777, please start to use the correct forums for posting your questions. I'll attempt to anawer for you but this topic will be going into the Tech Log forum.
If the weather at the departure airfield is below landing minima a suitable departure diversion must be established within 250nm. The forecast conditions at this airfield must be equal to or better than the minimum values for the available approach aids but never less than those given for a CAT I approach in the SAOM. An update on the latest actual weather conditions at this airfield should be obtained prior to departure to re-affirm its suitability. Due regard must be paid to en-route terrain in selecting the take-off alternate.
For planning requirements the selection of a take-off alternate must satisfy the following conditions:
[list=a][*]Met reports or forecasts must indicate that the weather at the aerodrome will be at or above the applicable landing minima for ± 1 hour of the a/c's ETA; and
[*]If only non-precision or circling approaches are available, ceiling (broken or more cloud) must be taken into account; and
[*]Any one engine inop limitations must be taken into account. e.g. Loss of CAT II or III capability[/list=a]
Now that is according to my ops manual and I am sure it is a legal requirement for any company operating on a UK AOC.
So, if you can't land within 250nm of your departure (approx 30 minutes in a jet) then you may be liable for the death of your pax if a landing within that time would have increased their chances of survival.
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Capt PPRuNe
The Professional Pilots RUmour NEtwork