Investigators probing the crash of a police helicopter in Scotland are questioning why both engines flamed out despite the aircraft still having 10-15 min. of fuel left. The Airbus—formerly Eurocopter—EC135T2 operated for Police Scotland by Bond Air Services had been on a routine police mission and was returning back to its base at Glasgow City Heliport when it crashed onto the Clutha Vaults public house late in the evening of Nov. 29, 2013. All three crew onboard the helicopter were killed as were six patrons in the bar when the aircraft crashed through the roof. A seventh person later succumbed to his injuries.

Investigators have few clues to go on. The aircraft did not have, nor was it required to have, a crash-worthy flight data recorder, which has forced the U.K. Air Accident Investigation Branch (AAIB) to examine recorded data collected by the onboard aircraft systems and contained in non-volatile memory in several key aircraft systems. But the majority of the information recorded leaves no form of time-stamp. “While the order of some of the snapshots can be determined, their relative timing is unknown,” say investigators.

In late January, Airbus Helicopters CEO Guillaume Faury said that the company was working closely with the AAIB but predicted a “long investigation.”

In an update to its progress, published on Feb. 14, the AAIB said that data recovered show that in the final stages of the flight, the right engine flamed out followed shortly after by the left, and that the rotor blades and the fenestron-shrouded tail rotor were not rotating at the moment of impact. Flame-out data were provided by the Full Authority Digital Engine Control (Fadec) computers on each engine.

The aircraft had taken off with 400 kg (880 lb.) fuel, giving an expected endurance of 1 hr. 35 min. Checks of the fuel systems after the accident found that the main tank still contained 76 kg and that the pipe work of the fuel systems and associated transfer passages had not revealed “any pre- or post-impact failure and all paths still permit uninterrupted fuel flow,” according to the AAIB report.

Meanwhile, examinations of the engines showed that apart from minor damage from the impact, there were no signs of failure or blockage, while engine control units were found to be serviceable. Rotor systems, including the main gearbox and the fenestron, had “superficial external damage, but were free from leaks,” the report said.

The aircraft's warning unit had recorded “low fuel” alerts triggered by thermal sensors in the tanks. Investigators believe the system provided intermittent “Low Fuel 1” warnings for the left fuel supply tank, and then a permanent “Low Fuel 2” warning for the right-side supply tank. The report points out that in the EC135T2 configuration, warnings indicate when there is approximately 32 kg and 28 kg of fuel remaining in the left and right supply tanks, respectively. “On receipt of these warnings, the manufacturer's flight manual for the helicopter instructs the pilot to: 'Land within 10 min.'”

The AAIB said it also wanted to find out “why no emergency radio transmission was received from the pilot, and why, following the double engine failure, an auto-rotative descent and flare recovery—a controlled landing—was not achieved.”

Three weeks after the accident, Bond Air Services updated its EC135 flight manual, issuing notice to pilots that they must carry final reserve fuel levels of 90 kg when completing their mission fuel calculations. This change was prompted when Bond engineers discovered supply-tank fuel gauging errors on some aircraft. According to Airbus, Bond's tests found that the fuel sensors, which measure the fuel levels in the supply tanks of another aircraft operated by Bond, had incorrectly stated the available fuel in the supply tanks. Airbus then called for worldwide checks on the fuel systems of the EC135, affecting around 1,000 aircraft. The majority of checks are now completed; only a handful of fuel sensors were found to have issues.

At its annual press conference, Airbus announced it was fitting Appareo Systems' Vision 1000 flight data collection and cockpit imaging system into more of its single-engine helicopters, and was planning to expand its use across the product family.

Police Scotland's EC135, was a 2007-built Turbomeca Arrius-powered aircraft, registered G-SPAO and was operated by Gloucestershire-based Bond Air Services, which supply and operate aircraft for a number of U.K. air ambulance trusts and other agencies around the U.K. The EC135 is the most commonly used helicopter for law enforcement operations in the U.K. and much of Europe.