Is there any way of knowing the FlightAware flight-id w/o knowing the exact dep. time.
Airline + flight# + date + dep_city - should always result in a unique flight.
No airline has more than 1 flight that meets the above uniqueness requirements - dep-time is not needed.
The problem with the departure time - is that it can change based on delays - and when that happens - no way to find the flightId.
You could call ** FlightInfoEx** with an ident and that will return faFlightID, among other things.
https://flightxml.flightaware.com/soap/FlightXML2/doc#op_FlightInfoEx
For example
https://flightxml.flightaware.com/json/FlightXML2/FlightInfoEx?ident=UAL3&howMany=1
gives me
{“FlightInfoExResult”:{“next_offset”:1,“flights”:{“faFlightID":"UAL3-1453476900-schedule-0000”,“ident”:“UAL3”,“aircrafttype”:“A320”,“filed_ete”:“04:31:00”,“filed_time”:1453476900,“filed_departuretime”:1453649700,“filed_airspeed_kts”:332,“filed_airspeed_mach”:“”,“filed_altitude”:0,“route”:“”,“actualdeparturetime”:0,“estimatedarrivaltime”:1453665960,“actualarrivaltime”:0,“diverted”:“”,“origin”:“KORD”,“destination”:“KSNA”,“originName”:“Chicago O’Hare Intl”,“originCity”:“Chicago, IL”,“destinationName”:“John Wayne”,“destinationCity”:“Santa Ana, CA”}]}}