Fuel tank sending unit. and KABOOM

[Dmichels]

I have always wondered about this. Your sender fails now you have a little spark between the brake in the wire. You have about 5 volts heading to the tank from the gauge. A little spark is all it takes. I have never worried much about it because there must be something I am missing that prevents sparks from inside the sending unit. But I have always thought it was a little nuts having an electric device inside a tank of gas.
Dave

[flyinlow]

I think it is because the air / fuel ratio it the tank is too rich to burn.

[Dmichels]

I thought about that too. Sounds like bad odds to me, too many what ifs

[Bob T]

True, a little spark could well do it I guess.
I seem to remember reading somewhere that the IRA used to run a wire from a sparkplug down the filler tube as a ignition source as a cheap way to blow up the next person that started that car

[Dino]

There is no wire on the sending unit, only a ground on the outside.  The part that makes the needle move works on résistance.  A small metal tab travels over a coiled metal strip inside the housing whenever the float moves, this measured resistance goes to the gauge.  Nothing in there to spark.

[Dmichels]

Dino
It is competing the circuit to the gauge There is juice going threw it
Dave

[flyinlow]

When I fly from N.Y. to Tokyo I start with about 50,000 gallons of fuel in eight tanks. On arrival 13 hours later four tanks will be  empty except for a few gallons in the fuel pump wells you can't get out.  The pumps run on 115V AC ,two or more per tank.  For years the wisdom was the mixture was too rich for combustion even in the mostly empty tanks. Then TWA lost flight 800 ( 747 also) . The NTSB said a spark ignited the fuel vapors building pressure in the tank until it  caused structural failure. Now they are looking at Nitrogen
systems to fill tanks as the fuel is burned off.  

If you left the gas cap off and had an ignition source near the filler neck.    maybe. The only fuel tank ignition events I know of personally, were because of the car being in an accident.


As always, good maintenance and operational practices will minimize mechanical difficulty's on any vehicle.  

[myk]

What about cars with in-tank electric pumps? 

As I was dropping the tank and extracting the in-tank electric pump from my 90's Mustang, it didn't occur to me until that moment that I was possibly looking at the last moment in my life. 

I'm told that there has to be a proper mixture of air and gas (vapor) for an explosion to happen; didn't make me feel any better about it though...

[Nacho-RT74]

What about cars with in-tank electric pumps? 

As I was dropping the tank and extracting the in-tank electric pump from my 90's Mustang, it didn't occur to me until that moment that I was possibly looking at the last moment in my life. 

I'm told that there has to be a proper mixture of air and gas (vapor) for an explosion to happen; didn't make me feel any better about it though...

I was to talk about that. Once I went to a meeting and exposition given by Local Federal Mogul representants, and they explained that the electric fuel pumps work with both poles ( - and + ) and never will get an explosion, because the gas itself without oxigen won't blown ever. In fact the fuel pum requieres the gas inside to keep it lubricated

[John_Kunkel]

Then TWA lost flight 800 ( 747 also) . The NTSB said a spark ignited the fuel vapors building pressure in the tank until it  caused structural failure.

Was the source of the spark ever determined in that incident? Modern jets usually have capacitive fuel level sensors that are explosion proof....always thought a pump or wiring was a more logical source.

[flyinlow]

Then TWA lost flight 800 ( 747 also) . The NTSB said a spark ignited the fuel vapors building pressure in the tank until it  caused structural failure.

Was the source of the spark ever determined in that incident? Modern jets usually have capacitive fuel level sensors that are explosion proof....always thought a pump or wiring was a more logical source.

I thought the NTSB said the probable cause was fuel indicator wiring. Heat build in that tank  on the ground helped vaporize the JetA making it easier to ignite.
You know how high energy the ignition systems are on turbofans just to get the fuel to light off , its hard to believe that big of spark would be produced by the fuel quantity?

Since that accident they have replaced /inspected all fuel pump wiring. They are three phase / 115v pumps with 30 amp breakers (if I remember correctly). I would suspect that first personally. TWA 800 changed our procedures, we can not reset those breakers in flight if they trip.

[Bob T]

I'm guessing here, but wouldn't the breakers be of a type that would measure imbalance or leakage to earth in milliamps to initiate primary trip rather than relying on full load 30A to trip? Got involved with some chemical plant/explosive area jobs a while back and a lot of the control gear also relied on magnetic barriers which were intrinsically safe to negate sparking at the contacts. Not sure what and how aircraft setups are configured, interesting though, as its usually an event which then results in a systems change
...just trying to add value or personal experience here, not trying to pick a fight or promote myself as a know all       

[flyinlow]

I'm guessing here, but wouldn't the breakers be of a type that would measure imbalance or leakage to earth in milliamps to initiate primary trip rather than relying on full load 30A to trip? Got involved with some chemical plant/explosive area jobs a while back and a lot of the control gear also relied on magnetic barriers which were intrinsically safe to negate sparking at the contacts. Not sure what and how aircraft setups are configured, interesting though, as its usually an event which then results in a systems change
...just trying to add value or personal experience here, not trying to pick a fight or promote myself as a know all      

You mean more like a ground fault breaker? Not sure. The part I can see, the trip indicator, looks like a regular breaker .   

[Bob T]

I'm guessing here, but wouldn't the breakers be of a type that would measure imbalance or leakage to earth in milliamps to initiate primary trip rather than relying on full load 30A to trip? Got involved with some chemical plant/explosive area jobs a while back and a lot of the control gear also relied on magnetic barriers which were intrinsically safe to negate sparking at the contacts. Not sure what and how aircraft setups are configured, interesting though, as its usually an event which then results in a systems change
...just trying to add value or personal experience here, not trying to pick a fight or promote myself as a know all      

You mean more like a ground fault breaker? Not sure. The part I can see, the trip indicator, looks like a regular breaker .   

Ground fault, yes, was'nt sure what you guys call them

[John_Kunkel]

GFI's are for AC circuits, not DC.

[Bob T]

GFI's are for AC circuits, not DC.

Good point, I hadn't considered what the supply was.

[flyinlow]

I stand corrected. Yesterday I got the chance to  ask one of our Tech Ops Supervisors if there was anything special about the fuel pump circuit breakers. His reply was that the breaker on the flight deck was a simple control breaker that was used with the remote 3 phase AC  GFCI style breaker in the avionic compartment....  Learned something.