Re: REFLECTOR: N19DW Accident, Probable Cause

["James F. Agnew" <Jim_Agnew@ibm.net>]

Simon, changing to 3/8" vent lines will not help a thing if you don't replace the
1/4" aluminum tubing that is used as the vent line entering the tanks.  You will
also have to increase the size of the overboard vent line.  That is the reason
that I have two 3/8" overboard vent lines going to the manifold that I designed.
This  to ensure that each tank had a full size vent line and as a backup should
one of the lines become plugged.  The manifold provides a full 3/8"  common
connection among all vent lines to ensure a balanced source of air and cross feed.

Jim

Simon Aegerter wrote:

> >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?
>
> Dave:
>
> 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
> plausible:
>
> 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.
>
> Best
> Simon
>
> Simon Aegerter, Winterthur, Switzerland

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