Mag Failure
I had my magnetos overhauled by a reputable organisation near MK approximately 2 years and 200 hours ago. I installed a new ignition harness at the same time. They are timed to fire at 20degs BTDC. Because I time them myself and check them at annual, they are within a hairs breadth and I normally see a 40rpm drop on both. I have an electronic tacho and rarely do a dead-cut as the tacho will tell me if either mag does not ground out during mag checks at the hold.
Yesterday I flew 5 eventless sorties and had three more planned. I arrived at the hold and went through my pre-takeoff checks. 1700 rpm Left mag - a standard 40 drop and commensurate engine note change. Right mag - engine cut.
Silly senile old Stik, I thought - could not have had the mags on BOTH initially on the key type starter. So turned the key one click further counterclockwise suspecting it would not go but it did and visually it showed mags as OFF, throttle back to idle and three clicks clockwise back to BOTH and she was running like a sewing machine.
I powered up to 1200 and repeated my earlier check - LEFT (the key grounds out the right mag and forces the engine to run on the left mag) a slight drop and minor engine note change. RIGHT (the key grounds out the left mag and forces the engine to run on the right mag) = total dead cut. I taxied back to dispersal and shut down.
I was slightly confused as in 30 years I have never had this! I've had faulty plugs, faulty leads, decaying condensers, etc.
My initial thoughts were, perhaps the P lead to the right mag has abraded and is permanently grounding out. After uncowling the engine it became clear what had occured. The Bendix mags are effectively three parts, the ignition harness screws into the casing. The casing is two parts, held together with 4 machine screws and these are sealed at overhaul.
The machine screws had vibrated loose and the casing had split by about 3/8". I can only assume that with the engine started (on the left impulse couple mag) the spark could bridge the gap and fire at the plugs, thus causing the audible/visible mag drop when I ground out the right mag but when I ground out the left mag, it had insufficient spark to keep it running.
Anyway the moral of the story is that perhaps had I given the top of the mag casing a shake when replenishing oil earlier in the week, I may have detected it as being loose and could have prevented yesterday's inconvenience. From now on I'll be giving them a shake - hopefully for the next 30 years!
I am heading back to the airfield where I left the aeroplane this evening with a spare zero-timed magneto, a tool box and my trusty 1942War Department mag timer in it's wooden case.
Stik