A Stansted question, though about airlines and their movements. Over the last few days I've noted perhaps 30 or more test flights of both Ryanair and Jet2 that power up, taxi, takeoff, do a circuit of no more than 10mins, land and power down again. These are obviously test flights, but bearing in mind that they are so many, all with different aircraft it piqued my curiosity as to why. Are they just being cycled through to keep them in airworthy condition bearing in mind the UK winter climate, or something else ?