Freezing water ballast

[Mark Mocho]

What is a good rule of thumb to decide whether to load water ballast when sub-freezing temperatures are expected at altitude? On a recent flight, I really wished I had water on board because of strong conditions and wind, but temps at 17,000 were only 21 degrees. I am most concerned about freezing the tail ballast, since a block of ice you can't get rid of could move the CG aft of limits. How long does it take for water in the wings and tail to get to a point where freezing becomes a factor?

[christopher behm]

I always wonder about this also. I hear you can add chemical to the water to lower the freezing temp, but that seems like it would be bad for the environment and I have bags, might be bad for the plastic also. 🤔 

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From: rasp...@googlegroups.com <rasp...@googlegroups.com> on behalf of Mark Mocho <markm...@gmail.com>
Sent: Tuesday, May 12, 2026 6:18:24 AM
To: RAS_Prime <rasp...@googlegroups.com>
Subject: [RAS_Prime] Freezing water ballast

 

What is a good rule of thumb to decide whether to load water ballast when sub-freezing temperatures are expected at altitude? On a recent flight, I really wished I had water on board because of strong conditions and wind, but temps at 17,000 were only 21 degrees. I am most concerned about freezing the tail ballast, since a block of ice you can't get rid of could move the CG aft of limits. How long does it take for water in the wings and tail to get to a point where freezing becomes a factor?

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[John Sinclair]

I  had one wing that wouldn’t dump in my Nimbus-3. To my surprise the heavy wing didn’t  drop until the very last part of my landing roll! A frozen tail tank could be a big problem, some have added antifreeze to the tail tank.

[Craig Funston]

On May 12, 2026, at 9:04 AM, 'John Sinclair' via RAS_Prime <rasp...@googlegroups.com> wrote:

I  had one wing that wouldn’t dump in my Nimbus-3. To my surprise the heavy wing didn’t  drop until the very last part of my landing roll! A frozen tail tank could be a big problem, some have added antifreeze to the tail tank.

On Tuesday, May 12, 2026 at 6:18:24 AM UTC-7 Mark Mocho wrote:

What is a good rule of thumb to decide whether to load water ballast when sub-freezing temperatures are expected at altitude? On a recent flight, I really wished I had water on board because of strong conditions and wind, but temps at 17,000 were only 21 degrees. I am most concerned about freezing the tail ballast, since a block of ice you can't get rid of could move the CG aft of limits. How long does it take for water in the wings and tail to get to a point where freezing becomes a factor?

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[Chip Bearden]

I had the same experience with my LS3 when a dump valve malfunctioned, (albeit at Cordele, where freezing isn't the problem). The heavy wing didn't go down until the last few meters of rollout.

Many years ago, I knew pilots who added antifreeze to their water ballast in the spring. Some solutions were more environmentally friendly than others, IIRC.

I don't have experience flying with ballast in cold temps. I have, however, done many long marathon training runs on well-below-freezing days. The danger there was not that the water bottles themselves would freeze solid, even when exposed to temps in the teens, probably because of constant shaking and proximity to my body. But water sloshed up into the super-chilled drinking valves from below. It only took a few droplets freezing in a valve to jam it beyond my ability to loosen it with my fingers.  

Glider ballast systems, on the other hand, have water above the valves, so the constantly sloshing water in the tanks might tend to keep valves from freezing for a while. That said, all it takes is for a thin coating of ice to jam a valve and you've got a serious problem, more so if it's the tail tank (unless the wing tanks also fail to dump, in which case your CG is OK even if you're stuck at a higher gross weight).

Older flexible tanks (bags) have enough extra room in the wings that even freezing solid would probably not overstress a wing. I'm not sure about rigid tanks.

The only experience I have with freezing liquids in a glider involved a pee tube. That was exciting for a few moments until the blockage melted, releasing the tension as well as the pressure in the rapidly expanding external catheter. :)

Chip Bearden

ASW24 JB

[Stéphane Vander Veken]

The only author I remember giving some data in this respect is the late Alain Blanchard in his work "Les Pyrénées en planeur" (p. 122). Altough he doesn't advocate the practice of ballasting a glider fort flight below freezing, he cites two types of antifreeze that are sometimes used: 

1) 95% denatured alcohol (ethanol) mixed with water in following proportions (with freezing point in degrees centigrade/ Celsius): 20% > -9°C; 30% > -17°C; 40% > -26°C; 50% > -38°C.
2) Calcium chloride mixed with water: 20% > -20°C; 30% > -45 (to be mixed in separate canisters long before being used, as the reaction is exothermic).

In both cases, after use, you will have to rinse the ballasts with pure water. And never leave the ballasts filled with these mixes...

[Dan Daly]

In "Ridge Soaring the Bald Eagle Ridge" 6th edition (2007), Tom Knauff had a chart on page 50:

Methanol Alcohol antifreeze chart

1 unit Methanol

            water

50%    2 units  -50C  -58F

35%    3 units  -25C  -13F

25%    4 units  -16C  +2F

20%    5 units  -12C  +9F

17%   6 units   -10C  +14F

14%   7 units   - 8C   +17F

12%   8 units   -7C    +19F

11%   9 units   -6C    +20F

10%  10 units -5C     +23F

He notes not to use common automotive antifreeze which is poisonous to all living things when dumped.

I miss Tom, Doris, and the Ridge.

[Kelvyn Flavall]

I regular fly wave in NZ where the OAT will drop to around 10°F and I almost always carry ballast in the wings (ASG 29 - no bags) but never fill the wing completely.  I don't put any ballast into the tail on wave days.  My main concern is the ice expansion in the fin/dump value area causing serious structural problems.  Perhaps I'm being over cautious.  Some pilots will put anti-freeze into the tail but I just would rather avoid the hassle.  If I do ballast for a thermal/ridge day with water in both the wings and tail then find myself getting into wave I will do a quick dump (<30secs) to get rid of most/all the ballast in the tail and keep as much in the wings as I can.

The ballast in the wings will freeze especially around the valves.  In the smooth wave there's not much turbulence to keep the water sloshing around.  I did have one experience of opening the dump valves as I entered the circuit and quickly realized that only one tank was draining at the full rate.  I tried to close the valves again but the slow draining valve had ice blocking it from closing.  An expediated landing onto the airfield and, as others have reported, the heavy wing didn't cause any issues until the last few seconds of the landing roll.

Now I regularly carry ballast to the airfield (grass strip about a mile long) and haven't had any issues.

On my last flight in Washington State I ballasted both wings (EB 29 - with bags) and the tail.  I didn't notice until I was about 7k' MSL that the OAT was already well below freezing (the freezing level surprisingly being only about 5k').  Not being able to see the tail I was concerned about being unable to see if it was draining if I tried to dump so I kept all the ballast.  I did spend some time up at 11k' and it was really cold.  I carried all the ballast to the ground and it took several minutes for the water in the wings to melt enough to drain.  Everything seems okay, but I don't intend to do that again.

So my rule of thumb is to ballast the wings and have nothing in the tail.  How much to put into the wings is determined by how far forwards I comfortable with having the CoG.

Kelvyn

[Daniel Mockler]

Flying New Mexico has spring time conditions consistently up to 18k with temperatures between 10 and 20 degrees F.  We don’t see temperatures above the mid twenties until the middle of June.

With most flights having percent thermal down near 10%. I spend a lot of time at altitude.

I have remade my tip filler tubes for dumping into a 5 gallon can. I also plug the tail and plan on landing fast. 

This allows me to carry a full load and dump the main tanks before landing. The tips and tail are collected are reused on the next flight.

On extremely cold days, I will dump the main tanks to 90% capacity. 

Dan

3M

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From: rasp...@googlegroups.com <rasp...@googlegroups.com> on behalf of Kelvyn Flavall <kel...@gmail.com>
Sent: Saturday, May 16, 2026 4:31:15 PM
To: RAS_Prime <rasp...@googlegroups.com>
Subject: Re: [RAS_Prime] Re: Freezing water ballast

 

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[Mark Mocho]

Kelvyn-

Thanks for the information. As best you can recollect what was the temperature at the field, what was the temp at altitude and about how long were you above the freezing level? And about how much water were you carrying?

[Kelvyn Flavall]

For the Washington State flight I mentioned I had a look at the IGC file OATs in the K records (via SeeYou).  Looks like ground (1300') temperature was ~59F,  temperature at altitude (10,000+') 10F.  I was above freezing for 3 hours.  The freezing level seems a bit strange as the heights reported as I crossed over 32F (0C) (went below early on and then I immediately glided down to warmer air before catching a good thermal and ascended into the cooler air above again - so 4 crossing points with the final glide) varied between 5000 and 7000 feet.  Perhaps there's some lag in the system or the probe isn't exposed directly to the outside air and it takes a while to settle on a changing temperature.

The NZ flights are often 8 hours pretty much all of which are below freezing temperature.

K

[Ian Molesworth]

I use 'thermal transfer fluid' intended for use in solar water heating systems to prevent freezing in winter. 

The basic material is Ethylene Glycol which is non toxic ( As opposed to Propylene Glycol which is nasty stuff ).Polarised DTX is good down to -20°C and turns to crystallised Gel before becoming a solid. 

Other concentrations ( 1:2 ) are good to -30°C and pure EG will go to around -50°C

So tail tank is safe. 

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[David S]

Thanks for the tip, but you reversed the names: Ethylene Glycol is toxic whereas Propylene Glycol is a food additive. 

   …david

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[Jim Lee]

Mark,

We have similar conditions to Moriarty here in Minden of course.  I ran several experiments in the JS1 on super cold high altitude thermal days with lots of time in the high teens.  I would dump my water at low altitude just before landing, and then close the dump valves.  The next day I would open the dump valves, and no water came out, even in the tail.  Hence, no ice in the wings or the tail.  The tail especially surprised me due to less mass.

The wings and wingtips would be filled to the max, then I would do a 5 second dump to leave some air in the tanks.

I never open the dump valves at altitude for fear of damaging the valves.  I have had leaking valves which produced icicles extending back behind the trailing edge out at the tips, interfering with the flaperons.  Periodic jerks of the stick broke them off. 

I do not carry any water in wave.  A forward cg is advantageous in wave so it doesn't matter.  I use two 50 lb lead plates bolted behind the seat to take me to max allowable of non-lifting parts.

Glider manufacturers use coatings to prevent water migration into the wing structure (mainly foam).  It is my understanding that it takes 3 plies of glass to be impervious to water and I don't think too many manufacturers use 3 plies on the inner skins of the wings.  None of these coatings have been tested with various antifreeze so there is no way to know what antifreeze will do once it gets inside a glider wing.

These coatings do not do the job of keeping water out of the wing structure. There are cracks and voids in the coating in the tanks. Our wings gain weight through the long term use of water ballast.  I shudder to think what antifreeze in the structure will do to it.

After the glider is put in the hangar I drop each wing for 5 minutes and then use a horse syringe to suck it out.  A cup of water comes out of each wing which could not be sloshed out of the dumps.  Do you think fans can dry out a cup of water?  I don't.  Then the fill caps and dump valves are left open to further dry out the wings. 

I bet we could learn a lot from information and/or experiments by Fidel and Steve Hill.  The manufactured weight of a wing is in the glider documents.  I would be interested to know what a 15 year old wing which has had lots of water ballast in it weighs now.

Cheers,

Jim

[Mark Mocho]

Jim-

Thanks for the information. I remember you saying that you had not experienced freezing ballast in the wings, but since we both flew hang gliders together for years, I did not believe you. ;-) I don't worry about the fiberglass and foam absorbing water, since I have bags instead of wet wings. Same with the tail, as it has a bag also. I was more worried about the dump valves freezing while draining, but I wouldn't be dumping at altitude anyway. The tail bag has a 12-volt solenoid valve with a small orifice that would be much more susceptible to freezing because of the slow dump rate, so I would consider propylene glycol as antifreeze. I would also use it in the wing bags. I don't ever expect to have to drink from them and wouldn't even consider it since the water at Moriarty probably isn't fit for human consumption in the first place. And after 15 or 20 years of the residue fermenting in there, there are probably a couple of life forms evolving that could be the subject of a PhD Biology thesis. 

I raised this question because of a flight in April where temperature at 17,500 was 21 degrees, freezing level was 12,000 and I spent a much of the four-hour flight between 15,000 and 17,500. I was flying dry, but the lift and wind conditions had me wishing for a few more pounds of wing loading. Now that summer is getting here, I won't be worrying about it as much. Flying in late fall and early spring is where this information will be helpful.

Now, if I could find a way to keep my feet and knee joints warm without a 12-volt deep-cycle marine battery.....

[Tango Eight]

My JS-1C POH is real specific:  plain water only, no additives of any sort.  

T8

[Ian Molesworth]

Oops! :)

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[Nicholas Kennedy]

If you want to add anti freeze thats non toxic use the RV winterizing formula known as The Pink Stuff

Fairly inexpensive/ works well

Nick 

MS

[Mark Mocho]

That's the propylene glycol I have seen others use. If you want to go with a more expensive route, I understand that propylene glycol is also one of the primary ingredients in vaping fluid. Which really makes me wonder about some people who emit clouds of smoke to avoid smoking.

[Eric Greenwell]

RV Antifreeze provides "burst" protection, because it contracts as it's temperature drops, instead of expanding like water does when it freezes. It goes through a "slush" stage as it cools, then ultimately freezes. Your tail tank won't burst, but it might not drain, either. The Google says: 

"Undiluted standard RV antifreeze (usually propylene glycol-based) remains a pumpable liquid down to about \(+20^{\circ} \text{F}\) to \(+16^{\circ} \text{F}\). Below these temperatures, it will begin to turn into a slushy gel, but it is formulated to provide "burst protection" down to \(-50^{\circ} \text{F}\) without expanding and cracking pipes. [1, 2, 3, 4]"

[Mark Mocho]

Well, I did a scientific test of water ballast freezing potential. My procedure was as follows:

1-Filled the water ballast bags in the wings and tail.

2-Went flying.

3-Got high and cold (15 degrees F at 17,500)

4-Dumped the water at 12,000

Results: Wing water dumped OK. Tail ballast froze enough that I still had over 1/2 gallon that did not dump. (I loaded 1.25 gallons)

Conclusion: Wing Ballast is probably OK, even in sub-freezing conditions, within reason. Tail ballast could probably use some antifreeze.

Other conclusions: As a side note, I discovered that the pee system is another critical factor to consider. I neglected to fully clear the urine dump tube. It froze, and my subsequent attempt to relieve myself resulted in an inflation and sudden release of the contents of the catheter. Discomfort ensued. Some embarrassment on egress from the cockpit. More discomfort until I got home and changed pants. Possible airing out of the cockpit may be necessary, although I believe the seat cushion remained free of contamination. It was enough of a reminder that I do not feel it will be necessary to add it to a checklist. I doubt I will ever forget the lesson, and I am sure my buddies will remind me endlessly.

[Charles Mampe]

Gotcha on the pee tube. A few years ago (southern NY) we had a drought. We were around 8000'  MSL a lot (ground about 600' MSL). I was new to external catheters (used to 1 qt zip lock bags). Used system at cloudbase (about 50*F ground} and left it connected. Later, used again until I had a softball sized catheter. Interesting to pinch the catheter, disconnect the dump tube, put on glareshield to warm, push to outside and blow, keep doing until clear (while still flying), then reconnect and let catheter empty.

Now, when doing a use, I disconnect for a minute and pull external tube (3/8" polyethylene) back inside. No more freezing issue.

[John Sinclair]

I found the baggy to be the best solution with an apple core included so that anyone finding it will think the baggy contains apple juice!

[Mark Mocho]

I prefer not to litter, but if I was so inclined, including an apple core in the ziploc bag is an excellent suggestion

[Daniel Mockler]

And think of what might come from the falling apple core

The Historical Context: During the plague in 1665, Newton left Cambridge and retreated to his childhood home at Woolsthorpe Manor. While sitting in his garden, he witnessed an apple drop.

The "Aha!" Moment: Instead of just wondering why the apple fell, he realized that whatever force pulled the apple also had to extend far into space to keep the Moon locked in its orbit around the Earth.

The Culmination: This realization became the foundation of his law of universal gravitation, formally published in 1687 in his

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From: rasp...@googlegroups.com <rasp...@googlegroups.com> on behalf of Mark Mocho <markm...@gmail.com>
Sent: Saturday, May 23, 2026 1:28:21 PM

To: RAS_Prime <rasp...@googlegroups.com>
Subject: Re: [RAS_Prime] Re: Freezing water ballast

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[Matt Herron]

Don't dump ballast in freezing conditions.  I did it without realizing it was below freezing outside once.  The water streamed back and froze on the root of the vertical stab (about 1.5 inches thick when I landed).  It also froze the static ports closed in the air.  This caused the instruments to slowly go crazy on me to the point i thought I was going crazy.  Airspeed started to climb, so i pulled back.  it kept increasing, but then i realized i heard no wind noise so I choose to ignore the instruments.  At that moment (right above Reno International) i didn't know what to trust as I didn't know what the failure mode was, so i ignored airspeed, altimeter, varios, etc.  Getting back in to Truckee was stressful to say the least.

also wet wings can freeze up surfaces (most likely flaps as they are inboard).  Check function while you still have altitude.

Matt H

[Dave Nadler]

How to prevent pee-tube from freezing: 
https://www.youtube.com/watch?v=QEJJb_ufo8A