You have quoted the most famous (or is that infamous?) example of the point I am making, BOAC. In that case, the Capt still retained his CAP371 authority to extend duty hours but had no power so to do. If CAP371 gave Capt's a prescriptive power as opposed to a permissive power, then nothing that BA or the CC unions negotiated or signed would have been able to interfere with or restrict that power. The very fact that it did goes back to what I describe as the wholly permissive wording of CAP 371 on this issue. The fact that the Capt has only to assure himself that the crew is fit to work the extended hours to place himself in a position where CAP371 permits him to authorise the extension does not mean that, having decided the crew are fit, the Capt can require them so to do. This, to me, has the same sort of didactic position as the: I eat meat; dogs eat meat; therefore I am a dog conundrum. So, with respect, whatever approach we choose to adopt as individuals, that, in itself, is not the meaning of Capt's discretion.

What a Capt then does, on the ground, when he wants to exercise his discretion follows exactly as Parabellum describes - most companies will have an SOP or other instruction that prescribes the contractually based rights of the Capt to act as the Crew's de facto employer and require them to work the hours he sees fit to authorise within CAP 371. All the problems I have seen/heard of arise because most Capts do not appear to realise that the power to require crews to go beyond normal FTL limits and work the hours he wishes to authorise arise from the contract of employment and not from a statutory authority granted by CAP 371 - but try to act as if it does. This is further illustrated by considering the sanctions available against a crew member who refuses, point blank, to work into discretionary hours. Were this a statutory authority, there would be legal sanction - there is not. That crew member will be dealt with iaw company disciplinary procedures for the breach of company employment rules.

Where that practically leaves us is, a little, between a rock and a hard place. If an individual crew member is unwilling, offload them and work light to the limits laid down in the Ops Manuals. If the crew as a whole are uneasy, then that is a test of a Capt's command ability - if you can't carry a crew with you it's best not to blow the whistle and launch yourself over the top, it can get very lonely in no-man's land. If the whole crew are saying they are not willing, that is such a CRM issue that a Capt's duty of care to the crew and pax probably precludes the exercise of that authority to extend and it may be that, in using his/her discretion, a Capt would decide it is better not to go.