Hugmonster,
I really don’t think that you are right that the first sector of a multi-sector day cannot depart if the final sector will go into discretion.
HSL and others.
It is quite reasonable and legal for a flight to depart will the possibility of using the first two hours discretion (see CAP 371 18.2) however commanders discretion is exactly that and only exercised when it is required i.e. normally before the last sector.
During the day when a pilot has slept all night it is not unreasonable for him to use discretion. However, if prior to a night flight where changes in sleep pattern has made decent sleep difficult to achieve, it would be quite reasonable and proper not to use discretion at all.
Likewise it is the responsibility of the airline to produce a reasonable and workable programme. If flights out of the UK are scheduled to operate with a normal turnaround time and no allowance for ATC delays or slippage during the day it would be quite unreasonable to schedule a flight to depart out of the northern UK immediately prior to the 2200 local duty period cut-off and expect crews to operate the delayed flight to the Canaries and back at night. It therefore follows that the history of that rotation would indicate whether a schedule is a reasonable or not.
If the late evening Canaries repeatedly become a night one, the rotation is not realistic and the company should change it (if necessary with pressure from the CAA).
As far as the use of discretion is concerned surely if a F/O is not fit to extend his duty he needs to tell the commander of the aircraft. If the commander then exceeds the maximum allowable he surely cannot sign the declaration that the crew were fit for the extended duty; and if he did, the F/O is quite entitled to report the commander for so doing.
Finally if a pilot is likely to have to extend duty to carry out a flight he would be both reasonable and helpful if he told the crewing department that it is unlikely that he would be fit to extend duty, and for the crewing officer to either change his programme or arrange hotel accommodation at the last airport. It also allows him to arrange hotel accommodation for the passengers.
Unreasonable and unrealistic schedules will not be changed if pilots operate them using discretion when they are unfit to do so. It is also clearly illegal and dangerous;
See
http://www.aaib.dtlr.gov.uk/bulletin/jan99/gbbaf.htm in particular the discussion of crew duty period.