It's the workhorse of Britain's fleet of air ambulance and police helicopters, but questions are being raised over the safety record of the Eurocopter EC135 Type 2, one of which crashed into the Clutha pub while it was packed with people on Friday night.
It emerged yesterday that the helicopter, which was operated for Police Scotland by Bond Air Services Ltd, was one of dozens of aircraft grounded in 2012 over safety fears.
With a cruising speed of up to 158mph, the twin-engined EC135 has become popular with police and ambulance services, but the Type 2 variant has been the subject of two recent European Aviation Safety Authority (EASA) emergency air-worthiness directives.
On 23 September, the body warned of "stiffness" in the "main rotor actuators" of the variant, which could lead to "reduced control of the helicopter".
In May last year, it reported a "crack detected" on parts of the "main rotor hub shaft", which could "lead to loss of the helicopter". This problem was first reported on an EC135 aircraft operated by Bond Air Services for the Scottish Air Ambulance Service.
The EASA alert led to the temporary grounding of all the 22 EC135 aircraft operated by Bond Air Services in the UK, including the Police Scotland aircraft that crashed on Friday, then operated by Strathclyde Police.
EASA has the authority to ground helicopters that are found to have technical design flaws. A spokesman said that the agency is "working closely" with Eurocopter and investigators but has not ruled out grounding the EC135 Type 2. He added that it is "prepared to take any action based on facts to ensure that the type of helicopter in question continues to be operated safely".
Solicitor James Healy-Pratt, head of aviation at Stewarts Law, said "Obviously, there are various potential causes of this tragedy. However, previous emergency airworthiness directives for the EC135 are of real relevance where no obvious immediate cause is apparent.
"The May 2012 EASA advisory directive does highlight a problem that, if not solved would, in Eurocopter's words, result in 'the loss of control of the helicopter'. I would like Eurocopter to comment on whether all main rotor hub mast assemblies on the EC135 fleet are 100 per cent safe."
Aviation analyst Chris Yates added: "After a crash like this, it would be obvious to check how well inspections have been conducted, and whether the appropriate checks and balances between the outsourced operator and police service are sufficiently robust to maintain this enhanced safety regime."
Bond Air Services and Police Scotland refused to confirm last night who was responsible for maintenance of the helicopter. However, The Independent on Sunday understands that Bond Air Services has retained independent air crash investigators to examine one or more of these previous incidents and the firm's safety practices.
Earlier this month, the transport select committee launched an investigation into the safety of helicopter flights in Scotland. Following the crash, a spokesman for the British Airline Pilots' Association said that the number of number of recent helicopter incidents in Scotland was a "matter for concern" and called for the committee to "look into the circumstances around" the accident.
On 23 August, four people were killed when an AS332L2 Super Puma crashed into the sea about 1.5 miles off the Shetland Islands
That aircraft was a similar one to one operated by Bond Offshore Helicopters which crashed a few miles off Peterhead in Scotland in April 2009. All 16 people on board were killed, and investigators concluded that the main cause of the accident was the catastrophic failure of the main rotor gearbox.
The aircraft was manufactured by EADS, which owns Eurocopter, and the company is concerned that fingers are already being unfairly pointed at it for the Glasgow crash. In the Super Puma incident, the Air Accidents Investigation Branch later found that there was no technical fault to blame.
Phil Giles, a former air accident investigator, said: "One of the first things you notice from the images from Glasgow is one of the rotor blades sticking up in the air, which suggests there was very little power on the helicopter when it crashed. So it looks like the engines had probably stopped. From everything I've seen, it suggests the pilot had more of a problem on his hands than just a power failure, though, as the aircraft doesn't seem to have entered autorotation, which is the helicopter equivalent of a glide, and it dropped liked a stone."
Last year, police helicopters in England and Wales, but not Scotland, were reorganised in a £15m cost-cutting measure into a single National Police Air Service. Its fleet now comprises 23 helicopters, including 14 of the EC135 Type 2 aircraft involved in the latest crash.