Background
The helicopter involved was a MBB BK-117 A3, registration HB-XNX, built in 1983. It was privately owned and operated. The pilot held a commercial helicopter license with an IFR endorsement and had about 2,700 total helicopter hours. The passenger was a friend who also held a student pilot license.
Flight Events
On 11 January 1989, the pilot flew from Würenlingen to Zurich-Kloten for customs clearance. After refueling, he departed Zurich-Kloten at 1103 UTC with the passenger, intending to fly to Munich via Friedrichshafen. Weather at departure was poor with 2 km visibility.
At 1131 UTC, the pilot contacted Friedrichshafen control, reporting his position 18 NM east on radial 090, flying above clouds, and heading to Altenrhein. The controller informed him twice that Friedrichshafen had clear skies. The pilot then decided to proceed directly to Friedrichshafen. This was the last radio contact.
Radar showed the helicopter vanishing over Lake Constance. A witness saw a helicopter flying slowly near Egnach, about 2 km south of Romanshorn, hovering and then turning toward Romanshorn around 1130 UTC.
Search and Wreckage
Authorities initiated a search involving police boats, an army helicopter, and a civil aviation helicopter. Personal effects and helicopter parts were found in Lake Constance, but the main wreck was not located. A submarine from the Max Planck Institute searched an area of 7 km² over 40 dive hours without success. A second search in 1991 with a deeper-capability submarine also failed. The official report notes that the exact cause of the accident could not be determined due to the lack of wreckage.
Conclusion
The helicopter is believed to have crashed approximately 4 km north/northeast of Romanshorn. The occupants were never found and are presumed deceased. The investigation could not establish a probable cause.