What happened
On Thursday, August 23, 1984, at approximately 19:00 local time, a Robin DR400-180R bearing registration HB-EZT departed from Grenchen Aerodrome. The pilot was accompanied by three colleagues for a short scenic flight over the Jura mountain range. The initial route covered several towns including Oberbipp, Moutier, and Tavannes before heading toward Sonceboz.
Upon entering the St. Imier Valley, the pilot flew upstream along the northern slope of Mount Chasseral at an altitude of roughly 1,000 to 1,100 meters. Near Cormoret, intending to cross west of the summit (1,607 m) to reach the southern side before returning to Grenchen, the aircraft entered a climb close to the terrain. Shortly after initiating this maneuver, the pilot noticed a sudden loss of altitude and experienced vibrations indicating an impending stall. He immediately pushed the control stick forward and attempted a right turn away from the slope. However, there was insufficient height remaining. The right wing struck a fir tree, causing the aircraft to crash into the forest at La Vieille Vacherie (1,170 m). The impact destroyed the plane.
One passenger died instantly upon ejection from the wreckage. The pilot and two other passengers sustained serious injuries but were rescued shortly after by ambulance. Minor forest damage occurred, but no fire broke out.
The investigation
The Swiss Transportation Safety Investigation Board (SUST) examined the flight dynamics, pilot qualifications, and aircraft status. The pilot held a valid commercial pilot license and flight instructor certificate with 955 total flight hours, including 37 on the Robin type. The aircraft was airworthy except for a reported failure of the stall warning device.
Weather conditions were clear with visibility over 10 km and light southerly winds. The accident site showed debris scattered down a steep (approx. 80%) forested slope. The propeller's high degree of destruction suggested the engine was producing power at impact, indicating the pilot had not shut it down prior to collision.
Findings
The investigation concluded that the primary cause was insufficient safety distance from the slope. Contributing factors included inadequate flight tactics and insufficient flight monitoring. The aircraft likely encountered downdrafts or entered a stall due to low airspeed while maneuvering close to the terrain. The pilot's reaction, while immediate, could not recover the lost altitude in time.
Safety action
The report highlights the critical importance of maintaining adequate terrain clearance during low-level maneuvers in mountainous areas. No specific regulatory changes were mandated in this summary, but the findings underscore risks associated with inadequate situational awareness near ridges.