Eurocopter is recommending worldwide checks of the fuel systems on its EC135 twin-engine light-helicopter.

Two Eurocopter alert service bulletins (ASB), followed by emergency airworthiness directives from the European Aviation Safety Agency, are calling on operators of the type to carry out mandatory one-time checks of the aircraft's fuel supply system and report the results to the manufacturer. This will provide an up-to-date fleet status regarding the type's fuel tank sensors; a second ASB introduces a revision of the flight manual with regard to the low fuel warning and the fuel pump caution indications.

The checks affect more than 1,100 of the popular twin-engine helicopter, which is in service with law-enforcement agencies, air ambulance operators and militaries globally.

Concerns have emerged in the wake of a fatal crash in Glasgow city center on Nov. 29. Ten people died when an EC135 T2, operated for Police Scotland by U.K. operator Bond Air Services, crashed through the roof of the Clutha Vaults public house. All three crew—a civilian pilot and two police officers—died in the aircraft, while six patrons were killed within the building; a seventh patron succumbed to his injuries in the hospital later.

Within two weeks of the accident, Bond unilaterally grounded its EC135s—disrupting operations with many of the U.K.'s air ambulances—when engineers discovered supply-tank fuel gauging errors on some aircraft. According to Eurocopter, Bond's tests found that the fuel sensors, which also measure fuel levels in the supply tanks of another aircraft operated by Bond, had incorrectly indicated the amount of fuel available.

The sensors in the supply tanks of this aircraft did not work properly; it was determined that an amber “caution” signal linked to the sensors was “not triggered.” However, the red “low fuel” alert still worked correctly because it operates on a completely decoupled system.

Eurocopter says fuel-system functionality tests subsequently performed by two other EC135 operators in Europe have revealed possible similar supply-tank fuel gauge errors on some aircraft. Of the 36 supply tank sensors tested in the U.K., one was found to have an issue.

“Following discovery, the faulty sensor was cleaned. When retested, the sensor was fully functional,” the company reported. LPR, which flies EC135s for air ambulance operations in Poland, also found an issue in one of their 23 aircraft.

On Dec. 20, Eurocopter revealed that checks on more than 100 fuel sensors from 50 aircraft have been completed; three were found to be defective. “The faulty sensors were cleaned. When retested, only one was still faulty while two of them were fully functional again.”

Bond has said that while all of its aircraft have now returned to operations, its EC135s will carry a minimum of 90 kg (198 lb.) of fuel at all times, which could have an impact on the payload capability of some heavier-mission-equipped aircraft such as those used by police forces.

The fuel issue does not, however, represent a clear line of inquiry for the accident probe. So far, investigators have said that the aircraft struck the roof of the building with a high rate of descent and low or negligible forward speed.

“All main rotor blades were attached at the time of the impact, but neither the main rotor nor the fenestron tail rotors were rotating,” the U.K. Air Accident Investigation Branch (AAIB) stated in a preliminary report released Dec. 9.

Investigators drained 95 liters (25 gal.) of fuel from the tanks, while further examinations found that all components were present and there were no signs of “major mechanical disruption of either engine.”

Only a few clues will be gleaned from the aircraft's onboard mission system as it was not fitted with a flight data recorder, nor was it required to be by the U.K. Civil Aviation Authority. Investigators said they were planning to make use of record fault codes in various systems onboard the aircraft. Onboard systems that capture images and audio also will be reviewed.

The accident comes at an already challenging time for Eurocopter in the U.K. The manufacturer is attempting to overcome adverse public perception over its Super Puma family following a set of five accidents in four years in the North Sea—two of which were fatal. While the most recent of those losses, the crash of a CHC AS332L2 on Aug. 23, seems to be the result of pilot error (the AAIB has not yet finished its report), people in Scotland have been quick to make the link between Eurocopter-helicopter involvement in the North Sea and Glasgow tragedies.