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musang
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Why 744 Fuel Dump Flap Restriction?

Thu Sep 17, 2015 12:37 pm

Apparently on the 744, one shouldn't move between Flaps 1 and Flaps 5 (I.e. Outer leading edge extension/retraction) whilst dumping fuel. Turbulence happens as the flaps are in transit, clearly affecting the dump stream directly behind.

However, can anyone tell me what bad things actually happen?

Any links a video of the disruption to the fuel stream? I think the 'tube is restricted on the wifi I'm on.

Thanks - musang
 
oly720man
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RE: Why 744 Fuel Dump Flap Restriction?

Thu Sep 17, 2015 8:12 pm

Flaps 5 is still pretty streamlined and given that the fuel dump is out near the winglet and well away from the flaps, I can't imagine any aerodynamic/turbulence effects close to the aircraft that would be a cause for concern. Once the fuel's gone it's gone and any effects due to the LE slats and TE flaps on the downstream fuel dispersal will be minor.

The only things I can think of, as a non 744 driver, are

that dumping fuel and deploying the flaps may trigger flutter, depending on the mass distribution of fuel within the wing.

possible electrical problems with flap actuators and fuel drain pump

moving parts as a very slight risk of an ignition source.
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musang
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RE: Why 744 Fuel Dump Flap Restriction?

Thu Sep 17, 2015 11:29 pm

Correct, Flaps 5 is pretty streamlined. However, the restriction is dumping with the flaps in transit, as there's a transient buffet as the flap goes through the mid part of the extension or retraction.

musang
 
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747classic
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RE: Why 744 Fuel Dump Flap Restriction?

Fri Sep 18, 2015 11:45 am

FCOM 744 indeed states : Do not extend or retract flaps between position 1 and 5 during jettison.

I don't recall the same restriction at the older 747-100/200/300SP wing platform.

With the introduction of the 744, the wingspan has been increased and one more LE panel has been added at the outboard position, exactly in front of the air flow pattern over the jettisson pipe. Also the wing fuel distribution is altered at the 744, changing the flutter pattern of the wing with different fuel loads.

Note : the jettisson pipe remained in the same position as the previous 747 series and has not been relocated further outboard, only a new ourboard wing section + winglet has been added.

During flaps transistion this added outboard LE panel has show to produce some minor wing flutter during certain airspeed/weight conditions during certification flight tests and the combination jettisson plus LE flapmovement 1-5 is therefore not allowed by the FAA.

[Edited 2015-09-18 05:39:21]
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CCA
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RE: Why 744 Fuel Dump Flap Restriction?

Sat Sep 19, 2015 4:30 am

It's not a flutter issue, this is straight from an old manual.

"Extending or retracting flaps between 1 and 5 is prohibited whilst fuel jettison is in progress due to the possibility of fuel being drawn into contact with the wing trailing edge"

[Edited 2015-09-18 21:53:13]
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musang
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RE: Why 744 Fuel Dump Flap Restriction?

Sat Sep 19, 2015 9:33 am

Thanks CCA,

thats the definitive answer I was hoping for.

To take it a step further, fuel periodically gets into contact with wing exterior surfaces anyway (leaking access panel, over-wing fuelling etc.). So why is fuel on the trailing edge such a no-no?

Thanks - musang
 
CplKlinger
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RE: Why 744 Fuel Dump Flap Restriction?

Sat Sep 19, 2015 1:47 pm

Quoting musang (Reply 5):

To take it a step further, fuel periodically gets into contact with wing exterior surfaces anyway (leaking access panel, over-wing fuelling etc.). So why is fuel on the trailing edge such a no-no?

Could it have to do with the lights on the wing tips? Granted the chances would be low, but you have a light with fairly high power requirements, even if it's an LED lamp. By then the fuel would be more vapor than liquid, and jet fuel is most dangerous as a vapor than a liquid.
 
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CALTECH
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RE: Why 744 Fuel Dump Flap Restriction?

Sat Sep 19, 2015 2:23 pm

Fuel is not allowed to reattach itself to the airframe during fuel jettison operations.

http://www.gpo.gov/fdsys/pkg/CFR-201...f/CFR-2011-title14-vol1-part25.pdf

Page 478

'(d) During the flight tests prescribed
in paragraph (c) of this section, it must
be shown that—
(1) The fuel jettisoning system and
its operation are free from fire hazard;
(2) The fuel discharges clear of any
part of the airplane;
(3) Fuel or fumes do not enter any
parts of the airplane; and
(4) The jettisoning operation does not
adversely affect the controllability of
the airplane. '
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musang
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RE: Why 744 Fuel Dump Flap Restriction?

Sat Sep 19, 2015 5:13 pm

Many thanks CALTECH. Now I'll look for videos of testing, as I'm intrigued by the airflow modification.

Regards - musang
 
Max Q
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RE: Why 744 Fuel Dump Flap Restriction?

Sat Sep 19, 2015 9:06 pm

Not to mention while dumping fuel that stream is under pretty high pressure, between what 2-4000 ppm on the B747 ?


Not going to do the wing much good if it is impacted by that.
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CALTECH
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RE: Why 744 Fuel Dump Flap Restriction?

Sun Sep 20, 2015 7:32 pm

Quoting Max Q (Reply 9):
Not to mention while dumping fuel that stream is under pretty high pressure, between what 2-4000 ppm on the B747 ?


Not going to do the wing much good if it is impacted by that

Don't think it is much higher than 55 PSI or so, just like the pressure fueling max PSI allowed. Some of the same components such as manifolds, are used in both refueling and dump modes. More that you'd be coating the outside of the airplane with fuel in a emergency.
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Max Q
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RE: Why 744 Fuel Dump Flap Restriction?

Mon Sep 21, 2015 10:24 am

Has to be higher than that to achieve that dump rate, it's not a fire hose
but its a powerful stream, not going to do any good if it hits anything.


Which is why its designed not to !
The best contribution to safety is a competent Pilot.


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mmo
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RE: Why 744 Fuel Dump Flap Restriction?

Mon Sep 21, 2015 12:31 pm

Quoting Max Q (Reply 9):

Not to mention while dumping fuel that stream is under pretty high pressure, between what 2-4000 ppm on the B747

Dumping with all pumps and full tanks will get you 6,000 Lbs/min.
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CALTECH
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RE: Why 744 Fuel Dump Flap Restriction?

Mon Sep 21, 2015 7:22 pm

Quoting Max Q (Reply 11):
Has to be higher than that to achieve that dump rate, it's not a fire hose
but its a powerful stream, not going to do any good if it hits anything.

From the 747-400 AMM, pretty sure all the aircraft are this way, and the boost pumps will not exceed this limit either,

" CAUTION: MAKE SURE THE FUEL PRESSURE USED TO REFUEL THE AIRCRAFT IS NOT MORE
THAN 55 PSI (379.2 KPA). IF THE PRESSURE IS MORE THAN 55 PSI (379.2 KPA),
DAMAGE TO THE FUEL SYSTEM CAN OCCUR.

(3) Make sure the refuel pressure is between 35 and 55 psi."



It only takes 4-7 PSI to extinguish the low pressure light light.
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AA737-823
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RE: Why 744 Fuel Dump Flap Restriction?

Tue Sep 22, 2015 9:35 am

Quoting oly720man (Reply 1):
Once the fuel's gone it's gone and any effects due to the LE slats and TE flaps on the downstream fuel dispersal will be minor.

The thing is, the 747 has NO slats. Only kruger flaps.
Therein lies the problem. Transient kruger flaps are... ugly aerodynamic messes.

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