LE flap retract during reverse B747

[flyburg]

Question,

During reverser operation the leading edge gr. 2 and 3 flaps retract, does anybody now the specific reason?

And, when in secondary mode for those groups, do they still do this as is secondary mode the flaps move much slower? Is there consequently a limitation or procedure for the use of reverse in this condition?

Thanks

[MarkerInbound]

"Automatic Leading Edge Flap Retraction

Group A leading edge flaps are automatically retracted during ground
reverse thrust operation in order to improve structural life of the flap
panels. The function is armed when the airplane is on the ground and
the flaps are operating in the pneumatic mode.
Group A leading edge flaps retract when armed and engines 1 and 4
are placed in reverse thrust or engines 2 and 3 are placed in reverse
thrust. The flaps re-extended when the reverse thrust signals are
removed. "

Since it says the automatic retraction is only armed when the flaps are driven pneumatically and secondary mode uses electric motors, I'd say no.

I don't see any limitation on the use of reverse in any of the ORH flap checklists.

[Alpha Sierra]

Have a look at this thread. The question was asked a few years ago.

There is a video showing it too.

http://www.pprune.org/tech-log/276980-reverse-thrust-kreuger-v-c-l-e-retract-why.html

[Old Fella]

On the CX Classic with RR RB524's the LED's DID NOT retract on application of reverse thrust. QF Classics DID. The reason they used to on some B747's was because, I believe, the possible damage to the LED's by hot stream gas flow being reversed. On the RR only the cold stream is reversed. CX believed that it was less costly to prolong pneumatic drive motor life by not retracting the LED than repair any damage they may incur from reverse operation. QF obviously did not share the view of CX, just as they differed on Rudder Fine Steering. Active on the CX aircraft, de-activated on the QF aircraft I was given to understand.

[flyburg]

MarkerInbound,

Thanks, that solves that question. That piece of text is missing in my manuals.

Greetings

[main_dog]

Old Fella, are you sure you're not referring to CX's GE-powered classics? Those indeed did not have retracting LE flaps, but CX's RR classics did (or at least the four RR freighters we had at the end did). Here's the excerpt from my Vol 4:

(RB211)
When thrust reverse is selected after landing all leading edge flaps retract to prevent damage.
With one and four engines in reverse group “A” leading edge flaps retract. With two and three engines in reverse group “B” leading edge flaps retract. The retraction of the leading edge flaps is actuated as the thrust reverse levers move through 25° from the reverse idle position.

[spannersatcx]

You are both right, they didn't used too but were modded to do so.

[SMOC]

The damage to the leading edge flap panels was referred to as fatigue cracking due to sonic vibration.

[Dan Winterland]

CX's GE powered aircraft were the ex Air Hong Kong 747s which (I think) came from VARIG, so weren't to the orignal CX specs.

BA have disabled the LED retraction on reverse because of an incident at Jo'berg when the system operated inadvertantly on the take off roll on one aircraft.

[Old Fella]

main_dog, not in my time on them they didn't. I retired in 1991 so anything could have happened since. spannersatcx has given us both a tick so as he says, we are both right. Hope you enjoy CX as much as I did. Thanks for the clarification spanners!!!

[main_dog]

Thanks Spanners and Fella, didn't know that.

OT From a pilot's point of view, CX is not quite the company it was when you left; having said if I have to be honest with myself there's nowhere I'd rather be :ok:

MD