Electrical wiring problem

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GAHorn
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Electrical wiring problem

Post by GAHorn »

Careless wiring or overlooked maintenance of wiring installation can lead to a bad day.

The wing tip lights electrical wiring was not routed properly through rubber grommets through the wing frames. The pilot selected the on position for his wing tip strobes prior to T/O and the wing blew up. Had the potential to ruin his day had this occurred in flight.
outside.JPG
inside wing.JPG
rear view.JPG
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons. ;)
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KG
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Re: Electrical wiring problem

Post by KG »

WOW.... makes me glad I put a rubber grommet around that wire that I found that a mechanic had left unprotected last year.
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bagarre
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Re: Electrical wiring problem

Post by bagarre »

Holy smokes! Talk about the best place for a bad thing to happen. (Down here instead of Up there) With that kind of luck, I'd be taking the next flight to Vegas.

Is there going to be more information coming on this?
I imagine there are quite a few strobe installations that don't have the wires running thru grommets.
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jrenwick
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Re: Electrical wiring problem

Post by jrenwick »

I don't understand why this didn't result in just a popped circuit breaker. Were there fuel vapors in the wing? Wouldn't that be a hazardous condition by itself?
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blueldr
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Re: Electrical wiring problem

Post by blueldr »

IT APPEARS TO ME THAT THERE MAY HAVE BEEN A SPARK IGNITION, BUT THERE ALSO HAD TO BE A SERIOUS ENOUGH FUEL LEAK TO PROVIDE AN EXPLOSIVE MIXTURE. I WOULD KIND OF EXPECT THAT MUCH FUEL LEAKAGE TO BE DETECTABLE.

(i struck that damn key again, didn't i)
BL
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GAHorn
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Re: Electrical wiring problem

Post by GAHorn »

It doesn't take that much fuel vapor to make an explosive mixture. I recall a fire demo where the demonstrator took an eyedropper and placed 7 drops of gasoline in a 5-gallon tin-pail and swirled the pail around a bit..then hooked it up to a sparkplug and a coil. He took a step behind a wall and flipped the switch ....and that pail went 30 feet into the air and split apart like it had a stick of dynamite in it.

A wee bit of fuel vapor must have been in that wing. (Navajos have bladder tanks.)

Think again about repairing your overhead fuel vent line when you occasionally smell fuel (and what kind of hose you want to use to do it.)

A circuit breaker will not "pop" until AFTER the open-short has occured and AFTER the current has been sent down the line. :?
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons. ;)
HA
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Re: Electrical wiring problem

Post by HA »

we had an identical problem on a Duke we owned that was on it's way to the Middle East back in about 1991- it blew up the RH wing on takeoff in the Azores but the pilot managed to set it down, then I got elected to go to Santa Maria and put on a new wing to get the airplane going again. These pix look very similar to the ones I have of our Duke wing, caused by a chafed strobe light wire and an apparently cracked vent hose in the wingtip area.

taught me to look real carefully at wiring runs ever after, I still keep the chafed wire in my toolbox.
'56 "C170 and change"
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n2582d
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Re: Electrical wiring problem

Post by n2582d »

In the early '80s I worked with a mechanic at Mission Aviation Fellowship (MAF) whose father was the first MAF pilot to die in a fatal accident. If I recall correctly they thought that the C-180 exploded when he keyed the mike to his HF radio. A short circuit and fuel vapor were a lethal combination.

On the Navajo is the outboard bladder tank just inboard of where picture # 2 was taken? I see the fueling decal in picture #3. It's surprising that the explosion didn't ignite fuel in the outboard bladder tank. In fact, picture #2 doesn't show any heat or fire damage that I can see--must have been just vapor without any liquid fuel to burn. Almost looks like the wing was over-pressurized. I wonder if fuel vapor could have got in the wing if the fueler accidentally over-fueled that outboard tank.

The 170 didn't originally come with rubber grommets in the "D" ribs forward of the front spar. The nav. and landing light wires pass through aluminum grommets much like the ones you'd see on a tarp. They give more surface area than the edge of the rib skin and also have a round edge so chaffing is minimal. It would be extremely difficult to add rubber grommets inside these aluminum ones. If one is concerned about wires chaffing here a dab of RTV at each aluminum grommet might be the easiest solution to eliminate relative movement between the rib and wires. A more pressing need in many of our planes is the old wiring that needs to be replaced with Tefzel wiring.

For my strobe lights I bought a A413A power supply. This is the 6" square one that mounts in the fuselage. From a safety standpoint I think the two wingtip mounted power supplies (A490) would have been a better choice since they eliminate the long run of high voltage wire through the wings.

My problem is that the grommets in the "D" ribs are too small to run all the wires through the left wing. In addition to the strobe light cable there are wires for the nav. lights, landing lights, stall warning, and pitot heat. I was thinking of running a Poly-Flo tubing through the lightening holes in the ribs and then using the Poly-Flo as a conduit for the wiring to the wingtip. But AC43.13-1B 11-96(k) says, "Insulating tubing must be kept at a minimum and must be used to protect wire and cable from abrasion, chafing, exposure to fluid, and other conditions which could affect the cable insulation. However; the use of insulating tubing for support of wires and cable in lieu of stand-offs is prohibited." I'm guessing the reason for keeping insulating tubing to a minimum is to avoid having the wires get too hot. How have you others that have wingtip strobes run the wires for them?
Gary
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falco
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Re: Electrical wiring problem

Post by falco »

What is required to replace the old wiring with new?

Just a logbook entry?

Asking about regs, not best practice, modifications or philosophy.
bagarre
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Re: Electrical wiring problem

Post by bagarre »

Replacing existing wires is a log book entry but not an aircraft owner thing. A&P required.

My strobe wires run thru the open D section in leading edge as well.
I imagine there are more installed like this than not.
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Bruce Fenstermacher
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Re: Electrical wiring problem

Post by Bruce Fenstermacher »

falco wrote:What is required to replace the old wiring with new?

Just a logbook entry?

Asking about regs, not best practice, modifications or philosophy.
It would be a log entry by an appropriately rated individual.
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minton
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Re: Electrical wiring problem

Post by minton »

Brings to mind the transport of propane/gasoline and warm weather. Same senario. Large flash and no airplane!
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GAHorn
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Re: Electrical wiring problem

Post by GAHorn »

n2582d wrote:...My problem is that the grommets in the "D" ribs are too small to run all the wires through the left wing. In addition to the strobe light cable there are wires for the nav. lights, landing lights, stall warning, and pitot heat. ... How have you others that have wingtip strobes run the wires for them?
There's no telling what kind of cable was used for the strobe light installation in that Navajo, but Whelen's official part for the job is a double insulated, and then shielded, 3-wire cable with an integral bare ground. It's difficult to imagine that Whelen cable ever being a problem.
Image

To illustrate the aluminum grommets:

Here''s my left wing, outboard of the land/taxi lights, with Whelen Strobe cable and nav light wire (my airplane has been completely re-wired with modern Tefzel) running thru the factory grommets in the D-rib.
Image

Here's the right wing with the same wiring as above...plus additional two wires for aux land/taxi lamps. No room for anything more obviously.
Image
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons. ;)
steve grewing
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Re: Electrical wiring problem

Post by steve grewing »

There is an old AD on Whelen strobe flash tubes which addresses the same issue. My plane had these old style flash tubes (pre-1974) when I refurbished it. There may still be some in the fleet.
I also completely rewired my plane with modern wiring.

Here's the AD for those interested.

75-05-04 WHELEN ENGINEERING COMPANY INCORPORATED: Amendment 39-2098. Applies to aircraft incorporating Whelen Engineering Company Incorporated A427 strobe light flash tubes manufactured before November 1, 1974. Compliance required within the next 100 hours time in service unless already accomplished.

(a) To preclude possible ignition of flammable fluids or vapors by arcing at the strobe light flash tube, install on the base of Whelen Engineering Company Incorporated A427 flash tubes a pressure sensitive vinyl label conforming to Whelen Engineering Company Incorporated Drawing A-30052, Revision 1, dated October 15, 1974, or latter FAA approved revision. Scotch Brand Type 33+ vinyl plastic electrical tape or equivalent tape approved by an FAA Maintenance/Avionics Inspector can be used in lieu of the vinyl label. If vinyl plastic electrical tape is used, it must be formed to cover the rivet at the rear of the flash tube without covering the identifying part number. If the flash tube incorporates a label, the new label or tape may be installed directly over the old label. Install the label or tape only when the label or tape and the flash tube are at a temperature above 50 degrees F.

(b) The installation required by this AD constitutes preventive maintenance and may be performed by persons authorized to perform preventive maintenance under FAR 43.

(c) Upon request with substantiating data submitted through an FAA Maintenance/Avionics Inspector, equivalent methods of compliance with this AD may be approved by the Chief, Engineering and Manufacturing Branch, Federal Aviation Administration, New England Region.

NOTE: For the requirements regarding the listing of compliance and method of compliance with this AD in the airplane's permanent maintenance record, see FAR 91.173.

This amendment becomes effective thirty (30) days after publication in the Federal Register.

Steve
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GAHorn
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Re: Electrical wiring problem

Post by GAHorn »

In a later issue of the Aircraft Safety Network, additional information was released about the Navajo incident which began this thread. It appears the pilot had actually begun his TAKE OFF ROLL and when the airplane had progressed about 700 feet down the runway is when the right wing blew-apart due to a fuel-vent line leak and an arcing strobe-light wire which had not been properly secured and insulated with grommets.

That pilot doubtlessly said some grateful prayers that day!

https://aviation-safety.net/wikibase/157073
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons. ;)
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