On Sun, 16 Mar 2008 12:14:53 -0700 (PDT),
wrote:

On Mar 15, 11:49 am, "Bob Gardner" wrote:
Depends on where you live and the design/condition of the fuel caps. Some
older Cessna fuel caps were notorious for letting water into the tanks.


There's an AD on those caps. The gaskets must be inspected every
year. The gasket (an O-ring) shrinks and cracks with age and exposure
to the sun's heat. If the gasket gets bad enough, the low pressure
atop the wing can suck air out of the tanks faster than the vent can
replace it and bladder tanks will collapse, forcing the fuel overboard

You don't need bladder tanks for that. Some years back I took off on
36. Where 36 crosses 06/24 there is slight rise and that is right
where I rotate. I did a rater agressive rotation into a steep climb.
That was the point where I discovered the caps on both tip tanks were
in need of adjusting. (They felt fine in the preflight, they are like
a thermos bottle top). Both caps blew off at the same instant. Both
tanks were *DRY* by the time I could make it around a tight pattern to
land. Each holds 15-16 gallons. At today's prices that pattern took
over $150 worth of gas not counting what went through the engine.

while the rising bladder bottom lifts the fuel sender float and makes
the gauge read full. Things can get quiet in a little while.
There are kits available to replace the flush cap with a
raised cap, like those found on the 172, and they don't let water in
nearly so much. They have a decent rubber gasket, not an O-ring.

Tanks that aren't full can promote condensation. Mostly it's a
problem on airplanes parked outside where the wings suffer wide
temperature swings between day and night. As the tanks cool (radiative
cooling into a clear sky can really aggravate it) moist air is drawn
in. As cooling progresses further, the moisture condenses out of that
air onto the tank walls, where it runs down and under the fuel. Next
morning, the sun heats the tanks, drives some of the now-drier air out
and leaving the water behind, and makes room for another load that
evening. Airplanes that sit outside for many weeks in humid areas will
get water in the tanks.

Some airplanes don't have sump drains. Sounds stupid, but the
manufacturers can get away with it. Both of the Citabrias we bought
had plugs in thos sump drains, which we replaced with drain valves.
The fuel selector on most Cessna 172/182 and others has a plug in it
too, that's supposed to come out every 100 hours. Most won't bother.
It gets a drain valve, too. Look under the belly, about under the
front of the copilot's seat.

Sometimes you don't get the water out. Bladder-type tanks
can have lateral wrinkles that prevent the water's flowing to the
sump. There's another Cessna AD on that issue. If the wrinkles trap
enough water, turbulence can dislodge it and it ends up filling the
strainer to the point that the carb gets it. Silence ensues.

Some carbs have really small metering jets that won't pass a
droplet of water, due to its surface tension. Not a good scene at all.
More silence. One drop can ruin your whole day. Many carbs have drain
plugs that should come out once in a while to clear out the
accumulated small bits that get past filters, and any water. The
metering jet is a little above the bottom of the float bowl, so some
water can exist in there until it becomes enough to cause trouble.

Water that sits in a tank long enough can absorb the blue dye and
some of the odor. Beware.

Water left in aluminum tanks will corrode them. We've found
corrosion products and pitting in fuel strainer bowls, indicating that
some owners don't bother draining them, and their mechanics never take
them apart. False economy of the worst sort.

Water in fuel come in three forms: dissolved (all fuel has a
little), entrained (suspended water droplets) and free water, the
stuff we find in the test cup. Dissolved water can precipitate into
what looks like "snow" in the fuel in cold weather and plug filters.
Entrained water will do that, too. Neglected free water can freeze in
drain valves and fuel lines, or just plain stop the engine.

Winter mogas, the stuff "without" ethanol, has a little ethanol
in it to prevent line freezup in cars. Seems to work ok in airplanes,
too. It amounts to less than 1%, they tell me. I wouldn't trust it so
much that I don't check for water. Automobile tanks are a different
setup than in airplanes. Controlled venting through filter canisters,
underneath the car where radiative cooling is no hassle, filters that
won't pass water, and so on.


Dan
Roger Halstead (K8RI & ARRL life member)
(N833R, S# CD-2 Worlds oldest Debonair)
www.rogerhalstead.com