CIRCUMSTANCES: "The flight was scheduled as a Supplemental Airline (SAL) flight number 421S, but due to the unavailability of a SAL qualified pilot, the flight was rescheduled as a charter flight. Both passengers were booked on airline flights out of Brisbane, one of which was scheduled to depart at 0905. When the pilot and passengers walked out to the aircraft it was raining, and the passengers put their umbrellas and a brief case in the right wing locker. The pilot then closed the locker door before boarding the aircraft. The pilot transmitted a taxiing call to Brisbane Flight Service at 0801 local time. Seven minutes later, he reported lining up on runway 11 and requested an airways clearance. After the aircraft became airborne, witnesses saw it make a left turn and proceed downwind, close to the runway, at about 150 feet above ground level. The landing gear was down, and there was a heavy rainshower in progress at the time at the aerodrome. When the aircraft was abeam the downwind end of the runway, it was seen making a steeply banked turn to the left. As the turn progressed through south, the bank angle became steeper and was estimated by witnesses to be approximately 90 degrees. The aircraft then became inverted and descended into trees. A piece of cleaning rag, found later on the runway, was identified as having been in the left wing locker when the aircraft returned from the previous flight. The rag had apparently fallen from the wing locker after the aircraft became airborne. It could not be determined if the door was incorrectly secured before flight or if it came open of its own accord as the aircraft became airborne. However, it does appear that the pilot noticed the open door soon after takeoff and was returning to land and fasten the door when the accident occurred. The resultant delay would have caused the aircraft to arrive in Brisbane late and could explain the pilot's apparent haste to land. Witnesses near the accident site said it was raining very heavily at the time of the accident. This could have caused the pilot to temporarily lose sight of the runway. The aircraft had passed through the extended runway centreline by approximately 200 metres, when the aircraft stalled and struck the trees. There were no mechanical defects found which may have contributed to the development of the accident."