pitts_pilot(at)bellsouth.
Guest
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 Posted: Mon Feb 27, 2006 10:49 am Post subject: fittings (was Re: Fuel tank bulletin) |
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G McNutt wrote:
| Quote: |
Hi Donald
Thanks for sharing that information.
Reading about pipe flares today and found that over tightening can cause
damage to the tube possibly causing failure
Very true. AN tubing fittings should be really 'snug' without acting
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like a gorilla. Somewhere (I don't remember where!) I saw some
recommendations on torque ...... and they were surprisingly small.
| Quote: | , also cracks and deformities in a flare are not acceptable.
An understatement, to be sure. Getting a good flare takes the same
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practice that good riveting does, and shares some of the same
operations. I use a file to 'square off the end' of the tubing, making
it flat, and removing most of the 'pinch' caused by the tubing cutter.
I use the 'reamer' to further clean up the inside of the tube end.
Making sure the cone of the flaring tool is centered in the tube, I
turn the handle a little and back it off, much like using a hole tap.
Sometimes I use a silicone lubricant, sometimes not. I know it can't
hurt!!!
After the flaring tool bottoms out ..... think snug again ..... take the
flaring tool off and inspect the inside of the flare fore cracks, the
flare being off-center, and any scoring that may be there. If the flare
isn't perfect .... make another piece.
Also, inspect the backside of the flare too, and the grooves left on the
tube by the flaring tube. The nipple should slide up to the flare
without any dragging. Dress the ridges a little if you need to.
Here's what I find (IMHO) screws up a flare: Cracks are caused by
flaring too fast; give the aluminum time to flow a little ...... and
leaving too much tubing stickiing out of the tubing holder ..... along
with not properly dressing up the end of the tube.
Having the flare cone off center will pretty much guarantee a leak ....
which folks solve by using a pipe extension on their wrench an over
tighten the nut. I've never really seen folks do that ..... but it's a
good (bad) picture!
As for the problem with flop tubes in the tank. IMHO (and you need to
remember that!) I can't for the life of me understand how a properly
flared tube and properly tightened nut work their way loose. I'm sorry,
but from my armchair-quarterback position, I think the nuts weren't
properly tightened. I'm not familiar with the flop tube arrangement, so
there may be something that I'm missing here. Do a simple test. Put
flares on a piece of tubing ..... say 4" long, and tighten a flare
fitting in each end. Hold one end in a vise and put a wrench on the
fitting on the other end (not the nut!) Now turn the fitting (twisting
the tubing) How much torque did you have to apply to make either
fitting slip???? Whatta you mean the tubing failed??? OK. Now put a
wrench on the neu (either one will do, but use the nut on the vise end).
and tell me how much torque it takes to loosen the nut just a little.
Alot? Hmmm. Now, ask yourself just where did all the rotational force
come from to loosen a properly tightened nut??? Vibration???? Ya gotta
be kidding. Vibration that would loosen your flare fittings would be
unmanageable, and your gyro based instruments would fail rapidly as an
indicator.
Now, before you flame me, just remember that I'm making observations
here. I'm not highly trained in tubing/flaring/tightening ...... just
the 25 years of building/owning/maintaining an experimental airplane.
It's so easy to forget to tighten a screw, nut, or any other type of
fastener, while you're building or repairing or ...... Just think of
all the fittings in airplanes all over the globe .... and ask yourself
what would happen if there was even a small percentage of fittings that
loosen under use. They'd have places already drilled for safety wire,
cotter pins, or some other safety method.
OK, now someone else can use the soapbox. All I ask is for y'all to be
methodical and safe out there!!!
Linn ..... I can stand the heat
| Quote: | Two questions for you - did the investigation find that the AN 818 nut
came completely off the bulkhead fitting allowing the pickup tube to
drop off, or had the nut just loosened allowing the fuel pump to suck air?
Was there any previous symptom of a fuel flow problem?
Thanks,
George in Langley BC
>There have been at least 2 accidents due to the loose flare nut fitting. I
>totaled my RV-6 31/2 years ago due to this problem. There was 6.5 gallons
>left in the tank, not 3.5. I informed Vans about it and never received a
>reply. There was a short note in the RVator 2-3 years ago addressing this
>problem. It was done by a contributor, not Van's staff. I prosealed the nuts
>on my current RV-9.
>Don Piermattei
>RV-9 90411
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