In the air Mag check

[RV7AV8R]

I recently worked on my ignition and afterword my ground run up mag check was fine. On my test flight I decided to check the mags again in the air. I had never done that before and was surprised to find one ran real rough. Back on the ground the mag check was good again. I am not sure what the problem was but after reseating the spark plug wires (auto plugs) it went away. I probably had one not all the way on. The point of this post is to advise you to consider an inflight mag check after working on them as well as a ground mag check. Maybe this is already standard practice by A&Ps, but I have always just assumed if they work on the ground they will work in the air.

[flyingriki]

Mike Busch did an article on this in the Sport Aviation, December 2010 issue.

He may address it in this:
http://www.savvymx.com/index.php/all-about-magnetos

Seems the In-Air check, at altitude and leaned, tells you a lot about the health of the ignition system. Best if done with good instrumentation.
Runup check only indicates really bad issues, which is good!

[jrs14855]

If the mixture is leaned the in flight mag check will often result in roughness. This is how Max Conrad leaned on most of his long distance flights. Turn off one mag, lean to roughness, back to both and it runs smooth. No small airplane egt gauges in Conrads era.

[flyboy1963]

....read somewhere on runup or shutdown to do an idle mag check...same reason, will show up a stumble more dramatically than at 1700 rpm.
...or if you carboned up a plug taxiing back to the hangar/tie-down!

[pierre smith]

I have a Lightspeed on one side and I had two cylinders that had higher EGT's and lower CHT's than the others because a coil went TU. It is a shared coil and Klaus said to check it at cruise power because at 1700 the problem didn't show up.

Best,

[RV10inOz]

If the mixture is leaned the in flight mag check will often result in roughness. This is how Max Conrad leaned on most of his long distance flights. Turn off one mag, lean to roughness, back to both and it runs smooth. No small airplane egt gauges in Conrads era.

Almost...........but not quite!


If you do a mad check in the air, leaned say 10-20F LOP as I did yesterday, no roughness to speak of, but a power reduction and speed loss. Effectively you have retarded the combustion process a bit. Note I said retarded the combustion process not ignition timing as its fixed at 25DBTDC.

Now if all is hunky dory, the engine will not run rough, unless it is leaned very much so that when on one mag the fuel air mix is so lean its almost impossible to ignite, then when it gets retarded even further its almost quitting.

Yesterday while smooth on both mags I have detected No.2 Left mag plug is ever so slightly off peak performance just by watching the EGT's. The likely cause will be when I regapped them the other day I bet one of the two outer electrodes is a bit closer at some point. Very hard to pick, but I did.

So when doing the LOP MAG/Plug check, you really need all cylinder monitoring, you need to be lean of peak or as lean as you can go and still be smooth (above 8000' if you are not able to do LOP properly) and when you switch to a mag leave it there for 10-15 seconds and watch what happens. Do not be alarmed by EGTS that progressively climb up to 1500 or more. This is OK and normal. They all should rise together and if one falls, you have a crook one and it will be running rough. If it flickers, possibly the plug gap is too large.

Plug gaps must be kept between 16-18 thou when being gapped and ideally after 50 hours they will not be beyond 22 thou.

I have probably left out something important...

Ohh yes........ Hands up who does not have an all cylinder monitor?..........Go out and buy one NOW. If you don't....