RAt 5,
putting the gear down early before landing is like using almighty brakes fans! The chill factor of a 200mph wind is great for reducing the brake temps - after all thats exactly what brake fans do, albeit at a slower speed!!
Like early starts/late finishes, its all about culmulative fatigue or in this case temperature. If you have just done a sector, quick turnround and off again the brakes will 'store' a lot of heat in them - they may be, say, 180C when the gear is retracted as opposed to, say, 80C after the first take off of the day. If you then go on to do another 3 sectors with 20 min turnrounds the brake temps will increase more and more and the store of heat could get to the magic 300C. Used to be a problem going from Skiathos to thessaloniki - and that was after an hours turnround.
Agreed, take-off performance will be limited if gear is left down after takeoff, quite significantly I recall, somewhere in the order of 7 tonnes. but at the weights we fly around at it could still be possible.
A chap did a study once, in parallel with Airbus, and its such a complicated subject. very involved with considerations of OAT, speed brakes applied, what temp were brakes before application determining how much of a skim was taken off the carbon fibre discs etc etc etc. The upshot was to get as much heat into the brakes as quickly as possible and do so in one contimuous application (carbon fibre brakes, NOT steel) and so NOT to use autobrake!
Take cover!!!!