneto is the heart of the ignition system. The magneto internally generates an electric pulse and has a distributor to send the pulse to the correct spark plug. The magneto is driven by the engine and as such is mechanically synchronized (timed) to the functions of the engine. The magnetos on TR are of the FIXED ignition type meaning the timing of the electrical pulses are fixed relative to the RPM of the engine (in automobiles, for example, the timing is designed to vary with the RPM for performance reasons - a feature not needed in airplanes). Automobile ignition by comparison derives its power from the electrical system, has variable timing and no redundancy more complicated; less reliable. Magnetos are self contained and work anytime they are spinning. To stop them from working, an external short has to be introduced into the magneto s circuitry. The Pilot has this control. The control feed into each magneto is called the P Lead. Any magneto not grounded (i.e. not having this induced short circuit) is considered to be a hot mag. Note to FEs: Rolling the props, or rotating an engine for any maintenance reason with a hot mag is a serious safety hazard don t do it. Check the mags first! 1

The TWO ignition systems per engine on TR are almost identical. The left magneto services the rear spark plugs; the right magneto services the front spark plugs. Only the right system is engaged to start the engine. Once started, both systems are engaged for all operations Left and right magnetos are not interchangeable. The only difference is the orientation of the mounting bracket on the back of the magneto (the flange piece that actually mounts to the engine). Note to FEs: A memory aid The words left and rear each have four letters. Right and front each have five letters. The magneto generates the electrical pulse by spinning a permanent magnet through coil creating a magnetic field. It first generates low voltage, transforms that into a high voltage jump all timed with breaker points, and sends that through the distributor onto the appropriate spark plug one spark at a time. The faster the spin the greater is the initial voltage, and vice/versa. This means at slow spin (like during engine start) the magneto has difficulty putting out enough voltage to jump the spark plugs. To augment this, the magneto is helped during engine start by a pulsed signal coming from the Induction Vibrator. The Induction Vibrator is a component completely separate from the magneto. There are Four one for each engine. The Induction Vibrator takes sufficient but steady voltage supplied by the electrical system batteries, converts that voltage to a pulsed signal via a vibrating coil / switch, and sends that into only the RIGHT magneto. The magneto transforms this to the high voltage and on to the appropriate spark plug same as if the voltage origin was the magneto itself. The only difference, the Induction Vibrator process showers the appropriate spark plug with sparks, whereas the magneto would only send one spark - a sloppy way to run an engine but it works well for starting. Once the engine starts, the Induction Vibrator is turned off via Pilot control and the engine then runs only on the magnetos (both). The Induction Vibrators are located on the back side of the fire wall just behind each engine. Note to FEs: If an engine is hard to start, a definite candidate for the problem is a non-operating Induction Vibrator. These things buzz when operating. With the engine off, check for the buzz sound. MAGNETO CHECK during RUN-UP: The magnetos are checked during run-up before every takeoff. During run-up each magneto is checked independently. The question is which magneto is actually being checked during this test. When the switch is on BOTH, both magnetos are operating, and when it s on OFF both magnetos are grounded and the engine quits. When the switch position is to the LEFT, the RIGHT magneto is grounded. The engine is operating on, and checking the performance of, the LEFT magneto. And vice/versa. 2

The pilots perform these tests. The FE observes. The test is to observe the RPM drop as the magnetos are switched. The engines are less efficient operating on just one magneto and a RPM drop should be observed. All engines are going to have some amount of drop running on a single magneto First is a rather instant drop referred to as the fast drop followed, maybe, by a further slow drop.. The maximum allowable total drop for TR is 100 RPM. A drop in excess of 100 RPM is a no-go for take off. The procedure is repeated for the other magneto. Note: The pilot usually calls out the engine and switch positions, i.e. #2 - RIGHT - BOTH - LEFT - BOTH. so that the co- pilot can direct his attention to the appropriate engine instruments.. The FE should be looking at the nacelle and prop dome to determine if any excess engine shaking is noticed or anything else that is unusual. When the drop is excessive, generally a fast drop is indicative of a fouled / bad plug(s) or a defective ignition harness. On the other hand a slow drop is generally indicative of faulty timing (magneto or distributor) or incorrect valve adjustment. Comparing the amounts of the two drops is a part of the check; they should be roughly comparable to one another. The difference between the two magneto drops should not be significant (pilot s judgment) even though both are less than the allowable 100 RPM. If there is absolutely no RPM drop on the magneto selected (if it stays absolutely the same) it means there is a bad P lead. In other words, that magneto is hot and unable to be grounded. That situation is also a no-go for take-off. MAGNETO FAILURE in flight: A complete magneto failure can (and has) occur in flight. That will affect engine efficiency (and ultimately the aircraft s range) but the loss probably will not be noticed by the flight crew. A failed magneto disables all the spark plugs on one side so the symptom of a rough running engine does not occur (Oil temperatures and cylinder head temperatures (CHT) should stay the same, and the RPM will be maintained by the prop governor). The failure will be first detected at the next engine run-up (or engine start if it is the right magneto). In flight magneto checks on TR are not done. TR can fly and land perfectly well with a failed magneto, BUT, all eight magnetos must be operational before there can be a take-off. The significance is that an in-flight failed magneto must be repaired / replaced at the next landing site - before the next flight. FEs replace magnetos. MAGNETO INSPECTION: 3

Pulling the cover off the back of a magneto reveals the points, condenser, the cam lobe, cam follower and the P lead connection. Check for cleanliness oil or water near the points is a problem. (TR left outside in heavy rain with the engines uncovered has caused problems before). (Oil mist from the running engine has collected on the P lead and then seeped into the magneto) Check the point s condition. A small amount of pitting and burning is normal. Do not use sandpaper or emery cloth to smooth up or clean the points. Best is to use thin cardboard to clean the points (thin cardboard without any printing on it - like a piece of a manila folder). If the points are severely burnt or pitted, the problem is most likely the condenser replace if defective. Check the cam and the cam follower for wear. Replace if worn excessively, A worn cam follower changes the point gap which alters the timing. The cam requires lubrication but only enough to exhibit an oil film feel and no more. Check the synchronization. Both set of points should open at exactly the same time. (a magneto sends the spark to the spark plugs when the points OPEN not when they close). (use the synchronization tool and roll the prop by hand to check this one) Check that the magnetos are securely fastened to the engine and each has a ground wire attached. MAGNETO MAINTENANCE: Points can be cleaned / adjusted, condensers replaced, the timing / synchronizing re-set, cam follower replaced, distributor block checked / cleaned, ignition wires checked / changed in place. All else requires a certified shop to fix MAGNETO REPLACEMENT: Faulty magnetos are replaced not fixed by FE s. Not all of the entire magneto needs to be removed to replace. To swap out a magneto, the P lead stays, the covers over the distributor block stays and the distributor block stays with the ignitions wires still connected. The distributor block is the semi-circular part that all the ignition wires tie into. Note to FEs: The distributor block has numbers by each ignition wire tie-in point (nine locations). These numbers for some reason do not correspond to the cylinder the ignition wire leads to. The maintenance manual is needed to sort that one out. Should a magneto need to be replaced, expect the replacement time to be between three to six hours. If on a Mission, advise the MTD of this downtime estimate. MAGNETO SETTING: (SETTING means installing a magneto onto the engine usually this will be a new or Yellow Tagged magneto) 4

There are two concerns when setting the TWO magnetos on an engine - TIMING and SYNCHRONIZATION. TIMING is appropriately clocking one magneto to the mechanicals of the engine. SYNCHRONIZATION is setting the second magneto timing relative to the first magneto. TIMING has to do with sending the magneto s electrical pulse to the appropriate cylinder at just the right time to ignite the fuel charge in to create the maximum power stroke (push on the piston). Fuel takes an instant to burn and the piston is moving fast. To account for this, the spark timing is set at 20 degrees before TDC (That s 20 degrees of crankshaft rotation angle before the piston reaches the top of the cylinder). The intent is to have both piston position and fuel burn climax at exactly the right time, which happens at near TDC). TIMING is done relative to Cylinder #1 (the one on top). The engine reference is the #1 piston on the compression stroke (not the exhaust stroke, otherwise all would be 180 degrees out of whack) and 20 degrees before TDC (top-dead-center). The 20 degrees is read through an observation port on the front of the engine. The magneto reference is timing marks on the circumference of the magneto housing aligned with the flats across the timing collar on the magneto rotor (the alignment is done with a straightedge across both magneto components). The magneto points are set to just open when this alignment is attained. The procedure is: 1) the engine is set according to its reference 2) the magneto is set according to its reference 3) then the magneto is inserted into the engine via a spline connection.4) the magneto is rotated finely to detail adjust the points to just open at this setting.. All of this is best demonstrated rather than verbally described here SYNCHRONIZATION is done on the second magneto again with the straightedge method and then dialed in precisely with a synchronization tool which has timing lights. Use of the synchronization tool requires that both magneto P leads be disconnected. The intent of synchronization is to have both magnetos ignite both spark plugs at the exactly same time. The procedure is: 1) the engine is set according to its reference 2) the magneto is set according to its reference 3) then the magneto is inserted into the engine via a spline connection.4) the magneto is rotated finely to detail adjust the points opening as prescribed by the synchronization tool.. Again, all of this is best demonstrated rather than verbally described here Note to FEs: Should a faulty magneto need to be replaced, the new magneto needs only to be SYNCHRONIZED to the remaining magneto not TIMED to the engine. The correct timing is retained by not removing the other magneto. Also when working in place on the airplane, SYNCHRONIZING is an easier task than TIMING - a good thing since this is the most probable repair action. Note to FEs: When being precise on these settings be aware that SYNCHRONIZING is more important than TIMING. SYNCHRONIZING puts efficiency into the engine and can be done precisely. TIMING (the 20 degrees before TDC) is somewhat of a compromise of a number of variables (like fuel 5

octane, inlet air temperature, variations in RPM) so there is some fuzz in the need for exactness of the setting. Note to FEs: The synchronization tool requires the prop to be hand rotated somewhat to activate the tool (i.e. both lights on the tool go on when both point sets are synchronized). Rotate to prop counterclockwise (same as when rolling the props) to eliminate any gear lash. Check that the mags are not hot before doing this. Note to FEs: The synchronization is routinely checked during 30 hour inspections. Only the P leads need to be disconnected to do this with the synchronizing tool. The P LEAD: The P lead connects the magnetos to the Pilot operated mag switch. Failure of the mag switch leaves a magneto hot. Removing the P lead form the magneto during maintenance however does not leave it hot. There is a fail safe mechanism in the magneto wiring that shorts the magneto when the P lead is pulled out. In the photo below, the P lead attachment is the male threaded connection on top - next to the inspection cover. (one for each mag) 6

Magnetos mounted on a engine ( P leads are not attached) The Ignition Wiring Harness with crossover are shown connected to the top of the mags. 7

Magneto without the wiring harness and the metal cover over the distributor removed and the distributor block removed. Note the Bakelite connector (the magenta colored item on top) is not used on TR. These mags are generic used on other engine setups - this item, although not used on TR is still required to remain because it was certified that way (FAA Rules). The magenta colored item in the center of the mag is the rotor. The rotor spins. In the photo, the rotor shows TWO protruding studs. One of the studs on the rotor aligns with a stationary stud on the distributor block (the other stud is for the non-used FAA required feature), all timed with the points opening. This distributes the spark to the proper spark plug. Actually there are four studs on the rotor (two pair each pair 180 degrees apart on the rotor. Two are for the spark distribution; the other two are for the non-used FAA required feature) 8

Distributor block without the ignition wires attached. The block can (and usually is) removed form the distributor without disconnecting the ignition wires. Note there are NINE studs (i.e. nine spark plugs). These align with the studs on the rotor. 9

Magneto with the inspection cover removed. The item in the center is the cam. Riding on the cam is the cam follower which pushes on the points to open. The cylinder at 2:00 o clock is the condenser. At 10:00 o clock is the P lead (here shown installed). Note the P lead is pushing down on an S shaped piece of spring steel. If (when) the P lead is removed, this spring steel moves up and contacts the body of the magneto. This shorts the magneto and is the fail-safe that allows the engine to be rolled during maintenance without the P leads in place. With the P leads in place, the mags are only hot per pilot control. Pull the P leads and the mags are cold. 10