Or, put an extra bit in the time calculation that if arrival time is less than departure time, add 24 hours.
Departure and arrival times and elapsed time all in hh:mm. Dep in col A, Arr in col B
Flight time is computed by =IF(B1>A1,B1-A1,B1-A1+24)
The catch is, of course, that if you put in a silly time you'll get a silly result.