Mandala Airlines flight 091, a Boeing 737-200, crashed during an inadvertent flapless takeoff from Medan-Polonia Airport, Indonesia, killing 100 of the 117 occupants and another 49 on the ground. Flight 091 was to depart Medan-Polonia Airport (MES) for Jakarta-Soekarno-Hatta International Airport (CGK). The previous flight was from Jakarta and arrived at Medan uneventfully. The same crew have flight schedule on the same day and returned to Jakarta. At 09:52 local time, MDL 091 asked for push back and start up clearance. After receiving the approval the air traffic controller the crew began starting the engines. At 09:56, the controller cleared MDL 091 to taxi into position on runway 23 via taxiway Alpha. Six minutes later MDL 091 received clearance for take off with additional clearance from ATC to turn left heading 120ø and maintain 1500 ft. The airplane failed to become airborne and overran runway 23, impacting several approach lights. It traveled through a grass area and over a river, and impacted several buildings and vehicles before coming to rest on a road. A post crash fire destroyed much of the airplane. One hundred passengers and crew members on board the plane died. Fifteen passengers seriously injured and two passengers (a mother and child) survived without any injuries. 49 persons on ground were killed and 26 people were seriously injured. Medan-Polonia has a single 2900 x 45 meters asphalt runway (05/23). The National Transportation Safety Committee determines that probable causes of this accident are: – The aircraft took-off with improper take off configuration namely with retracted flaps and slats causing the aircraft failed to lift off. – Improper checklist procedure execution had lead to failure to identify the flap in retract position. – The aircraft’s take off warning horn was not heard on the CAM channel of the CVR. It is possible that the take-off configuration warning horn was not sounding.