What happened
On May 27, 1967, at approximately 15:04 CEST, a Piper Cub (HB-OSU) was performing a takeoff from the Biel-Kappelen airfield. Following a series of short flights and a passenger shuttle to Grenchen, the pilot attempted a new departure with a passenger. During the takeoff roll, the aircraft struggled to gain altitude, resulting in an unusually long and shallow climb.
As the pilot attempted to identify the cause of the poor performance, they discovered that the carburetor air preheater had been left in the on position. While attempting to maneuver the aircraft at extremely low altitude to fly under nearby electrical lines, the pilot had to bank to avoid a utility pole. During this maneuver, the left wingtip struck a concrete road sign marking the Lyss-Biel route. Despite approximately one meter of the wing being torn away by the impact, the aircraft remained controllable, and the pilot successfully landed the aircraft 3mueters further on without further incident.
The investigation
The investigation focused on the aircraft's configuration and the pilot's pre-flight procedures. The Piper Cub was found to be in an airworthy condition, with its weight and center of gravity within permissible limits. The investigation confirmed that the airfield, which had recently opened, featured a grass runway. Weather conditions were not a contributing factor to the event.
Findings
- The primary cause of the incident was inadequate takeoff preparation.
- The pilot failed to verify that the carburetor air preheater was closed (pushed in) prior to departure.
- The active preheater caused a significant reduction in engine power, which led to the shallow climb profile and left the aircraft at a critically low altitude near obstacles.