B4/2009L Hard landing for helicopter in Porvoo on 7 May 2009

An accident occurred at Ali-Vekkoski near the city of Porvoo on Thursday, 7 May 2009 at 12:21 Finnish Daylight Saving Time. A Textron-manufactured Bell Helicopter, the Bell 412EP Medium Transport Helicopter, registration OH-HVK, owned and operated by the Finnish Border Guard, made such a hard landing during an operational exercise that its landing gear fractured and the sub-floor structure of its fuselage buckled. The helicopter had three crewmembers and a six-man team from the Counter Terrorist Unit (CTU) of the Finnish Police as passengers. Two members of the team sustained minor injury.

The task of the exercise team was to conduct a search in order to detain fugitives in the terrain. Once the pilot-in-command (PIC) of the helicopter spotted their target he decided to land the team in the terrain in front of the target. The crew was unfamiliar with the clearing that was designated as the landing site; it was to the right of where the helicopter was making its approach, behind a spit of trees. It was elemental to the success of the task that the team deplane in front of their target. The PIC began his approach to the landing site by accelerating from the search airspeed to 107 kts while maintaining the approximate altitude of 250 ft. When there was approximately 300 m to go to the landing site he began to reduce airspeed for landing.

The pilot-in-command reduced the airspeed of the helicopter by simultaneously lowering the collective and applying aft cyclic. At this time the main rotor RPM rapidly surged and exceeded 104.5%, at which time the warning system gave a high RPM warning. Once the airspeed had bled off and the collective lever had been raised, rotor drag rapidly increased which resulted in the main rotor RPM dropping below 95%. Consequently, the warning system sounded a low RPM warning. The PIC had to suspend raising the collective lever for a moment, which increased the helicopter’s rate of descent. The main rotor was entering into a developing vortex ring state. When the engines spooled up, rotor RPM also resumed and the PIC continued to raise the collective lever.

Raising the collective lever in the state did not stop the helicopter from sinking. According to the DFDR the maximum sink rate was approximately 1100 ft/min. The helicopter collided with the ground approximately four seconds after the low RPM warning.

When the PIC realised that a crash was unavoidable, he warned the rest of the crew of an impending hard landing and straightened out the helicopter. It hit the gently rising ploughed field in the nearly optimal position for minimising damage.

The investigation found no technical fault or malfunction in the helicopter. The helicopter was airworthy. Its weight and centre of gravity were within the permissible range. The meteorological conditions were suitable for the task. Crew ratings were valid and they had the required experience to fly the mission.

The cause of the accident was the helicopter’s main rotor entering into a developing vortex ring state during landing in conditions that prevented any further corrective actions.

Contributing factors included

• A high approach speed and the late reduction of airspeed combined with sizeable power setting changes with a heavily laden helicopter,
• Inadequate Multi Crew Co-operation, for example, as regards checklists as well as airspeed, altitude, sink rate and wind direction monitoring,
• Insufficient attention to the helicopter’s state of flight as well as the unfamiliar landing site during the time when the flight crew was deeply focused on the tactical mission, and
• Deficient standard operating procedures (SOP) and guidelines for this particular type of task.

The Finnish Border Guard’s statutory tasks require flight operations in nonstandard situations and demanding conditions, moreover special tasks carry more risks than normal flight operations. The Air Patrol Squadron is responsible for carrying out the Border Guard’s flight operations, including special air operations related to cooperation with the authorities.

The investigation commission gave the four following safety recommendations for the Finnish Border Guard:

• The Air Patrol Squadron should review their flight procedures and training syllabi as regards the observed shortcomings. Special attention should be given to Multi Crew Co-operation-related monitoring techniques.
• The Air Patrol Squadron should improve their pilots’ awareness of the risks of the vortex ring state as well as associated corrective actions.
• In order to improve flight safety, the Air Patrol Squadron should analyse and consider the applicability of the ICAO’s Safety Management System (SMS) in their operations.
• Related to their operational duties the Finnish Border Guard should specify their requirements for the transport of dangerous goods by air. Following this, the Border Guard should revalidate the relevant permits and modernise the regulations.

In addition, the investigation commission gave a recommendation for Trafi Aviation, i.e. the Finnish Transport Safety Agency with regard to improving the requisites of investigation. The investigation commission further proposes that Trafi Aviation amend the national Aviation Regulation AIR M16-1 (initial aircraft inspection) so as to extend the JAR-OPS 3’s requirement of readily available recorded flight data to all helicopters fitted with flight recorders.

B4/2009L Report (pdf, 3.2 Mt)