The aircraft was approaching runway 21R at an altitude of 1500 feet when it entered a severe thunderstorm. The HS-748 was caught in a downdraft, struck the ground slightly banked to the right, slid for 510 feet and broke up. PROBABLE CAUSE: “The pilot directed the aircraft into a thunderstorm in an early dissipating stage. The aircraft was severely affected by a downdraft and lost altitude rapidly. At that time the altitude of the aircraft was only 1500ft and the pilot could not maintain altitude of the aircraft. The aircraft crashed into the ground and ran forward. Factors which caused the pilot to direct the aircraft into the thunderstorm were: 1. During flight prior to the accident, the pilot did not tune his radio to the ATIS frequency, so he did not receive the special weather report (SPECI) broadcast four minutes before the accident advising that there was a thunderstorm in the area he would soon encounter. 2. The pilot did not make use of airborne weather radar. 3. While the pilot was approaching at high altitude, he could see the thunderstorm, but it was not in his flight path to Bangkok International Airport. When the pilot was descending in compliance with Bangkok Approach Control instruction, he was not aware that the rain area which he was entering was a thunderstorm which was moving and developing severely. The thunderstorm which had developed to the severe stage was moving and intercepted the aircraft. 4. The pilot assumed that flying by radar vector would be safe as he thought that the Approach Control Radar could detect the thunderstorm and Bangkok Approach Control would not vector the aircraft into the thunderstorm.” (Thai Accident Investigation Committee)