Mag Checks after landing

[Ross]

Today after I completed my BFR I saw an early model lonely Jab LSA on the tarmac with the top cowl removed with the owner in attendance.

 

It turned out that this cross country pilot who had just landed to refuel always makes it a practice to check his "mags" before shut down as well as before take off.

 

Today the check found a faulty one.

 

It was initially thought the fault was possibly in the coil or it's earth lead had been accidentally grounded somewhere in the coil wiring system. The "mag" switches normally stop the motor by closing both switches thus earthing the lead from each of the coils. This kills the available charge in the coil. This is opposite of what we normally expect from a switch in a lamp circuit.

 

After shutting down the good coil with the ignition switch and trying the faulty one it was firing but could not maintain the running motor without the other coil as well.

 

The distributor cap was removed revealing the inside of the cap marked around about 180 degrees of its circumference in line with the rotor head and a fine coating of brown plastic or Bakelite like dust around most of the inside of the cap.

 

On removal of the rotor it was found that the shaft could be easily rotated using two fingers through a full 360 degrees but still had some erratically slight connection to the rest of the engine.

 

The pilot decided not to continue the trip or return home on one ignition system.

 

 

[Guest Graham Lea]

An idle mag check can save the day for the next flight if you discover it then rather than when about to start. It should be part of the shutdown procedure. Likewise checking the plug leads prior to starting.

 

 

[philipnz]

I always check the mags as i shut down.

 

 

[Deskpilot]

I was not taught to do this with the Jabiru but have been with the SportCub. Different planes, different owners/instructors. Seems a good idea but.

 

 

[icebob]

I do this too, goes back to my first flight training in a Cub in the '60's.

 

Bob

 

 

[slartibartfast]

Ditto here. A short stop in Port MacQuarie turned into a longer stop after the shutdown check revealed a problem in one circuit a few months back. Just a loose lead, but I'd rather find it on shutdown. As most people do the mag check during runup, it can save a lot of taxiing and time. Doesn't cost anything either (a first in aviation?).

 

 

[BigPete]

I do it as part of shutdown too. When things are hot they may break down but work fine when cold.

 

So when I fly I check when cold (runup) and later when hot (shutdown).

 

regards

 

 

[Ross]

The cover over the gears was removed to see why the rotor was rotating freely.

 

Inside the housing the back of the rotor is normally pop riveted to the non metal drive gear that picks up the drive presumably from the camshaft.

 

Two of the four steel pop rivets are missing/failed eventually possibly allowing the gears to "de-mesh" therefore eventually stopping the left distributor firing in the correct part of the 4 stroke action. The remains of most of the missing two pop rivets was sitting on the bottom inside edge of the cover.

 

Whether the failed pop rivets were a result or a cause of the failure is certainly not clear at this stage. The factory said they had never seen a failure of those rivets in the past.

 

The owner says that he thought the "free play" experienced in the rotors when felt some time in the past at a scheduled inspection was excessive.

 

His opinion of there being excessive backlash was dismissed as unlikely at the time!!

 

 

[BigPete]

Sometimes you need to listen to "gut" feelings. I picked both of my valve failures way before they made themselves known by "leaking audibly thru the exhaust".

 

regards

 

 

[Guest AusDarren]

When I learnt to fly (GA) I was taught to shut down with mixture control..

 

When I was trained in the Jabiru, fuel was left on and Mag off to shut down..

 

and have fown with at least 4 instructors since in Jabiru aircraft, and have not been challanged or queried about it..

 

what have you been shown in Jabiru aircraft?

 

Regards,

 

AusDarren

 

 

[Ross]

As of yesterday,Saturday, the LSA55 is back home in its Northern home having flown there on borrowed parts so that it had two working distributors. The owner was still waiting on parts from Bundaberg but he was advised that they had already been despatched.

 

 

[Guest ROM]

We will be very polite!

 

A couple of years ago I saw in the hands of a Jab owner who collected his bits from the very well connected to Jabiru maintenance person who repaired his engine, the same rotor drive with those same rivets pulled or shorn off from the drive cog.

 

We oohhed and aahhed about the use of pop rivets to hold the the rotor drive shaft to the fibre cog while we examined the failure.

 

Just lets say that Jabiru is pulling that old, old stunt of "this is the first one we have seen!"

 

 

[Ross]

There is no manual mixture control on the Jabiru motor so the only normal option is to ground the ignition system with the "mag switches". The throttle stop is supposed to be set so that the 2200 motor idles at 800 rpm.

 

If that does not work, like I saw at a recent fly in (not on a Jabiru motor) then the normal remaining option is to shut off the fuel with the main fuel valve and that can take some minutes before the carburettor fuel level is low enough to stop the motor.

 

Maybe you could also possibly flood the warm motor with the choke and stop it if the throttle stop had been set correctly. This would be quicker than waiting for the carburettor to empty but probably not very good for the motor.

 

 

[Ross]

ROM I am not surprised to hear that. There do seem to be a number of communication failures in the organisations involved.

 

It is sounding like the old flywheel using 1/4" set screws and with no dowels story to me.

 

 

[Ross]

More follow up from the owner of the LSA55.

 

His new parts from Jabiru arrived on Monday and he has returned the borrowed parts to their owner.

 

I quote below from his email.

 

I cut the old Jabiru oil filter open yesterday. Quite a lot of what looks like aluminium swarf. When the rivets broke, they rubbed between the rotor spindle and the aluminium distributor housing, so that's where that came from. It all seems to have stayed on the right side of the filter. Checked compression, all still on 175 /173 PSI (I don't have a leak down), valve settings all OK, and oil pressure hasn't changed.

 

 

[Guest skipd]

Make it part of the run up procedure at idle? Quick on/off/on....could catch a problem before leaving homeplate.