Originally Posted by higney85

So you land flaps 40, suck everything up to flaps zero (slats up too). Lots of moisture in there, plus crushing ice along the way of retraction, then only set flaps to 5/13/18 (if memory serves) for the next T/O. That’s a lot of extra potential crud iced up into the system. Land at 40 flaps, and all the gunk gets collected/frozen. Easier for the next crew to have all of that cleared out of the flaps/slats at full droop/mess and start fresh after. Not on the fleet anymore, but that’s the answer that likely hasn’t changed since the airframe design of the 60’s.