Mag switch failure

[Bluey]

After a long flight yesterday, I went to shut down the engine and one of the mag switches failed and the engine failed to shut down. I flicked the switches on and off 3 or 4 times before the engine succesfully shut down. After a little while I restarted and tested the switches on at a time and they both worked fine. I did this numerous times. I test flew and landed and the engine shut down normally. Before each shut down, I normally test each switch to ensure proper working order. Anyone ever have a problem like this? I have heard of a few instances of random failures that never repeat themselves. I have purchased two replacement switches but have been told that unless the failures happen again then it might be pointless replacing the switches. I have even heard of a couple of instances of failures on a new aircraft.

 

Bluey.

 

 

[facthunter]

Switches wear out. Sometimes the feel of it will indicate that something is not right. You can never guarantee the switches working. That is why you always consider the prop to be "alive." . Nev

 

 

[Guest davidh10]

I've not heard of such an issue, but have had similar failures with GPO PowerPoint due to plastic molding faults or floating plastic particle in the mechanism.

 

As you have already obtained a replacement switch, I'd replace the one that failed, but leave the one that worked ok.

 

One of my startup checks is to observe the drop in engine revs when each mag switch is activated and return to normal when deactivated. Each switch is tested in turn.

 

If you check on shutdown, how do you prove both ignition systems are operative before takeoff?

 

 

[Bluey]

David, I always do a pre take off check to 4000 rpm when the oil temperature is above 40 degrees celcius. The shutdown check is at idle and is just an extra mag circuit check that really is not required but I do it anyway.

 

Bluey

 

 

[Guest Maj Millard]

Bluey , From memory you have a 912 ?. Even though the IGN switch only grounds out the IGN modual, rotax does specify a certain grade and capacity of switch. As facthunter mentions switches do wear out eventually, but make sure you have a quality switch of the correct capacity..............................................................Maj...

 

 

[Bluey]

Maj, I bought the Narva single throw switch (12V, 20A) which are apparently the same as those used by airborne.

 

Bluey

 

 

[facthunter]

20A should be more than enough, but today relays, switches etc are so miniaturised I cannot see how they meet the stated current rating. None of this stuff is really aero grade and there may be brand "faking". It's not worth your time to keep something there that has been an intermittent failure. It doesn't stop the engine running, of course. Doing a quick "cut " of both at low revs after a flight is a good idea. Not much point in finding it out next time you want to go flying, that a coil is U/S.

 

 

[cscotthendry]

After a long flight yesterday, I went to shut down the engine and one of the mag switches failed and the engine failed to shut down. Bluey.

Bluey: It would be highly unusual for BOTH switches to fail simultaneously. Since the mag switches shut the engine down by grounding the mags out, I'd run a continuity check between the ground connection at the switches and the engine. If that wire is broken internally (say from engine vibration) then that would cause the symptoms that you mentioned. BTW, this is a "fail safe" design in that if a broken circuit were to happen in flight with a circuit that ENabled the mags rather than DISabled them, the engine would stop and cause an emergency landing. As it is, it only means that the engine cannot be shut down. In the manual, Airborne reccommend that if this is the case and you cannot shut the engine down, that you use the choke to kill it.

 

 

[Bluey]

Hi Scott, it was only one mag that temporarily failed but I don't know for sure which one. I think it was the second switch but I didn't get to check this because the engine shut down before I could systematically fault find. This I am sure of because I recall the rpm drop when I switched one of the switches off at shut down (I always do a mag check before shutdown every time). Anyway, I have spoken to some pretty experienced people who have told me they have had similar intermittent problems and have just ignored them with no long term issues. This doesn't mean I would do the same but this has been the experiences of others. In the case of the CHT issue I was having, I noticed the connection to the probe was a little off centre. I moved the connection for better contact and have since flown more than 5 hours with virtually identical CHT's. I will keep a close eye on the temperatures in any case.

 

Bluey