Couple of fuel questions

Frogs97

Frogs97

Pre-takeoff checklist
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Frogs97
A long time ago, I had an instructor tell me that one way to help keep water out of the gas tanks was to keep them full. The idea being condensation couldn't form if it was filled to the brim. Is this pretty common practice? Sure seems to me that, no matter how "full" you get the tanks, there's still exposed surface for the condensation to form (if it even would). If it is common, how does this fit with choosing how much fuel you will take on a trip?

I also read a lot on here about the pros of mogas. Obviously, one major con to using mogas is it's availability. Has anyone been following that enough to know if that's likely to change (either for the better or worse)? It sounds like, at least in the case of some of the 182s, that the mogas STC is just paperwork. Some of the pipers look like there's a fuel pump swap, but in both cases it sounds like the same engine can run avgas and mogas. Is that correct? If you've got half a tank of mogas and top it off with avgas, are you causing any issues?

Thanks!
 
After I'm done flying, I top off the tanks. Most everyone else I have flown with does it as well. I have no experience relating water in a tank directly to the tank being partially empty after the last flight. I have drained water from a tank after a storm.

I have no experience with MoGas.
 
IMO most condensation is caused by leaking fuel caps, especially in older 182's with the old style caps. They attracted and held water until it could seep past the dry rotted seals into the tanks. :D
I used to sump water out of our 182P almost every time I did a pre-flight.:mad2: It was tied down outside and had the old caps. Now our 182 has the newer style umbrella caps and lives in a hangar, I don't think I have ever gotten water out of the sumps. :dunno:
 
Gas can hold water in solution, about a teaspoon per gallon if I remember right. It can come out of solution as the temps fall. This is what you have to look out for and sump the tanks. The engine can and will ingest a small amount of water and you will never know it, maybe that is what causes those hickups at night? :eek:

Leaking gas caps are a larger problem for water contamination. I cover mine with duct tape or plastic if I know it is going to rain.

I use mogas exclusively in an IO-360 and IO-540. 800 hours now, running strong. That is a saving of approximately $15,000. Most builders in the experimental world set their planes up to burn mogas from the start.

YMMV. :yesnod:

Condensation is more of an issue in the crankcase rather than fuel tanks. IMHO.
 
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Frogs97 said:
A long time ago, I had an instructor tell me that one way to help keep water out of the gas tanks was to keep them full. The idea being condensation couldn't form if it was filled to the brim.
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I've heard that innumerable times over the last 45 years, but I've yet to see any evidence of it actually happening. Only times in 8000 hours of light plane flying I've ever found water in the tank was with a leaky fuel cap and hard rain or a bad fuel supply (e.g., leaky truck/storage tank).
 
Filling them up is cool if weight isn't a concern.

When I was doing training in grummans, we would default the tanks to 1/4, for possible weight issues, never got water in the tanks.

On my own plane I default it to full

On the work plane we default to half tanks.


I'd say leaking caps would be a larger and more likely cause of water.
 
You can mix 100LL and non-ethanol auto fuel all day long if you have the appropriate paperwork.

Some very few of us can mix in E-10, but for many aircraft that would (and has) caused problems with hoses. If you are asking, it's pretty safe to assume you are not one of these exceptions.
 
Condensation will not form if the caps are tight,making a good seal. Hated the caps on my polder 172. People who have bladder tanks like to keep them full ,to keep the rubber wet.
 
frfly172 said:
Condensation will not form if the caps are tight,making a good seal. Hated the caps on my polder 172. People who have bladder tanks like to keep them full ,to keep the rubber wet.
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Bladders do just fine so long as there are a few gallons in there keeping the vapor up and O2 down.
 
Planes left outside in the weather risk water more than any. The AIM I believe makes reference to topping off tanks to minimize condensation.

I use 91E10 exclusively unless I have to get some 100LL on a trip, then i use Decalin to dump the lead.

Mogas is not available on airfields. I use a transfer tank trailer to bring it to my hangar.
 
where do bladders almost always fail? at the quick drain attachment, the only spot that is always covered with fuel for the life of the bladder.
 
Thanks, all. I knew there were times you would want to be light on fuel, for weight reasons. I didn't know how this concept fit in with the "full tanks" issue. Filing this all away if (sure hope it's "when") I get a plane of my own. This working around other people's schedule business is for the birds!!

I've seen a few airport locations that show mogas availability. Most seem to be in very urban airports where I'm guessing farmers are also coming to fill up. I was just curious if anyone knew if this was up or down from years past.
 
where are you?

nevermind, I googled your zip. Ft. Worth, you're pretty far away from the nearest ethanol free gas on the street which is typically about $1 cheaper than MoGas on the field...at least around here it is.
 
Frogs97 said:
Thanks, all. I knew there were times you would want to be light on fuel, for weight reasons. I didn't know how this concept fit in with the "full tanks" issue. Filing this all away if (sure hope it's "when") I get a plane of my own. This working around other people's schedule business is for the birds!!
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tankering around unneeded fuel is one of the biggest wastes in aviation. I can't think of but 2 or 3 times I've flown with brimmed tanks in the last decade.
 
I agree, I used to fly with full tanks all the time, when I started flying twins that changed drastically!:D I won't say never, but I very seldom top off an airplane any more. It just doesn't make sense, unless I'm light and the fuel is cheap, even then I will usually just add a few extra gallons. ;)

JHW said:
tankering around unneeded fuel is one of the biggest wastes in aviation. I can't think of but 2 or 3 times I've flown with brimmed tanks in the last decade.
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Jeff, there's not much difference in my POH between the lowest weight and max gross performance numbers. A few percent, in fact.

Not doubting you, but for my plane the extra weight doesn't appear to make a big diff. What are you referring to?
 
With my Mooney I leave the fuel where it is until my next flight, then I add only what's needed for my trip with required reserves. Keeping that plane full would be folly. It holds 102 gallons - that's 306 lbs in each wing. All that dead weight squatting on my mains would flatten the gear donuts in no time.

I keep 10 gallons a side in my RV-8 because I only fly it locally and in the pattern and that's the perfect amount of fuel to light up and go for about 90 minutes.

In 30 years of flying I think I've detected water in the fuel once... and never with any of my planes.
 
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Mistake Not... said:
Jeff, there's not much difference in my POH between the lowest weight and max gross performance numbers. A few percent, in fact.

Not doubting you, but for my plane the extra weight doesn't appear to make a big diff. What are you referring to?
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Much of your fuel burn and enroute time for a trip is dictated by the climb.
 
Mistake Not... said:
Jeff, there's not much difference in my POH between the lowest weight and max gross performance numbers. A few percent, in fact.

Not doubting you, but for my plane the extra weight doesn't appear to make a big diff. What are you referring to?
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Extra time to climb and extra drag throughs the flight add up to significant fuel waste. Why do you think the airlines don't tanker fuel? On transoceanic flights they file initial flight plan to an intermediate island also they don't have to carry reserves they most likely won't need (and if they did, they could stop and get them) but are mandated if they file the full route. It saves across the industry huge amounts of fuel every year not having to fly around that extra weight.
 
I learned to fly in Denver in a 152. We avoided topping the tanks. We'd add six gallons after each lesson.
 
Henning said:
Why do you think the airlines don't tanker fuel?.
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Your question prompted me to wonder about fuel / empty weight ratios and me and the Big Guys.

From wikipedia:

Boeing 777-200 (Operating Empty Weight): 297300lbs.
Max fuel capacity: 31000 gal

JetA lbs/gal: 6.79, so at full fuel, a 777-200 weighs 297300 + 210490 = 507800lbs.

that's a % change of 507800 / 297300 = 1.70 -> 70% more.

My empty weight: 1851lbs, weight of full fuel (64gal * 6lbs/gal): 384 lbs, total weight: 1851 + 384 = 2235lbs. Or, 20% more.

But, if I leave HALF my fuel on the ground, my weight only changes by 10%. If they leave half on the ground, their weight changes by 35%.

I'd say the biggest reason they leave fuel on the ground is that they're essentially ginourmous flying fuel tanks, and they simply don't need it. And in that case, sure, it doesn't make sense to carry it. OTOH, leaving an hour of fuel (13.5 gallons) out of mine changes my weight by 4%.

4%.

For small planes, I can't see fuel making a big performance or cost difference. I don't carry enough that leaving any behind saves me enough to worry about.
 
Big commercial jets have the same problems with W&B as any other aircraft. W&B is calculated for each flight based on passenger, fuel, and baggage loads.

The computed fuel needs are adjusted for W&B and for the amount needed to make the various flights during the workday.

Sometimes baggage is either not put onboard, or taken back off again dependent on W&B.

Also, these large aircraft have water monitors in their fuel tanks. Removing all water from fuel is impractical; therefore, fuel heaters are usually used on commercial aircraft to prevent water in fuel from freezing.
 
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Henning said:
Bladders do just fine so long as there are a few gallons in there keeping the vapor up and O2 down.
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Agreed as long as the plane is usually parked where the sun doesn't shine on the wings when the OAT is high.

I believe that under the right (wrong?) conditions such as a hot cloudless day in Florida, the temperature of the wing's top surface can get high enough to shorten the life of a rubber bladder sitting just under that wing top. Having fuel in contact with the top of the bladder should keep the temp considerably lower and prevent long term damage.

But other than that, I've been told by "experts" that a cup of fuel in a tank provides enough vapors to keep the entire bladder protected from deterioration just as well as keeping it full.
 
Mistake Not... said:
Your question prompted me to wonder about fuel / empty weight ratios and me and the Big Guys.

From wikipedia:

Boeing 777-200 (Operating Empty Weight): 297300lbs.
Max fuel capacity: 31000 gal

JetA lbs/gal: 6.79, so at full fuel, a 777-200 weighs 297300 + 210490 = 507800lbs.

that's a % change of 507800 / 297300 = 1.70 -> 70% more.

My empty weight: 1851lbs, weight of full fuel (64gal * 6lbs/gal): 384 lbs, total weight: 1851 + 384 = 2235lbs. Or, 20% more.

But, if I leave HALF my fuel on the ground, my weight only changes by 10%. If they leave half on the ground, their weight changes by 35%.

I'd say the biggest reason they leave fuel on the ground is that they're essentially ginourmous flying fuel tanks, and they simply don't need it. And in that case, sure, it doesn't make sense to carry it. OTOH, leaving an hour of fuel (13.5 gallons) out of mine changes my weight by 4%.

4%.

For small planes, I can't see fuel making a big performance or cost difference. I don't carry enough that leaving any behind saves me enough to worry about.
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Maybe throw a couple sandbags in the baggage too. After all it's a small % and you can carry them.
 
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