What happened
On February 13, 2006, a Robinson R22 BETA II helicopter, registration OK-LEA, operated by NISA AIR, was performing a VFR flight to inspect a gas pipeline. During the final leg of the flight, the aircraft entered a snow shower with visibility between 1 and 1.5 km. During this period, the pilot observed a low rotor RPM warning light.
In response to the decreasing rotor speed, the pilot attempted to increase engine power via manual controls, managing to raise the RPM from 90% to 95%. Simultaneously, the pilot noted that the carburetor temperature was approaching the limit of 8–10°C and set the carburetor heat to maximum. Due to the lack of expected engine performance recovery, the pilot initiated an emergency landing on a nearby flat area.
While descending at an altitude of approximately 15 meters, the pilot attempted to transition the helicopter into a hover to clear snow being kicked up by the rotor wash. However, due to insufficient engine power, the pilot lost control of the aircraft's attitude. The landing skid and subsequently the rotor blades struck the snow-covered ground, causing the helicopter to capsize onto its side. The crew and passenger escaped the aircraft without injury.
The investigation
The ÚZPLN investigation examined the wreckage and the mechanical operation of the carburetor heating system. While the pilot stated that the carburetor heat was set to maximum, investigators found the control linkage was positioned such that only a minimal amount of warm air was being supplied.
Technical analysis revealed that the carburetor heat assistant mechanism, which is linked to the collective pitch lever, can cause the heat control linkage to move. It was determined that the movement of the collective lever during the emergency landing likely caused the heat control linkage to slide toward the closed position, effectively overriding the pilot's manual setting.
Findings
- The primary cause of the accident was the pilot's inability to maintain control during the landing in snowy terrain.
- This was significantly influenced by the objective cause of the helicopter's inability to maintain a hover due to insufficient rotor RPM.
- The drop in engine performance was likely caused by inadequate carburetor diffuser heating, leading to a reduced fuel-air mixture flow.
- The mechanical linkage between the collective lever and the carburetor heat control allowed the heat setting to be inadvertently reduced during the landing maneuvers.