Ps76
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Do All Airliners Have Live Wires In The Fuel Tanks

Mon Jun 01, 2009 1:13 pm

Hi,

Just had a quick question as I'm a nervous flier and one of the things I worry about is a situation similar to TWA800 happening. Was just wondering if all airliners have electrical wiring in their fuel tanks or what is there to prevent an incident to TWA800 happening again. I know that jet fuel won't ignite easily but anything else to help put my mind at rest would be cool.

Any info welcome.

Many thanks,

Pierre.
 
474218
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RE: Do All Airliners Have Live Wires In The Fuel Tanks

Mon Jun 01, 2009 1:40 pm



Quoting Ps76 (Thread starter):
Was just wondering if all airliners have electrical wiring in their fuel tanks

Yes.

The fuel quantity probes have electrical wires.

The fuel boost pumps (which are electric motors) have wires.

The fuel fill switch's have electrical wires.

The secret is to keep the wires submersed in fuel so they can't spark.

The reason TWA 800's tank exploded was that there was no fuel in the tank, only fumes.
 
Okie
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RE: Do All Airliners Have Live Wires In The Fuel Tanks

Tue Jun 02, 2009 1:39 am



Quoting Ps76 (Thread starter):
Was just wondering if all airliners have electrical wiring in their fuel tanks or what is there to prevent an incident to TWA800 happening again

You have fuel pumps and fuel gauges (wiring) in your automobile as well.

The trick here is what is referred to as lower explosive/ignition limits and upper explosive/ingnition limits. If the fuel to oxygen ratio is to low the vapors will not ignite, if the fuel to oxygen ratio is to high the vapors will not ignite.

Speaking generally taking maintenance functions out of the equation then the only time there should be an issue is the first time you fill an empty tank that you pass through the point where a spark can cause a fire or explosion.

The problem with TWA800 is that they let the tank get empty. A few hundred pounds of Jet-A would have saved the day. Newer aircraft today come with an inerting system that displaces the oxygen in the fuel tanks with nitrogen so it can not ignite.

Okie
 
Woosie
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RE: Do All Airliners Have Live Wires In The Fuel Tanks

Tue Jun 02, 2009 7:13 am

C urrently, Yes. I believe the 787 does not have electrical wiring inside the fuel tanks; possibly the A380 doesn't either. Fuel quantity gauging systems insert low current, low power into the tank, to measure capacitance across fuel probes. They are then summed and junction probes and this value is read in the cockpit indicators. Some airplane types also run electrical wiring for fuel pumps in the tanks too.
 
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HAWK21M
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RE: Do All Airliners Have Live Wires In The Fuel Tanks

Tue Jun 02, 2009 9:19 am

The Insulation has to ensure no arcing reaches the fuel vapours.
There are constant call out to check the insulation sleeve ends of wiring attachment to components within the fuel tank.
Also a proposal to fill the Fuel tank with an inert gas was considered.

regds
MEL
I may not win often, but I damn well never lose!!! ;)
 
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Starlionblue
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RE: Do All Airliners Have Live Wires In The Fuel Tanks

Tue Jun 02, 2009 12:54 pm



Quoting Ps76 (Thread starter):
I know that jet fuel won't ignite easily but anything else to help put my mind at rest would be cool.

Jet fuel is VERY hard to ignite. Vapors are easier, but still hard. IIRC TWA800 only happened because of some pretty far out circumstances.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
Ps76
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RE: Do All Airliners Have Live Wires In The Fuel Tanks

Tue Jun 02, 2009 7:57 pm

Hi,

Just to say many thanks for the replies. Today I had a flight and hardly worried about the fuel tank exsploding at all so maybe there's hope for me still(!)

Many thanks,

Pierre.
 
tdscanuck
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RE: Do All Airliners Have Live Wires In The Fuel Tanks

Thu Jun 04, 2009 9:29 pm



Quoting Ps76 (Thread starter):
Was just wondering if all airliners have electrical wiring in their fuel tanks

Generally, yes.

Quoting Ps76 (Thread starter):
or what is there to prevent an incident to TWA800 happening again

After TWA800, the FAA issued a special federal aviation regulation (SFAR88) that required all the OEM's to totally redo their fuel system safety analysis. The details would go on for pages but, suffice to say, it resulted in what is probably the biggest upgrade/retrofit program in the history of commercial aviation. Some models had upwards of two dozen modifications to decrease the possibility of a fuel tank ignition.

Quoting 474218 (Reply 1):
The secret is to keep the wires submersed in fuel so they can't spark.

Although that does work, it's not the design criteria. The only acceptable criteria today are:
1) Ensure insufficient energy to generate a spark or
2) Ensure that a spark is contained and cannot ignite the fuel tank ullage

Almost everything in the fuel tank is designed to 1) except the boost pumps. They need so much power than 1) isn't an option, so they use 2).

Quoting Okie (Reply 2):
The problem with TWA800 is that they let the tank get empty. A few hundred pounds of Jet-A would have saved the day.

TWA800 had a few hundred pounds of Jet-A (probably more than that). The dead volume in a fuel tank that size is considerable. The only way to get a tank truly empty is to sump it, then crawl inside with absorbers. Neither one of those things was done prior to TWA800.

Tom.
 
474218
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RE: Do All Airliners Have Live Wires In The Fuel Tanks

Thu Jun 04, 2009 9:43 pm



Quoting Tdscanuck (Reply 7):
Although that does work, it's not the design criteria.

Shortly after the TWA800 incident the FAA mandated that all aircraft equipped with fuel tanks in the center wing box be operated minimum amount of fuel in the tank. This was to insure all wiring would be submerged. I am sure that rule has been rescinded but when it was I don't know.
 
soon7x7
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RE: Do All Airliners Have Live Wires In The Fuel Tanks

Fri Jun 05, 2009 12:37 pm

Yes they do, the scavenge pumps and fuel probes are of the low voltage type...pictured below is a center wing tank fuel probe from a 747/200. The end with the black cap is the end that is submerged in the fuel. Generally speaking, this is the bottom end of the probe, it is rather long (tall).
Big version: Width: 1024 Height: 683 File size: 550kb
747/200 cntrwing tank probe...
 
boeing767mech
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RE: Do All Airliners Have Live Wires In The Fuel Tanks

Fri Jun 05, 2009 6:26 pm



Quoting HAWK21M (Reply 4):
Also a proposal to fill the Fuel tank with an inert gas was considered.

It is fitted on our new 737-800's, check out ATA 47 for the new Nitrogen Generating System (NGS)

Quoting 474218 (Reply 8):

Shortly after the TWA800 incident the FAA mandated that all aircraft equipped with fuel tanks in the center wing box be operated minimum amount of fuel in the tank. This was to insure all wiring would be submerged. I am sure that rule has been rescinded but when it was I don't know.

We run our tanks empty.

David
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