Although the causes of the fatal 29 November crash of the Police Scotland Eurocopter EC135T2 in Glasgow are as-yet unknown, analysis of the type's accident history suggest a helicopter that has, so far at least, a strong safety record.
Data from Flightglobal's Ascend Online Fleets database reveals that since the 2.9t helicopter entered service in 1997, there have been just 45 accidents covering minor, major and total hull-loss incidents.
The total figure comprises 23 total hull-losses for the type, 13 major accidents and nine minor incidents, including several instances of arson.
Ascend Online additionally records that the accidents, excluding the Clutha Vaults incident, have caused a total of 32 fatalities – 13 flightcrew and 19 passengers.
Eurocopter says the entire EC135 fleet has accumulated over 2.9 million flight hours since service entry, suggesting an extremely low accident rate of one incident every 64,400h.
Collisions with objects, high ground or level ground account for 12 of the 45 accidents, with loss of control in flight blamed for five incidents – the same number as hard landings, the Ascend Online data suggests. In addition, no loss of control accidents have been recorded since 2010.
Clearly that does not preclude a catastrophic failure of the helicopter in Glasgow. However, information released by the UK Air Accidents Investigation Branch into the crash through the roof of the Clutha Vaults public house at 22:25 local time has so-far been limited.
In the days following the crash David Miller, deputy chief investigator at the AAIB, confirmed there had been no "emergency transmission" prior to the crash, and that the helicopter had descended vertically. He also stated that the helicopter was intact when it came down and that all four rotor blades were attached to the rotor head after impact.
This was a direct reference to an emergency airworthiness directive issued by the European Aviation Safety Agency early in 2012 after cracks were found in the lower hub-shaft flange of the main rotor-hub. If allowed to propagate, EASA warned, this could lead to a failure of the hub "and consequent loss of the helicopter". After checks revealed a further four helicopters were suffering from the problem, EASA mandated a regime of pre-flight visual inspections for the type.
However, as the AAIB makes clear, the rotors were attached and, as Eurocopter is at pains to point out: "As the four main rotor blades are still attached to the main rotor mast, a crack in the mast is very unlikely to be at the origin of the accident."
A further airworthiness directive for the EC135 was issued by EASA on 23 September, covering the main rotor actuators of newly delivered aircraft. But, as the AD states, if the problem has not manifested itself within 20h, the aircraft is unlikely to be affected.
In the Glasgow crash, the aircraft (G-SPAO) was a 2007-built example that had accumulated 4,400h, putting it outside the scope of the AD.
Bond Air Services, which operated the helicopter on behalf of Police Scotland, has a further 22 EC135s – the majority of which are the T2 variant – in its fleet. The vast bulk are employed in the medical evacuation role for ambulance services across the UK, and these aircraft continue to fly.
Only two customers – Scottish & Southern Energy and the Northern Lighthouse Board – have asked for operations to be suspended with the type, says Bond.
The company has paid tribute to the pilot of the ill-fated EC135, Capt David Traill, who was killed in the incident alongside Police Constables Kirsty Nelis and Tony Collins.
The company describes Traill as "the epitome of the consummate professional", who was one of its most experienced pilots. He was formerly a pilot of Boeing CH-47 Chinooks in the Royal Air Force – including as a member of type's display team – and served in both Gulf Wars. He joined Bond as a commercial pilot in 2008.
The wreckage of the helicopter has been recovered and transported to the AAIB's headquarters at Farnborough in the south of the UK for analysis. Although it was not equipped with cockpit-voice or flight-data recorders, the agency is confident it will be able to extract some data from its systems.