Last night I spent a good four hours trying to figure out how I had screwed up my perfectly fine airplane engine. What started out as a 500 hour Mag inspection turned into a troubleshooting exercise that finally turned up a maintenance procedural error on my part - I timed the Mags to the wrong mark on the flywheel! Now I?ve been working on these kinds of engines for years - how did I make such a fundamental mistake? I always like to look at errors (whether they be mine or others)to see if there are any lessons I can learn from them , and no good lesson is complete until it has been debriefed, so here goes?.
Although I have installed Mags from scratch before, I have only done it a few times over 30 years. I have checked timing a lot, but had never done any internal mag maintenance on my own before. I?d watched it done, but that was a long time ago. Consequently, I asked questions here on the forum, got a copy of the latest maintenance manual, picked up a couple of special tools, and really studied the procedure, step by step until I understood it. Removing the mags was easy (although I never seem to have the right Torx tip screw-driver to remove the harness caps!), and quite frankly, inspecting the internals of the mags and setting the E-gap timing was a piece of cake. The Unison manual was clear (although it has to be read carefully because it covers several different models, and you have to ignore a lot of paragraphs that don?t apply).
Before removing the Mags, the Unison manual said ?Rotate the engine to locate TDC for Cylinder #1, and leave it there.? You then take the mags off and do the internal stuff, and end with them at firing position with the pin in??and then assumes that your engine manual tells you how to install them. Of course, the engine manual says ?Install and time the mags?.? Now, I am not going to blame my mistake on the manuals - it was my mistake - but non-integrated procedures can be a source of errors! (There are many such examples in the aviation world - engine and acessory manufacturures tend to dance around the interface parts...)
I think my biggest mistake was relying on step by step instructions and not just stepping back and looking at the ?big picture?. If you stop and think about it, you want the cylinder to fire at a certain number of degrees before TDC, so that the spark has time to ignite the charge before it gets to TDC. The mag fires when the points open, and the timing light tells you when that happens. Therefore, you want the timing light to open at 25 BTDC! Pretty dang simple - so why did I miss it? Maybe it?s because I normally think like an Engineer (?how does it work??) rather than like a Technician (?how do I work it??). When I get to following rote instructions, I get lazy and forget to think about what I am doing. A contributing factor is a little fear of screwing up something as expensive and vital as my airplane engine, so I didn?t want to move things very far from where they were when I took them apart. But in this case, since I had disassembled the mag anyway, moving the engine shouldn?t have been any more traumatic!
When I finally thought about it, I realized just how darn simple these things are - with the mag rotor locked in firing position by the pin, you can insert it into the engine anywhere in the engine?s cycle that the gear teeth allow. So move the engine to (approximately) where you want it to fire, then fine tune it with the timing light. Something else that I walked into was the fact that the original anti-tamper lacquer from assembly was still on the engine and the mag, and when they lined up really close, I figure that I MUST be close to OK?.but that makes no difference, as that is just setting the relative position of the Mag and the accessory case - not the mag and the crankshaft.
So, for me, the lessons are to use the procedures, but make sure I think about what they mean?.make sure I understand that the procedures are frequently written for someone that already has been taught how to do the task (and aren?t necessarily written as a tutorial), and, oh yeah?.when I run into something that doesn?t make sense, go knock on my friend?s door (he?s an A&P/IA and lives next to the hangars) and say ?Mike, It?s been a long time since I?ve done this?can you check my work??
OK, so now that I?m current, does anyone need mag help?.before I forget how?
Paul
Although I have installed Mags from scratch before, I have only done it a few times over 30 years. I have checked timing a lot, but had never done any internal mag maintenance on my own before. I?d watched it done, but that was a long time ago. Consequently, I asked questions here on the forum, got a copy of the latest maintenance manual, picked up a couple of special tools, and really studied the procedure, step by step until I understood it. Removing the mags was easy (although I never seem to have the right Torx tip screw-driver to remove the harness caps!), and quite frankly, inspecting the internals of the mags and setting the E-gap timing was a piece of cake. The Unison manual was clear (although it has to be read carefully because it covers several different models, and you have to ignore a lot of paragraphs that don?t apply).
Before removing the Mags, the Unison manual said ?Rotate the engine to locate TDC for Cylinder #1, and leave it there.? You then take the mags off and do the internal stuff, and end with them at firing position with the pin in??and then assumes that your engine manual tells you how to install them. Of course, the engine manual says ?Install and time the mags?.? Now, I am not going to blame my mistake on the manuals - it was my mistake - but non-integrated procedures can be a source of errors! (There are many such examples in the aviation world - engine and acessory manufacturures tend to dance around the interface parts...)
I think my biggest mistake was relying on step by step instructions and not just stepping back and looking at the ?big picture?. If you stop and think about it, you want the cylinder to fire at a certain number of degrees before TDC, so that the spark has time to ignite the charge before it gets to TDC. The mag fires when the points open, and the timing light tells you when that happens. Therefore, you want the timing light to open at 25 BTDC! Pretty dang simple - so why did I miss it? Maybe it?s because I normally think like an Engineer (?how does it work??) rather than like a Technician (?how do I work it??). When I get to following rote instructions, I get lazy and forget to think about what I am doing. A contributing factor is a little fear of screwing up something as expensive and vital as my airplane engine, so I didn?t want to move things very far from where they were when I took them apart. But in this case, since I had disassembled the mag anyway, moving the engine shouldn?t have been any more traumatic!
When I finally thought about it, I realized just how darn simple these things are - with the mag rotor locked in firing position by the pin, you can insert it into the engine anywhere in the engine?s cycle that the gear teeth allow. So move the engine to (approximately) where you want it to fire, then fine tune it with the timing light. Something else that I walked into was the fact that the original anti-tamper lacquer from assembly was still on the engine and the mag, and when they lined up really close, I figure that I MUST be close to OK?.but that makes no difference, as that is just setting the relative position of the Mag and the accessory case - not the mag and the crankshaft.
So, for me, the lessons are to use the procedures, but make sure I think about what they mean?.make sure I understand that the procedures are frequently written for someone that already has been taught how to do the task (and aren?t necessarily written as a tutorial), and, oh yeah?.when I run into something that doesn?t make sense, go knock on my friend?s door (he?s an A&P/IA and lives next to the hangars) and say ?Mike, It?s been a long time since I?ve done this?can you check my work??
OK, so now that I?m current, does anyone need mag help?.before I forget how?
Paul
