var data = from fd in FlightDetails

           join pd in PassengersDetails on fd.Flightno equals pd.FlightNo into joinedT

           from pd in joinedT.DefaultIfEmpty()

           select new {

                         nr = fd.Flightno,

                         name = fd.FlightName,

                         passengerId = pd == null ? String.Empty : pd.PassengerId,

                         passengerType = pd == null ? String.Empty : pd.PassengerType

                       }