5 aircraft are required for the Summer operations. As someone pointed out, on certain days per the current released schedule, 5 aircraft are needed to cover the PM wave of flights. None of these flights appear to be W patterns indicating based a/c operation. EDIT: With HEL this means that the PM bank of flights require 5 aircraft every day.
According to some of my colleagues on the Flightdeck, the U2 320's can just about make SSH and this route has been rumoured for a while but still hasn't materialised. Looking at the current schedule, there is now no basis for SSH to be launched without a 6th aircraft. I guess there is always a possibility for next Winter.
Other rumours around the base include.....
Rome... Could be launched and operated by FCO
LCA..... Looking at the schedule, unlikely.