Landing gear retraction times
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?