The pilot and two medical crewmembers were on a return flight to a hospital helipad. During the approach to the helipad, the crew heard a noise outside the helicopter. The medical crewmembers reported they felt a slight "shudder" on the helicopter but attributed it to the wind as they approached the helipad. The pilot heard the noise, but there was no effect on the controls or flight characteristics, so he continued the landing without further incident. After engine shutdown, the oncoming pilot noticed that the helicopter's tail rotor drive shaft cover was missing. The surrounding area near the helicopter's approach path was searched, and the missing cover was found. An examination of the helicopter noted substantial damage to the tail rotor drive shaft. The day before the accident, scheduled maintenance was performed on the helicopter's tail rotor pitch change mechanism, which required removal of the drive shaft cover. The cover was reinstalled, and no problems were reported during a subsequent preflight inspection. It is likely that maintenance personnel did not properly secure the cover fasteners after the cover was reinstalled.