Landing gear retraction times

[chalfontim]

On my daily commute passed Heathrow I've started to notice the odd aircraft which seems to keep its landing gear down for several minutes after take off, as opposed the vast majority of aircraft which retract their gear literally within seconds of take off.
Is there any reason for the difference?
I saw it on a Pakistani 747-200, and several times now on A319/320/321s. In fact flying back from Faro on a A321 (GB Airways) I actually heard the gear go up after about 3-4 minutes into flight, and an altitude of 3000ft+. This seemed very unusual to me. Is it? Why would you want to keep your gear down for an extended time after take off? Surely it begins to effect the aerodynamics as your speed / altitude increases?

[Rainboe]

A few minutes with the gear down in the air is equivalent for getting onto an hour of ground cooling just sitting still with a very minor draft blowing through the undercarriage, or flying with the gear retracted (very poor air circulation). As a result, some short legs with quick turnarounds can cause the wheel brakes and tyres to start getting really hot. For the Classic 747, hot brakes are quite a problem. A landing at max landing weight alone can easily cause very hot brakes, and that is only using light braking.

[mutt]

Apart from brake cooling, there is also a MEL that permits 747s to takeoff with one or two brakes deactivated provided that the gear is left down for two minutes.

In my part of the world, its very common to see aircraft leaving the gear down after takeoff due to cooling purposes. You have to remember that all takeoff calculations are based upon an engine failure, so in the case of brake cooling you have an un-failed engine to account for the increased drag from the gear.

Mutt