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1. 12:57 AM - Still on the "Case of the missing fuel" (MPPalmer@aol.com)
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| Subject: | Still on the "Case of the missing fuel" |
Thanks all for your suggestions so far. Others have emailed me separately,
off the list, suggesting I do the obvious and check our totalizer for accuracy,
so that I'm chasing a red herring. (implied.)
Yes, I agree that's an obvious step. Unfortunately, in our plane, it's not so
easy to do. Not accurately. See, I originally plumbed the Flowscan transducer
after the FRAM fuel filter, just as it passed into the firewall and made a 90
degree turn in that old Glasair I fuel fitting they used to supply. It's a
street type fitting (as opposed to a smooth mandrel bend) and proved to be too
turbulent. We never got steady fuel flow readouts. So, in my brilliance, I
moved the transducer to aft of the electric fuel pump, ahead of the FRAM fuel
filter. The Flowscan is about 10 to 12 linear inches aft of the Faucet electric
fuel pump. This works real well.... except when the electric fuel pump is on.
Then the pulses from the pump pulse the transducer, causing it to read 2 GPH
higher than nominal. (I can turn the pump on with the engine off and watch all
day as the FF indicates about 2 GPH with no fuel moving through the system.)
The RV guys have this problem too and someone sells a $50 snubber that you
Tee into the fuel line to mitigate the problem. Since we only run the electric
pump on take off and landing, it's not been a real problem, and I've thought it
a "feature" to have the totalizer read a little lower than actual remaining.
So that's the long story for why I haven't done the obvious and confirmed the
totalizer is still accurate. But, I think I will fill the tanks and go for an
hour flight without using the electric pump and see how much fuel I use on
refill. When this whole "error" thing started a few months ago, I wondered if
there wasn't some "fuzz" build up (a.k.a. "belly button lint") in the
transducer. (I've seen this before in the finger screens way back in first flight
days.)
I should probably bite the bullet, disconnect the lines to the transducer and
reverse flush it. (It's in an awkward position, it's hot outside! Even in our
nights. And I can't see well enough with only one eye.)
I pulled one wing tip tonight and saw that all our wing tip vent fittings are
behind the end rib. We had put a small inspection hole there and everything
looked fine on that side.
I also ran the electric pump while setting the prop at 8 stations, twice
around. I waited 2 minutes for each station, checking the header tank for any
change in level or for any leaking fuel at the mechanical pump or any fumes or
obvious leaks. None found.
I'm quickly running out of ideas. I will plan to top the main tank off
tomorrow and blow the tip vents out again with the pressure of some fluid behind
them. (Hard to explain in text, but we have a combination GI/GII wing tip vent
system. I think they issued a SB or Advisory suggesting the topology we had come
up with on our own... vent to the tip and tee off that to vent down to the
center and stand pipe.
I haven't pulled the cowl yet to look around the carb... tomorrow's plan. But
I didn't see any leaking for the half hour the pump was on tonight,
pressurizing the carb.
One more possible "clue" I forgot to mention before ... when we ran out of
fuel in flight on the main a few days ago, the main tank fuel gage was showing
about 10 gallons remaining. (Which is what we expected was in there, but upon
fill up, the tank was empty.) I should have looked at the gage again on the
ground... that cork float hasn't been the most reliable thing. Maybe it stuck in
flight and vibrated down on landing? But I'm wondering if I had some kind of
weird suction in the stand pipe that could pull the float up (and somehow pull
gas out too)? I know that when I changed out the FRAM filter this year,
suction didn't break like it should have, and I ended up dribbling fuel for about
15
minutes after I broke the seal at the filter housing. It usually doesn't do
that. (You know, I haven't done a visual inspection to see that the vent lines
down the back of the wing are intact. Since one reporter reported a
failure.... )
Keep those ideas coming, guys. No matter how obvious... or bizarre.
Thanks,
Mike Palmer <><<BR><BR><BR>**************<BR>Gas prices getting you down? Search
AOL Autos
for fuel-efficient used cars.<BR>
(http://autos.aol.com/used?ncid=aolaut00050000000007)
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