What happened
On May 3, 1972, a pilot operating a Wassmer WA40 A, registration HB-DCC, departed Bern for a private VFR flight to Freiburg, Germany, accompanied by four passengers. The outbound leg of the flight proceeded without incident. During the return flight to Bern, approximately 40 minutes into the journey near Oensingen, the pilot observed a gradual failure of all electrical instruments. Initially fearing an engine failure, the pilot attempted an emergency landing near Kestenholz after failing to establish radio contact with Grenchen.
Upon preparing for landing, the pilot realized the issue was an electrical malfunction rather than engine failure. This failure also prevented the normal deployment of the landing gear. The pilot proceeded to Bern-Belp, maintaining an altitude of 4,000 feet. While flying over Langenthal, the pilot attempted to deploy the gear using the mechanical emergency system, referencing the aircraft's French-language operating instructions. To alert ground personnel to the potential landing gear issue—as the electrical indicators were also non-functional—the pilot flew over the Bern control tower several times. After receiving landing clearance, the pilot landed on runway 32 at 19:02. Upon touchdown, the landing gear collapsed, causing the aircraft to skid 260 meters on its belly. There were no injuries among the five occupants, though the aircraft sustained heavy damage.
The investigation
The investigation focused on the cause of the electrical failure and the subsequent failure of the emergency gear deployment. Investigators examined the aircraft's maintenance history and found that a previous inspection in August 1971 had noted a loosely secured capacitor. It was determined that vibrations from this capacitor, which remained improperly secured or had become loose again, caused a wire to break, interrupting the electrical supply.
Findings
- The primary cause of the accident was the incorrect operation of the landing gear emergency system.
- An electrical power failure, caused by a maintenance deficiency involving a loose capacitor, necessitated the use of the emergency system.
- The failure to fully deploy and lock the gear was influenced by inadequate type-specific training for the pilot, as the instructor lacked sufficient flight hours and the training process was not thorough.
- The manufacturer's operating instructions were deemed insufficiently clear regarding the specific manipulations required to mechanically lock the landing gear and the emergency deployment lever.