View Full Version : Underwing Refuelling

9th Jul 2002, 08:12
Why can you fit more fuel in by underwing (pressure), rather than overwing (gravity) refuelling?

This would suggest that the tanks are pressurised.

Any explanation ??

9th Jul 2002, 11:37
How does this sound: When you refuel from below, (pressure), you allow the displaced air to escape via the vents that are a part of the surge tanks, it is a natural process, the fuel level rises the air moves out.

Alternatively, if you fill from above, (gravity), somehow air gets trapped as there is no significant pressure differential?

That said, I remember on the BAC1-11, when operating Tenrife to Manchester, we would refuel at the panel until cutoff, then ask the refueller, (now armed with several packets of fags), to complete overwing.

An interesting question.

Young Paul
9th Jul 2002, 11:57
The first bit sounds implausible, although I suspect that this is very type dependent as the last replier suggests.

How about, because of dihedral, it is not possible to fill above the level of the overwing refuelling point with gravity feeding, so there is a volume in the wing tanks that can't be reached by gravity fuelling. However, with pressure fuelling, the tank can be filled up as it vents at the outer edges.

9th Jul 2002, 13:32
Young Paul is quite right. Were you (hypotheticaly) to open the over wing fueling cap (on most a/c) having fueled to full wings under wing, then a small avtur fountain would dump the fuel above the cap back onto the ramp.


9th Jul 2002, 23:52
Young Paul seems rather knowledgeable for his (young?) years....for the B707 and TriStar...quite correct.

11th Jul 2002, 01:38
Actually, it is only because the pressure refueling rigs have bigger hoses and pump at higher rates.

Overwing/open/gravity fueling is limited by the ability to handle the unrestrained end of the hose, as well as the danger potential of fuel spills. With a closed system, these limits are nonexistent.

Automation in tank vents, level switches, and shutoff valves allows pressure refueling with high flow rates, and the closed nature of the system helps prevent fuel spills.

Flight Detent
11th Jul 2002, 11:20
Run that past me again!!!

I agree, Young Paul is quite right!


Capt Byrd de Styrke
11th Jul 2002, 20:58
Some clarrification...

Pressure Refueling Rigs do not pump!!! If you mean hydrant dispensers/cart they rely on the pressure of fuel released from a hydrant system...upto 150PSI, reduced by point of delivery at wing. It is possible to achieve fuel flow rates of upto 4,000 litres+ using hydrant pressure refuelling

Refueller/bowser vehicles (big tank vehicle with wheels) do pump fuel and can still fuel underwing but typically at lower flow rates.

11th Jul 2002, 22:24
Capt BdS:

There's gotta be a pump somewhere, either on the truck or somewhere underground... :D

Flight Detent:

Take a look at the diameter of the hoses used for typical overwing refueling and for typical pressure fueling. I think you'll find the pressure hoses are much bigger, as are the openings in the couplings ("nozzles").

If you've ever handled a fire hose, you might know how much of a problem they are to handle. That's why there are typically 2 or 3 firefighters on each hose end. If you had similar pressure and flow rate in an overwing fuel hose, you would risk spills, splashes, and losing control of the hose end.

As far as Young Paul's statement regarding dihedral, it is correct as far as it goes. If the overwing cap is at thr outer/top end of the tank, there may be little unusable space in a tank. However, even with pressure refueling you have to deal with internal float switches and vent valves, so it may or may not be possible to completely fill a tank.

12th Jul 2002, 05:36
Young Paul has the correct explanation for why you can get more fuel in by pressure refuelling.

On the good old VC10, Fuel Quantity System probes were changed through overwing panels. You had to be careful to check the fuel in the tank using the MLIs before opening the panel, or everything above the panel drained onto the concrete. I learned about maintenance from that

Through difficulties to the cinema

12th Jul 2002, 14:18
All the arguments put forward here are more or less correct.

The main consideration to be allowed for when open line refuelling ie overwing is that the fuel vapour that exists within the tank being refuelled will endeavour to exit to atmosphere from the open overwing fuelling cap rather than via the normal vent system.

It is for this reason that you cannot fuel so quickly; as the fuel entering the tank will tend to be blown back out of the filler cap orifice by the exiting fuel vapour.

Flight Detent
13th Jul 2002, 10:38
Thanks Intruder,
Yes, I have handled quite a number of fire hoses, for exercise, in my time, as well as many, many overwing refuelling hoses - Caribou, Vampire, Canberra, C-47/DC-3, Neptune, F-86, Mirage, during my many years as a Ground Engineer, prior to starting flying!
No system, as far as I am aware, fully fills the available tank space with fuel, all leave varying amounts of airspace for thermal expansion, THAT'S WHY the overwing filling access is never right at the top of the tank, because then you would be able to completely fill the tank, and the designers don't want that!

I appreciate that the 'Pie-cart', as the hydrant truck is referred to where I come from, only dispenses, regulates and measures the fuel into the aircraft, and the fuel pressure and flow rate is increased because the system is 'closed' and is therefore much less liable to spill, exept for shut-off malfunctions in the aircraft fuel tanks.