Incident Overview
On 13 February 2006, a Robinson R22 BETA II helicopter (registration OK-LEA) operated by NISA AIR spol. s r.o. was conducting a visual flight rules (VFR) flight for a monthly gas pipeline inspection. While returning to its base, the helicopter entered a snow shower with visibility reduced to 1–1.5 km. After several minutes, a low rotor RPM warning activated. The pilot responded by lowering the collective lever and increasing engine power via the throttle. Rotor RPM increased from 90% to 95%. The pilot then observed the carburetor temperature indicator showing 8–10°C, approaching the yellow caution range, and set the carburetor heat to maximum.
Forced Landing Attempt
As expected RPM recovery did not occur, the pilot decided to perform an emergency landing on the nearest flat area. The helicopter was at an altitude of 80–100 meters above ground. After selecting a suitable area, the pilot initiated descent with the landing light on, aware of snow cover. At approximately 15 meters, the pilot attempted to transition to a hover. During this phase, snow was blown up by the rotor downwash, causing the pilot to lose spatial orientation. The helicopter first contacted the snow with its skid, then the rotor blades struck the snow, and the aircraft rolled onto its side. The pilot shut off the fuel valve and master switch before evacuating with the passenger.
Aircraft Damage and Examination
The helicopter was destroyed. The forward fuselage shell was deformed, the vertical firewall near the main rotor mast was collapsed, and the fuel tanks were deformed but not ruptured. Main rotor blades were deformed, and the engine sustained minor damage. Post-accident examination revealed that the carburetor heat control rod was pulled out about 20 mm from the closed position, indicating very little warm air was being supplied to the carburetor. This contradicted the pilot's statement that he had set carburetor heat to maximum. Further investigation showed that when the collective lever is moved, the carburetor heat assist mechanism can move the control rod, potentially reducing the previously set heat. This likely occurred during the landing flare.
Probable Cause
The accident was attributed to the pilot's failure to maintain control during landing in snow, fundamentally influenced by the helicopter's inability to achieve a hover due to insufficient main rotor RPM. The reason for the RPM drop was not fully determined, but the circumstances strongly indicated inadequate carburetor heat, leading to reduced fuel mixture flow caused by improper warm air supply adjustment.