PDA

View Full Version : Fuel transfer for trimming


witwiw
2nd Aug 2016, 11:44
Given a number of large aircraft can transfer fuel to trim/adjust the CG in flight, what is the status of that fuel that is shifted? By that I mean, is there an amount of fuel that can only be used for that purpose and not consumed? Or, as the fuel disappears from the main tanks, does the need for a transfer to adjust/trim etc become less and the fuel that was earlier used for that purpose now available to be used?

TurningFinalRWY36
2nd Aug 2016, 12:10
On the 330 fuel will be pre loaded into the stabiliser tank on long range ops to increase fuel load and keep CG within acceptable limits. Passing through FL255 fuel will be automatically transferred to stab tank to provide a more rearward CG. Once fuel load reduces to 4000kg in the main tanks the stab fuel will be automatically be transferred to the main tanks and no longer be used for CG control

BuzzBox
2nd Aug 2016, 12:22
All of the fuel is useable on the A330/A340. Fuel is initially transferred aft, but small quantities are then pumped forward throughout the flight to keep the CG close to the aft CG target as fuel is used from the centre/inner tanks. Any remaining trim tank fuel is transferred forward towards the end of the flight.

Keg
2nd Aug 2016, 21:09
The A380 had an optimal take off CG and an optimal in flight CG. Fuel was moved around as required in flight to maintain as close to the optimal in flight CG as possible. All useable. Quite a clever concept really.

Snakecharma
2nd Aug 2016, 21:30
If I recall correctly the A350 doesn't use a trim tank but manipulates the leading and trailing edge flaps to manage the trim. Saves weight on the plumbing etc but adds complexity in other areas.

The trim fuel is clever and works well in my experience but it will be interesting to see how the system works on the 350.

Found this after a Google, so give it the same amount of credibility that any Google info should be given :)

The A350 is the first Airbus wide body aircraft to not have the trim tank in the tail that uses fuel to improve cruise performance by adjusting the center of gravity (though I should note that early A300 and A310 variants also lacked a trim tank). On the A350, a variable camber wing that adjusts minute deflections in the flaps accomplishes the same functions without the complexity of a trim tank system.

BuzzBox
3rd Aug 2016, 00:25
That's correct Snakecharma, unlike other Airbus widebodies, the A350 doesn't use a trim tank to control the CG during flight. It uses a variable camber system known as the 'Flap Cruise Deployment Function' to control the wing's centre of lift. The system achieves the same drag reduction (if not better) as the trim tank system, by reducing the down force produced by the tail and improving the drag characteristics of the wing. That is achieved by deploying the flaps a small amount during the later part of the climb and during cruise. The inner flaps can deploy up to 1.5 down, while the outer flaps can deploy up to 2 up or down. The deployment is controlled automatically by the flight control computers and there are no indications to the crew.

It sounds complex, but most of the complexity is in the flight control computer software. The flap system itself is actually a more simple design that, according to Airbus, is about 400kg lighter than older systems. The lack of a trim tank and associated plumbing provides a significant additional weight advantage.

But I digress...

Potsie Weber
3rd Aug 2016, 00:45
All of the fuel is useable on the A330/A340. Fuel is initially transferred aft, but small quantities are then pumped forward throughout the flight to keep the CG close to the aft CG target as fuel is used from the centre/inner tanks. Any remaining trim tank fuel is transferred forward towards the end of the flight.

Except when the main pumps are off and 2,000kg in each inner tank becomes unusable. This has led to some interesting outcomes in the simulator when dealing with low fuel level ECAMS where pumps are switched off and then attitude changes in go-arounds etc have then generated pump low pressure ECAMS.

BuzzBox
3rd Aug 2016, 01:50
Well if you want to complicate things...

halas
3rd Aug 2016, 06:23
B777 droops the ailerons 5 degrees in cruise to do the same thing. Crude but simple.

halas

BuzzBox
3rd Aug 2016, 08:51
B777 droops the ailerons 5 degrees in cruise to do the same thing.

I think you'll find the aileron droop on the B777 is only 2. The feature was first designed into the B777-300ER and was later made available as part of a Performance Improvement Package for older versions of the aircraft. According to Boeing: "This software-based modification reduces drag by creating higher aerodynamic loading on the outboard part of the wing and making the spanwise loading more elliptical. As the aileron droops, the increased loading also causes a wing twist change that reduces the local flow incidence toward the wingtip. This reduces the shock strength on the outboard wing, thereby reducing drag even further."

plhought
4th Aug 2016, 02:02
Old 727 boys used to occasionally do the as the new-fangled 350. Couple CBs opened (to avoid slat deployment) and the flaps would be selected to 1*-2*. On those heavy freight hauls at altitude, would dip the nose down a bit and get the airplane on "the step" as the FE would call it.

*Told from an aged 27 driver - never personally witnessed*

BuzzBox
4th Aug 2016, 02:34
That unauthorised procedure nearly caused the demise of a 727 back in 1979 (TWA Flight 841), when the crew allegedly tried to extend the trailing edge flaps 2 independently of the slats while cruising at FL390. The crew never admitted it, but the investigators believed the slats were inadvertently extended and one of the slat segments remained extended when the crew tried to retract them. The aircraft suffered an uncommanded roll, entered a spiral dive and lost 34,000ft in a little over a minute. It rolled through 360 twice during the dive and suffered significant structural damage. :eek:

witwiw
4th Aug 2016, 05:49
Good, everyone, thanks for the responses.