As reported elsewhere:
An RAF spokesman said: 'We start planning the trail four weeks in advance and while we've been conducting trails for many years each one is different.
'When we get tasked we plan the route and determine where each air-air refuelling will take place. This then gives us the information we require to obtain diplomatic clearances and to book airspace.
'When trailing fast jets over long distances we need to remain within 90 minutes of a diversion airfield lest we have any problem with one of the aircraft.
'We also have to factor in the sea temperature to ensure that in the unlikely event of a pilot having to eject that they are able to survive in the water for as long as possible.'