Leave the fuel tanks full

[Genghis the Engineer]

I was taught, many years ago, when parking an aicraft outdoors to leave the tank(s) full so as to minimise condensation. And, largely, I still do.

But, here's the thing - is there actually foundation in fact of this?

G

[Adrian N]

No foundation at all for a typical GA aircraft, if my distant memory of chemistry is correct.

Let's suppose you park your wonderplane with 100 Litres of empty capacity in its tank, on a summer day when the temperature is 30C. Overnight it cools to 10C.

At 30°C, 3.04 grams of water can be held in a 100 litre volume, and at 10°C that falls to 0.94 grams. So, worst case scenario, if the relative humidity is 100%, you could have 2.1 cc of water at the bottom of your tank if all the water vapour condenses onto the surface of the fuel tank and you refuel to the brim while the temperature is still cold.

There would actually be a bit less water than that at the bottom of your tank, as some water can be dissolved in Avgas - I don’t know how much. As your fuel cooled at high altitude, more water could come out of solution.

I don’t know if the saturated vapour density of water will be affected by the presence of other gasses in your fuel tank; we need someone who’s been a scientist more recently than me! If there is an effect, I would anticipate it would be a reduction in the amount of water held.

Conclusion: if your engine fails due to water contamination, it’s not likely to be because water condensed overnight in your fuel tank.

[ronnie3585]

I was thought to keep the tanks because the fuel vapors are a lot more flammable than the fuel its. The more fuel the less vapors within the tank.

Re minimising condensation; I not too sure. Maybe if the aircraft is out side of a hot day with half full tanks the heat may cause the traces of water with the fuel to evaporate and the condense on the inside walls of the tank, this in turn may lead to water contamination of the fuel.
Please correct me if Im wrong.

[mcgoo]

there must be something in it, when i was in the u.s training on the warrior, regardless of the plane whichever one wasn't refuelled at the end of the day always had water in the fuel tester in the morning and all the ones that were refuelled had just neat fuel come out!

[cirrus01]

30 dec C.......... cor !.......... if only !

There again that's way too much if like me you are using Mogas !

:ok: :ok:

[helicopter-redeye]

Interesting study done on this last year next door in Rotorheads (http://www.pprune.com/forums/showthread.php?s=&threadid=171368&highlight=gaseous)

Keeping the tanks full to avoid condensation is also a JAA ATPL question in AGK (Airframes & Systems) so one assumes there "must be something in it" (even if it is just Avgas...)

h-r:)

[markflyer6580]

Whilst on trade training for a aircraft technical related trade in the RAF,we were told that all a/c were to be full fuel after the last flight of the day.:)

[aluminium persuader]

My understanding -

If there is an excess of air in the tank, condensation can occur in any outside temp. Witness a car windscreen. The water falls through the fuel and collects at the base of the tank. The engine is fed from near the base. Sod's law clearly states that there will be enough "clean" fuel for you to start up, taxi, complete power checks and commence the take-off run. At the most critical point a dollop of water will be sucked up and deprive your ehgine of power for precisely long enough to spoil your day.

Obviously it's avoidable by carefully draining the tanks pre-flight.

Also, I for one HATE getting to an aircraft to find I need to start, taxi to pumps, fuel etc before I can do anything. As a courtesy to the next user I make sure the tanks are reasonably full. Why not full? Short strip, perf-critical I'm afraid.

Oh well!
:\

[djpil]

I for one HATE getting to an aircraft to find I need to start, taxi to pumps, fuel etc before I can do anything.
I just hate having to drain fuel from the aircraft before I can go flying. Two up in most two-seaters I fly means only half tanks. Same problem if I want to take a few friends for a local flight in a 172.

[vancouv]

I have sympathy with the 2 last posts. Yes, it is a real pain having to fill up before flying, which I have to do nearly every time even if I'm just doing a short local. Inevitably there will be a queue at the pumps, or they're shut for some reason or other. It all eats into your precious time that the plane is booked for, and is a right fag if you have non-flying friends with you who have to hang around.

Not to mention the fact that some planes I've flown have been reluctant to start again after a stop at the pumps.

On the other hand, arrive with four of you for a PA28 that has been filled to the top - you've had it.

[Kolibear]

And that, I'm afraid, is the inevitable collision between sensibilitiy & practicality.

Being sensible means filling them after flight to minimise the risk of condensation.

Being practical means leaving them half empty for the next person to decide how much fuel they need.

Pre-flight, I used to shake the wings to move any water droplets down into the sump and then drain the fuel, by which time the nice chappie with the fuel bowser had arrived.

As a matter of interest, how many pilots have had water in their fuel testers? Is the problem more due to badly fitting fuel caps than condensation?

The Koliber had a very effective rubber bung sealing the filler neck and a metal flap over the top of it. We never had water in the fuel.

[Flyin'Dutch']

Typical old wives aviation tale.

Adrian has it right. The rest of you seems to prefer to indulge in keeping the old tales going.

Few questions:

Q: How often does your car engine stop due to water contamination as a result of condensation in a non full tank?

A: Never.

Q: Can I fill up an empty metal container with water by leaving it in a hangar for a few days and nights?

a. Yes of course, that is how we fill them anyway!
b. No, are you stupid?

Water in fuel tanks in aeroplanes is a result of dodgy seals or contaminated fuel being put into those tanks.

Nothing more, nothing less.

[Yorks.ppl]

The same advice is given to boat owners regarding keeping tanks full, and I have seen the evidence of condensation my self but that is with gas oil and in a more humid marine enviroment, not sure how that translates to aircraft.

That said, all modern marine engines are fitted with water seperators so unless there is gallons of water there is never a problem. couldn't these be fitted to aircraft fuel systems as a backup to draining the tanks before flight?

[Studefather]

Chemistry apart, on many a/c, full tanks gives a chance to positively check fuel levels against gauge readings before engine start ( no chance of (a dipstick) misreading a dipstick).

Personally, I prefer to top up fuel and other fluids as part of the 'post flight / wind down / rosy glow', rather than as an additional 'pre flight wind up'.

Anyway, the Chipmunk only carries 9 gal per side, has better '2 up' CG margins with full fuel, and it's kinder on the flexible tanks to keep them full.

The fuel and oil uplift is also logged directly against the last sortie and a quick mental check of flying time/type against consumption is no bad thing (this especially applies to oil consumption on the Gipsy Major!!).

[Windy Militant]

Coming at this from a slightly different perspective I've always been told that it's best to leave any vehicle with full tanks and over many years tinkering with cars, both fuel tanks I've had to replace were corroded from the inside out. Both vehicles were used by the previous owners as second cars and only part filled as they only did short runs.
Another thought occurs to me that bladder tanks lose flexibility if allowed to dry out, as you don't know how long it will be before the A/C is used again it would seem to be a sensible precaution to leave the tanks full.
Yet another thought occurs that the old natural rubber seals on riveted tanks would dry out if the tanks were not full.

So lots of reasons why it was good practice and may well still be. Perhaps another example of doing the right thing for the wrong reasons.

P S I don't have many cans in my shed overflowing with condensation. However I do have a whole buch that are really rusty! ;) ;)

[IO540]

A really really good reason to fill up straight after a flight is because it enables a quick departure later.

Once one is airborne, one has to land anyway (sounds stupid I know). Whereas a departure depends on all sorts of stuff, starting with the weather and, in many places abroad, involving the filling in of a pile of forms. The last thing one wants to do is mess about with all that and then not go because they can't find the avgas bowser or nobody can (or wants to) speak English.

In the same vein, all large airports want to get the occupants off airside really quick. You land and usually within minutes a van or bus turns up to get you out of there. This is a brilliant time to fill up. Just politely refuse to leave the aircraft until you have fuel. It's amazing how quickly they "find" the fuel then :O

[J430]

The issue of tanks full or empty will always err on the side of fill them up, however if you leave a plane in humid conditions for a period of a month or more the condenstion becomes more significant.

Now for the MOGAS users and in Europe this is more common than here in OZ, the bigger problem is Premium ULP is enhanced with octane boosting products like Benzene, which easily evapourates, before you know it, you are back to regular ULP, so fresh is best with PULP.

Just a thought....

J:ok:

[S-Works]

I cant leave my tanks full. My 152 leaks fuel from the breather pipe check valve (despite being replaced a couple of months) faster than it burns it. This is a common problem on all 152's.

I would be very unpopular if I burnt the hanger down from a fuel leak!!

[zepthiir]

From Flyin Dutch
Few questions:

Q: How often does your car engine stop due to water contamination as a result of condensation in a non full tank?

A: Never.

Q: Can I fill up an empty metal container with water by leaving it in a hangar for a few days and nights?


firstly car engines dont stop due to water contamination probably because they generally have water seperators in them nowadays as said by yorks. This is an innovation which has sadly not been adapted to GA engines in general.

Secondly you will never fill a metal container with condensation by leaving it in a hangar because it is not a sealed environment like a fuel tank, the water is able to evaporate out of it once the temperature rises again. A fuel tank not only traps the water inside once it condenses, but due to the fact that it is vented and not air tight extra moisture is able to get in over time.

Overnight the amount of moisture you are likely to encounter is minimal although it still warrants draining. But if the plane isnt used over a period of time then you would be surprised at the amount of water you can find.

just my two cents..
Zepth

[Confabulous]

WARNING: thread drift....

What about heating the fuel? Would that make the atomisation/combustion process more efficient as a benefit?

Confab

[Mariner9]

Not much Benzene in European Mogas these days (tis carcinogenic) Dunno about Oz specs but prolly the same Dunnunda

Benzene's not the most volatile component in any event, butane is, and if that evaporates, you actually get left with a higher octane fuel than before (though less of it of course ;) )

[Adrian N]

FD - one can lead a horse to water (even in a fuel tank), but one can not make it drink.

If the fuel tank is sealed, then the only water that can condense to cause fuel contamination is the water that is in a gaseous state in the tank. Even at 100% relative humidity, on a warm day, the amount of water that can be held is quite small. (Look up "saturated vapour density" on google, and you'll find all the science you need to calculate exactly how much.)

If you leave the tank for days and days, then if it is sealed you can not get more water into it. Each time the temperature cools, some water will condense out. As the temperature increases during the day, it will return to the gaseous state.

100% relative humidity is probably not common inside a fuel tank. Temperature variations greater than 20C are probably very unusual in the UK. And in the typical GA aircraft that most of us fly, you're not likely to have more than 100 litres of empty capacity (although some of you may be lucky enough to have more). The very worst case, therefore is that the 2.1cc of water condenses onto the surface of the tank. If you then fill the tank to the brim, and wait long enough, that water can settle to the bottom of the tank. For various reasons, you won't actually get that much.

At takeoff power, the engine of my aeroplane (O-360) consumes approximately 20cc per second. So, in my very worst case scenario, if all that water condenses out and enters the fuel line, I might get a 1/10th of a second power loss.

I do check my fuel regularly for water, which can come from contaminated supply, or poorly sealed tanks. Everyone should do the same. But even if I have got the science quite badly wrong, the water is not going to come from condensation.

[B2N2]

I'm not a scientist by a long shot, but here we go:
Left for weeks at a time, fuel tanks do rust from the inside out.
Left for months, it will actually build nice
quarter inch stalagtites of rust in your tank.
That would be one reason for leaving the tanks full.
Rubber bladders dry out and wll crack and leak of not kept wet by fuel.
That's reason #2.
Fuel tanks in aircraft are vented ( exposed to outside air circulation) for obvious reasons.
100% humidity outside will mean 100% humidity inside the tank.
Cycles of heating up and cooling down will draw fresh(moist) air into the tank.
depending on amount of cycles and humidity variations this could mount up to more then 2.1cc's over time.
Water evaporating again when the tank heats up does not mean it will escape the tank through the vent pipe.
Cooling down it will draw in fresh air again. Cycle repeats itself.
reason #3
Engines will draw in fresh(moist) air when cooling down, through the airfilter and through the exhaust.
One reason why engines will rust inside when left over time.
Car exhaust( when cold) will drip water till the pipes heat up and it evaporates.
That's not water in the fuel, that's water condensed overnight.
Check the disc of the brakes on your car after you haven't driven it for a day, that's rust,from moist air.
So unless you live in Arizona......

[Onan the Clumsy]

I have sympathy with the 2 last posts. Yes, it is a real pain having to fill up before flying, Maybe the before fill up should be the standard, that way we know that on getting to the airport, it's on the list of tasks to do and afterwards, we can leave it for someone else.

[djpil]

before fill up should be the standard If that was the case I wouldn't fly - given the choice between flying overloaded or draining the fuel. Ever tried removing half a tank of fuel from an aeroplane? Where does it go? Do you put it back in again when you get back? If not, do you get to pay for the fuel that some-one else put in unnecessarily?

Some good points above - rusted fuel tanks in an aeroplane is not one of them however - just plain Aluminium.

[kookabat]

I can remember a few years back when we had major bushfires where I lived that the aircraft owners at the local airstrip were reported as having filled their tanks UP to reduce the fire risk - the idea being that fuel vapour, of course, is more flammable than liquid fuel so by filling with liquid fuel there's no vapour left to go BOOM!...


Or something like that.

[Windy Militant]

rusted fuel tanks in an aeroplane is not one of them however - just plain Aluminium.

You obviously haven't been near an Auster or Tiger Moth Lately Then. ;)

I 've just had another one of those blinding flashes. That's three in a day I'll run out of thunks at this rate.

3) Fuel and water when churned about sufficently will emulsify. However if you leave it over night the water settles out of the fuel and into the drain sumps rather than freezing in the carb jets on take off. which is an aspect of carb icing people often forget about.

[vector4fun]

I believe the caution about fuel bladders drying out. On the other hand, since my home airport has been without fuel for 6 months now, there's been numerous times my plane has sat for a week in it's hangar with 2/3rds full tanks, and I've never found a drop of water in either tank. I'm much more suspicious of the fuel I've gotten from unfamiliar pumps while away from home.

[eharding]

quarter inch stalagtites of rust in your tank.


just plain Aluminium.


Ah. Apparent rust formations in aluminium tanks. Very
bad news - these are in fact the larval form of the lesser
genus Aetheris Diabolus...the common Gremlin.

Transformation from the larval stage to the adult is
initiated by a strong transient voltage gradient,
combined with a low solar ambient UV... so typically,
they will emerge from the tanks in IMC, just after a
nearby lightning strike - traditionally, all your fuses &
circuit breakers will also have blown. Infestation
at this stage is invariably terminal.

The only known means of disinfection is to fill the tanks
with Gordon's Export Gin (domestic variants are too
diluted), generally obtained from overseas NAAFI outlets.

Best of luck.

Ed.

[djpil]

You obviously haven't been near an Auster or Tiger Moth Lately Then. That'll teach me to make general statements without my usual qualifiers and disclaimers.
I actually go out of my way to avoid Austers and had forgotten the Tiger's tank.

[B2N2]

:p

This thread will carry on for a looong time..LOL

Not all tanks are aluminum.
And I've never seen corrosion on a Cessna either. Wonder what that bubbly stuff under the paint is......
In any case I have seen tanks removed from a C150 that were filled with brown crunchy ferrous like material, I won't call it rust for now.
I have seen an Aztec parked outside for a couple of months with empy tanks.
You could stick your finger down the filler neck and feel the build-up of, once again,
brown crunchy ferrous like material.:confused:
You have more chance of finding water after a nice rain shower, but that doesn't mean that condensation does not occur.
Cheers
:ok:

http://marinesurvey.com/yacht/fueltank.htm

http://www.stormingmedia.us/79/7915/A791524.html

http://www.brokeboats.com/badtanks.html

http://www.emeraldinsight.com/Insight/ViewContentServlet?Filename=Published/EmeraldFullTextArticle/Articles/1270750507.html

Now don\'t come back saying that this doesn\'t apply \'cause it\'s a boat-tank...:E

[Rod1]

My tank is carbon fibre. A assume it is immune from most of the problems and will not provide a cold surface for the water to condense out on, so would reduce this as well. Any of our budding physicists care to comment?

Rod1

[Kolibear]

The worst thing that I've ever seen in a fuel tank was a pencil stub. Someone had tried to dip the tank (C152), dropped the stub in and couldn't get it out. Nor could I.

[Mariner9]

Rod, surely your carbon fibre would be at ambient temp, just as aluminium would?

My Pioneer's got aluminium tanks, and it lives under cover so no chance of rain ingress. Have only seen water once in the wing tank drains, and that was when the tanks had been left half full for 8 days. On other occasions, no water at all despite slack tanks for similar periods. Depends on climactic conditions probably.

As for all the calcs of possible water quantities done previously by our resident physisists, I would say the calculations are flawed, as they assume only a single atmosphere would be present in the tank ullage space. With diurnal temperature variations, the atmosphere would be changed frequently.

Another suggested reason for keeping your tanks full would be an improved surface/volume ratio for the fuel. The rate of evaporation of the fuel's volatile components depends upon this.

[Windy Militant]

My tank is carbon fibre. A assume it is immune from most of the problems

I was told that one of the worries with carbon fibre is that it's electrically conductive. Micro cracks in the resin allow moisture to impinge around the fibres so that if the aircraft is struck by lighting the charge tracks down the fibre and the heat generated as it does so flashes the moisture into steam which turns the composite into cotton wool. :ooh:
So as long as you stay away from CBs you should be OK! ;) ;)

[Rod1]

Thanks Windy,

I will add it to the list of reasons to keep away from CB's.

It is quite a long list.

Rod1

[Kolibear]

At takeoff power, the engine of my aeroplane (O-360) consumes approximately 20cc per second. So, in my very worst case scenario, if all that water condenses out and enters the fuel line, I might get a 1/10th of a second power loss.

I'm having a bit of a problem dealing with this. Its not that I dispute the figures, just the effect.

If 2 cc of water can enter the fuel line, get swallowed by the engine in 1/10 of a sec with no effect, how come melting carb ice, which is probably a smaller volume over a longer time, has such a major impact on the smooth running of the engine?

[Flyin'Dutch']

If 2 cc of water can enter the fuel line, get swallowed by the engine in 1/10 of a sec with no effect, how come melting carb ice, which is probably a smaller volume over a longer time, has such a major impact on the smooth running of the engine?

How about 'cause it obstructs the airflow through the carburettor?'

It is not the volume of ice but the obstructing effect it has.

FD

[Gerhardt]

I'm not sure of the exact science behind it but I can tell you that I used to keep a 150 in an unheated hangar, tanks full after flying almost all the time. No problem with condensation unless the pumps were closed when I'd landed the prior flight and couldn't fill the tanks.

[aytoo]

To answer the question about pre-heating fuel: it is a valid technique used on some gas turbines. Both the Apache and the Wessex (quite an age range then!) use a heat exchanger whereby engine oil is cooled by fuel on its way in.

I once had an engine failure in Yorkshire due to condensation - though thankfully not in an aircraft. Still, scary enough when it is in a narrowboat on the tidal stretch of the Ouse heading for Hull and Rotterdam on an 11 knot ebb tide...

I drained over 10 litres of water from the fuel tank after that little adventure, following years of neglect by the previous owner of the boat. Consensus was that it was almost certainly caused by condensation over a period of years.

Prior to 21 years as an army pilot, I was trained as a refueller, so am better placed that many to understand the problems of water and fuel. Fuel bowsers have very efficient coalescers that extract water from fuel, yet still the annals of aviation are peppered with stories of contam fuel. One such was a Scout AH1 that failed shortly after take-off having just been refuelled. The bowser had been correctly set up for the day, with all the fuel inside flushed through the coalescers. However, the extension hoses which were later used were not flushed. Scout had about a gallon of water in the fuel tank, if memory serves me right. :eek:

[B2N2]

Found this on the FAA website:

http://www.faa.gov/aircraft/safety/alerts/saib/media/CE-06-06.pdf


Deals with rust forming in metal fuel tanks.

From the same website:

http://www.faa.gov/ats/afss/newyork/ACFTOPS.htm

That's for all of you into winterflying
;)

I'll just stay here:

KAPF 271553Z 34007KT 10SM CLR 16/04 A3017 RMK AO2 SLP216 T01560044

KAPF 271120Z 271212 35004KT P6SM SKC
FM1500 03005KT P6SM SKC
FM1800 00000KT P6SM SKC


KMIA 271553Z 01007KT 10SM CLR 17/06 A3016 RMK AO2 SLP212 T01670056

KMIA 271120Z 271212 34006KT P6SM SKC
FM1500 35005KT P6SM SKC
FM1800 00000KT P6SM SKC
:ok: :ok: :ok:

[BigEndBob]

I know steel motorcycle tanks rust up if not kept full of fuel over winter if laid up. My old Honda Superdream succumbed to this fate.

Wasn't there also some concerns over not keeping bladder tanks full as they may shrink or crease reducing their volume or trapping water.

[B2N2]

C'mon C'mon.... what happened to all the scientists here?

:8

out- scienced maybe ?

[Flyin'Dutch']

No, we were just narked by your comment!

I'll just stay here:

KAPF 271553Z 34007KT 10SM CLR 16/04 A3017 RMK AO2 SLP216 T01560044


:{