What happened
On September 20, 2003, an AS-355 F2 helicopter, registration OK-MIA, was conducting geophysical measurements near the village of Soseň, Czech Republic. The aircraft was flying with a probe suspended from a cargo hook. The flight began from Rakovník airport, following a pattern of westward and northward maneuvers to cover the survey area.
While flying at a predetermined altitude of approximately 200 feet above terrain, the pilot attempted to follow the rising contours of a forested slope. During this maneuver, the pilot felt a sudden jolt from the cargo hook. Upon checking the mirrors, the pilot discovered that the probe had struck a tree and become detached. The pilot subsequently performed a turn to record the location via GPS before returning to Rakovník. There were no injuries to the crew or the operator on board, and the helicopter sustained no damage.
The investigation
The ÚZPLN investigation focused on the flight conditions and the technical performance of the altitude-sensing equipment. The investigation established that the pilot was fully qualified and medically fit for the mission. The weather conditions were favorable (CAVOK), though the sun was positioned low on the horizon.
The inquiry examined the use of the onboard radar altimeter, noting that while used to maintain terrain clearance, the device provided altitude readings with a delay relative to the aircraft's actual position over the terrain. The investigation also looked into the impact of the terrain's topography and visibility during the flight path.