H2F Weekly Mayday #30

WEEK 30

H2F brings you the ‘helicopter mayday of the week’ with a short accident report summary. I tell you what happened. You think more about why it happened. We all learn from it. Because that’s what accident reporting is for.

TITLE

Power line strike by HEMS aircraft on approach results in fatal collision with terrain.

WHAT?

An EC135 was dispatched from Lørenskog, Norway to a road traffic collision. During the final approach to an improvised landing site beside the road in question, the helicopter struck an unmarked power line. The main rotor was severely damaged and the helicopter fell approximately 80ft to the ground.

WHERE?

Buskerud, Norway.

WHEN?

Mid morning on 14 January 2014. The final report was published in June 2015.  

HOW?

En route, the crew received the accident coordinates and examined the area on the helicopter’s moving map. A power line was displayed and acknowledged by the crew. Attempts to establish radio contact with emergency personnel on the ground were unsuccessful.

After an initial reconnaissance at approximately 800 ft AGL, the crew selected an emergency lay-by close to the road accident as their landing site and commenced an approach. During the final stages the left-hand seat crew member changed the multifunction display from the moving map to flight instruments, removing the obstacle depiction. All three occupants were visually searching for obstacles on the final descent to the site at approximately 8–10 kt. The crew member opened his door to check clearance from a visible line between roadside lamp posts. Shortly after closing it, the helicopter struck the higher 22-kV power line crossing the approach path.

CONDITIONS?

The flight took place in day VMC. The grey wires were exceptionally difficult to distinguish against a complex background. The supporting poles were largely concealed by trees and terrain, and there was scarce visible evidence of a power-line corridor. The line was not physically marked. Although it was depicted on the moving map, its height was not shown.  

OUTCOME?

Contact with the power cable, despite the low speed, extensively damaged the main rotor and the helicopter descended almost vertically from approximately 80 ft. The aircraft was destroyed. The pilot and HEMS doctor were fatally injured and the crewman seriously injured.

WHY?

The investigation went into greater depth than simply treating the accident as a failure to see a wire. Investigators concluded that multiple defensive barriers were absent or ineffective:

• The wire was exceptionally difficult to see and had no physical markings.
• Although the crew knew a power line was depicted on the map, they apparently did not positively identify its location in the real environment before committing to the approach.
• The obstacle display was no longer visible during the critical final approach. The database was known to contain inaccuracies, reducing crews’ confidence in it, while the warning was visual rather than audible.  
• Failed air-ground communications prevented police officers, who had identified hazards around the landing site, from warning the helicopter.
• The TCM’s workload combined obstacle lookout, landing-site assessment, systems and clearance duties, illustrating the vulnerability of crew cross-monitoring during an unknown-site approach.
• Investigators identified latent organisational weaknesses in standardisation, crew task-sharing, use of obstacle warnings and management of the risks associated with unknown landing sites, particularly under time pressure.

The core lesson was of the Threat and Error Management philosophy of defence in depth. There is very little margin when only that final barrier remains of a crew visually detecting a thin wire during a high-workload HEMS approach.

REFERENCE?

Accident Investigation Board Norway. (2015). Report on air accident at Sollihøgda in Buskerud, Norway, 14 January 2014 with Airbus Helicopters EC135 P2+, LN-OOI operated by Norsk Luftambulanse AS (Aviation Report 2015/06). 

Note:

Accident reports selected from the following open source databases: US NTSB; UK AAIB; Flight Safety Foundation’s Aviation Safety Network; Australia’s ATSB. Ireland’s AAIU; Taiwan’s TTSB; France’s BEA; Spain’s CIAIAC. Germany’s BFU.