[{"_id":ObjectId("3f94fb7faff0b71ae5e3cb8d"),"flight_number":"SWA4146","departure_airport":"SNA","arrival_airport":"SMF","fuel_consumption_gallons":700.4,"flight_distance_miles":450},{}]

db.collection.aggregate([{"$group":{"_id":{"arrival_airport":"$arrival_airport"},"departure_airport":"$departure_airport","total_fuel_consumption_gallons":{"$sum":"$fuel_consumption_gallons"}}},{"$project":{"_id":0,"route":{"$concat":["$_id.departure_airport","-","$_id.arrival_airport"]},"total_fuel_consumption_gallons":1}},{"$sort":{"total_fuel_consumption_gallons":-1}},{"$limit":5}])