Old fuel in airplane tanks

[digits_]

It's a slow day, so time for a somewhat theoretical question/discussion.

Avgas seems to have a shelf life of about one year for the consumer. You buy the avgas, you should burn it up in one year. Ok, easy.

Now let's consider our private pilot, Barry.

Barry doesn't fly very often. When he fuels up with avgas, eventually there will be old fuel in his tanks.

If Barry always keeps his tanks full, and if he burns off 25% per flight, we can calculate:

If Barry does't fly, 100% of his tank will be expired avgas by the end of the year
If Barry flies 4 times a year, 25% of his tanks will be expired avgas by the end of the year
If Barry flies 7 times, the tank will contain 10% expired avgas by the end of the year

Barry would need to fly 15 times to get to less than 1% of expired fuel, or about 24 times get to less than 0.1% of fuel.

The average private airplane doesn't fly much. 10-50 hours is not uncommon. Only flying 7 times a year, isn't that farfetched. So Barry constantly flies around with 10% of expired fuel in his tanks. How will this affect his plane?

Obviously expired fuel doesn't turn into water, otherwise Barry would have serious issues, but let's assume that this is Avgas that does go bad after one year. The worst possible avgas that you can legally buy. What would be the effects?

Is it as simple as saying that the old fuel mixes with the new fuel, so you'd get a new octane number, something like 95LL instead of 100LL?

I've never liked chemistry very much, hoping someone can shed a light on this.

[CruiserNU]

There's a number of factors at work here:

First, if the airplane sits all winter in the cold (and I mean really cold) then whatever chemical reaction that causes the fuel to deteriorate will be slowed down considerably.

Airplane tanks are vented, but the vents are small. It's not like you're leaving a bunch of avgas in a big tub open to the air. So again, the deterioration will be minimal. Hot weather will change the picture somewhat.

In my experience, avgas (100LL) has a life much longer than a year, even in a vented tank. Most engines can tolerate quite a loss of octane number without serious consequences.

100LL lasts a good long time in sealed drums. Much longer than a year I hope because I have a dozen drums in stock that will take me a few years to use up!

[ahramin]

Homeopathic flying, very interesting. Avogadro would be proud.

For your theoretical example, my guess it that an engine burning a 10% expired fuel mix would run poorly, but still run.

In actual practice, there are thousands of planes that sit around for years and then go flying without doing anything to the fuel. I have not heard of a single situation where an engine lost power because of old fuel. I see this often enough with mogas to know that it's a serious problem, but for avgas as long as the plane is flown once a year the fuel would be the least of my concerns.

[Scuderia]

I'm currently running a related but non-aviation experiment but with different grades of mogas to see how the octane rating and Reid vapour pressure decay over time. Far from scientific, I'm doing it with COTS gas cans and old motorcycle tanks. Testing will happen next year, I'll see if I remember to share the results here.

https://apps.dtic.mil/dtic/tr/fulltext/u2/a174091.pdf is a good read
Page 2 says "The longer the fuel remains in vented tanks, the more likely the fuel will lose the more volatile components. The loss of the more volatile components may alter the octane rating of the fuel. In addition, the more heavy ends (higher molecular weight and, conversely, higher boiling point) the greater the tendency to form gum deposits as the fuel sits in the tank. A typical aircraft will be operated about once a week, whereas a typical car is operated at least once a day. As a consequence, fuel tends to sit in general aviation aircraft for longer periods of time than in an automobile. "

Page 19 has notes on the RVP decay of a blended mogas over two years. Mogas begins decay sooner than Avgas.

If Barry always keeps his tanks full, and if he burns off 25% per flight, we can calculate:

If Barry does't fly, 100% of his tank will be expired avgas by the end of the year
If Barry flies 4 times a year, 25% of his tanks will be expired avgas by the end of the year
If Barry flies 7 times, the tank will contain 10% expired avgas by the end of the year

Barry would need to fly 15 times to get to less than 1% of expired fuel, or about 24 times get to less than 0.1% of fuel.

Just curious on how these numbers were calculated? I can't seem to replicate them with any iteration I've formulated.

[ahramin]

I'm currently running a related but non-aviation experiment but with different grades of mogas to see how the octane rating and Reid vapour pressure decay over time. Far from scientific, I'm doing it with COTS gas cans and old motorcycle tanks. Testing will happen next year, I'll see if I remember to share the results here.

https://apps.dtic.mil/dtic/tr/fulltext/u2/a174091.pdf is a good read
Page 2 says "The longer the fuel remains in vented tanks, the more likely the fuel will lose the more volatile components. The loss of the more volatile components may alter the octane rating of the fuel. In addition, the more heavy ends (higher molecular weight and, conversely, higher boiling point) the greater the tendency to form gum deposits as the fuel sits in the tank. A typical aircraft will be operated about once a week, whereas a typical car is operated at least once a day. As a consequence, fuel tends to sit in general aviation aircraft for longer periods of time than in an automobile. "

Page 19 has notes on the RVP decay of a blended mogas over two years. Mogas begins decay sooner than Avgas.

If Barry always keeps his tanks full, and if he burns off 25% per flight, we can calculate:

If Barry does't fly, 100% of his tank will be expired avgas by the end of the year
If Barry flies 4 times a year, 25% of his tanks will be expired avgas by the end of the year
If Barry flies 7 times, the tank will contain 10% expired avgas by the end of the year

Barry would need to fly 15 times to get to less than 1% of expired fuel, or about 24 times get to less than 0.1% of fuel.

Just curious on how these numbers were calculated? I can't seem to replicate them with any iteration I've formulated.

I get 32% after 4 flights and 13% after 7.

[digits_]

Just curious on how these numbers were calculated? I can't seem to replicate them with any iteration I've formulated.

Before the first flight, we start with 100% fresh fuel. Let's assume a tank of contents 100 (whatever unit you want) to make it easy.

Start: 100

After first flight:

Fresh fuel: 25
One cycle old fuel: 75

After second flight:

Fresh fuel: 25
One cycle old fuel: 18.75 (=25*0.75)
Two cycles .. : 56.25 (=75*0.75)

After third flight:

Fresh fuel: 25
Once cycle: 18.75
Two cycles: 14.06 (18.75 * 0.75)
Three cycles: 42.19 (=56.25*0.75)

After fourth:
Fresh: 25
One: 18.75
Two: 14.06
Three: 10.55
Four: 31.64

After fifth:
Fresh: 25
One: 18.75
Two: 14.06
Three: 10.55
Four: 7.91
Five: 23.73

After 4 flights it could be 32% as well, depends if Barry fuels up right after his flight, or just before his next one.


I appreciate all the replies. Is the assumption that old fuel will mix with new fuel to create a new fuel with averaged octane rating correct though? Or will there be a point where the fuel is so old it will sink to the bottom or float to the top?

How can you know if the fuel still has enough energy left: are you safe if you get the engine started? Or if it passes the run up? Would you expect trouble at take off power, or more at higher altitude?

[Scuderia]

Ah, OK.

I just did 100 x 0.75ⁿ, n being the number of flights. Gives the same results ahramin came up with.

Is the assumption that old fuel will mix with new fuel to create a new fuel with averaged octane rating correct though?

I think yes, insofar as octane is all that you are considering. Eg. "89" gas at the gas station is just a blend of the "87" and "91" at most gas stations, mixed right there in the pump above ground.

Or will there be a point where the fuel is so old it will sink to the bottom or float to the top?

How can you know if the fuel still has enough energy left: are you safe if you get the engine started? Or if it passes the run up? Would you expect trouble at take off power, or more at higher altitude?

As far as my understanding goes, the density does not appreciably change although certain components evaporate leading to the reduced RVP. Certainly one may experience a poor running engine due to less volatile fuel but I think at that point the greater concern comes from the oxidation that could have occurred leading to gum deposits etc.

[freakonature]

And come spring the fuel supply you are refreshing from might be older than the fuel in your tanks. Some of the larger fuel suppliers have a refresh program for their fuel. But it doesn’t hurt to ask how old their fuel is. Especially in these times of less flying.

[RedAndWhiteBaron]

Well, I don't own a plane, but I do own a shitty, leaky boat with a little 1980ish 7.5HP two stroke Mercury outboard. Long term gas storage is a similar concern. It's a lesser risk factor, but you can by no means disregard the risk of engine failure due to bad gas in a boat either, especially when weather conditions are iffy - and weather conditions tend to be more "iffy" in the spring, when you're running on last year's gas.

I can say without doubt that I notice the difference between fresh gas and old gas. I have a two stroke motor, the kind that leaves a smoking trail of wisps in your wake that you can smell, so it's easily noticeable. With a four stroke (that doesn't need oil mixed in), you generally pour whatever gas is left at the end of the season into your car to avoid the problem. You can't do that with mixed gas, and gas ain't entirely cheap, so long-term storage it is.

How bad it is depends on how much gas is left in my marine tank - I mix the gas with oil only when I put it in that tank, and I'll have a couple of jerry cans with pure gas waiting to be mixed. So, if at the end of the season, that marine tank was mostly full, I'll see a marked difference from the previous season in the new year. But if it was nearly empty, and I've now refilled it with fresh gas, there is much less of a difference. The major differences between old and new gas are:

• The exhaust from old gas "smells" worse. Can't quite pin it down, it just smells more greasy and dirty.

• It's harder to start the motor with old gas, especially in cold conditions.

• The engine sputters more at idle with old gas

• Old gas fouls the plugs a helluva lot more.

I've been sitting at the cabin or somewhere in the middle of nowhere cleaning the plugs with a toothbrush once or twice (no, I didn't then brush my teeth with it) because old gas and cold conditions fouled the plugs too much. Noticeably, this doesn't happen if I'm using mostly fresh gas. I now carry a soft wire brush in the boat toolbox.

One thing that seems to make a big difference is making sure that marine tank is completely sealed off before winter storage. I believe this is because octane has a lower evaporation temperature than heptane - so if it's allowed to vent, the octane, which has a lower flash point than heptane, will evaporate out of the tank more so than the heptane, which has the effect of lowering the octane rating. I think by sealing it off I'm building enough of a vapour pressure in the tank to stop that process. I notice the difference also with jerry cans of fresh gas; if I seal them off, the gas seems to be "better" the next year, than if I allow them to vent. This would not be the case with vented tanks in airplanes.

I typically also add a fuel stabilizer for winter storage, which I would not recommend for an aero engine. It lowers the flashpoint but is designed to break down after a few months.

Now, to answer the original question - in my experience, if I have a quarter tank of fuel in my 20L marine tank and top it up with something I just bought, I don't need to worry. I notice it, but I don't need to worry about it. It performs almost as if I'm using nothing but fresh gas. It does appear to mix very well, giving consistent, if not reliable performance. With less than 10% left in the marine tank and topped up with fresh gas, I can't tell the difference - and as I mentioned, with a dirty 2 stroke motor, any difference is very easy to notice. If I have more than half a tank of old gas and top it up with fresh gas, I do need to worry about engine failure in cold conditions. In warm conditions, pretty much everything is peachy aside from the more blue smoke with old gas and that smoke smelling worse than last year.

That's my experience with the "old gas" issue. The risk factor is quite different, but the chemistry remains the same, aside from proprietary additives. Old gas boils down to (hah, see what I did there?) the octane evaporating faster than the heptane.