Fuel tank wire testing - best practices
Fuel tank wire testing - best practices
(OP)
What would be the best fuel tank wire testing for Insulation to ground and insulation between conductors, considering that meggering is very risky?
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Fuel tank wire testing - best practices
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Fuel tank wire testing - best practicesFuel tank wire testing - best practices(OP)
What would be the best fuel tank wire testing for Insulation to ground and insulation between conductors, considering that meggering is very risky?
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RE: Fuel tank wire testing - best practices
Do you not have an AMM to refer to for this? What is the a/c type?
Remember that fuel vapour explosion risks relate to values of a number of factors, which if known and controlled can enable you to carry out such tests safely.
RE: Fuel tank wire testing - best practices
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RE: Fuel tank wire testing - best practices
You wouldn't want to meg into a probe anyway.
FQIS harnesses are wet, but pump/valve wiring is usually in conduit. A DMM will tell you if there is a problem in the conduit.
RE: Fuel tank wire testing - best practices
RE: Fuel tank wire testing - best practices
If you find that you cannot find an alternative to using a Megger, then purge the tank with dry nitrogen and ensure that all of the oxygen is out before you do.
B.E.
RE: Fuel tank wire testing - best practices
The PSD/GTF type FQIS test sets MEG the wires and probes and can be accomplished with the systems installed. The reason I would not use a Megger is that I do not know at what level the PSD test equipment operate at. A megger can supply 250, 500 or 1000 VDC (low amp). I do not know what output the qualified FQIS test equipment operates at and therefore would not recommend using one unless there were procedures stating what settings to use. Damage to probes or even electrical arc could occur.
Another thing to think about is that if there is a wiring problem in the tank, chances are your only option is to replace the wire/harness. All aircraft I ever worked with do not allow splice repairs to wire harness inside the tank and harnesses are potted.
RE: Fuel tank wire testing - best practices
I agree that you are unlikely to be able to do a worthwhile repair, even by returning to the manufacturer, so as you've obviously got the tanks open already to vent them (or will shortly do so,) you might want to do a visual inspection.
You might be lucky and find it's just a loose connection. If you rely on the Megger (or alternative) you might end up ordering a replacement harness unnecessarily in this case. Fuel harnesses aren't cheap of course.
If you end up Meggering, don't forget to be sure to isolate the harness from the FQIS. You don't want more problems than you have already.
RE: Fuel tank wire testing - best practices
RE: Fuel tank wire testing - best practices
Maybe this will help you
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Regards
Fernando
RE: Fuel tank wire testing - best practices
RE: Fuel tank wire testing - best practices
RE: Fuel tank wire testing - best practices
To MiketheEngineer. Without doing a very long post that goes way off thread, there are finally a lot of long overdue issues with wiring in tanks and aircraft wiring in general now being addressed. It will though be a while yet before we see the disappearance of in-tank fuel harnesses on new medium and large aircraft, and a lot longer before the last in-tank harness leaves the sky.
RE: Fuel tank wire testing - best practices
RE: Fuel tank wire testing - best practices
When I was younger, and even stupider, I decided to fix a pinhole in the bottom of a terneplate 14 gallon gasoline tank by soldering a penny over it.
I chose a windy day, drained the tank, removed the filler cap and the sender unit (which left a decent size handhole, swabbed the remaining liquid out of the bottom with a rag on a stick, and wiped down all the accessible surfaces, until there was no liquid fuel remaining, and no detectable odor of fuel. Then I let it sit for an hour so the wind could further ventilate it.
You can see where this is going.
You know how you learned in college that a little lighter fluid dribbled on the mouth of an empty beer bottle makes an amusing display as flame shoots out of the mouth while the flame front proceeds down inside the bottle in search of more oxygen? Same thing, on a much larger scale.
First application of the torch to the pinhole set off the vapor and air in the tank. Got a really pretty blue flame out of the sender hole, and a smaller one from the filler cap, and a loud acoustic noise, sort of a cross between a moan and whoosh. Not some gentle lick of flame either; pretty decent velocity, extending out maybe five feet. It burned out in about a minute, while I was wondering about the tank seams.
It cooled off enough to touch in half an hour, by which time the air supply in the tank had been replenished. The second flame was just a little less violent than the first, and didn't last for quite as long.
Recurse eight more times until the fuel supply was actually exhausted, and I was able to sweat the penny on nicely.
Next time I'll just use a sheet metal screw and a little Permatex.
Mike Halloran
Pembroke Pines, FL, USA
RE: Fuel tank wire testing - best practices
The above says it all. Nitrogen purge during the test should be mandatory. I think you need to make sure that the insulation is not cracking anywhere in the wire run. If the wire is in a bundle it may have short path through to another wire, not necessarily airframe ground. Take a look at whatever gaseous residuals or fuel is in the tank, obviously these things will have different dielectric constants, which is something to think about.
RE: Fuel tank wire testing - best practices
As a last resort, high potential testers may be used in fuel areas provided they are drained, purged and maintained at less than 20% LEL.
Funny thing about the military....they have tech data for everything!
RE: Fuel tank wire testing - best practices
RE: Fuel tank wire testing - best practices
Sorry, the company I work for gets these books for me through a contract that does not allow redistribution of any data.
I would like to, but can not.
RE: Fuel tank wire testing - best practices
Eng3000: almost nobody has a TDR. They are soooo expensive - but sweet. You are right about the tank wiring. People have a bit of a cavalier attitude towards fuel tank - we burned out two aircraft in a hangar because we didn't bother with bonding the (plastic) bucket or earthing the aeroplane that we were draining the AvGas from.
Onemike: You are right to question. Your situation may have seemed obvious - consult the AMM and do that BUT you must question everything. Good Man!!
RE: Fuel tank wire testing - best practices
Copy from referenced T.O.:
a- High potential voltage or insulation breakdown testing of aircraft electrical wiring and cabling will be conducted as a last resort after all previous methods of detecting faulty cabling have been accomplished. These methods shall consist of continuity, visual inspection, common voltmeter and ohmmeter checks, and other methods not involving high potential voltage tests.
WARNING
Potential explosive hazards may exist. Equipment operators shall become thoroughly
familiar with operational characteristics of the testers prior to use.
b- The following procedures will apply where tests, employing high potential testers, are made to check aircraft electrical cabling and wiring involving those areas where fuels, fumes, or explosive vapors may possibly be present.
(1) Providing fumes or vapors can be removed by isolating fuel from the areas involved by means of shut-off or bypass
valves, only that portion of the fuel system which is isolated must be drained, purged, and maintained at less than 20% LEL (Lower Explosive Limit).
(2) Providing adequate isolation cannot be accomplished, the entire fuel system will be drained, purged, and maintained at less than 20% LEL (Lower Explosive Limit).