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747 Center Fuel Tank Explosion

Sat May 05, 2001 10:53 pm

Years ago, a beechjet 400A, was at Washington DCA refueling for its trip back into canada. The pilots shut down the aircraft while the FBO fueled both the wing fuel tanks and the center fuel tanks, including the 4 foot high aft fuselage tank. When the pilots returned to their aircraft, they found it filled with the cabin. When they opened the aft end of the airplane, they found that the Aft fuel tank had in fact, exploded.

So, what exactly happened to this beechjet? Through exhausting investigations, the answer was finally found. The Beechjet had fueled up with Jet B in Canada and Jet A in Washington. When Jet A and Jet B are mixed, the two fuels are both usable in the airplane, but not the same. In fact, these two fuels' vapors rise at a different rate. When you have some fumes rising faster than others, static electricity is created, quite the same way rising air in a thunderstorm makes lighting.

With 4 feet in the aft fuselage for the vapors to rise, there was quite a "thunderstorm" occuring in that tank full of volitile fuel vapors. In fact, its been said that the static electricy would have been so great, it would have lighted the entire tank enough for a person to see clearly. With all of this static electricity, and volitile fuel vapors, explosion was eminent; luckily it was while on the ramp and not in the air.

Recap the recipe for dangerous fuel tank scenario:

Must have:

-Tall enough fuel tank to allow vapors to form static electricity

-Mixture of two fuels with different vapor rates (Jet A and Jet B)

TWA - 800

-Returning to Europe after arriving in JFK, refuelling and reloading.

-Center fuel tank is tall enough for a person to stand in.

-Aircraft just returned from Europe where it was fueled with Jet B.

-Aircraft refuelled at JFK with Jet A.

-At the time of the explosion, fuel would be minimum in center tank, leaving vapors with most height to rise and cause static electricity

This all adds up PERFECTLY! Here is a report from the NTSB:

"Based on evaluation of the recovered wreckage and a detailed evaluation of the sequence of events, we have determined that the fuel/air vapor in the center fuel tank exploded and that the explosion of the tank initiated the breakup of the airplane. Wehave not yet determined what ignited the fuel vapor in the center tank. "

The answer to TWA-800's demise might be found in a nine passenger Beechjet 400A.



*Note: After the Beechjet incident, all aft fuselage tanks were filled with a pourous foam to keep vapors from rising enough to create static electricity. The tank holds the same amount of fuel, but takes a long time to fuel.
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RE: 747 Center Fuel Tank Explosion

Sun May 06, 2001 3:29 am

The center fuel tank on the 747 was never fueled at JFK. It departed with only residual fuel.

Jet fuels as far as I know are mixable. I have never read any where or been made aware of a problem mixing jet A and jet B.

If there was a potential for such a problem wouldn't the FAA have instructed the airlines that this practice of mixing fuels was potentailly catastrophic?

Static electricity isn't enough to light jet fuel vapors. That's been proven.

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RE: 747 Center Fuel Tank Explosion

Sun May 06, 2001 6:23 am

Do you just go through the threads to find my name to contradict me?

Static electricity can carry serious voltage. There is no limit to static electricity, unless you truly think lightning doesn't carry enough voltage to ignite fuel vapors near stoiciometric efficiency.

Jet fuels are mixable, but they vaporize at different rates. Different gasses rising at different rates cause friction and static electricity.

This is called "Thinking outside the box." A theory, not constrained by what we "think" to be true. The beechjet aft fuel tank exploded due to static electricity. Perhaps you missed that part of the original post when you said its been proven that static electricity cannot ignite fuel.
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RE: 747 Center Fuel Tank Explosion

Sun May 06, 2001 7:12 am

I have just one thing to add.

Here in Europe we use JetA-1.

The Military uses JP8, almost similar to jetA-1, but with higher flame point.

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RE: 747 Center Fuel Tank Explosion

Sun May 06, 2001 7:57 am

The article below is from the Association Of Licenced Aircraft Engineers magazine "Tech Log". It descibes the length the writer went to in an attempt to start a fuel fire with a spark.

The website of the organisation is : -

TWA 800. Accident or Error?

By Gerry Evans.


Some years ago I was asked, together with the electrical/instrument group of my Squadron, to give a demonstration of the volatility of Aircraft fuel. It was summer time and we were based in central Europe. The landmass there always produces high ambient temperatures, often so hot that you are unable to touch the upper surfaces of the aircraft. The clay we chose to carry out the demonstration was like this, the temperature being in the high eighties and touching 90 through mid afternoon.

Initially we used a 90-gallon drum containing about 6 inches of aviation fuel, I cannot remember the fuel specification, but it was similar to what is used today. The drum haying been left standing in the sun was already too hot to touch, it cooled with the added fuel, but quickly returned to a high temperature giving rise to considerable vapour. We then applied, via two probes. a continuous spark the source being from a 250 volt megger. The probes would only work when close together, but the spark was strong. There was no immediate result and so the probes were placed gradually nearer to the fuel at the bottom of the tank Again no result, and so we progressed through a 500-volt megger and on to a 1000-volt megger. The probes were varied in their construction and the sparks generated were magnificent. But no explosion or fire resulted. A lid was applied to concentrate the fumes and eventually holes were made in the side to improve air supply. This went on for some hours and by this time the tank temperature was over 100 degrees. We got fed up and tossed in a lighted rag, it simply went out. A week later we had another go. This time with a galvanised 400 gallon tank, the ambient temperature again in the high eighties. A similar procedure was followed, but using only the 1000 volt spark source as this gave the best sparks across a larger gap Apart from becoming adept at generating sparks we achieved no fire or explosion. This was what we had expected before we started.

When I first heard that an Aircraft fuel tank had exploded I was immediately interested. When it was suggested that the cause was due to a stray spark from some aircraft wiring source, I was amazed and, quite simply, disbelieving. I was sceptical because a spark, per se, has no energy. To be maintained it has to be fed from a source, and it would require a large voltage spark to be effective. Given the right conditions a spark can ignite a fuel vapour but not this fuel, in these circumstances In the case of the TWA 800 the catastrophic failure that engulfed this aircraft happened in only a fraction of time. Such a failure required the instantaneous dissipation of a large source of energy. If it is deemed that the fuel tank exploded, it could only have done so if there had been applied across the tank a discharge of considerable energy such that the entire volume of vapour in the tank ignited virtually instantaneously. In other words a detonation took place. Combustion has a slow rate reaction, and would require precise conditions and volume to achieve an explosion. I am talking about the burning reaction of the fuel vapour. The fuel residue would not have contributed to the explosion, only to any subsequent fire. Liquid fuels only burn as vapours. Surface temperature and fuel temperature would control the vapour volume and as more vapour is generated so it mixes with the unchanged volume of air within the tank.

Reaction in vapours is governed by heat, this may seem obvious, but chemical explosives are different, they are metastable compounds and so behave in a way quite unlike that of a vapour It is essentially the rate of reaction that separates combustion from detonation. That is not to say that the eventual damage occurring from the two reactions will be dissimilar, it is more a question of probability. In this particular case we have to ask, did the volume of this tank have the capacity to generate the pressure required to cause this explosion, having been initiated by a low intensity spark from aircraft wiring? The answer from me, at this time, is no.


In combustion the reaction rate is relatively slow and therefore requires precise conditions to result in an explosion of such a magnitude as that which occurred to the TWA 800 aircraft. Considering the fuel vapour as the only explosive medium present, and given that the present argument provides for a spark having generated a fire in that vapour then the final energy dissipated would be dependent upon the rate of advance of the reaction through the vapour That reaction would be relatively slow to start and would accelerate as the heat increased, that slow start would of course burn any air in the tank and at the same time remove some of the fuel vapour. The reaction would be enhanced by the confinement provided within the tank and the tank temperature. The reaction would however be restricted by the divisions in the tank, this would have the effect of slowing down the chain reaction through the vapour and the subsequent build of pressure The build time of the reaction would also rob energy from the eventual explosion.

The heat source would have to be a fire, started, as suggested by a spark. The heat generated by that fire would vaporise more fuel, but there would not be any additional air. The rate of reaction would have to be immense in order to generate the energy required to disintegrate the tank. An explosion has a transitory effect, splitting the tank would be one consideration, breaking through the fuselage would be another.

It makes the argument for the rate of reaction and therefore the source of detonation very strong in favour of another option.


Detonation of the fuel vapour would be a different story.

Remember that it is the reaction rates that effectively separate Combustion from Detonation.

In Detonation the reaction accelerates through the material/vapour like a shock wave, the resulting explosion is instantaneous.

Detonation would require the dissipation of a large energy source across the tank, such that the fuel vapour ignited instantaneously, the rate of reaction giving rise to a very large pressure within the tank and subsequent disintegration of the tank taking place. The instantaneous pressure generated would be immense, and so we have the explosion we are looking for. But where do we look for the source of detonation?

From the aircraft, such sources can only be the HF radio and engine ignition, both unlikely.

The idea that a spark generated from the break up of a fuel pump is untenable. Sparks from such a source are transitional and without energy. Sparks from Aircraft wiring faults would have to be external to the tank and again without the potential that I believe would be required, to trigger an explosion.

Static however, I have reservations about, this subject deserves more research, especially cabin generated static.

We are therefore looking at an external source, external in the sense that it was not generated within the Aircraft.

Firstly, the most perfect catalyst would be a high heat source such as an explosive device, secondly. fork lightning.

From here we move into the world of phenomena, or perhaps something more sinister.
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RE: 747 Center Fuel Tank Explosion

Sun May 06, 2001 8:09 am

If that article were true in difficulty of a spark igniting fuel vapors, then I have three questions:

1. How do spark plugs work?
2. How did it happen on the Beechjet?
3. How do you explain the following video? (in real Player Format)

Author even admitted in above article that static charges require more research.

I'm not saying this theory is right, nor am I saying yours are wrong. I'm just playing devil's advocate.
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RE: 747 Center Fuel Tank Explosion

Sun May 06, 2001 8:15 am

Re spark plugs, petrol is more volatile than Jet A-1.

Top Gun
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RE: 747 Center Fuel Tank Explosion

Sun May 06, 2001 12:29 pm

Why don't you think Diesels have spark plugs?
Diesel is the same chemical make up as either Jet A or B, not sure which though.

A spark wouldn't do much in Diesel, there for they don't use em.

PS.Glow Plugs are not spark plugs.
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RE: 747 Center Fuel Tank Explosion

Sun May 06, 2001 1:57 pm

If I'm correct, a spark plug in Deisel would destroy the engine. Deisel engines use no spark plugs because the mixture can be ignited by simply compressing it. Wouldn't that make it more volitile? I don't know much about deisel engines so comments are appreciated.
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RE: 747 Center Fuel Tank Explosion

Sun May 06, 2001 2:15 pm

Kerosene & Diesel fuel have similar properties, they both need high press & temp to ignite. Precisely the opposite of what you find in a fuel tank.
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RE: 747 Center Fuel Tank Explosion

Sun May 06, 2001 2:26 pm

The fuel tank of that 747 got really hot didnt it? I am not too sure how much pressure was in the tank though.

It is kind of wierd that it didnt go off on the ground though instead of in the air.
Chicks dig winglets.
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RE: 747 Center Fuel Tank Explosion

Sun May 06, 2001 2:34 pm

Not as hot as a 747 parked up in Saudia Arabia. If that was all it was, they would be blowing up all the time out there.
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RE: 747 Center Fuel Tank Explosion

Sun May 06, 2001 3:56 pm

I refered to one of my collegues' who was an accident investigator for the FAA and worked on TWA 800 recovery and investigation team.
First and foremost there where no traces of residue nor evidence to support any type of anti-aircraft missile or bomb that caused the explosion.
The center fuel tank and the surrounding structure showed metal stress and fatigue consistant to a relatively slow overpressure or shockwave.

There was no evidence that found any metal fragmentation from a high speed shockwave.

However this is what they found, using a test airplane and putting it under similar circumstances.

Utilizing a 747 using approx the same volume of fuel in the center fuel tank and approximating the same conditions, They allowed the ACM's to run for approx. the same amount of time. The center fuel tank which had temp probes installed, recorded tempetures in the tank to almost 400F.
This condition occurs due to the close proximity of the ACM's to the center fuel tank.
This combination of temp and fuel would provide sufficient vaporization of the fuel and if the right combination of fuel to air ratio is met, all that would be needed is an ignition source.
The source is still unknown, however it is suspected that it came from damaged wiring that ran through the fuel tank. This wiring also ran along side of some High voltage wires in another portion of the aircraft, Which leads the investigators to believe that this is the source of the electrical energy for the spark.

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RE: 747 Center Fuel Tank Explosion

Mon May 07, 2001 4:23 am

Just a question.
Are wires that run through fuel tanks in larger a/c in conduit? If so, are they bonded (conduit) to the structure? I would imagine that they would be, but I'm not totally sure seein' that I don't usually end up the belly tank of a 747.
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RE: 747 Center Fuel Tank Explosion

Mon May 07, 2001 8:55 am

I'm under the impression the wires don't actually run through the tank, but rather on the outside of it. Enough voltage running through those wires when they shorted out on the fuel tank could cause arcing in the tank, hence igniting the vapors.

In response to all the missle theorists out there:

I did some rather interesting reading last night on the missle theory. There are some very convincing arguments, including the 34 witnesses. What doesn't hold up though is that the NTSB has found no evidence whatsoever.

Even if it was an FBI coverup there would be no way to silence the hundreds of people who studied the wreckage. Most of the wreckage was found after the FBI handed the investigation over to the NTSB. There really isn't any way to tamper with it.

Who knows? I've heard theories on things from minature bombs in the fuel tank to a pocket of methane gas that was released from the ocean before the 747 flew through it. Its just your modern-day Bermuda Triangle  Smile

RE: 747 Center Fuel Tank Explosion

Mon May 07, 2001 10:33 am

I would have to agree with VC-10. Here are a few thoughts.

1) The fuel tank would rupture long before it would hold enough pressure to cause spontaneous combustion.

2) As temperature increases vapor pressure also increases. Since the vapor is heavier than air, it will dispace the air (via vents) overboard. Thus the mixture is too rich for ignition. If the fuel was closer to the OAT, it would not have been generating enough vapor to sustain ignition.(too lean) If I recall correctly, the window between too rich, and too lean is about 4 %.

3) If a fire occured within the fuel tank, It should have blevied. This also would have caused the fuel tank to rupture at the weakest points. The investigators would have had evidence of a sustained fire.

4) Not ALL of the evidence has been recovered. There is still a portion of the aircraft missing!!

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RE: 747 Center Fuel Tank Explosion

Tue May 08, 2001 12:59 pm

As far as the question about wiring running through tanks without conduit, the answer is yes, at least in the Lockheed product I deal with. It is fuel quanity wiring. Fuel level control valve wiring (115VAC) is enclosed in conduit however. Boost pump wiring (115VAC) is external of the tank. We have frequent (several times a year) problems with the in tank fuel quanity wiring, which requires the entire harness to be R&R'd. The C-5 is quite different than most aircraft however. The vapor area of the fuel tank is inerted with nitrogen gas, so fuel/air mixture is not there to support fire/explosion. The fire suppression system is a maintenance problem to say the least, so it is not without drawbacks.
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RE: 747 Center Fuel Tank Explosion

Wed May 09, 2001 2:08 am

As a matter of fact on some aircraft such as the 727, fuel boost pump wiring (3 phase 115VAC) run through the fuel tank inside a conduit. Basically, each fuel boost pump is mounted in its own dry housing. Dry meaning that when I remove the access cover for a particular boost pump under the wing, its a fuel free environment with the entire pump, electrical plug and wiring into the plug visible. Sometimes its referred to as the doghouse.

The way the wires are routed is from the fuselage to the leading edge of the wing in front of the forward spar (this is still a fuel free area). As a matter of fact the wires are visible under the drooping leading edge flaps. Now, the wires at this point must pass through the front spar, into the fuel tank and to its respective pump in the doghouse. But the wires aren't actually in the fuel itself but running through a conduit running from the front spar to each pump doghouse. So its the conduit keeping the wiring dry.

But their was an AD complied with 2 years ago concerning all 727s. I think Continental or United on an inspection found these wires running through this conduit to be arcing. The insulation was worn off these wires over the years from the wires rubbing in the conduit.

At FDX we complied with this AD and I did many of them in EWR. It was a fun job during the spring and summer.

What the AD entailed was the removal of all the wiring passing through each conduit (from wing spar to doghouse, 8 doghouses/pumps per aircraft) and thouroughly inspect wiring for chafing, arcing and wet spots as well as smell it for any sign of fuel. If there were the slightest bit of wear (as spelled out by the AD) the wire harness was to be sent to engineering for analysis and a new wire harness installed. The harnesses were all fabricated locally with new wire and connectors. The vast, vast majority of harnesses were serviceable but were replaced anyway (at least in EWR).

The next step was instead of just reinstalling the wiring back into the conduit as before, this time the harness was enclosed in a teflon tube and then reinstalled. The teflon acting as a barrier to prevent wear of the harness in the conduit.

If any of you mechs didn't participate in this AD, next time you see a 727 peek under the krueger flaps and you should see wire bundles in tie wrapped teflon tubes shooting into the front spar. Maybe 737s as well.
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