I'd like to ask one more question on this, if I may.
I have sometimes wondered what does an airline do when there is crew illness down-route at a "long-haul" (i.e. very far away) destination? Say, for example, a B777 crew are due to operate a flight from Singapore to Heathrow, and one of the first officers does not feel fit to fly, for whatever reason. The airline does not have any standby crew at the departure airport or indeed anywhere near it. All it has is the crew who have completed their "slip" and are due to operate back to LHR.
What happens? Does the airline put a standby FO at LHR on the next flight out to Singapore? They would of course then need to rest there before being able to operate back. Or do you simply have to wait until the FO is better and can operate the flight?
I hope you see what I'm driving at. I am just curious to know what contingencies an airline has for this particular type of situation. It's not the same as where, at LHR, you have a crew illness and you can call on anyone on stand-by to take over!
Nick