747-400 Wing Anti-Ice

[QAVION]

Just wondering if anyone can tell me why Boeing chose to inhibit Wing Anti-Ice with the leading edge flaps deployed on those 747-400's fitted with Auto WAI? (Yet not on the aircraft with Manual WAI).

Any advantages?

Thanks.

Cheers.
Q.

[Wide-Body]

Hi

On our aircraft with manual WAI it is a flying manual requirement NOT to use WAI when the LE flaps are deployed. I assume in a manual system it would be extra cost/complexity to install an inhibiting system. Much easier to design in an inhibiting system in to an auto controller. Hope this is of some help
Regards

Wide:p

[QAVION]

"On our aircraft with manual WAI it is a flying manual requirement NOT to use WAI when the LE flaps are deployed."

Thanks, Wide.
It's starting to make a little more sense to me now.

I see that the auto circuit is quite complex, involving all three Flap Control Units... as well as the normal Manual WAI CMC test and air/ground circuits.

Cheers.
Q.

[Cornish Jack]

Hi Q
Welcome to the club! :D Three of us at VS tried to work our way through the wiring diagrams for the WAI to make sense of what was happening. The other two guys were much more up to speed on these things than me and eventually could follow what was happening but it certainly ain't easy! :( (The logic of it is all too easy, it's the wiring interconnects that are a bit mind-boggling.) Quite obviously, one doesn't want to pump hot air into the atmosphere when the LEs are deployed and there's the run back problem to avoid as well.
The distinction made in the Boeing FM between Auto and Manual is 'inhibited' (for Auto) as against 'ineffective' (for Manual). Pushing hot air straight into the airstream would certainly be 'ineffective' ;) .

[Long Haul]

Normally a crew would only select WAI on when the LE flaps are up. Auto mode simply imitates this behaviour. The real question is why does Boeing allow you to open the valves even when the LE flaps are down, as is possible on aircraft with auto WAI by placing the switch in ON. Other than their basic philosophy of allowing automatic systems to always be able to be overridden, it is useful when one valve has stuck open to avoid asymmetrical bleed demands, or when you've flown into a volcanic ash cloud, where you want to keep the air flowing through the engines as much as possible. BTW in the last nine years I've seen the auto WAI come on once .

[QAVION]

"Quite obviously, one doesn't want to pump hot air into the atmosphere when the LEs are deployed ..."

I'm not sure what is meant by "pumping air into the atmosphere". The 747-400 WAI system does this anyway after the hot bleed air has circulated in the sealed area of the leading edge at the very front of the wing, irrespective of what position the LE flaps are in (on a non-Auto WAI system). I have the feeling that some of you are thinking that hot anti-ice air circulates in the wing cavity where the LE Flaps are stowed(?) It doesn't. The heated area and this cavity are isolated completely by sheetmetal (for the entire length of the wing LE (where the WAI is)).

http://members.ozemail.com.au/~b744er/WingLE.gif

I suppose some warmth will be conducted via the metal into this cavity, but this will probably be greatly offset by the coldness of the fuel in the tanks. You'd probably get far more heat coming off the bleed air duct itself, which resides in the same space as the LE Flaps.

After the air has circulated in the very front of the leading edge, it is vented c/o small slots under the wing LE just in front of the stowed Leading Edge panels. There are only six slots per wing and they are so small and widely spaced apart, it's unlikely that the vents have any secondary purpose, such as keeping the stowed LE's free of ice. As you can see from the following picture, there is not even one slot per LE flap panel.

http://members.ozemail.com.au/~b744er/LeadingEdge1a.jpg

Anyway, thanks for the great feedback :)

Cheers.
Q.

[SMOC]

When the flaps are deployed, the reason WAI is not selected with a manual system (SOPs) or selected off automatically with the Auto system, is the fact that the leading edge looses the majority of airflow over it, as it gets covered by the LE flaps.

http://www.airliners.net/open.file/228836/L/

http://www.airliners.net/open.file/289926/L/

All that ends up happening is the possibility of overheating the LE which as you probably know is only made of aluminium as opposed to the spray tube which is made of titanium, like the bleed ducts.

Hope this sorts it out.

[QAVION]

"All that ends up happening is the possibility of overheating the LE "

Thanks, SMOC.
It was this I was worrying about... and why the manual system didn't have any cutout circuits to prevent this (Boeing already went to the bother of automatically inhibiting WAI on the ground to prevent overheating). "Wide-Body", however, provided the missing link, saying that the WAI is manually switched off (at least on one airline) to prevent overheating in the air.

The Variable Camber LE Flaps are slotted, so I suppose the overheat problem is reduced by the airflow between the flaps and the leading edge.

Thanks to all.

Cheers.
Q.

http://members.ozemail.com.au/~b744er/WAIDucting.gif

[Cornish Jack]

Q
There has been a change by Boeing on this area of ops since the original 744 non-normal procedures. When I first started on 44s the non-normal, for a WAI valve stuck open, required that the air on that side of the aircraft had to be isolated within 30 seconds of touchdown - (obviously to prevent leading edge distortion). Now, however, there is no specified time limit, just 'after landing'. Slightly less caution perhaps, or more realism?

[etsd0001]

Looking at the VS M,M it says that when the a/c in the air mode with Auto selected and iceing conditions are detected the anti-ice sys is automatically activated.

The MM schematic backs this up.

[QAVION]

"Looking at the VS M,M it says that when the a/c in the air mode with Auto selected and icing conditions are detected the anti-ice sys is automatically activated."

Not sure what your point is, etsd0001
:confused:

Group A LE flap deployment overrides the ice detector signals.

Regards.
Q.

[HiDrvr]

Simple answer to why the system allows manual override to on with flaps deployed. Maintenance procedures sometime require the valves to be opened, with flaps deployed and on the ground, but with no bleed air to overheat the LE's.