Just out of interest, what is your understanding of fdp's and discretion? The flight duty period starts when you report for duty 45 mins or 1 hour before the flight depending on the contract and ends when you go off duty back at base or in the hotel if away. Discretion is used solely as a means of getting a duty finished that has ALREADY BEGUN!!! You cannot legally roster someone into discretion. It is illegal and Emerald would lose their AOC if found by the CAA to be doing such practices. This i know to be true from links i have to the CAA. They do not condone illegal rostering in any form and regular checks, which incidentally are often done without warning, are carried out. I think you are getting confused about what is and isn’t allowable. Ops/ Crewing can ask a crew to do a duty/ favour, and if this request is within 12 hours then they are perfectly within their rights to refuse. Normal practice for airlines is to give 12 hours minimum notice of a duty so technically anything within this could be seen as a favour. Your duty does not however start when you receive that call unless you are actually on standby. Ops could legally not give you a roster that jeopardized your rest at base etc as they simply aren’t allowed. All duty times and rest periods are entered into a database that flags up inaccuracies in fdp/ dp/ rest and this CANNOT BE FIDDLED! Discretion is only used once a duty has begun, and a max of 2 hours is allowable. Also anything over 1 hours discretion and/ or 1 hours reduction in rest must be submitted to the CAA in writing, and so regular rostering of the kind you suggest would instantly be flagged up and your company would be in deep ****. So therefore if wx is ****e or the aircraft is tech you can as a crew elect to use your discretion and continue. You also can elect not to as is within your rights, and i would suggest that if you feel you are being illegally rostered into discretion you tell your operations/ crewing department and let them sort it out.