According to Eurocontrol (
Frequently Asked Questions (FAQ) on route charges | EUROCONTROL):
"The MTOW declared should be the maximum certificated take-off weight of the aircraft. In the case of multiple certificated take-off weights, the MTOW to be declared must be the highest weight authorised by the State of registration."
Then what is the point of using flexible MTOM?
Eurocontrol is not the only charge which is assessed on the basis of MTOW - most airport landing fees are as well. Of course Ryanair negotiates their own deal at the majority of their airports, but having a flex MTOW allows them to shift the starting point for the negotiation downwards. They tell airports that the MTOW is 67T and conduct the fee negotiation on that basis.