Poor Mixture Control = Fire? I Think So

[BackPacker]

I think I made the chain of events and the causal factors pretty clear.

Nope. That's exactly what I dispute. Suppose they did not overprime when starting the engine for the second time. Would a fire have broken out then? I don't think so. So the plug fouling and everything else that did or did not happen before the second start did NOT contribute to the fire.

The only thing that the plug fouling did was cause the flight to be aborted, and that eventually caused a necessity for a second start. But it was the overpriming that caused the fire. And the fire would have happened too if the plugs were not fouled up, but there would have been another reason for the restart where the same sloppy priming technique would have been used.

I would guess that you aren't an advocate of flying lean of peak.

You show me an O-320 installation in a PA28 that can reliably and consistently be leaned to LOP, and you've made me a convert. But until then I'll just lean until rough running sets in, and then enrich a tad more to restore smooth running. Heck, most O-320 installations I've seen don't even have an EGT gauge.

Can't understand what this has to do with the present discussion though. Unless it's intended as a low blow with which to end the discussion. In that case I'd say: Bring in the Nazis. Godwin's law - Wikipedia, the free encyclopedia

[Victorian]

So the plug fouling and everything else that did or did not happen before the second start did NOT contribute to the fire.

With respect to Backpacker, I don't really agree with that. I think that there are lots of aircraft that are difficult to start, or re-start, or some other finnicky behaviour because of lazy maintenance.

These behaviours lead to equally innovative starting proceedures, especially (in the Warrior) throttle pumping, the results of which I've described on here before. Essentially, if you pump the throttle more than about 3 times with the engine stopped, you will have neat fuel runniing down the inside of the cowling. No amount of 'continue cranking' etc will put that out if it ignites, at least until the engine starts.

So there's the point - why does it ignite? Unfortunately, the badly maintained engine that won't start in the first place is the same one that's likely to backfire or have fuel burning in the exhaust. So if your engine is reluctant to start, get it fixed before someone - quite possibly an instructor - devises a 'sure fire' personal technique with catastrophic results.

[A and C]

Victorian

Just to put a little more accuracy to this, if you move the throttle from idle to full power once with the engine not turning you will get fuel running down into the air box below the carb, how much of that fuel gets into the bottom of the cowling depends on the temperature in the carb and cowl at the time.

[Desert185]

BackPacker

The only thing that the plug fouling did was cause the flight to be aborted, and that eventually caused a necessity for a second start. But it was the overpriming that caused the fire. And the fire would have happened too if the plugs were not fouled up, but there would have been another reason for the restart where the same sloppy priming technique would have been used.

Agreed. I never said plug fouling caused the fire. Error one was the taxi out with mixture rich, and subsequent fouling causing the excessive mag drop. If they hadn't done a restart after receiving some counseling on the mag drop, they would not have made a bad hot start with the resulting fire. I hope this is clear enough.

Quote:
I would guess that you aren't an advocate of flying lean of peak.
You show me an O-320 installation in a PA28 that can reliably and consistently be leaned to LOP, and you've made me a convert. But until then I'll just lean until rough running sets in, and then enrich a tad more to restore smooth running. Heck, most O-320 installations I've seen don't even have an EGT gauge.

Can't understand what this has to do with the present discussion though. Unless it's intended as a low blow with which to end the discussion. In that case I'd say: Bring in the Nazis. Godwin's law - Wikipedia, the free encyclopedia

The reference to lean of peak was because you apparently strictly adhere to the recommendations of the POH. POH's generally don't discuss running LOP, even though the practice is entirely accepted these days on fuel injected and the occasional carbureted engines. That was the reference intended.

I generated this thread in the spirit of educating those who might not understand the concept of mixture control, and the consequences thereof. If arguments are your preference, fine. They aren't mine, especially when referencing Nazis, which has nothing to do with the present discussion.

[BackPacker]

I never said plug fouling caused the fire.

You said poor mixture control caused the fire. In the title of the thread even.

If they hadn't done a restart after receiving some counseling on the mag drop, they would not have made a bad hot start with the resulting fire.

If they hadn't gotten out of their beds that morning, the fire also would not have happened. That's not the same as saying that their getting out of their beds that morning caused the fire.

I'm here on this forum to learn from the mistakes of others, because I don't have time (or enough lives) to make them all myself. That's why I'm interested in what really caused the fire. Which is still, in my opinion, the sloppy starting technique - overpriming - on the second (hot) start.

What caused the hot start to be needed is largely irrelevant to the cause of the fire. It could also have been a passenger forgetting to turn his cellphone off and putting it in a wing locker, or a forgotten pitot cover.

Having said that, I do agree to Victorians argument that people may start to develop "innovative" starting techniques on engines that are hard to start. And engines with fouled plugs may be harder to start than normal. Still, that's no excuse for overpriming - something that the POH warns against. If anything, a hot engine requires less priming than normal.

The reference to lean of peak was because you apparently strictly adhere to the recommendations of the POH.

I don't, but I do expect instructors to teach their students to fly the aircraft according to the POH in the first place. Only once they've fully mastered that, should I expect them to teach stuff beyond what's in the POH. So I don't automatically condemn an instructor for not leaning during taxi, if that's not something that's written in the POH.

[Desert185]

BackPacker

Quote:
I never said plug fouling caused the fire. You said poor mixture control caused the fire. In the title of the thread even.

Fuel makes fire...and fuel at an aircraft carburetor is controlled by...?

[mad_jock]

Live by the POH; die by the POH.

That must be one of the most stupidest statements on here in a long time.

If there was anything in the POH that is a risk to life the authorities would never allow it to be in there.

If there was such a procedure which could risk life. The POH would have been changed or a design MOD done to remove the risk.

Do you think they make these documents up for fun?

[Desert185]

Some of you folks are taking this too literally. The POH doesn't cover every eventuality, so if you strictly adhere to the recommendations as if they were limitations, you're doing yourself and your aircraft a disservice. There is more to operating an airplane safely and efficiently than adherance to the POH.

[tmmorris]

I agree the POH isn't the be all and end all. In the PA28 (owned by A and C) that I fly most often, I tried recently the POH engine start procedures instead of the ones I was taught as a student pilot.

Lo and behold... the engine doesn't start...

Back to what I was taught... fires every time.

[cockney steve]

PURELY CONJECTURE.....
The POH was formulated on the basis of a new aeroplane
It would NOT have taken account of today's pilots, who are not mechanically inclined or empathetic.
It would not have considered a carburettor ~40 years old, with worn float-needle and seat, worn throttle-butterfly spindle and Carb body, worn linkages..Maybe a bit of backlash in the Mag-drives, Plug-leads in less than A! condition....
Then there's the afore-mentioned Pilot Empathy.........Back in the day, they would have been enthusiasts , generally,- It was a rich man's hobby and the cost of a mechanic to keep everything spot-on was , overall, fairly insignificant.

The Aviator would know and really understand the operating -principles and demands of the petrol-engine....remember, Pre-war cars commonly had "mixture" "advance-retard" and "hand-throttle" controls for the driver to manipulate and keep his engine running efficiently.

Anyone who has read the Trevor Thoms series should be able to pass the exams....It's learning by rote! UNDERSTANDING is a different animal.
It's the understanding that enables someone to depart from the "one size fits all" POH GUIDELINES and operate their engine in an efficient and safe manner.

the "push a button" brigade will continue to have problems....An IC engine in reasonable condition (A defacto requirement of a C of A, i would think) will always start if it's given the right fuel-air mixture and the right ignition at the right time.......I'll admit that fixed mag-timing is set at optimum for cruise and therefore is not the ideal timing for starting -but that's what an impulse -coupling is for!

Know and understand your machinery apply logic to the symptoms and you will arrive at the correct solution

TMMorris is right!

[riverrock83]

Originally Posted by Desert185

Fuel makes fire...and fuel at an aircraft carburetor is controlled by...?

Both mixture control, throttle AND PRIMER .
Engine fire was caused (probably) by TOO MUCH PRIMING. Mixture could have been at ICO and it could still have caught fire.
Nothing to do with the mixture control.

You said in your original post that there was repeated cranking during the attempted hot start. You were over 100m away. Could the repeated cranking not be part of the engine fire during startup procedure? The POH says that if the fire doesn't stop while continuing to crank after a few seconds, the aircraft should be abandoned. By the time you had got your extinguisher and travelled more than 100m it will have taken at least 15 seconds, probably much more. Unless the FI says they didn't continue to crank (and they may have been in shock straight afterwards) then how can you know whether they did or not?

They were following the authorised procedures until probably over cranking.

Clearing mags is not part of most training (I only know about it because it happened during my training). Its not in any text book that I've seen. I think you are doing the FI a disservice.

I can't comment on their hot start procedure as there are lots of engine variants around, but this one http://www.ddac.com.au/documents/aircraft/PA28.pdf suggests not priming at all unless it doesn't start first time, the only difference between hot and cold start is a higher throttle setting for a hot start. If priming was the cause (and their aircraft is similar to this one), then they weren't following the POH...

[Desert185]

Riverock83

Clearing mags is not part of most training (I only know about it because it happened during my training). Its not in any text book that I've seen. I think you are doing the FI a disservice.

I learned that technique in 1967. Mediocre POH...good flight instructor. The FI screws the pooch multiple times...and I am doing him a disservice!? Seriously?

They didn't know about the fire until I was within about 50 feet of the airplane when the smoke started billowing. There was no attempt by them to continue cranking, even after seeing smoke emanating from the cowling. They had no idea there was a fire until that point.

We watched the entire series of events unfold while sitting in front of the hangar adjacent to the grass runway. It was a goat rope from the first start and runup.

[phiggsbroadband]

The problem was that priming was used. Even an engine that has only run for 40 seconds will have its combustion spaces sufficiently hot enough for no
priming to be necessary.

A taxi to the runway and back, maybe of over one mile, would have got the cylinders extremely hot... Maybe the crankcase and crankshaft and oil would not be up to cruising temperature, but the bits that matter would be very hot indeed (i.e. don't touch!)

I once saw an injected c172 spend about 5 minutes after re-fueling, trying to start a hot engine, there was a good cup full of fuel on the ground under his engine, which fortunately did not catch fire when it eventually started.

[JamieE]

When I learnt to fly we had some old knackered PA28's that were allowed to go 100 hours between checks by the authority as a concession.

If they had too much fuel in the system, we used to start them by fully opening the throttle, fully closing the mixture and then swapping the levers round once it fired.

This work every time as long as you managed the swap fast enough, and if not, a normal start worked after this procedure.

I have no idea if that's what we were supposed to be doing, but most of the instructors were ex-military test pilots!!

[cockney steve]

[QUOTE]If they had too much fuel in the system, we used to start them by fully opening the throttle, fully closing the mixture and then swapping the levers round once it fired.[QUOTE]

Yes, that's exactly the solution.....Now, If you had studied your "Trevor Thoms" assiduously, you'd know why it is correct and it works!


Too much fuel - air-fuel mixture won't ignite ....close mixture, shuts off fuel supply......cranking the engine dilutes the ratio of fuel to air (as you have given it an unrestricted supply of air to "mop up" te excess fuel Until you reach a combustible ratio,at which point the engine starts firing and rapidly consumes the residual fuel.....swap levers quickly and the balance is maintained....common sense and basic physics , really -It's got sod-all to do with "difficult...bad starter....temperamental...etc.

[172_driver]

Live by the POH; die by the POH.

That must be one of the most stupidest statements on here in a long time.

Not sure if it's so stupid..

A friend of mine had an engine failure at 8000 ft on a hot & dry day in SoCal. The POH drill said Mixture - Rich! But it wasn't until he leaned the mixture that it finally ignited for him again.

[mad_jock]

Which would have happened anyway when he had lost some height.

And you don`t just have an engine failure that then restarts. Usually you have to do something stupid outside the POH for it to stop

[172_driver]

Or a badly adjusted Fuel Control Unit that delivered too much fuel for the ambient conditions. But that's not the point…

It's not always black & white … someone who had not understood fuel-air ratio and flammable limits would perhaps not have lived to tell the tale. Lower altitude might have solved it.. yes.. maybe..

[The Flying Pram]

This is neither the first, nor last instructor, to not have a clue about the workings of an engine

That strikes me as more than a little worrying. When I learned to fly a microlight in 1990 the syllabus included basic engine (both 2 & 4 stroke) theory. I was, perhaps, fortunate that I already had years of practical experience with motorbikes and cars. But nonetheless I still ended up having a disagreement with my instructor (Hi, Mike) over fuel/oil ratios - it was 2 stroke pre-mix with the Rotax. The question was what is preferable: too little oil or too much? I said too much, as I had practical experience of what the other lead to. However he pointed out what I hadn't appreciated - more oil in the same volume of fuel mixture means less petrol, and consequently a lean running engine. I was initially not convinced, but then realised that it's very rare to have a car or bike operating at full power for more than a few seconds, unlike an aircraft taking off and climbing to cruising altitude, so the situation is rather different.

Back then full engine management systems were not universal, and most drivers had a reasonable idea what went on under the bonnet. Nowadays there is virtually nothing you can do with a new car, so even those who do have an understanding are helpless. But since the majority of light aircraft still rely on stone age technology, a good understanding of ignition and fuel delivery SHOULD to be taught to all students.

As to listening and learning from "old hands" - I've flown quite a bit with a local pilot in his C150 and remember one evening when he did the run up checks and found a significant rev drop on one mag. He spent some time trying to clear it with mixture leaning, to no avail, and we taxied back to the hangar and discovered a plug with a nice globule of lead across the electrodes. I am also aware of the "cut the fuel/mixture and continue cranking" advice if a fire breaks out whilst starting, and the "open throttle and close mixture" method of dealing with over priming is also familiar to anyone of my age. This sort of knowledge should be imparted more widely than seems to be the case in the OP's incident, even if it means upsetting a younger FI.

[Big Pistons Forever]

I would suggest that any engine that fouls the plugs under the stated conditions has the idle mixture set too rich.

An easy way to check is to note the momentary rise in RPM as the mixture is pulled to idle cut off. There should be a rise of 25 to 50 RPM. If the rise is excessive the idle mixture is too rich and if there is no rise the mixture is too lean.

A friend was recently complaining about his plugs fouling on the taxi. I found a rise of over 100 RPM so we had the mixture adjusted and his plug fouling problem went away.

BTW the proper way to do a "burn out", that is clear plug fouling, is to go to runup RPM and lean until the engine is so lean it is running rough. Hold that for about 15 second then return to full rich and check the mags. If that does not clear the plugs you have other problems.

Avoid excessive running with the mixture grossly leaned. If 15 seconds of lean running won't clear the plugs, more time won't help.