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Date: Sun, 07 Dec 2008 10:13:09 -0500
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From: "Robert Pastusek" <rpastusek@htii.com>
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Subject: RE: [LML] Fuel Tank vents
Organization: Holmes-Tucker International, Inc.
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Dan Schaefer wrote:

I installed similar check valves in my LNC2 following an interesting 
experience involving a mud-dauber who thought my left wing tank vent was the

ideal place for, well, mud-daubing.

On a trip where I had to park the airplane outside for a week, I took off to

return home and just by chance, glanced at the left wing while I pumped fuel

from it. The light was just right for me to notice that the left wing 
surface was showing it's ribs a bit - something I had never noticed before. 
I landed to check it out and when I opened the filler cap there was a 
noticeable "Whoosh". Long story short, I found the dauber's contribution to 
my fuel system just out of sight in the vent. Took a bit of digging out with

a length of wire followed by over-filling the tank to wash the debris from 
the vent as best I could.

To avert this in the future, I installed tee in all three of the vent lines 
and connected an in-flowing check valve to each one. A check-valve was used 
to retain the pressure recovery from the main vents plus the original ones 
were already protruding thru the skins and I didn't want to remove them. 
Over the open ends of the check-valves, I attached a couple layers of 
cheese-cloth to keep other industrious bugs (or ice or other stuff) from 
plugging them. My tank vent system allowed me to place the valves in the 
wheel wells.

I'm using Facet pumps for fuel transfer (which allows flow-thru when 
de-energized) so, during a climb, should the primary vents become blocked, a

rise in tank pressure relative to ambient, will merely push fuel thru the 
transfer pumps and into the header. Conversely, the inflowing check-valves 
will allow tank pressure equalization during a descent.

A flight test with the primary vents purposely plugged verified that the mod

worked as expected.

Incidentally, when I must leave the airplane parked outside now, I stick a 
pipe cleaner (dyed red with a marker pen) in each vent and pitot tube, just 
to add a belt to the suspenders.

Dan Schaefer
LNC2   N235SP 

Dan,

Having seen a few posts about this over the years, I am now motivated to
comment... Bobby Thomas, the DAR that inspected and signed off my IV-P, was
concerned about "exposure" of the fuel tank vents. Seems he investigated a
crash believed to be caused by exactly what you described above, and I once
took off without any airspeed because of this. 

The tank vents on my IV-P are fed from small NACA ducts in the winglets. I
flattened the ends of the 3/8" tubing that attach to the NACA duct so they
present an oval opening to curious mud-daubers and leaf-rollers. My thinking
was that these fellows are discriminating about their choice of nesting
area, and wouldn't give an oval hole a second look--but now I'm not so sure.
Bobby Thomas encouraged me to make some plugs from foam or similar, and to
use them as religiously as I do the pitot covers. I haven't taken his
advice--yet--but now wondering about the experience/views of others...

Bob Pastusek
150 hours and flying...