Gulfstream Aerospace Corp, the business jet unit of General Dynamics (NYSE:GD), said a Gulfstream G650 test aircraft crashed during takeoff performance tests on Saturday, killing the two pilots and two flight test engineers who were aboard at the time.The plane struck its right wingtip on the ground as it took off from Roswell International air Center at about 9:30 am local time, according to the FAA. The plane hit the ground, causing the landing gear to collapse, skidded along the runway and caught fire, finally coming to rest only 40 feet from the control tower.Gulfstream has become synonymous with large, luxurious private jets. The company’s models are popular with celebrities, athletes, major corporations and wealthy individuals. More than one quarter of Fortune 500 companies operate Gulfstream planes.The accident is an enormous blow to the G650, which has yet to be certified by the FAA for delivery. Before the crash, the company had expected to receive certification sometime this year, with initial deliveries expected to begin in early 2012.