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One thing I did not mention in my previous posting was the known problem of
drilling through the fuel sealant. The early Lancairs used an elastic fuel
sealant (dark brown in color), not the harder Jeffco sealant used after
c.1996. It was stated that a small hole or several small holes were drilled
to vent the fuel. If these hole(s) were drilled in the logical place then
they would be located forward of the fuel filler at the top of the main
spar. During application of the fuel sealant the wings are upside down and
the sealant tends to collect at the bottom (aircraft top) of vertical
surfaces and can result in a coating thickness of several millimeters. As
you are probably aware, cutting through an elastomer with a small diameter
drill does not leave a clean hole. Given the statement that they had to let
the tank settle for 15+ minutes indicates that the vent hole was tiny (less
than 0.010" diameter). It would be quite easy for this size hole to become
plugged or function like a check valve.

One possible scenario is this. If the fuel vent hole became plugged, perhaps
due to a change in temperature or pressure, while the tanks were less than
2/3 full then enough air would be trapped to cause less fuel to be available
than was indicating at the next fill up. It works like this. The D section
has two connections to the fuel tank aft of the main spar, the fuel cross
over tube at the bottom and the 1/4" vent hole at the top. If the fuel vent
is plugged then the D section is only connected at the bottom of the tank,
like a Cartesian Diver. On climb out the air in the D section expands and
air bubbles out of the lower crossover tube into the main part of the tank.

On decent the air contracts and draws fuel into the D section. This fuel is
no longer "usable".

Having the fuel drop several gallons more than usual would NOT necessarily
alarm the pilot. This is because the fuel probe location in the IV wing
results in a reading several gallons low when the aircraft is pitched down.
Experienced IV pilots will check fuel quantity and plan fuel use just prior
to decent because as soon as you start a decent the gages are not accurate.
The pilot would have no way of knowing that 2-6 gallons of fuel that was
usable at FL200 is no longer usable at 7,000 ASL. In this scenario it would
be possible to believe that you have 80 gallons of usable fuel for a flight
when you actually have only 68 gallons, cutting your endurance by 36
minutes.

Also, the shock of the crash could unplug the vent, allowing several gallons
back into the main tank.

A chill ran through my spine when this scenario occurred to me because
despite the pilot not doing anything stupid, the aircraft can run out of
fuel and then have fuel in the tank again once on the ground. A mechanical
problem that masks itself as pilot error. I would hate to see the default
"pilot error" cause assigned to this accident when there was a mechanical
problem at the root.

Please be assured that I am not accusing anyone of anything so, please, no
hate mail. I do not see any harm in bringing this possible scenario to the
attention of the investigators so that they may inspect the D section air
vent holes.

Regards
Brent Regan



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