I need to ask stephen what the basis for longest route is, exactly, for that measurement. my bet is it's round trip, plus gate time.
at present (unintentionally) you can push an aircraft up to -8 hours use in a day, so it's possible it's going too far in 24 hours.
also, there's (in the present route creation schema) a small speed boost to attempt to correct for the fact that though any plane is capable of reaching its range in 24 hours it is not necessarily capable of doing a round trip flight in 24 hours. for the next age (2000, give or take a month or two), there should be and almost definitely will be a new create route page capable of doing one-way flights and preventing negative hours.