So you would need the mother of all leaks in the cabin to produce a mixture in the cabin generally which might ignite.
That may take a bit of work to achieve, given that there is airflow through the cabin normally, plus any gases in the cabin (which are not outside) will act IAW the law of partial pressures i.e. will try to escape preferentially.
Yes but if there is a liquid leak, at some micro-distance from the liquid surface the correct air/fuel ratio will exist…just looking for an ignition source… So the main priority is to ensure no liquid leaks…
Peter wrote:
ne has to be very careful with safety – a “live and live” approach will get you eventually but the point I am making is that you need a substantial spill and an ignition source (which could be static i.e. from clothes!) to make it go off. That is why aircraft are grounded when refuelling.
The odd thing is that cars need not be grounded while MOGAS is more volatile than AVGAS. I’ve never heard of explosions in petrol stations either. Is there some other reason an aircraft needs to be grounded while a car does not?
dublinpilot wrote:
I wonder how those who use mogas, and need to arrange lifts to local petrol stations why flying elsewhere, manage? Do they try to borrow jerry cans locally? Empty cans wasn’t something that I’d given any thought to, figuring full ones would be the problem!
< why flying elsewhere? > Because flying the local circuit ends up in boredom, sooner or later? 
< borrow jerrycans locally > yes indeed, has worked very well for me on the rare occasions I had a need, and usually one can borrow a car at the same time, sometimes even with a drover. Especially if previously arranged it wirks like a chorm, in mist cases. The ultralight world is full of supportive sympathetic souls. However, against the ultimate day when everything goes wrong I carry a couple of inflatable 10L containers, they won’t survive with petrol permanently stored but may one day help me out of extreme misery. They are stored with the ground screws and their ropes that are equally unused, as yet.
< figuring full ones would be the problem > neither totally full nor totally empty are very dangerous – as has been duly explained. However a big quantity of fuel in whatever degree of vapourisation adds to the danger of any crash, if the recipient, either structural or carried, breaks.
Is there some other reason an aircraft needs to be grounded while a car does not?
The only one I can think of is a plane is more likely to build up static charge?
Yes – very much so.
Tyres are probably very slightly conductive, and also static dissipates via corona discharge (until the voltage drops off to a few tens of kV) so the charge is an issue only if you refuel right after landing, which is exactly what happens most of the time.
The only time I have ever seen a spark fly from the grounding point to the ground wire tongs was on a very dry, cold, sunny winter day after a long flight.
In my day job, I spent a number of years involved in fuel tank protection for aircraft – specifically explosive suppressant foam modifications and on board inerting gas systems. The science of flame front propagation was pretty staggering (and frightening) when listening to some of the boffins presentations.
As a fire investigator (and a floatplane pilot), I deal with some of these issues frequently. First, modern jerrycans are fairly safe. However, there are a few things to consider:
1) If there is a measurable amount of fuel in the can (and the cap is closed) the headspace will be fuel rich (above the upper flammability or explosive limit) and the container will not explode.
2) New designs (and high quality old designs) have flame/spark arrestors at the cap to prevent a flame front burning back into the can as it is emptied (and fresh air drawn in to dilute the fuel rich air mixture). The following video of jerrycans without spark arrestors makes it appear that a flashback to the can is more likely than I suspect it actually is in real life. I note that they appear to be pouring the last little bit of fuel out when the cans go ‘bang’ (see 1:08 in the video). It is likely they have done this many times in the lab to get it just right (for litigation purposes).
3) As has been mentioned in previous posts mogas has a terrible odour. A very small spill in an enclosed space will make it uninhabitable (to me, but perhaps I am sensitive ). This is well below the explosive limit. I would not want to spill any mogas in a nice aircraft. Although one poster mentioned ‘a perfect’ fuel-air mixture at some distance above a small spill, it is unlikely for this to be ignited. If it is ignited, there will be a soft ‘pop’, and then the fire will go out (similar to an open/closed cup flashpoint test apparatus). If the conditions are just right, the vapour coming off the spill might continue to burn, but if the cap is on the can or there is a flash arrestor, there will be no explosion of the container itself.
Because of the odour and oily residue from mogas, I prefer to use avgas in jerrycans for remote refueling. I find that it evaporates with much less residue on your hands, clothes, etc..
I believe the biggest risk from jerrycan use is a large spill at either end, for example when filling the cans, when pouring them into the aircraft, or when there is a fuel leak on the ground. As mentioned previously these can be ignited by static discharge from clothing, a poor electrical connection, an arc off a switch, etc. I have investigated quite a number of incidents where automotive technicians have been seriously injured when repairing a fuel leak from a car. I discussed this with my aircraft mechanic, and he admitted that several times he has been apprehensive when there was a large pool of fuel that had been spilled from a leaking wing tank. The video below shows the combustible vapour cloud in slow motion at (1:10). As a result, I am extremely careful when an open container of fuel is being handled! Also, as has been stated in a previous post, I don’t fill the cans to the brim. This gives you a bit of room to work with and helps prevent spilling.
Of course, sometimes people are not aware of the vapour cloud (as in the video above) and get caught in the results of the ensuing vapour cloud ignition. If the ignition is inside a structure which is strong enough for the pressure to build via the expanding hot gas, then an explosion will occur when the structure fails and the pressure is released. Here is another one.
Airborne_Again wrote:
The odd thing is that cars need not be grounded while MOGAS is more volatile than AVGAS. I’ve never heard of explosions in petrol stations either.
On a frosty winter night, after flying, I put two jerrycans of 4* into our wood and fabric Jodel, earthing the funnel to the metal can. No problems.
After shutting the hangar, as I sat in my car, in the dark before starting, I unzipped my leather jacket, with a nylon lining, which was worn over a wool sweater.
A display of electrical flashes lit up.
