I agree, there is no legal requirement (Government, authority, etc) with each country (Some do) , but MOST FOM will include it, thus making it legal for their operations.
I also clearly stated earlier that my above references were FOM / AOC references.
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USE OF ADEQUATE AIRPORTS FOR NON-ETOPS FLIGHTS
Normally weather for adequate airports are not required except:
- where the departure airport is below landing minima, a useable adequate airport must have a valid forecast from 30 mins prior to earliest time of use to 30 mins after latest time of use.
PRE-FLIGHT (non-ETOPS) and in the non-ETOPS segments of an international flight there must be an adequate airport above the landing minima within 60 min / 405 nm (180 min / 1200 nm ETOPS A/C) of the A/C for the entire route. This is a company (not regulatory) requirement to make sure that enroute airports are available for use. MEL reductions in ETOPS diversion distances do not affect this requirement (ETOPS affecting MELs apply to ETOPS flights).
INFLIGHT, there is no legal requirement to monitor/maintain adequate airport weather above landing minima. Airmanship dictates general airport proximity and associated weather awareness.
Again, Planning (Pre-Flight) and actual (In flight). There is a difference.
The original question....
on a non ETOPs route for a twin engine operation what's the weather requirements for an enroute alternate airport,
answer that I have in FOM (approved by local authority)
there must be an adequate airport above the landing minima within 60 min / 405 nm ...of the A/C for the entire route
R T F Q
As a side note:
...a suitable airport is an adequate airport with weather reports, forecasts or combination thereof, indicating that the weather conditions will be at or above minima as specified...
...there must be an adequate airport above the landing minima...
...by definition, implies a "Suitable" Airport.