Friday, August 24, 2012

Fragile

As I shut down the airplane I turn off the avionics, bring the power back to idle and then on each engine momentarily turn both magnetos off, then back on again as soon as I have verified that the engine would die without them. That means that when the off position is selected for the magnetos, turning a propeller by hand just to move it out of the way cannot produce a spark and start the engine. Then I return the power to 1000 rpm and shut down the engine by pulling the mixture levers all the way back, cutting off the fuel supply to the engine.

Considering the number of things I have to do right to start the engine when I want it to start, it seems a little ridiculous imagining the slight movement of a propeller during ground handling, with the fuel cut off, causing the engine to start. I wonder if this is a safety precaution that made sense long ago and is no longer relevant, but that we've never stopped following. Has anyone ever heard of a ground-handling propeller strike accident that occurred because of a live magneto, when the mixtures were in idle cut-off and the mags selected off? I have met a few American pilots and mechanics who have never heard of the check, so perhaps it lingers longer in the British Empire.

On this particular occasion I not only put the airplane in the hangar, but then climbed back on board and stowed all the oxygen masks and cables, took everything out of the seat back pockets, removed snacks and garbage and chart collections and generally cleared the way for maintenance to take out the seats and rip out the floor panels for a major inspection. That also means a few days off for me. The exact number will be determined by what they find on the inspection. I go home and put my phone on the charger. For a few days I am set free from the requirement to have it always with me.

The next day I happen to be sitting near the charger and it rings, with my boss' name on the display. I answer and recognize the boss' bad news voice. I wonder what horrible thing they have found with the airplane, and if there is some way I have caused it through poor airmanship, or missed finding it on my preflight inspections. But, it's nothing to do with the airplane. One of my coworkers has been critically injured in an accident. Nothing to do with work, just out with friends. It puts everything else into context. Life is so fragile.

12 comments:

Frank Van Haste said...

Dear Trix:

I'm sorry to learn that your colleague has been injured. I hope for the best.

I've always done the same P-lead check on the mags as you do. Taught to me by my first instructor, and I firmly believe it to be an important (and low cost!) safeguard. Although, I do find that I have to watch line persons carefully as they can be fooled by the apparently imminent shutdown.

Best regards,

Frank

Anoynmous said...

Fragile. Yeah. I've seen someone take his final breath, and "pulling the mixture levers all the way back" seems like an interesting analogy.

Angus said...

Sorry to hear about your coworker's accident, life is fragile indeed. I knew someone who was killed by a prop via a live magneto, so I'm inclined to say keep checking. The story is long and bizarre, and requires lunch for the telling.

Sarah said...

Sorry to hear about your coworker. Life is fragile and everything can change in the blink of an eye...

Which is why my first instructor insisted I do the mag check just as you describe. Not everyone does - I remember my first checkride, when my examiner flinched visibly thinking I was shutting down with the mag switch!

I've never personally found a broken P-lead, but have heard of those who have. It seems cheap insurance.

Aviatrix said...

I've had a broken P-lead at least three times (including the instance when I cut it myself in order to start the engine), so that part definitely happens.

Next time you're in town, Angus, lunch and tales.

D.B. said...

I do that once in a while, and I have seen it discussed on line on in ground training. But you're right, it isn't commonly taught in the US.

Personally, while it's a good thing to know if the P-lead is broken, I don't think it enhances safety by any appreciable margin (except perhaps on very small, starter-less engines like an an old Cub or Moth). If the mixture is fully off, the engine has no fuel and won't fire. But those old engines would be primed and pulled through by hand with the mags off, and *could* fire when not expected if a magnetos was hot.

Tina Marie said...

Happened at the airport I used to be based at:

NTSB

Anonymous said...

Sorry to hear about your co-worker, and thanks for noting that it wasn't aviation-related. Sad nonetheless.

I too was taught the p-lead check by my instructor back in the late '90s. Seems to make sense given the lethality of the prop.

Regards,
Marty

Chad said...

A live mag doesn't necessarily create a hazard for a full on engine start, but more likely it is possible for residual fuel fumes to create the conditions where its possible for the engine to "kick" through a combustion cycle if the engine stops at just the right spot in the stroke and the propeller is accidentally turned.

Also I imagine this is most likely to happen on smaller engines (O-200, O-320s, etc) that are equipped with impulse couplings (a mechanical device that allows the spark plugs to generate sufficient spark at low RPM to assist in starting the engine).

Echojuliet said...

My flight school was at the very top of straight-laced safety, and I think they would have a heart attack at the thought of shutting an engine down with the mags instead of fuel. (even if it did come right back on. Probably would get grounded for pulling a stunt like that)

From the maintenance side of things, I think its better to find a broken P lead on a pre-inspection run than a post inspection run (though its not a terribly difficult fix, its just one more thing to do when you think you are done). Regardless, I check it any time I run an engine.

Grant said...

It's always a good idea to keep checking the whys and wherefors of SOPs. Having installed an auto conversion engine in my aircraft, I had to adapt procedures to this new environment - i.e. Electric fuel pumps, electronic ignition... It also forces one to think through emergency procedures... I.e. will shutting off the Master sw. also shut down the engine? Oops...

Because the engine shuts OFF with ignition switches, and it starts so easily with the electronic ignition, I do pull the ignition breakers when parking the plane.

(so far I have failed to prove I am not a robot two times.. Sigh! Shall I go for three? Is it just iPad's small screen that makes this so hard? I guess I won't bother commenting any more... Too haaaaard)

Colin Southern said...

I can't say that I ever uncovered the reason for this - but - one day I did discover that when shutting down both engines on a GA7 by moving the mixture controls to idle/cut-off, if the engines were immediately cranked again - with the mixture still at ICO - the engines would fire - spin through a few hundred revolutions - and then die again. It certainly make me a lot more aware of mag checks.