Mag Checks

[jetboy]

for the engine to 'dead cut' when you test failed the L mag it means either the R mag was faulty or all the plugs run from the R mag were fouled and none of the plugs from the L mag were affected.

 

These "mags" are just a coil / ignition pack for lawnmower engines and some of the jabiru supplied versions will fail when hot and work again when cool

 

Replacing the R mag after these 2 occurences was the proper thing to do

 

I carry a spare in my aircraft because it can take time to get a replacement.

 

havent had to use it yet

 

 

[anjum_jabiru]

I agree with Jetboy that the mags may not run when hot and run once again when they are back to normal temperature. But what could have made them hot in the first place. Taxiing about 400m at a low rpm from start up is highly unlikely to make them that hot.

 

 

[facthunter]

They shouldn't run hot unless the are leaking (current) and if that was the case they are just about U/S. The insulation can deteriorate and allow leakage of the very high voltage. The secondary has many turns of very fine wire. Moisture and heat are the enemies of coils. Sometimes when you cut them open they are green inside. Corrosion of the copper wire due to moisture and breakdown of the lacquer they are infused with when manufactured. Nev

 

 

[anjum_jabiru]

{{ title }}

 

The above article seems to suggest that it is very much possible for fuel to foul the spark plugs.

 

 

[DWF]

FYI

 

After about an hour in the circuit in our J120C we taxied back for a break and shut down normally.

 

About 15 minutes later the student taxied out for solo circuits and got a dead cut on the RH mag. Confirmed by L2. Replaced RH coil. All good again.

 

DWF

 

 

[anjum_jabiru]

It seems to be an issue with the Jabs.

 

 

[jetboy]

The coil packs on Jabs are at their hottest during taxi and shutdown after flight because of heatsoak. They are mounted behind the cylinder cooling fins. Since about 2006 they are required to have blast tubes fitted to help cooling them, but these are ineffective on the ground. They also changed the brand of coil pack which reduced the failure rate so its uncertain whether the cooling tubes are needed.

 

 

[alpine aviation]

Hi GuysI had my passenger endorsement test done today. On late finals my instructor reached out and shut off both mags at around 1800 rpm, to check if I can bring it home safely. All went well until I decided to go up again after passing my endorsement rating. During the pre-take off run up and mag checks, as soon as I switched off the left mag on the Jab with engine running at 1800 rpm, the engine quit. Believing it to be some form of oversight on my part I repeated the checks and the engine quit again as soon as I switched off my left mag. I then decided to restart the engine using both mags as usual and end the flight.

 

My question is - Is it possible that if someone switched off both mags simultaneously with the engine running at 1800 rpm in mid flight, this could result in any form of damage to the magnetos or the engine?

 

Thanks again for your input.

Not sure if the RPM setting and cutout at the higher RPM could cause the damage but I am aware of a similar issue being experienced on checking the mags on shutdown after a navex flight. The fault was in one of the distributors where the rivets holding the drive had sheared. The failure was due to the use of rivets that were not recommended for this purpose.

 

 

[Teckair]

{{ title }}The above article seems to suggest that it is very much possible for fuel to foul the spark plugs.

Well you can think that if you like and it is true a flooded engine will not run but that was not your problem.

 

 

[anjum_jabiru]

The most likely cause then is probably an overheated magneto, which worked alright after it cooled down a bit.

 

 

[facthunter]

It's unlikely to fix itself if it has been overheated to any extent. If a coil shorts internally it's weakened. A track forms through the windings of the coil. It often goes visibly black in areas. A good magneto shouldn't make much heat. It could pole or get heat from surrounding sources, or from leakage of current, if it's developing a fault. Nev

 

 

[anjum_jabiru]

Yes, my instructor has now put a new one in as he did not wish to take any chances.

 

 

[Ultralights]

just wondering, how many people do a mag check at top of descent?

 

 

[anjum_jabiru]

Gosh No! Would not want to risk it lest the mag fails and the otherwise smooth running engine shuts down last moment. And if the mag was to fail, it could fail on the finals anyways, hence what would be the benefit of that check? Once you are up there you will have to come down at some point. It is possible to avoid take-off but not possible to avoid landing with a failed mag.

 

 

[dutchroll]

just wondering, how many people do a mag check at top of descent?

Engine diagnostics, testing and troubleshooting are best done on the ground if circumstances permit, as a general principle.

 

 

[Jaba-who]

This is an ignition coil problem.

 

I've seen it three times - and even had both on a brand new jab engine. First one died before even getting in the air. Exact same symptoms. Second at three hours use. I can't recall if they were Honda coils or Jabiru coils.

 

One had Consistent failure even when cooled. Second and third ones had failure when hot and ok when cooled.

 

All were changed out and worked fine.

 

In the first event I changed the plugs as I had a second set sitting there. Spoke to Rod Stiff himself. He was certain it was fouled plugs but had to agree if plugs were changed out and problem persisted then the coil was changed and it fixed the problem - it had to be the coil.

 

Second coil lasted three hours on the same engine and exactly the same problem. Second coil changed out - fixed.

 

Both coils on same engine tested by me using the specs in the manual. Both appeared ok. But sent them back to jabiru. Both failed their test.

 

Third one was a few years back. Had a few hundred hours on it. When it happened I just changed the coil and it fixed it so I didn't dwell any more on it.

 

 

[facthunter]

Jabiru plugs are both on the top and not far apart. This means the rev drop is likely to be less when you test the switches. You wouldn't foul up one set of plugs completely and not any of the others. Most unlikely on both counts. Nev

 

 

[anjum_jabiru]

Yes. No wonder I don't see much of a drop in revs on doing the mag checks. When I switch the first one off, there is almost no difference and the needle stays put. On switching the second one off there is a very slight deflection of the needle to the left, but not what I can call significant. It has had me wondering if the mags are working fine or not. But the reassurance of the engine revving alright only on one magneto at a time during the checks is enough to satisfy me that the mag check is ok.

 

 

[facthunter]

Some engines with the plugs situated far apart do drop RPM as they are having the effective timing retarded, so they drop considerable revs with no plugs outrunning on each magneto alone. Not all engines fire the separate magnetos at the same timing, and if this is not the case it will drop more on one than the other. The engine is designed to run normally on TWO plugs per cylinder. There are some engines where the fibre timing gear for the distributors can part shear and that magneto still fires but at entirely the wrong time to be of any use. It obviously will cause damage unless the affected magneto is switched off. This is one time you will do an inflight mag check but at lower power. If you have to fly on one mag (to complete the flight to a suitable aerodrome, Not take off to commence one) Don't lean out much. Nev