Given the lack of an answer it might be the way I asked, not to mention that it is not an exhilarating subject. I try rephrasing anyway:
On B777, the calculation of the required landing distance (RLD) at the alternate airport is made on the OPT with the choice of flaps 25 or 30 only.
This calculation, known as "landing-dispatch", gives the maxi performance landing limit weight at any airport (destination or alternate) taking into account the regulatory requirements at the flight preparation stage (factors 0.6, 1.15, etc.) and based on the following conditions: RWY, Missed Approach gradient, RWY condition, wind, OAT, QNH, Reversers, Autoland or Manual, A/C config, A/I config, MEL, CDL, NOTAM and FLAPS.
Before the flight, the crew must verify that the landing weight will be lower than this maxi performance landing limit weight in order to be able to retain the airport as an alternate.
The regulatory provisions, which I referred to in my previous post, make it necessary to take into account the "aeroplane configuration, environmental conditions and the operation of systems that have an adverse effect on performance."
The regulatory flight profile, departing with the minimum regulatory fuel, leads to land at the alternate airport with only the final reserve fuel on board.
This final reserve fuel is less than 4 tons on B777.
With less than 4 tons of fuel on board, Boeing's procedures (QRH NNC "Fuel Low Qty") impose on the crew to land flaps 20.
Again, it is a matter of regulatory planning and of interpretation of regulations, not of in-flight fuel management and what may happen during the flight or what will do the crew once airborne.
Therefore, you already know at preflight that you're supposed to land flaps 20 at the alternate airport but you can only check the flaps 25 or 30 RLD.
According to the "Dispatch" requirements, regulatory accessibility of the alternate airport is thus verified with flaps 25 or 30, whereas the regulatory flight sequence is based on landing at the alternate with only the final reserve fuel on board and consequently with a mandatory Flaps 20 configuration.
So, what (and how) do you do, and why?
Thank's!