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Re: REFLECTOR: N19DW Accident, Probable Cause

>This is priceless knowledge!! If your theory is correct, it would explain the
>uneven fuel flow observed by several other builders, as well as the
>potentially serious nature of this condition.

>One question: Did N19DW have a vent line CONNECTING the two main tanks?


you may be stirring up a hornets nest here and one that has remained
undisturbed for way too long. When I first read Don's report, it didn't
sound very plausible to me for two reasons: 1. why would the boost pump get
the engine to run another "couple of minutes" if the sump was dry? 2. The
new venting systen, introduced several years ago, connects the two wings.

However, I have myself observed a worrying difference between the two
gauges and was unable to find a reason. The difference was not the same on
every flight and sometimes there was none. I too assumed the fuel in the
fuller tank to be unusable once the other one ran dry, so my wife in the
back seat had to call out the readings every 10 minutes or so. The
non-reproduceability of the effect may point to the fuel caps. Don's report
led me to do some order of magnitude estimations to see if it is at all

The pressure differential between the two tanks one empty ond one half full
is about 0.1 psi. Can a leaky fuel cap suck that much pressure? We have a
wing loading of about 15 lb/sft. If I understand the system correctly that
translates to 0.1 psi. So: just barely possible since not all of the wing
load is taken up by sucktion on top; some is pressure from the bottom. We
have about 3 to 4 feet of 1/4" tubing between the tanks. Now the question
is: can a leak in the fuel cap be big enough that 0.1 psi is not enough to
push the same amount of air through these 4 feet of tubing? The amount of
air is proportional to the pressure difference,  the inverse length and the
square of the cross section (the fourth power of the diameter).

So, if the leak is 1" long, it is 100 times shorter than the tubing. The
fourth root of 100 is about 3. That means, if your vent tubing has an inner
dia of 3/16" you need a 1/16" hole in the fuel cap (give or take a factor
of two). Am I missing something?

Considering the wide margin of error, I must conclude that the scenario, as
unlikely as it might seem, could just be plausible.

You know what the first thing is I'm gonna do when I meet my dream machine
again (well, other than fit the new wings and hang the engine)? Yes, I'll
replace the cross-feeding vent lines with 3/8" ones (i.d.!) - at least. I
don't think I can bet my life on sealing the fuel caps each and every time.

Don, thanks for being frank and open about your accident; and Dave, thanks
for raising the issue. This shows again what a wonderful instrument the
reflector is. It is just too bad that it seems to take accidents to get our
brains moving.


Simon Aegerter, Winterthur, Switzerland