lehpron
Posts: 6846
Joined: Tue Jul 10, 2001 3:42 am

Fuel That Never Gets Used...

Tue May 04, 2004 3:09 am

This is what I'm thinking: Every plane gets filled b4 flight and then extra is added in case the landing is delayed. After landing, it gets serviced and filled up again. BUT, I figure that no airliner, at least, has flown on dry tanks. I hope you know where I'm going with this. It's like if you fill a barrel with oil or water but the suction hose only goes so far down and at a point of 'empty', there is still stuff down there by the time it is refilled.

What I want to know is what happens to that stuff that doesn't get used right off the bat, or does it decay or does its potency go down?

The meaning of life is curiosity; we were put on this planet to explore opportunities.
 
Mender
Posts: 249
Joined: Wed Feb 04, 2004 9:36 am

RE: Fuel That Never Gets Used...

Tue May 04, 2004 3:34 am

It gets mixed with the fresh fuel. That's it.

If you pour some rum in a glass the add some coke you can never drink the neat rum.

If you put a really thick green liquid in a glass, then, really carefully pour another really thick red substance on top of it the two will don't mix.

Now stir them with a spoon clockwise one full circle. Now stir it back anti clockwise one fully circle. You will never fully "un-mix" the two substances. I believe it's called the chaos theory.
 
SlamClick
Posts: 9576
Joined: Sun Nov 23, 2003 7:09 am

RE: Fuel That Never Gets Used...

Tue May 04, 2004 3:39 am

What you are talking about is highly theoretical, that there might be a molecule of jet fuel that grows old in the tank. Consider the reality.

When the plane is serviced there may be five to twenty thousand pounds or even more remaining in the tanks, depending on which type aircraft we are talking about and some other variables. The fuel remaining is normally cold soaked to well below the freezing point of water. The fuel being added is tankfarm temperature. Presumable they will not mix until the temperatures come together. I think this is called "affinity" in chemistry.

Eventually the old and new fuel will mix. After all, it is being agitated constantly once the plane starts moving. The notion that some fuel will never take its turn down the pipe and somehow "age" is just not worth considering.

Here is a concern! When you drain fuel from a tank, it is replaced by air. Fuel down, air up, fuel up, air down. The airspace above the fuel in the tank increases and decreases. Seen another way, the tank is breathing. Now all air has some moisture content. Even in LAS or PHX there is some humidity. What happens to all that water that is drawn into the tanks?

You can be sure that some very smart people have addressed both our questions.

Yours is a little like the question that goes: If electricity is the flow of electrons, why does electric wire age since the matter within it is being constantly replaced?
Happiness is not seeing another trite Ste. Maarten photo all week long.
 
citationjet
Posts: 2259
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RE: Fuel That Never Gets Used...

Tue May 04, 2004 4:04 am

There are two categories of fuel that cannot be used:
1. Trapped fuel (unuseabale & undrainable fuel)
2. Unuseable, drainable fuel

Trapped fuel cannot be removed (drained) from the airplane without disassembly of the aircraft.
Unuseable fuel cannot be used by the engine, but can be drained from the low points in the fuel system.
Item 1 is usually a smaller quantity than item 2.
Boeing Flown: 701,702,703;717;720;721,722;731,732,733,734,735,737,738,739;741,742,743,744,747SP;752,753;762,763;772,773.
 
Mender
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Joined: Wed Feb 04, 2004 9:36 am

RE: Fuel That Never Gets Used...

Tue May 04, 2004 4:13 am

Water does build up in fuel tanks. Every airplane from a Cessna 150 to a 747 has water drains in the lowest point of the fuel tank for this reason.

Boeing actually go one better than Cessna though.

They BURN the water in the engines. They fit "auto sumping jet pumps" in the bottom of the fuel tanks which suck up the water and feed it to the engines together with fuel. This prevents large accumulations of water forming in the tank in the first place.
 
lehpron
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RE: Fuel That Never Gets Used...

Tue May 04, 2004 5:03 am

Jet engines take in water when it rains, it doesn't burn it per se, just knows how to deal with it. Still though, that stuff they never gets drained, I suppose, does it erode or decay at all? Or is it like a stray whirlpool in a stream, it's just there.
The meaning of life is curiosity; we were put on this planet to explore opportunities.
 
SlamClick
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RE: Fuel That Never Gets Used...

Tue May 04, 2004 5:08 am

I think your question has been answered. The old gets mixed with the new. There is no pocket of old fuel anywhere.
Happiness is not seeing another trite Ste. Maarten photo all week long.
 
AUAE
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RE: Fuel That Never Gets Used...

Tue May 04, 2004 5:10 am

The C-5 and L1011 are excellent examples of a very effiecient fuel pick up system. Aside from the boost pump, both planes have tubes that reach into the lowest parts of the tank to draw up fuel. You could litterally run the tanks dry on those aircraft.

Boeing is not as intricate, but the amount of trapped fuel and unusable is very very small.

As far as this being any type of concern, I agree with all the above posts. The fuel mixes enough that you really don't get "old fuel". Water also just gets fed right into the engine. And at 50 psi or so, water isn't going to freeze in the pipes.

The only thing you really worry about in a tank is debris (paint flakes, old sealant) and fungus. I think all fuel is treated now, so the fungi problems are very very rare these days.

Shawn
Air transport is just a glorified bus operation. -Michael O'Leary, Ryanair's chief executive
 
aloges
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RE: Fuel That Never Gets Used...

Tue May 04, 2004 5:59 am

What exactly is "trapped fuel"?

I imagine that it is fuel that found its way to a place somewhere in the tanks where the fuel pick up system cannot pick it up. Is that anywhere near correct?

And what happens when one or all of the tanks have to be drained for an inspection? Is the trapped fuel simply left to evaporate or what to mx people do?
Don't cry because it's over, smile because it happened.
 
air2gxs
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Joined: Mon Jun 18, 2001 1:29 pm

RE: Fuel That Never Gets Used...

Tue May 04, 2004 7:51 am

Trapped fuel is indeed fuel that is in an area of a tank that, due to design of the tank, rarely if ever leave that area. Consider a stream or pool of water with an isolated area. That area typically harbors growth (moss, fungus, etc.) because there is little or no movement of the water.

The same thing occurs in fuel tanks. There are certain strains of bacteria that thrive on hydro-carbons. Normally they do not present a problem, because fuel in a tank is not static, it is constantly moving and mixed, except where the trapped fuel is and in the sumps if maintenance has not been doing there job.

If allowed to thrive, this bacteria will start to cause fuel quantity problems because it will eventual propagate to the probes. We try to prevent this problem by sending off fuel samples at regular intervals and by treating the tank with a product called Bio-bor or some similar name.

So, manufacturers try their best to limit the areas in a tank in which fuel can be trapped and maintenance programs are designed to sump tanks at regualr intervals to limit this growth. Don't be mistaken though, sumpings primary function is to clear water from the tank.
 
VC-10
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RE: Fuel That Never Gets Used...

Tue May 04, 2004 8:46 am

What nobody seems to have mentioned is that the fuel boost pumps are located at the lowest part of the tank so they are always taking what fuel ever is in that part of the tank so there shouls be no 'old' fuel around.
 
FDXmech
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RE: Fuel That Never Gets Used...

Tue May 04, 2004 9:02 am

Lehpron

A good example of what you're talking about is the outboard tanks on the A300-600.

These tanks are sequenced to burn fuel last to keep weight in the wingtips. They hold about 8,600 lbs each.

But the the fuel in these tanks are indeed trapped. Often times they can remain full or nearly so indefinately. There is no water scavenge system to speak of.

Because of this potential stagnant pool, it's part of our service check to pump the outer tanks dry. We transfer the fuel into the inner tanks which are nearly always burned low.
Then when the airplane is fueled, ther outers get fresh, new fuel.
You're only as good as your last departure.
 
citationjet
Posts: 2259
Joined: Sun Mar 09, 2003 2:26 am

RE: Fuel That Never Gets Used...

Tue May 04, 2004 9:19 am

Unusable Fuel definition:
Fuel remaining after a fuel runout test has been completed in accordance with governmental regulations.

Trapped Fuel definition
Fuel which is not capable of being drained from the aircraft after a fuel runout test has been completed in accordance with governmental regulations.

From the above definitions, it appears that trapped is included in unusable fuel.


Boeing Flown: 701,702,703;717;720;721,722;731,732,733,734,735,737,738,739;741,742,743,744,747SP;752,753;762,763;772,773.
 
MD11Engineer
Posts: 13916
Joined: Sun Oct 26, 2003 5:25 am

RE: Fuel That Never Gets Used...

Tue May 04, 2004 1:42 pm

Lehpron,
Having spent several weeks ongoing in fuel tanks I´d like to state the following:
If you´ve got to enter a tank for an inspection or other maintenance, at first you pump all fuel out which can be picked up by the boost pumps. Then you attach an adapter with a hose at the drain valve at the lowest point of the tank and let the rest drain out (usualy a few hundred liters).
Afterwards you´ll open two fuel tank access panels, if possible the two most distant from each other and connect two big hoses, one attached to a suction pump (usualy an air injector pump, no electrics due to the danger of an explosion) and the other one leading fresh air in. After about a day you´ll check the gas concentration with an explosimeter. If it is ok, you can enter the tank. You´ll be wearing some cotton coveralls without pockets or metal zippers, and special tank socks, no shoes. You´ll be wearing a respirator as well. You crawl into the tank and mop up all the leftover fuel (usualy about one or two buckets full). Then you start with your work. You´ll use explosion proof lights and keep your tools in a plastic tray, as not to damage the fuel tank coating. Electric tools are a big NO NO. Another guy will stand at the access hole, fully dressed with a resprator to help you in case you faint. Many tanks are nothing for people with claustrophobia (speaking about the wing root of a 737 or a 727!), other tanks a big enough to have a party inside (747 center). This is what the book says.
At LH in the hangar the sheeties had to mark cutting areas on the surface of the wings, where to cut causing the least damage in case somebody collapsed inside. We also always had a tank rescue trolley with air cutting tools, respirators and advanced first aid equipment, like an oxygen cylinder beside the plane. In other companies I worked for, you were lucky if some colleague stuck his head into the hole every few hours and asked you if you were still alive! I was working in a 737 wing root, changing the check valves on the boost pumps, I didn´t like the respirator with the air line in this tight space ansd I caught myself singing dirty songs in German rather loudly, I was like drunk!

Jan
Je Suis Charlie et je suis Ahmet aussi
 
MD11Engineer
Posts: 13916
Joined: Sun Oct 26, 2003 5:25 am

RE: Fuel That Never Gets Used...

Tue May 04, 2004 1:45 pm

Another thing: When you get out of the tank after a day´s work you´ve got a smell of fuel stuck to you which even taking a shower three times won´t kill. When I first came back home like this, my ex asked me to sleep on the sofa instead of the bed.

Jan
Je Suis Charlie et je suis Ahmet aussi
 
DC-10Tech
Posts: 291
Joined: Fri Jun 15, 2001 6:40 pm

RE: Fuel That Never Gets Used...

Tue May 04, 2004 2:36 pm

I hear that Jan, you just can't get that smell out of your clothes, and anything you wash with those clothes comes out smelling like fuel!

I used to have a laundry quarantine for fuel and skydrol soaked clothes.

My last adventure where I spent serious time in a tank was rewiring the Lo Z to the #4 aux on a 747-100... one of the most unpleasant jobs I've ever done.
Forums.AMTCentral.com
 
pilotpip
Posts: 2821
Joined: Fri Sep 19, 2003 3:26 pm

RE: Fuel That Never Gets Used...

Tue May 04, 2004 3:52 pm

That lovely smell is the reason that I keep all of my work clothes separate from my street clothes. I won't even put my jacket on our office chairs at work because it will smell. If I spill fuel on myself, those clothes will more than likely end up in the trash.
DMI
 
SlamClick
Posts: 9576
Joined: Sun Nov 23, 2003 7:09 am

RE: Fuel That Never Gets Used...

Tue May 04, 2004 11:42 pm

There has been some good info posted here. Answered some old questions I had, even. But back to the original question:

There is NO old trapped fuel.


All those little catch basins get sluiced out every landing, every couple of flying hours. References to moss or fungus growing in stagnant pools of water simply does not apply. Every time the plane is fueled, these pools get sluiced out and the fuel in them replaced with brand new fuel. If you sluiced out a dead pool of a creek with fresh water every couple of hours do you think it would get all mossy?

There are only dead pools of trapped fuel when the tanks are emptied, either partially or fully. If you are thinking that this trapped fuel is there forever you are deluding yourself. It becomes part of the general fuel load once the tanks are serviced at the next landing.


Happiness is not seeing another trite Ste. Maarten photo all week long.
 
air2gxs
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RE: Fuel That Never Gets Used...

Wed May 05, 2004 1:01 am

Slamclick,

You are mistaken. There is trapped fuel. There is the very real possibility of bacteria, not moss, growing in this fuel. This bacteria can cause all sorts of issues including but not limited to fuel quantity problems and corrosion. We treat for bacteria regularly. I have seen this bacteria. It can only grow in fuel that has been stagnant for some time.

It probably isn't more than a couple of gallons total in a tank, but it is there. I have seen it, with the growth on the structure around it. I've seen it on B747, B727 and DC8 aircraft.

When I get to work this weekend, I'll see if I can find some info on the bacterial agent we use to kill the stuff.
 
AUAE
Posts: 292
Joined: Fri Apr 09, 2004 4:41 am

RE: Fuel That Never Gets Used...

Wed May 05, 2004 1:32 am

OK,

I think we are beating this issue up.

Here is a link to microorganisms that live in fuels.

http://www.conidia.com/hormonconis_resinae.html

Now, with that, lets re-state and clear up some issues. Trapped and unusable fuel are design terms. They define quantities of fuel that can be drained/not drained, used/not used during flight, ect. These definitions do not mean that a particular molecule of fuel is never used, or becomes "old". The fuel will always stay in a constant state of mixing unless it is frozen. (definition of liquid)

Secondly, this constant motion of fuel molecules does not prevent microbial growth. Neither does the daily flight / refueling of an aircraft. This stuff can grow on the fuel and on the tank walls. This is why tanks are checked routinely for growth and why most fuel contains biocides.

Shawn
Air transport is just a glorified bus operation. -Michael O'Leary, Ryanair's chief executive
 
MD11Engineer
Posts: 13916
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RE: Fuel That Never Gets Used...

Wed May 05, 2004 1:41 am

There is a misconception here about microbial growth in fueltanks. The bacteria actualy live in water and feed on the fuel at the interface between water (specificaly heavier than fuel) and the fuel. This water accumulates at the lowest points of the tanks, and the bacteria form a kind of slimy goop on the tank structure, which promotes corrosion and can disolve the tank sealant. This is why fuel tanks should be sumped regularely. Most transport category jets have a water scavenging system containing jet pumps, which will continiously draw the fluid (fuel / water mix) from the lowest points of the tanks. Also, if you fuel the plane, the effect is like holding a fire hose into the tank, it will stir everything up, this the reason why you shouldn´t sump the tanks for water until the fuel has been standing still for at least 3 hours. On the MD-11the fuel is constanly moved as long as the pumps are on. The only plane I ever found a few drops of water in was the outer wing tanks of an A300-600. We´ve got a procedure now to move the outer wing fuel into the inner wing tanks at every weekly service check. It was explained before, that the fuel in the outer wing tanks of the A300-600 usualy doesn´t get used.

Jan
Je Suis Charlie et je suis Ahmet aussi
 
Mender
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RE: Fuel That Never Gets Used...

Wed May 05, 2004 2:39 am

I'm glad MD11 engineer pointed out that the bacteria needs the water.

The worst affected aircraft are private jets based in tropical countries. This is due to the low flying hours combined with the climate.

Aircraft in these conditions should be parked with full fuel tanks to reduce the risk of condensation forming inside the tank ABOVE the fuel. Hence the need for full tanks and regular draining of the sumps.
 
SlamClick
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RE: Fuel That Never Gets Used...

Wed May 05, 2004 11:48 am

Okay, for those of you who did not read the thread starter, the question was:

what happens to that stuff that doesn't get used right off the bat, or does it decay or does its potency go down?

Again, the answer is NO.
There is no old fuel.

He did not ask about colloid fungus or entrained water. He asked about jet fuel aging.

So those of you who disagree with my answers, please explain to us all what mechanism is in place to prevent liter of jet fuel trapped behind a wing rib from mixing with the rest of the fuel load once the plane is refueled and takes off once again into a turbulent atmosphere.

Happiness is not seeing another trite Ste. Maarten photo all week long.
 
MD11Engineer
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Joined: Sun Oct 26, 2003 5:25 am

RE: Fuel That Never Gets Used...

Wed May 05, 2004 12:44 pm

Slamclick,

Even ordinary thermal diffusion will make the leftover fuel mix with the new fuel over time.

Jan
Je Suis Charlie et je suis Ahmet aussi
 
Arrow
Posts: 2325
Joined: Wed Jun 19, 2002 7:44 am

RE: Fuel That Never Gets Used...

Thu May 06, 2004 12:29 am

I found this thread fascinating. But here's a question: how do you go about the "cleaning' process in tanks on small jets like CRJs, Embraers, etc? Surely you can't crawl into those tanks.
Never let the facts get in the way of a good story.
 
lehpron
Posts: 6846
Joined: Tue Jul 10, 2001 3:42 am

RE: Fuel That Never Gets Used...

Thu May 06, 2004 2:10 am

Yeah me too, I did not know stuff grows in fuel, of course oil comes from millions of years of decaying dinosaurs and whatnot so "life finds a way" to quote Dr. Malcom in Jurrassic Park.  Smile

I'm guessing when you folks clean the tanks, you can't assume that all fuels are less dense than water and just flush the fuel out? Or does your local EPA give you beef about fuels going down the sewer drain as if all dolphins live at the ends of sewers?

Okay Slamclick, we got the point, fuel doesn't decay in the plane.  Big grin How long would it have to sit there to happen? Hypothetically speaking, unless it has, like in USAF squadrons during the 1990's...  Wink/being sarcastic
The meaning of life is curiosity; we were put on this planet to explore opportunities.
 
challengerdan
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Joined: Sat Sep 27, 2003 3:17 pm

RE: Fuel That Never Gets Used...

Thu May 06, 2004 2:28 am

On the CRJ, a regular to small size person can crawl into the most inboard part of the wing (the first 4 or 5 acces panels, from the center) but it's a tight fit. Outboard of that, everything is pretty much within arm's reach, even tough you sometimes need a really strange position.
if your flight goes MX in YUL, I might be called to fix it!
 
Dalmd88
Posts: 2447
Joined: Fri Jul 28, 2000 3:19 am

RE: Fuel That Never Gets Used...

Thu May 06, 2004 3:51 am

When the fuel is drained for maintenace it doesn't go down the drain. Any fuel that doesn't get pumped into the defuel truck is drained out the sumps into a large portable tank. It is considered to be 'offspec fuel'. At Delta we burn it in the buildings boilers. At one commuter airline I worked at one mechanic kept a bunch of five gallon drums in his truck so he could take the stuff home and burn in his oil furnace.

The EPA has some strict rules regarding uncontained fuel. A spill of over five gallons has to be reported, even if it never reached a drain.

If the inside of a tank needs to be cleaned we don't use water. Usually you use a solvent like Acetone. Of course any rags we use while doing this have to be disposed of in an EPA compliant manner. The days of everything goes down the drain or in the trash are long gone.
 
AUAE
Posts: 292
Joined: Fri Apr 09, 2004 4:41 am

RE: Fuel That Never Gets Used...

Thu May 06, 2004 3:58 am

Red cans, Yellow cans, Trash cans....hey what can do I throw this rag in????  Big grin
Air transport is just a glorified bus operation. -Michael O'Leary, Ryanair's chief executive
 
dc10hound
Posts: 460
Joined: Fri Dec 15, 2000 4:18 pm

RE: Fuel That Never Gets Used...

Thu May 06, 2004 12:49 pm

As far as "stirring everything up" or 'sluicing out" a fuel tank, I respectfully submit the following AMM excerpt:

Reference: A300-600 AMM 28-25-00-0

"The refuel/defuel system network includes a line routed through the center tank to connect the LH and RH wing and trim tank systems. In each tank the lines terminate in diffusers which disperse incoming fuel at a set rate over a large area to prevent formation of fuel mist or foaming and to ensure a calm fuel surface."

Even after sumping A300 inner tanks, we always found several gallons of fluid filled with unidentifiable gunk at the inboard bulkhead when we entered the tank.

At AAL, the A300 sump system is notorious for hiding water in its tanks. The fuel drain valves are not simply located at the lowest point in the tank. There are drain tubes connected to them, and water sumping depends on the weight of the fuel above it:

Reference A300-600 AMM 28-25-00-0:

"Water that collects below the fuel in the lowest parts of the main tanks is drained away, clear of all aircraft structure, through small bore pipes connected to manually operated self-sealing water drain valves.

The minimum weight of fuel required in each tank to force any collected water through the small bore pipes to the water drain valves is as follows:

- Outer tank 660 lb (300 kg) - Inner tank 2200 lb (1000 kg)
- Center tank 2200 lb (1000 kg) - Trim tank 2200 lb (1000 kg)"


There is liquid left in the tank when it is "empty" and "sumped", just not much.


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