Yesterday I landed at the Mckenzie River to take a run on the McKenzie River Trail. We finished our run and got back to the plane. I shut the engine down with the magnetos, which I sometimes do if I am going to fly again in the next few hours because it starts so readily when I do this, saving wear on the starter.

When I started it, there was a loud pop and a healthy puff of smoke from under the cowling. I thought that there was going to be a fire, but it ended right away. I tried to start it again, and it would only run on prime, then stop running right away.

We pulled the cowling to find the the forward crossover induction tube had blown off and bent forward in the explosion. Apparently the fuel vapor was ripe for ignition within the induction tubes.



The induction air circuit was under a tremendous amount of pressure for an instant. Has anyone ever seen or heard of this? Could the ignition source have been through a partially closed intake valve? Is there any other possible damage that anyone can think of that should be considered due to these conditions?

After refitting the piece, it ran perfectly as usual.

When you shut down with mixture you void any fuel vapors from the induction system. When you shutdown by turning off the mags fuel air mix is still in the intakes, you start up and it backfires in the induction side with that mix. check airbox and airfilter.

Glidergeek wrote:When you shut down with mixture you void any fuel vapors from the induction system. When you shutdown by turning off the mags fuel air mix is still in the intakes, you start up and it backfires in the induction side with that mix. check airbox and airfilter.

How can this vapor be ignited though? The intake valves of the cylinders igniting should be completely closed, right?

Not sure of your exact situation but we've all had, heard or seen a backfire at some time. Did it kick back when this happened? If so it might have rolled back far enough to open an intake valve enough to send ignited mix back into the induction.

Glidergeek wrote:Not sure of your exact situation but we've all had, heard or seen a backfire at some time. Did it kick back when this happened? If so it might have rolled back far enough to open an intake valve enough to send ignited mix back into the induction.

Wasn't watching for anything unusual, as it behaves so predictably, but it must have rolled back.

Scolopax wrote:I shut the engine down with the magnetos, which I sometimes do if I am going to fly again in the next few hours because it starts so readily when I do this, saving wear on the starter.

Sure glad no major damage was done!

I think you've already figured out shutting down with mags isn't the best for the engine. Backfiring an engine like a 450hp Pratt & Whitney can destroy the supercharger, which when ingested can destroy the engine. Best not to develop bad habits.

I'm glad you posted this cause once or twice I've shut off my 185 with the mag switch - and I should know better since I also fly behind a 450.

Thanks

That's a fairly standard result of a backfire in a 470/520, I would always suggest checking the induction following a kick back or obvious backfire... And maybe not shutting down with the mags! But at least your doing a dead cut check! Good you sorted it so quick, I've seen supposedly talented engineers struggle to find a problem like that...

My 182 did that this spring (O-470R), but I didn't shut down with the mags. I had landed at the Tatalina long range radar site, shut down and after loading up my passenger, fired up within 2 minutes. It cranked a second, backfired, and wouldn't start. With a few more primes it tried to start, but wouldn't. I popped the cowling and there it was, exactly like yours, left side balance tube. I was able to get it back together and flew it to McGrath. We replaced it with a new one we had on the shelf and never had another problem.

FWIW: I shut down my 90 hp Cub with the mags anytime I am planning a quick restart. Never had any problems-maybe just lucky? It has an Armstrong starter, and will start on the first blade in this situation.

The only time I have had backfire problems is below freezing when engine is cold with no preheat. I do avoid this at home base by preheat, but out n about it usually isn't possible (I don't carry the gear to preheat, and I am usually where there are no services-usually the temps for this are just under freezing-not Alaska temps)

Comments?
(Other than I should carry preheat gear-2 up plus gas I'm gross)

lc