Fuel tank explosions in aircrafts

[siddharth]

I read about incidences of fuel tank explosions in commercial airliners recently. (For example, this http://www.cnn.com/2006/US/07/14/twa.main/index.html" ). I'm aware of four incidences of fuel tank explosions on commercial airliners. All four were on Boeing airplanes.

- The first was in 1990 when a Boeing 737-300 operated by Philippine Airlines exploded on the ground while taxiing [1]

- Then in 1996, was the tragic incident when a Boeing 747-131 operated by Trans World Airlines exploded in midair some 12 mins after take off from JFK [2]

- The next one was in 2001, when a Boeing 737-400 of Thai Airways exploded, half an hour before scheduled take off. [3]

- And the last one I'm aware of happened in 2006, when the left wing fuel tank of a Boeing 727 of Transmile Air Service exploded in Bangalore while being towed on ground. [4]

The Nation Transportation Safety Board says that available evidence from these incidents suggests that the cause is the ignition of fuel-air mixture in the center wing fuel tank. Also, the Caltech Explosion Dynamics Laboratory performed a series of experiments which highlighted the flammability of fuel/air mixtures of jet fuel [5].

I have many comments and questions on this. It would be great if someone could help.

First of all, why only boeings? I read that a problem with the boeings is that the heat sinks of the air conditioning system are located beneath the center wing fuel tanks which could rise the temperature and flammability of the fuel mixture [5].

Does anyone know for which models, the location of the AC heat sinks is an issue? Does this affect the newer 777's? Did Boeing make any changes regarding this issue in the newer aircrafts after the TWA disaster?

Also, I read that there was going to be an inerting system introduced in some 747's which reduces the oxygen content using nitrogen. Why only the 747's?

Then, the FAA introduced some regulations to reduce the occurrences of ignition in the fuel tanks. Were these implemented globally, or only on US aircrafts?

Is there any reason that three of these four incidents occurred when the plane was on the ground (radiant heat from tarmac)? Or was it luck?

I also read that some airbus models (the 320 & 340) were also susceptible to center wing fuel tank explosions. What is airbus going to do on this?

Refs:
[1]-http://aviation-safety.net/database/record.php?id=19900511-1&lang=en"
[2]-http://aviation-safety.net/database/record.php?id=19960717-0&lang=en"
[3]-http://www.ntsb.gov/Events/twa800/"
[4]-http://www.ntsb.gov/Pressrel/2006/060712a.htm"
[5]-http://www.galcit.caltech.edu/EDL/projects/JetA/background.html"

* This sudden interest is due to my (now intense) dislike of flying. I'm aware that statistically, flying is the one of the safest form of travel per km of distance traveled. Yet, I've got a boeing 777 flight to catch soon, and I'm scared *

 

[Astronuc]

Here is a discussion of FAA/DOT research into this matter - http://www.fire.tc.faa.gov/systems/fueltank/intro.stm
http://www.fire.tc.faa.gov/systems/fueltank/papers.stm

As to why only Boeings, Airbus apparently uses a different wiring method, which they argue is safer.

I believe that the airlines have implemented ground based inerting (GBI) methods, but I can't confirm it.

TWA 800 (N93119) was an old plane - apparently built in 1971 so it was 25 years old in 1996. The age of the wiring was considered a factor in the accident. The location of one of the air-conditioning units was a consideration. AFAIK, while the aircraft are on the ground, the AC is provided by ground-based systems.

Boeing 777's are younger aircraft, and I believe the wiring for the tanks was redesigned. I'll be flying in one tomorrow, and then again on Thursday. I expect to be around for many more years.

 

[FredGarvin]

I can't really comment on why just Boeing aircraft at this point. It may be shown statistically that Boeing simply has more aircraft handling the very long flights and thus a higher probability of this happening.

One major sore spot in a lot of investigations has been the continued use of Kapton wire in aircraft. There is a lot of talk about the nature of this wiring/insulation and it's supposed proclivity for becoming faulty and causing fires or arcing. Some reports are more doomsday than others so it is difficult to get an accurate picture on the real size of the problem. I will say this that the DoD has, to a large extent, reduced or eliminated Kapton from it's aircraft. Take that for what you will. That is the most likely culprit for the TWA flight off of Long Island (combined with a high fuel vapors present in the tank).

It's not rocket science to state that fuel air mixtures are flamable. That requires a big DUH! Jet A, which is the most widely used fuel type in commercial turbine aircraft (Jet A1 in Europe & Canada), is very stable in the pure liquid phase. You can drop a match into it and it will not ignite. It does have a flash point of around 120°F (it's minimum flash is about 100°F). The one thing about all fuels that makes this matter worse is that as the fuel temperature goes up, the temperature band in which the fuel will ignite widens at lower altitudes. It makes a lot of sense that an aircraft sitting on the tarmac in the Phillipines or some other very warm climate to see very high internal fuel tank temps. It gets compunded by the fact that aircraft do not always fill up with fuel completely for every flight. The fuel load is dictated by the load carried and the distance for that particular flight.

Some recommendations have been made from the NTSB to look at using JP-5 in stead of Jet-A for all commercial aircraft. JP-5 is the fuel that is made for the Navy for use on carriers because it has a higher ignition point and is safer for storage. That is a more expensive fuel to manufacture, so I don't see that happening any time in the near future.

Here is a pretty good analysis of the TWA flight that has some good references to some tests Boeing has done since the accident. Most noteably that the center body tank's temperature increases drastically when less fuel is present in it.

http://tc.engr.wisc.edu/UER/uer00/author2/content.html#6

I can't say anything that will change your mind about disliking air travel. In cases like this we tend to fix on the negative cases. I know I do. Sometimes ignorance is bliss. I know some people that absolutely refuse to fly. So be it. If you have to, well, hopefully you can feel a little bit at ease knowing that a 777 is the latest and greatest. It's a very nice aircraft. It's not going to be a 30 year old airframe with a 100,000 hours on it.

EDIT: Added link for Kapton issues.
http://www.pogo.org/p/contracts/ca-021102-wiring.html

 

[siddharth]

One major sore spot in a lot of investigations has been the continued use of Kapton wire in aircraft. There is a lot of talk about the nature of this wiring/insulation and it's supposed proclivity for becoming faulty and causing fires or arcing. Some reports are more doomsday than others so it is difficult to get an accurate picture on the real size of the problem. I will say this that the DoD has, to a large extent, reduced or eliminated Kapton from it's aircraft. Take that for what you will. That is the most likely culprit for the TWA flight off of Long Island (combined with a high fuel vapors present in the tank).

EDIT: Added link for Kapton issues.
http://www.pogo.org/p/contracts/ca-021102-wiring.html

That link was some read!
By the way, I found this bit strange.

Block said Boeing uses Teflon-Kapton-Teflon, which he considers the best wiring on the market right now, in its 737 and 757, but uses another wiring insulation called cross-linked Tefzel that is known to be smoky, flammable and toxic on its large twin-aisle transports, the 777, 767 and 747. Boeing originally only used cross-linked Tefzel external to the fuselage (in non-pressurized areas) because of flammability concerns, but in 1993 started to use it across-the-board on the large transports, Block said.

If T-K-T is indeed the best wiring, how come boeing doesn't use that on it's modern jets, but only on the 737 and 757?

Regarding faulty wiring, I came across http://www.wsws.org/news/1998/may1998/737-m12.shtml" which says

Newer 747s, 767s and 777s and all European Airbus jets have redesigned fuel supply systems which avoid having wires passing through the fuel tanks. But in the older jets, the wires pass directly through the fuel tanks, inside copper conduits.

If true, this could be a possible link between the wiring and the incidents on the 737's and the early model 747.

Here is a pretty good analysis of the TWA flight that has some good references to some tests Boeing has done since the accident. Most noteably that the center body tank's temperature increases drastically when less fuel is present in it.

http://tc.engr.wisc.edu/UER/uer00/author2/content.html#6

Another disturbing read. The TWA flight was from JFK to CDG? That's around 5,900 km. If the CWFT was empty for that flight, does this mean the fuel tanks on the wings are always completely filled first?

<begin irrational fear>My flight is about half that distance </end>

I can't say anything that will change your mind about disliking air travel. In cases like this we tend to fix on the negative cases. I know I do. Sometimes ignorance is bliss. I know some people that absolutely refuse to fly. So be it. If you have to, well, hopefully you can feel a little bit at ease knowing that a 777 is the latest and greatest. It's a very nice aircraft. It's not going to be a 30 year old airframe with a 100,000 hours on it.

I know. Given the safety records, my dislike is irrational. I'm trying to overcome parts of it with some knowledge, but it seems to be backfiring.

 

[siddharth]

Here is a discussion of FAA/DOT research into this matter - http://www.fire.tc.faa.gov/systems/fueltank/intro.stm
http://www.fire.tc.faa.gov/systems/fueltank/papers.stm

As to why only Boeings, Airbus apparently uses a different wiring method, which they argue is safer.

I believe that the airlines have implemented ground based inerting (GBI) methods, but I can't confirm it.

Thanks for the info astronuc, the information about ground based inerting is new to me. If http://www.findarticles.com/p/articles/mi_m0UBT/is_29_14/ai_63557904" is true, then all US airlines should have implemented GBI.

I have some questions on this as well. I've read the directives issued by the FAA. Are these applicable only to US airlines? Or are these directives (such as the GBI) to be implemented by Boeing and Airbus on all of it's aircrafts?

The location of one of the air-conditioning units was a consideration. AFAIK, while the aircraft are on the ground, the AC is provided by ground-based systems.

Do you happen to have any information on whether the location of the AC units is an issue on the 777 ()? Specifically, the location of the ECS pack near the CWT on the B-747 seems to be a major factor in increasing the fuel vapour temperature (http://www.tc.faa.gov//its/worldpac/techrpt/ar98-26.pdf" , published 1998). Was there any modification in the design regarding this (like different location of the ECS pack, heat shields, etc)? Yeah, from what I read, when the plane is on the ground it is powered by a separate ground power unit.

For those interested,
http://www.epa.gov/fedrgstr/EPA-GENERAL/1997/April/Day-03/g8495.pdf" - A notice on fuel tank ignition prevention measures released by the FAA in 1997

 

[Astronuc]

AFAIK, any aircraft operated in the US must be certified (authorized or approved) by the FAA, which means that the aircraft must meet FAA requirements for safe operation. Similary national (and now perhaps EU) agencies approve aircraft operated in other countries, but that could mean one agency accepts the approval of another one, i.e. UK could accept FAA approval.

The FAA requirements apply to any aircraft operating (fly to and from) the US. Presumably, Europe and most industrial nations have similar requirements.

Do you happen to have any information on whether the location of the AC units is an issue on the 777 ()? Specifically, the location of the ECS pack near the CWT on the B-747 seems to be a major factor in increasing the fuel vapour temperature (source, published 1998). Was there any modification in the design regarding this (like different location of the ECS pack, heat shields, etc)? Yeah, from what I read, when the plane is on the ground it is powered by a separate ground power unit.

I'm looking for a cutaway view showing the internal details of a 777. So far I've only found this -
http://www.asminternational.org/images2/cof/0903B777Fig3b.jpg

or try this - http://members.ync.net/jam/cutaways.html - and scroll down to 777 - cutaway is javascript app.

I think the ACS has been relocated in the 777, and I believe there was a plan to insulate the ACS from the CWT, or alternatively, not to use the ECS pack under the CWT.

ASM International has a really cool page - http://www.asminternational.org/Content/NavigationMenu/Magazines/AdvancedMaterialsandProcesses/Centennial_of_Flight/Centennial_of_Flight.htm

 

[AlephZero]

The European equivalent is the JAA (Joint Aviation Authority) and its regulations are JARs, similar to the FAA's FARs. See http://www.jaa.nl/

AFAIK the JAA and FAA certification requirements for civil aircraft are pretty much harmonized (i.e. the standards are equivalent). The "rest of the world" aviation industry also uses the same requirements, otherwise they can't operate their aircraft in either Europe or the US.

 

[FredGarvin]

The thing about FAA or any kind of certification is that it mostly centers around design prove out and flightworthyness issues (to my knowledge). What is done in the cert process is to demonstrate the required tests to the FAA's satisfaction to prove one's design. There really isn't anything the FAA can do PRIOR to an accident to eliminate a fault that may pop up in 10 years. The best they can do is to learn from every single mishap and institute the appropriate changes.

 

[only.corvette]

Obiggs

If it makes you feel any better about fuel tank explosions there has been a new 'system' invented to help prevent fuel tank explosions and it is being used on quite a few military and commercial planes. It's called OBIGGS short for Onboard Inert Gas Generation System, and it removes the oxygen from the air above the fuel in the fuel tank and replaces it with nitrogen. This causes it to be more difficult to ignite the gas, and also get rid of flammable gaseous fumes.

 

[smithto]

It's well known that the Boeing 747 suffers from the problem of having a heated center wing tank.

The Airbus A380 avoids this problem, because it has no center wing tank.

What's the situation with the Boeing 777?

 

[S-1A Cadet]

Folks,

Please, the word aircraft is singular or plural depending upon context.