On August 28, 1998, at 1053 central daylight time, a Messerschmitt BK-117, N230H, operated by St. Louis Helicopters, dba Topeka Lifestar, collided with the terrain during a landing approach at the Topeka-Forbes Airport, Topeka, Kansas. The helicopter was returning from a maintenance test flight when a "loud bang" was heard and it began rotating to the right. The pilot was unable to control the helicopter and it descended to impact with the terrain. The helicopter was substantially damaged. The pilot and two mechanics on board were not injured. The local flight originated from the Topeka-Forbes Airport, at 1040 cdt. The pilot reported the purpose of the flight was to obtain engine matching adjustment measurements after having had a new No. 1 engine installed. The pilot reported that they felt a "severe bump like turbulence" when the helicopter was approximately 2 miles west of the airport. He reported that he and the mechanic in the front left seat, scanned the instruments and everything seemed normal. Within seconds there was "...a loud bang, yaw and the aircraft started to oscillate violently." Another mechanic who was in the back of the helicopter, facing rearward, informed the pilot that the tail rotor had departed the helicopter. The pilot reported the helicopter immediately began to rotate to the right. He reported that they were at an altitude of about 300 to 400 feet above the ground at this time and that he had already picked out a field in which to land. The pilot reported his main objective was to try and maintain as much control of the helicopter as possible, and to land in a level attitude. The helicopter touched down in an upright attitude then rolled onto its left side. Post accident inspection of the helicopter by Federal Aviation Administration Inspectors from the Wichita Flight Standards District Office and from the Rotorcraft Certification Directorate revealed the left engine cowling separated from the helicopter inflight. According to the FAA Inspectors the cowling separated from the helicopter which resulted in damage to the main rotor blades and the separation of the tail rotor gear box. Inspection of the cowling latches revealed the forward over center Hartwell latch was deformed and bent. The aft over center Hartwell latch was not damaged. All three wing head stud fasteners were missing. According to the inspectors the two forward wing head stud fastener holes were distorted/elongated. The wing head stud fastener hole on the aft portion of the cowling did not appear to be damaged. On June 26, 1989, MMB-Helicopters issued Service Bulletin ASB- MBB-BK 117-20-104, to "...ensure secure fit of main transmission cowling access door and engine cowling access door in the event of a hinge failure... ." This Service Bulletin addressed removing and modifying various latches of the transmission access doors and the engine cowling and the installation of new locks, the wing head stud fasteners. This service bulletin was made into Airworthiness Directive, AD 95-08-12. On June 16, 1997 Eurocopter issued Service Bulletin, SB-MBB-BK 117-20-109. This service bulletin calls for the installation of a hook system which will prevent the transmission and engine cowlings from fully opening if the fasteners on the cowlings are not correctly latched. This service bulletin was not incorporated on the accident helicopter. According to a technical representative from American Eurocopter, the service bulletins resulted from several occurrences where the engine and/or transmission cowlings inadvertently opened during flight. He stated that unless pressure is placed on the cowling when the Hartwell latch is closed, it is possible that it won't catch the bottom latch. In which case it appears as though the latch is secured when in fact it is not. He stated that due to the changes in airflow the cowlings have typically opened during the descents and not during cruise. The manufacturers BK-117 Flight Manual states that checking the security of the engine and transmission access doors is a preflight item which should be checked prior to every flight.