The airline we take from Luanda to the USA ( TAAG ) uses a somewhat aging Boeing 747 that has been
modified to carry enough fuel for the 13 or so hour flight. A few weeks ago, we left on one of these trips.
Shortly after takeoff the pilot announced we needed to return to the airport for some unspecified problem.
Looking out the window I noticed a white mist which I assumed was the pilot dumping
the jet fuel.
We flew around for perhaps 45min before pilots said we were landing.
What concerned me was the chance slim that the fuel in a vapor form might ignite. What say the experts ?
As noted, jet fuel is basically the same as diesel fuel, home heating oil or kerosene. None of them ignite that easily and they won’t ignite at all without the proper fuel/air mixture. Airliners fly really quickly. That means that a trail of dumped fuel is atomized and left far behind before it can ignite even if there was a source of ignition. It is impossible for it to turn into a fireball that can follow in either direction quickly enough.
Try lighting a bucket of diesel fuel with a lighter while blowing a hair dryer across it. All you will get is a blown out lighter. That is very different than a post crash fire. Concentrated jet fuel will burn very hot but a little slowly if it is well distributed over a small area and then ignited.
This isn’t really my area of expertise, but I know it’s possible to catch dumped fuel on fire because you can do it intentionally in a fighter jet with an afterburner. It’s called a “zippo”, named of course after the cigarette lighter.
Here’s a video of an F111 doing it:
There’s no real danger to the plane since the fuel won’t burn unless it is mixed with air (oxygen), and the fuel coming out of the plane is all fuel and no air. So it won’t start burning until it has had a chance to spread out through the air a bit. From what I recall, the rate of flame moving through fuel is also slower than the airspeed of a typical plane. The fighter in the video is constantly lighting the fuel with the afterburner. Without that constant ignition, the flame can’t catch up to the plane. The plane just moves too fast for it.
Without an afterburner, you are going to have a hard time getting the fuel to light. It needs some sort of source of ignition. But again, even if you do manage to get it to light through some sort of static discharge or something, all you are going to do is make a fancy flame show. It might scare the bejeezes out of the passengers (FIRE!!!) but other than that, no biggie, as far as I’m aware.
In the summer, my wife and I often have a burn pile that is just collected wood/trees that has fallen down. It’s often a bit wet. It’s proactive, to keep fuels away from our house.
I always have some diesel on hand, and gasoline. The diesel burns slower and longer to get the wet wood started. I’ll often use gasoline to get the diesel started burning.
I’m not getting an Eagle Scout badge for doing that, but yep, it can be hard to start diesel/kerosene.
This is unrelated but it is almost impossible to light anything with a cigarette. I have a standing bet that I have never lost. It is supposedly possible with nearly perfect kindling and a long smoldering time but I have never been able to do it either.
For that matter, you can toss lit cigarettes into pools of gasoline all day long and they won’t ignite let alone explode either. The movies lied to us. You don’t need to try it yourself. I already did it for you hundreds of times and it simply doesn’t work. The fuel/air ratio isn’t favorable even if you ignore the temperature problem. Kerosene/diesel fuel/jet fuel is even harder to ignite than that.
It isn’t nearly as easy as many people assume. The cause of most of those is a long, smoldering in a piece of stuffed furniture like a couch that eventually combusts. That is hard to recreate intentionally. I don’t smoke anymore but I have bet countless people money that they can’t light something like a napkin, newspaper or anything else with a cigarette and I have never lost. I know it is possible somehow. It is just a whole lot harder than people think.
Try dousing the kindling in liquid oxygen first. Or have the stuff underneath the kindling doused in LOX.
Anyway, any number of fire reports will put you right. My aunt nearly died when she fell asleep with a cigarette in hand and it had almost ignited her chair when others intervened.
It’s not impossible, just really really really really hard. So hard it would take you days to do it on purpose. The only reason it happens is that one time in a million it starts a fire, and there are millions of smokers who smoke dozens of cigarettes 365 days a year. Those one in a million odds start to add up at that point.
The TAAG 747 was probably not “modified”. They came from the factory with that kind of range. Not every airline used the max range available.
There is a general prohibition about circling too closely while dumping fuel. Ingesting your own fuel cloud into your engines and air conditioning might not be good. Setting it on fire by your passage is not a concern.
There is a general prohibition about not dumping fuel below 5000 feet above ground as it might not completely evaporate before falling as a diffuse mist to ground level. Where it might create ground pollution, stink, and perhaps a fire hazard.
There is NOT any concern about the aircraft itself setting the dumped fuel promptly on fire. Yes, F-111s with fuel dumps and afterburners could do it. Airliners, not so much. We have no prohibition about dumping fuel following a cabin or engine fire.
Speaking in generalities, fuel dumping is slowly going the way of the dodo. Aircraft are powerful enough nowadays to fly just fine with an engine out under almost any credible conditions. And to land & go around with an engine out as well. Such that the time wasted dumping fuel to reduce landing weight is mostly viewed as increasing the hazard exposure, not reducing it.
There are exceptions, such as departing islands with short marginal runways and no nearby alternatives. Where for certain malfunctions that impede landing capability but don’t pose much threat against staying airborne it can be helpful to dump, or burn off, fuel before landing at that marginal airport.
These exceptions are rare, and becoming more so all the time.
I have won one of those bets. The trick to getting a napkin to light is to tear it slightly, so as to get a fuzzy edge, then exhale through the cigarette so the temperature of the cherry rises as you touch the napkin edge. The extra oxygen from the outflowing air will cause ignition.
But otherwise, no. And as far as forest fires, it needs to be in some sort of medium (Pine needles or somesuch) that will cause it to smolder and become a larger heat source that can then be oxygenated by the breeze. The drier the better, obviously.
And many smaller models of planes don’t come with a fuel dump mechanism as standard. A318/319/320/321 I think. So landing overweight can’t be that big of a deal.
This isn’t really my area of expertise, but I know it’s possible to catch dumped fuel on fire because you can do it intentionally in a fighter jet with an afterburner. It’s called a “zippo”, named of course after the cigarette lighter.
Here’s a video of an F111 doing it:
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Note that “dump & burn” is only possible when the plane’s fuel dump nozzle is situated near the engine exhaust - which is notoriously the case on the F-111 but IIRC on most jets the dump valves are directly on the wings (where most of the fuel is stored).
The SR-71 had an amusing design constraint as well : it was meant to fly very, very, very fast at super high altitude, but as a result its “skin” didn’t quite seal at ground altitude as it was designed to expand & fit together with the heat of continuous supersonic flight. So it leaked fuel all the way through take-off (they actually had to refuel in-flight immediately after take-off !). In some cases, as the pilot turned on the afterburner to achieve lift-off, he’d also light the runway on fire like Wile E Coyote on rocket skates
Not true! It is a very big deal. Serious inspections are required, I have done many of them. We do not just kick the tires. That aircraft is not going to carry passengers any time soon. Think weeks & months.
I would have to look it up, in the FARs, but IIRC: If a commercial aircraft has the ability to take off with a full fuel load weight that exceeds its maximum landing weight by 10 percent, it is required to have a fuel dumping system. IE: If full fuel, cargo, & passengers can be 110 percent or above the maximum landing weight, a fuel dumping system is required. Again, IIRC, The above mentioned aircraft do not have that issue.
The requirements for the aircraft’s landing system to be able to safely land overloaded is significant. I believe it is 150 percent of maximum landing weight. I do not recall what the exact number is at this time, & I am too tired to look it up right now. It is much higher than 110 percent. Any time an aircraft has exceeded 100 percent, an inspection is required. It is a big deal. It has cost many pilots their jobs.
Keep in mind that if the landing gear fails & punches through the wing, it will surely rupture the in-wing fuel tanks. Even on a 747 they will rupture the belly tank. This is bad, very bad.
I am not familiar with what the TAAG ordered installed on their 747s, but many of Boeing’s customers do order the long range tanks. When I worked for Boeing, very few airlines did not order the long range tanks.
Commercial airplanes are required to be able to fly across the ocean & still have enough fuel to divert to at least one alternate airport, in case the weather does not cooperate with their plans. Most airlines like a little more fuel than is mandated, dead passengers do not buy more tickets. It is also hard on airplanes to run out of fuel in flight.
That’s a bit strong versus current reality. It might have been true in the past.
I don’t know the specifics of when fuel dumping gear is required (or even if it’s ever required). But I know we have a batch of 767-300s with max takeoff weights of 408,000# and max landing weights of 310,000#. So max TO is about 130% of max landing. Some of those have fuel dump, some don’t. The aircraft are otherwise identical. So there’s clearly more involved than just the weights.
As other folks have said, dumping is not installed (or even available AFAIK) on smaller aircraft. 757s and below don’t have it. The 757 has the range to go transatlantic, trans US, and US-Hawaii. So it’s not just short-range aircraft with small fuel fractions which lack dumping.
I was tangentially involved in a hard landing incident once. I was getting ready to depart and suddenly the maintenance supervisor showed up and told us to stop boarding. The airplane had been hard-landed (by somebody not me) a few flights ago and the data recorder had just been read out and the incident uncovered. This was not an overweight landing but was a hard landing.
It took a team of four guys two hours to finish the inspection then we re-boarded and left. They did say they had work checklists for 3 levels of hard-landing inspection and this was the middle one. The big list would have taken a team of guys a full day in the hangar. Obviously had they found a problem the fix could have taken hours, days, or even weeks for a severe one.
Our guidance on dumping fuel or landing overweight is to land overweight if the runway’s long enough. Even at max takeoff weight, which is ~130% of max landing weight, landing that heavy is no big deal. *If *you make a halfway decent landing. The biggest issue is getting stopped without thermally overloading the wheel brakes. We’ve got tables to figure out all this stuff. Ideally we’d use reverse thrust & wing spoilers to reduce our speed down to about 50mph, then gently apply wheel brakes to finish stopping.
After an overweight landing a maintenance inspection is required. But again it’s the work of a couple man-hours unless you fry the brakes, set the tires on fire, or prang it on.
