Water in my tanks

JGoodish said:
I've heard this "condensation myth" alleged before, supposedly by HVAC "experts" and others who have "done the math." However, after decades, apparently no one has been able to convince the FAA or NTSB, both of which continue to promote the risk of aircraft fuel tank condensation.
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deltaindia said:
This is timely, as there was an accident analysis in this month's Flying magazine involving a Cherokee 6.

NTSB determined that the fuel had water due to condensation/moisture build up over those 2 months of inactivity
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How does the NTSB know he had sumped at any time in the previous 20 years either?

Where was this by the way?

You said it'd been sitting for two months in a hangar half full of fuel. So let's do the math.

The Cherokee 6 has a capacity of 84 gallons (from a google search) thus the air in tanks = 42 gallons.

Assumptions:

Assume a daily range of 20 degrees at 75% relative humidity, 95*F highs, 75*F lows (and that ain't gonna happen...@ 75% relative humidity air doesn't move in temperature that much on a typical day but we'll use 20 degrees as a worse case scenario. Plus air typically isn't 75% RH during the highest temps of the day, it's closer to 65% even in the sultry south)

Assume a 100% exchange of air in the tanks EVERY DAY for 60 days (and that ain't gonna happen either unless the caps were left off and even then I doubt it).

Conversions:

95*F = 35*C
75*F = 24*C
1 oz = 28 grams
1 lb = 0.45 kg
air weighs 1.2 kg / cubic meter
1 cubic meter = 264 gallons

Hard Data & Calculations:

95*F air at 75% RH holds 38 grams of moisture / KG of air
75*F air at 100% RH holds 20 grams of moisture / KG of air

see chart here

42 gallons = 0.19 KG of air

The above described air dumps 18 grams (38-20=18) of moisture per KG per night so 18 x .19 = 3.42 grams dumped per 42 gallons of "air space" in the tanks.

3.42 grams x 60 days = 205 grams

205 grams x .036 = 7.3 oz. of water (in both tanks).

Actual amounts would be a fraction of that...less than 25%...because the temperature in the tanks of a hangared airplane isn't going to fluctuate as much as the outdoor ambient and you'll never achieve a 100% change of air each day.

So, two ounces of water would be reasonable to expect...one ounce per tank.
 
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I can't remember ever finding any significant amount of water in a hangared airplane.

On the other hand, a Mooney 201 that was tied down outside repeatedly had significant amounts of water in the tanks when I sumped it. Changed the fuel cap seals, problem solved.
 
timwinters said:
So, two ounces of water would be reasonable to expect...one ounce per tank.
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And to Tim's well calculated point, if condensation was a significant driver of water in fuel, we'd be finding water in many planes, all the time when we sump -- not decades between events on only some planes, as my casual observation leads.

By some folk's logic, I should be able to set empty, capped and vented fuel cans in my hangar and watch them fill with water.


I sump to check for leaks in my caps and crap from the fuel truck.
 
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wow. I've seen a few drops every now and then, but even that is rare. a jar and a half--that is a lot.
 
timwinters said:
The above described air dumps 18 grams (38-20=18) of moisture per KG per night so 18 x .19 = 3.42 grams dumped per 42 gallons of "air space" in the tanks.
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Only if the tanks are sealed. Check your assumptions about what happens as moisture condenses in a vented tank.
 
timwinters said:
The above described air dumps 18 grams (38-20=18) of moisture per KG per night so 18 x .19 = 3.42 grams dumped per 42 gallons of "air space" in the tanks.
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3.4 grams of water converted from vapor to liquid is purd near a gallon of vapor condensed inside the tank which brings in more air/water vapor in the tank beyond the original 42 gallons and adds a few hundredths of grams of water to the total. I'm not going to bother to try to integrate over the change in temperature.

Better yet how about you disprove any part of what I posted...if you can.

Go for it, I'll wait right here! ;)
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Hope you didn't have to wait too long...
 
Capt. Geoffrey Thorpe said:
3.4 grams of water converted from vapor to liquid is purd near a gallon of vapor condensed inside the tank which brings in more air/water vapor in the tank beyond the original 42 gallons and adds a few hundredths of grams of water to the total. I'm not going to bother to try to integrate over the change in temperature.



Hope you didn't have to wait too long...
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I assumed a full exchange of air every day. Far more than just replacing the volume of the water that had condensed out.

Besides, when the water is condensing out, the air will be at/near the coldest temp of the day (and the ambient temperature likely colder than the internal temp of the tank since the fuel has more thermal inertia than air) thus there won't be much, if any, addl cooling and condensation until the tank goes through another thermal cycle.

Remember, I assumed a full exchange of air every day. That just ain't gonna happen even taking your assertion into account, thus you still haven't disproved anything in my post.

Still waiting.

Edit: even if your assertion did make a difference, it's only a 2.38% change to the calcs. An amount that's lost in the background noise.
 
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deltaindia said:
Jackson, MS

http://www.ntsb.gov/aviationquery/brief.aspx?ev_id=20121113X94546&key=1
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Interesting, the crash happened in mid-November. The average daily H/L for the previous two months in Jackson would be 78/54. The average daily afternoon humidity would be 50%. Overnight is 93%.

Cooler air can hold far less moisture than hot air...which my calculations were based upon. So...

78 degree air at 50% RH holds about 11 grams of water / KG.
54 degree air at 93% RH holds about 6 or 7 grams of water / KG.

So, worse case, 5 grams of water per KG of air per day is condensed.

That's 2 ounces over 60 days in 42 gallons of air.

Again, assuming a full exchange of air in the tanks every day and a full temperature swing...neither of which is going to happen especially in an enclosed hangar.
 
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Has anyone ruled out sabotage? What is the plane insured for? All the students who fly the plane have solid marriages? Check the life insurance for the pilot community.

This is an unusually large amount of water.

-Skip
 
I suppose a life insurance company could pursue that avenue, but it would be hard to prove. Any water in tanks should have been thoroughly sumped out, and that responsiblitiy lies with the PIC. Especially with the Cherokee 6 you are supposed to not only drain each of the 4 individual quick drains under the tanks, but also drain it with the sump lever on the left cabin wall for each of the 4 tanks.

The report indicated that, due to the 1 foot diameter fuel covering on the ground, that it didn't appear the pilot properly drained it since the recommended method is approximately 11 seconds per tank (i.e. 44 seconds total). A 1 foot diameter stain the ground would indicdate something far less than that.

Is it possible water was in one or all of the fuel lines already due to 2 months of condensation? Is it possible water was in fewer than one of the fuel lines, but then prior to take off he switched to a different tank, which used a fuel line with water in it? He would have had enough fuel in the system to accomplish the take off roll and get airborne before the water got into the system and choked the engine.

A similar incident happened with a Cherokee 6 owner I knew 15 years ago. He crashed shortly after take off when the engine failed. They found the fuel lever between tanks. They determined he likely either hit the lever with his foot on entry to the plane and never double checked it before take off, or he switched tanks right before take off and didn't verify that the selector was firmly set to a particular tank. He had enough fuel to get airborne and then the engine croaked when there was nothing left because the selector was between tanks....
 
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