A SENIOR avionics expert for the operators of the Clutha helicopter said fuel probe problems were becoming a "semi-regular" occurrence across the fleet, an inquiry has heard.

Martin Forster, Avionics Manager at Babcock, told a fatal accident inquiry that in the months before the Glasgow crash, which killed 10 people, problems with the fuel system were becoming a "concern".

Mr Forster said Babcock, formerly Bond, was "struggling to keep up with the demand" from clients for new fuel sensors.

The inquiry has previously heard that experienced pilot David Traill, who was flying helicopter G-SPAO with two crew, would have heard five low fuel warnings before the crash in November 2013.

Read more: Airbus 'put on hold' solution' to EC135 safety concerns on day of Clutha tragedy

It is mandatory, the inquiry has heard, that pilots land within 10 minutes of a low fuel warning but Mr Traill kept the Police Scotland aircraft in the air.

Lawyer David Adams, representing pilot David Traill's fiancee Lucy Thomas, asked if fuel sensor issues were a "regular occurrence".

Mr Forster said: "Semi-regular. The fuel probe replacements and problems with the fuel system were within the areas of the aircraft that we were concerned about.

"It is a big fleet, we do get failures on a regular basis, we've got lots of different systems but this had become more prominent and was creeping up our list of areas that needed further investigation."

The inquiry, in a temporary court at Hampden Park, has heard that the aircraft which crashed, an EC135, experienced problems with fuel readings in the months before the crash.

The court heard previously how on December 11, 2013, an air ambulance landed at City Airport, Manchester, with the pilot reporting contradictory fuel readings and low fuel warnings.

Following this, the entire fleet was checked for fuel sensor problems and the minimum level of fuel in the EC135 helicopter was increased from 60kg to 90kg.

The inquiry was shown technical logs which noted that on July 8, 2013 the fuel content indication in the aircraft which later crashed into the Clutha was found to be inaccurate and work was carried out to fix the problem.

The log showed another problem in October that year when engineers defuelling the tank found that the quantity indicator was stuck at 11kg when the tank was empty and again work was carried out to resolve the issue.

Read more: Clutha FAI: Low fuel warning 'was modified out of the Airbus EC135 helicopter'

The helicopter manufacturer issued an information notice dated January 21 2013 regarding water contamination of the fuel system, which stated that a "couple of drops of water" concentrated between the metal tubes of the fuel level sensor may be enough to affect the signal to the display shown to the pilot.

Asked whether he was aware of it as a problem when he started at Bond as an avionics engineer in 2008, Mr Forster said: "No not really, it seemed to be a shift into 2013 from late 2012.

"It seemed to be a more common occurrence.

"I was thinking about whether Airbus had changed supplier that was putting us in a position where the parts were not as reliable as we would have expected them to be and we were seeing more faults than previously."

More than 100 people were at the Clutha Vaults pub when the helicopter, returning to its base on the banks of the River Clyde, crashed through the roof.

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Mr Traill, 51; PC Tony Collins, 43; and PC Kirsty Nelis, 36, died along with seven customers who were in the bar, Gary Arthur, 48; Joe Cusker, 59; Colin Gibson, 33; Robert Jenkins, 61; John McGarrigle, 58; Samuel McGhee, 56; and Mark O'Prey, 44.

The inquiry, in front of Sheriff Principal Craig Turnbull, continues on Thursday morning.