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Which aircraft type is least likely to catch fire?

Peter, I’m sure that the exhaust headers do reach more than 450 C, but I guess only while the engine is running. Thin tube should cool quite quickly. I’ll maybe check with an IR gun next time the cowlings are off…

What lights a fuel fire must depend on the fuel. AVGAS, designed to resist pre/auto/compression ignition, probably needs a flame or spark. When teenagers shoot a milk carton full of gasoline with a varmint rifle (bullet travelling at about Mach 3), the gas just sprays everywhere like water. No ignition. For ignition the aforesaid kids need to put a tin of black powder behind the milk carton. Even tracer doesn’t work reliably on its own. (P.S. please don’t try this unless you live in a free country and even then, not in a suburban garden).

But I’m not arguing that AVGAS is significantly “safer” than AVTUR for GA, merely that neither theory nor fatal aviation accident statistics support the (“common sense”) contrary assertion.

Last Edited by Jacko at 04 Nov 20:11
Glenswinton, SW Scotland, United Kingdom

What triggers the fire are several factors.

There has to be a rupture in the fuel tank or system to let out fuel.
There has to be an ignition source such as sparks coming from metal rubbing on concrete or from severed electrical wiring or also contact with a hot exhaust.

Whenever a fuel tank is ruptured, it may take a short while for the fuel to leak and then vaporize into a flammable air fuel mixture. That is why it is very important in a situation where the integrity of an airframe is not assured to evacuate as fast as you can. Quite a few people survived crash landings and did not exit immediately with horriffic consequences. This goes for cars as well btw. I know at least one guy who got severe burns after he exited unhurt but went back to recover his luggage. the gas tank had leaked and opening the trunk, the fuel – air mixture became volatile and they reckon a broken light bulb set off the fire which consumed the whole car.

LSZH(work) LSZF (GA base), Switzerland

This is really interesting

the auto-ignition temperature of jet-A1 is about 230 C whereas AVGAS 91/96UL does not auto-ignite until 450 C

although I don’t agree with

well above the temperature of most of the engine and exhaust system.

because the exhaust can glow dull red hot which is well above 450C

The crankcase itself never reaches even +100C though.

So what triggers the fire in an accident?

If you put some fuel in a container (like a wing tank) and smash it against a rock at 150kt, would it ignite and, if so, how? I think it would not, so the ignition perhaps comes from interrupted/shorted live wiring.

Administrator
Shoreham EGKA, United Kingdom

I can’t currently find the article I recall – perhaps it was in one of my paper magazines. Packard claimed that one of the chief advantages of the engine was ‘the elimination of the danger of fire’ – which is obviously what they would say.

http://ox5.org/wp-content/uploads/Packard_brochureRotated.pdf local copy

You might expect it to be easier to quantify the risks for land vehicles, for which there is much more data. Those of us flying vehicles from 1963-1968 might find this report interesting:

http://www.iafss.org/publications/frn/836/-1/view/frn_836.pdf local copy

Page 10 discusses the ‘materials ignited first’ – 1963
For cars, it’s petrol in 1044 cases and diesel in none.
For lorries it’s petrol in 88 cases and diesel in 100 cases.
For vans it’s petrol in 412 cases and diesel in 4.

What’s missing, of course, is the denominator. As far as I can gather only a few percent of cars would have been diesel powered in the 1960s, but even so you might expect a larger number of diesel fires. There are a few (4 diesel versus 2300 gasoline) in 1968.

http://www.iafss.org/publications/frn/836/-1/view/frn_836.pdf

consistently rates Diesel fuel as being lower risk than petrol.

I can’t find any truly convincing analysis of whether petrol or diesel cars are less likely to catch fire though, which is surprising.

Last Edited by kwlf at 04 Nov 00:41

That would be an interesting article, though I wonder whether CI aero engines were or ever will be common enough for their fatal accidents to yield data rather than anecdote.

For instance, my first look at the US database was restricted to Robinson helicopters, for which I felt sure I could mine a rich vein of fatal accidents. But happily, while true of the R22 and R44, there are too few R66 (turbine) fatalities to be statistically significant.

As to what ignites fuel post-crash, whether at Leicester or Battersea, that might logically depend on the fuel. AVGAS is designed for spark ignition and to resist auto-ignition, while AVTUR is designed for reliable auto-ignition at relatively low temperature.

Incidentally, another safety consideration might be the amount of fuel which we carry. No point in filling the tanks for a half-hour burger run or just to tow a few gliders, but I know I usually carry too much.

Glenswinton, SW Scotland, United Kingdom

Jacko wrote:

Interestingly, the incidence of fire in fatal turbine and recip helicopter crashes is about the same, so perhaps ignition has more to do with impact speed than fuel type.

I once read an article about early Diesel aircraft engines in the 1930s (possibly Packard) which said that post crash fires were much less likely than with gasoline powered aircraft. In this situation, we would be comparing relatively similar AVGAS piston aircraft to relatively similar AVTUR piston aircraft, rather than slow AVGAS piston aircraft with faster AVTUR turbine aircraft. I’ll see if I can find the article again.

I would expect turbine engines to be more likely to go on fire, when showered with fuel of any type in a crash, than reciprocating engines. But then, what is it that sets off fires in crashes anyway? Engine heat? Exhaust pipes? Electrical sparks? Steel latches? I’ve often wondered?

Have you ever watched someone start a bonfire with diesel? I confess I’ve never watched anyone start a fire with petrol, but I’ve seen the aftermath many times.

@kwlf wrote:

Heavy fuel is significantly less likely to catch fire than avgas.

And @alioth wrote:

Light aircraft burning Jet-A – fuel that’s more difficult to ignite…

Which statements seem contrary to logic and such statistical evidence as can be gathered by a casual trawl of the NTSB database.

We all know, if we’ve ever bothered to read the MSDS for the squished dinosaur juice which we squirt into our aircraft fuel tanks, that the auto-ignition temperature of jet-A1 is about 230 C whereas AVGAS 91/96UL does not auto-ignite until 450 C – well above the temperature of most of the engine and exhaust system.

Ha, you say, poor old Jacko disnae ken that gasoline has a much higher vapour pressure and much lower flash point than kero. Well, yes, but as we know from Mike Busch’s webinars, our fuel vapour won’t ignite at all unless the fuel/air ratio is within a few percent, and even then it won’t burn worth a fart until we put a spark to it.

A few keyword searches of the NTSB database show that fire is roughly 1.5 times more likely in a fatal turbine-engined airplane accident as in a fatal recip accident. Interestingly, the incidence of fire in fatal turbine and recip helicopter crashes is about the same, so perhaps ignition has more to do with impact speed than fuel type.

Nor can I find much statistical evidence to support my personal prejudice against stinking, hose-rotting MOGAS or gasohol, with homebuilt recips not significantly more likely to catch fire than their certified counterparts.

Overall, with fire occurring in 30 to 60 percent of fatal airplane and helicopter accidents, what’s worth doing – short of switching fuel? I think kwlf is on the right track: fire retardant (or at least, natural fibre) clothing, and perhaps headgear of some kind.

Last Edited by Jacko at 03 Nov 18:26
Glenswinton, SW Scotland, United Kingdom

I wear a Nomex flight suit and gloves, but when I looked into causes of potentially preventable deaths fire was surprisingly low on the list – head injuries were more significant. I continue to wear fire resistant gear and would be more than happy to fly an aircraft less likely to go up in flames. However it is no longer as high on my list of concerns as once it was.

Peter wrote:

Is there any difference in fire risk between metal wings with bladders, and metal wings with no bladders

Probably? The Piper Pawnee used to have a glass fiber tank just aft of the engine. This caused lots of fire when it ruptured, burning pilots alive. They modified it with a bladder, which helped. They later changed the whole configuration to wing tanks close to the wing tips. Seems like it is more important to get the fuel away from the hot engine.

The elephant is the circulation
ENVA ENOP ENMO, Norway

I guess Diamond indeed does a good job of providing passive safety in its designs and active safety as well; docile handling that prevents getting into problems in the first place. A Cirrus has wet wings that make the ignition risk higher on hard impact, but isn’t the whole point of the parachute to prevent such impacts? Don’t know the statistics on burned Cirri but they may actually be quite low.

Last Edited by aart at 10 Aug 18:08
Private field, Mallorca, Spain
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