On 15 February 2013, the crew of a Eurocopter Helicopter MBB‑BK117 B-2 (BK117), registered VH-VSA, were conducting a trauma recovery flight from Port Pirie to Adelaide Hospital, South Australia. After reaching the cruise altitude of 5,000 ft above mean sea level, the crew observed fluctuations of the hydraulic system pressure gauges. Shortly after, the helicopter sustained an uncommanded and violent nose-up pitch and rolled left before descending. The pilot regained control at about 800 ft above ground level. Control checks by the pilot confirmed normal control had resumed and the pilot flew the helicopter back to Port Pirie Aerodrome. No injuries were reported by the occupants and the helicopter sustained minor damage.
What the ATSB found
The ATSB did not find any mechanical or system faults that could account for the hydraulic system pressure fluctuations. The ATSB found that the helicopter was being operated at a weight, density altitude and airspeed, and in meteorological conditions that were conducive to the onset of retreating blade stall. The uncommanded and violent nose-up pitch and left roll were consistent with the onset of that condition. The pilot’s instinctive action of pushing the cyclic control forward delayed recovery from the stall.
What's been done as a result
The operator issued an urgent Immediate Safety Notification advising all company BK117 pilots of the conditions conducive to retreating blade stall and the correct actions to recover from that condition.
This incident highlights the importance of pilot awareness of the factors conducive to retreating blade stall, including high all-up weight, high density altitude, high airspeed, manoeuvres that increase flight loads and flight in turbulence. Similarly, the importance of initially reducing collective pitch to optimise recovery is emphasised as incorrect recovery actions can result in loss of control of the helicopter.