Praying during a flight: Which way of purification is correct?

I think my question is somewhat related to:https://islam.stackexchange.com/questions/14475/what-should-be-done-regarding-salat-on-a-long-flight However I'd like to know what is the most correct choice for the purification during a flight (if one for example would miss a prayer if he/she wouldn't pray in the aircraft): Could we say tayamum is valid or would it need wudu'? I mean in the aircraft of course there are toilets with water, but isn't it an abuse to use this water for wudu' and therefore one could do tayamum or is wudu' necessary because water is available?