View Full Version : 747-400 fuel tank temperature question
Ok so in accordance with the fuel card logic the reserve tank will gravity feed into Main Tank 2 and Main Tank 4 when their respective quantity drops below 18,200kgs.
Ok got that part.
Fuel temperature is meaured in Main tank 1 which together with #4 are the smaller tanks so the temp in #2 and #3 and CWT will always be higher because of larger quantity and therefore coldsoaking will take longer. Not to mention hot hydraulic fluid being cooled in these tanks.
Ok got that part also.
https://s-media-cache-ak0.pinimg.com/originals/4d/ea/bc/4deabc67a5499e273317a80adf16b014.jpg
So here is my question.
Assuming we depart with full fuel, all tanks full.
Initially fuel from the center tank feeds all engines.
Then Main 2 feeds engine 1 and engine 2 and Main 3 feeds engine 3 and engine 4.
My point being you can spend a considerable amount of time at cruising altitudes before Main Tank 2 and 3 drop below the 18,200 for the gravity feed of the reserve tanks which are now cold soaked and at an unknown temperature.
I understand Main Tank 2 is still comnsiderable warmer because of the larger quantity an even Main Tank 1 is warmer as it still sits almost full.
So how do you prevent fuel of an unknown (colder then indicated fuel temp) temperature (and possible gelforming already taking place) from clogging pump inlets/filter screens and oil/fuel intercoolers aka the British Airways that flamed out on the ILS?
So basically 4000kgs of fuel in your wingtip is getting colder and colder and colder while waiting for space in another tank.
How long does it take to cool 4000kgs of fuel from 15C to -40C?
Yes, I have a tendency to overthink and miss the obvious...:}
So how do you prevent fuel of an unknown (colder then indicated fuel temp) temperature (and possible gelforming already taking place) from clogging pump inlets/filter screens and oil/fuel intercoolers aka the British Airways that flamed out on the ILS?
Has the 744 suffered from the same problem? I know the heat exchanger was modified to help prevent this problem on the 777.
My (non-expert) take on this:
The diameter of the transfer pipe probably reduces the flow of cold fuel from the reserves. There will be mixing as the fuel enters the tank.
The O/J pumps will initially be the pumps providing the engine supply. The inlets for these are around the 3~4 tonne level, not in the bottom of the tank. Assuming the coldest fuel from the reserves sinks to the bottom, the fuel being used probably won't be the coldest of the cold.
I've heard of pilots turning on the wing anti-ice in the belief it will heat up the fuel in the tanks. I can't see how it would make any noticeable difference. The anti-ice ducts only run in the very forward part of the wing leading edge which is several feet away from the tank. No heated air is vented into the cavity where the leading edge flaps are stowed. Only way heat can get to the tanks is through heat conduction through the skin. With cold winds blasting across the surfaces of the wing, I think this effect will be negligible. You already have some very large heat-generating pneumatic ducts in the leading edges that are heated all the time.
True.
But how do we know we're not dumping slush into 2&3?
Part of the problem with the 777 were stagnant 'dead' zones in the tank where slush could accumulate.
Does the 747 have 'stirrers' in the tanks?
I have no idea how long it takes from a full fuel load and a normal climb profile to burn the CWT empty and the 2&3 Main tanks down to 18,200kgs for the gravity transfer to take place.
Do these transfers lines run through tank 1&2 and therefore 'preheated' by warmer fuel?
How does Boeing prevent 4000kgs of slush dumping into 2&3?
But how do we know we're not dumping slush into 2&3?
You don't but TBH even if you have it's what silverado said - Tank 1 temp reports the lowest temp fuel onboard - and TBF it's a system that has worked OK on the 747 for a few decades so if there was a tank "slush" problem in the reserves I suspect we'd have known about it now.
I flew the 744 for best part of a couple of decades, obviously (?) iaw the Fuel temp limitations ( and yes, I know they also did that on the Hatton Cross 777) and I never saw any engine probs due tank fuel temp, despite doing a lot of ULH trans Siberain/ Trans Northern Canada in winter Ops. Don't forget the main problem on the 777 at Hatton Cross was the design of the Heat Exchanger in the RR engine, not fuel tank temperature or fuel waxing in the tanks.
Does the 747 have 'stirrers' in the tanks?
Not that I've ever been made aware of.
How long does it take to cool 4000kgs of fuel from 15C to -40C?
It's been a while but I think we used to quoted 3 degrees an hour towards the TAT, though if you got into really cold Siberian Temperatures it could by more like 5-6 degrees an hour or even more.........
Just found this which might be of interest:
http://www.iasa.com.au/folders/Safety_Issues/RiskManagement/cold_fuel.htm
Physically where does the reserve fuel enter the 2&3 tank?
Close to the hydraulic/fuel heat exchangers?
I'm just fascinated by what 'they' don't bother telling us.
I'm just fascinated by what 'they' don't bother telling us.
I think the days of a pilot needing to be able to describe the fuel system in detail are almost before my time.
I did my 74 Classic course well over 25 years ago, followed not long after by my 400 course and even then Fuel heat got little mention beyond what was in the "Flying manual" though maybe on the Classic the Flight Engineers studied it in depth, can't remember.
My type change to another Boeing a few years back was much more abbreviated than previous courses and really was limited to "if you can't control it from the Flight deck or it not an issue in a QRH" you don't need to know and it probably doesn't get taught for a TR/conversion.
Also true but the term 'system logic' seems to create a discontinuity in my understanding of a system.
I'd rather hear the number followed by "nice to know" then not hear the number.
I'd rather hear the system regulates between 30 and 60 then 'the system logic keeps it in the green'
That's just the way I learn lol.
From what I remember about the accident report on the 777 they had dead zones in the tank where waxy deposits accumulated.
I don't know if that was fixed through stirrers or flow deflectors in the tank.
I do recall they had unusually low temps over China. -67C or somewhere about there.
In the CJ2 I've seen ISA -13 in the lower 400's.
tdracer
1st Apr 2017, 23:49
B2N2
The 777 accident was not due to cold/freezing fuel as such - the fuel in the tanks, while quite cold, was still comfortably above the fuel freezing temperature.
The problem was WATER ice that formed on the heat exchanger. As noted earlier, the Rolls heat exchanger was redesigned to prevent water ice formation.
All Boeing aircraft have a passive 'sumping' system that draws fuel from the lowest point in the tanks (where water would accumulate) and sprays it in the vicinity of the fuel pump inlets. This is intended to prevent significant accumulation of water in the fuel tanks.
Old Fella
2nd Apr 2017, 01:40
Can't comment on the B744, however on the Classic we would normally leave the Reserve fuel transfer until shortly before TOD. It could not be xfrd until Gross weight was below around 290000 kg or until tanks 1 & 4 were below around 10500 kg each in any event. Normally maintained as long as possible for added structural strength. Never known of any fuel freezing in Reserve Tanks being an issue.
Never known of any fuel freezing in Reserve Tanks being an issue.
And I'd like to know the engineering/design reason that prevents it from happening.
Somebody with a slide rule and a pocket protector thought of this and came up with a solution.
I'd like to know what it is ;)
Capt Quentin McHale
2nd Apr 2017, 05:38
B2N2,
"Physically where does the reserve fuel enter the 2&3 tank?
Close to the hydraulic/fuel heat exchangers?"
Reserve tank fuel gravity feeds via a fuel line that passes through the outboard tanks 1&4 and then through the wing rib that divides the inboard and outboard tanks. The fuel then exits through a check valve immeadiately inboard of the dividing tank rib.
The 4 hydraulic system heat exchangers are located (1 in each main tank) in the aft inboard corners of each main tank (lowest point of each tank) so this places the heat exchangers in tanks 2&3 at opposite ends of their respective tanks in relation to the reserve tank fuel transfer outlets. Hope this helps.
From what I remember about the accident report on the 777 they had dead zones in the tank where waxy deposits accumulated.
I don't know if that was fixed through stirrers or flow deflectors in the tank.
I do recall they had unusually low temps over China. -67C or somewhere about there.
In the CJ2 I've seen ISA -13 in the lower 400's.
As tdracer has been said waxing of fuel in tanks was not the cause of the 777 accident, the problem was due to heat exchanger design at the engine end of the line and water in the fuel , rather than waxed fuel. AFAIK there was no need to modify tank design or tank components as a result. The accident report mentions low temps en route on that flight but any enroute issues were handled correctly iaw with the FCOM.
Unless an old bold Boeing engineer comes along with I don't think you are going to get a magic answer. As has been said Tank 1 is considered to give you the coldest fuel reading, that triggers the FCOM cold fuel procedures, and also I suspect the declared fuel low temp limit no doubt has a buffer before significant waxing occurs.....