747-400 Query...

[Geoff565]

Hello all,
I hope this in the correct forum so my apologies if it isn't, i've also had a search but had no luck finding my exact query, so here goes!
I was watching a 744 Video (http://www.flightlevel350.com/viewer.php?id=4051)on FL350 yesterday when i noticed something that i haven't seen before.
Upon touchdown, as Reverse Thrust is applied the 744 LE Kruegers retract automatically, could anyone please give me a brief explanation as to why? Is it a common attribute across most aircraft?
Just a matter of curiousity really; tried applying some ToF principles but couldn't reach a sensible explanation!
Any insight would be much appreciated!
Cheers,
Geoff

[SMOC]

The reason they retract is to reduce cracking of the leading edge flaps and linkage which has been attributed to sonic vibration and side loads resulting from engine exhaust stream impingement during thrust reverser operation while the leading edge flaps are extended.

SMOC

[spannersatcx]

Several sets of Leading edge devices (Kreugers and the L/E flaps between the engines) automatically retract on selection of Reverse Thrust on the 744 and the 747 (those that have been modded) to prevent damage from any FOD/dirt/etc thrown fwd by the pplication of reversers.

You'll also notice on the video that once the reverse is cancelled they come back out again.

[Geoff565]

Oh i see, I notice they do extend again. I must admit, I didn't think about potentital FOD/Loads from the Gasses. Thanks a lot for that chaps, much appreciated!
Cheers!
Geoff