fin/wing tanks freezing

On Thursday, June 6, 2013 3:41:20 AM UTC+12, Matt Herron Jr. wrote:
Can anyone share some wisdom on using water at high elevations for long durations? How do you know your fin or wing tanks will not freeze? If I am at 18K for 6 hrs in the Sierras, I really don't want my vertical stab splitting open in flight. Has this ever happened? Any guidance would be appreciated.



Matt

I fly at Omarama and we spend a lot of time high and cold. -10 to -20 C is common, sometimes less. As I mostly fly a Duo Discus I almost always have tail water and occasionally wing water.

I've never heard of any-one having a problem with wing water causing structural problems by freezing in flight. What does cause problems is the dump valves freezing and being unable to dump. You also see people having problems with leaking valves. They land with enormous ice sculptures under the wing.

Tail water does freeze, but I've never seen it break anything structural. The dump valves do fail when they freeze. Water going out the overflow holes freezes and can jam the rudder. I have seen a thin rod of ice forced out of the top overflow hole.

Some people do use anti-freeze in the tail, others prefer not to due to fears that the anti-freeze will damage the seals and such.

The real danger is if you load up with water and the tail valve freezes. The wings dump and you are left with the tail water and an aft C of G. In a Duo that isn't a problem because with two people you have the tail mostly full anyway for trim. In a single seater you might get into real trouble.

Different systems have different problems, but actual ballast turning solid and bursting the structure isn't a problem. Dump valves getting damaged and leaking does happen. That's just a nuisance.

Water getting into controls and freezing does happen. Being unable to dump the ballast does happen. That is much more serious.

--
Philip Plane

Had a Nimbus 3 in the shop some years ago with massive structural wing
damage due to freezing tanks. Took the upper wing shells clean off the
spars and/or cracked the shell right through just in front of the spar (the
width of the tanks), and the resulting 'new' aerofoil made flying it very
interesting according to the pilot. Interesting repair as well.

Damage can be massive, control problems and weight/balance problems would
be an issue too with freezing water. Most flight manuals specifically
state: water to be dumped before reaching freezing level. Even when not
using common sense these factors should be enough to make up your mind, I
would say.