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Fuel pump switch position raises questions in Glasgow helicopter crash

By Vertical Mag | February 14, 2014

Estimated reading time 6 minutes, 40 seconds.

As more evidence is collected for the helicopter crash that killed 10 people in Glasgow, Scotland, last year, investigators are seeking to determine why both engines on the Airbus Helicopters EC135 T2+ flamed out when 76 kilograms (167 pounds) of fuel remained in the aircraft’s main fuel tank.
According to a new special bulletin released by the United Kingdom Air Accidents Investigation Branch (AAIB) on Feb. 14, recorded data indicates that, in the latter stages of the accident flight on Nov. 29, 2013, the EC135’s right engine flamed out, and shortly after the left engine flamed out. The pilot failed to successfully achieve an autorotative descent and flare, and the helicopter fell out of the sky, crashing through the roof of the crowded Clutha Vaults bar in central Glasgow.
The EC135 was being operated by Bond Air Services on behalf of Police Scotland, and was returning to Glasgow City Heliport following a police operation when the crash occurred. All three people on board the helicopter — including pilot David Traill, and constables Tony Collins and Kirsty Nelis — were killed in the accident, which also immediately took the lives of six people who were in or adjacent to the bar. Of the 12 people who were seriously injured in the event, one person subsequently died of his injuries, on Dec. 12.
According to the AAIB bulletin, investigation of the helicopter’s two Turbomeca Arrius 2B2 turboshaft engines found no evidence of any mechanical failure or intake or exhaust blockage in either engine. The left engine fuel filter was found to contain a small amount of fuel, while the right engine was found to be empty of fuel. Both engine fuel control units were tested and found to be serviceable, and investigators determined that the engine fuel valve assemblies and engine control panel switches were in their correct positions. 
However, investigators determined that, while 76 kilograms of fuel remained in the main fuel tank at the time of the crash, the left (No. 1) supply tank contained only 0.4 kilograms of fuel, and the right (No. 2) supply tank was empty. According to manufacturer Eurocopter (now Airbus Helicopters) the two supply tanks are normally fed continuously by the main fuel tank, and should remain full until the main tank is empty. When the tank is full, an overflow channel allows fuel to flow between the main and supply tanks. Below a certain level, fuel is transferred from the main tank to the supply tanks via fore and aft transfer pumps.
In the accident helicopter, investigators found that the No. 1 and 2 prime pump switches for the supply tanks — which are normally used for starting — were set to the ON position. Meanwhile, the fore and aft transfer pump switches for the main tank — which are typically set to the ON position during flight — were set to the OFF position. All pumps were tested and found to operate correctly, and the pump inlets and outlets were free from blockage and contamination.
On the night of the flight, the helicopter departed Glasgow City Heliport at 2045 hours with 400 kilograms of fuel on board, which the AAIB said gave it an operating endurance of approximately 1 hour and 35 minutes, plus required reserve fuel. By the time Traill made his last radio transmission at 2218 hours, the helicopter had been in the air for around 1 hour and 33 minutes. The AAIB said one focus of its investigation will be to determine why Traill did not make an emergency radio transmission prior to the crash. According to the report, at that time, Bond’s “final reserve fuel” at night was 85 kilograms, and the operator advised its pilots that an emergency condition could be considered to exist if the commander believed that the helicopter would land below its “minimum land on allowance” fuel (Bond later revised its procedures to replace the “minimum land on allowance” wording with “final reserve fuel”).
In December, Eurocopter issued an Alert Service Bulletin, alerting EC135 operators to the fact that supply-tank fuel gauging errors on some aircraft could lead to the quantity of fuel being overestimated. In the worst case, the manufacturer said, the aircraft’s amber fuel caution lights could fail to illuminate prior to fuel reaching critical levels, although the helicopters’ red low fuel warning lights would continue to operate correctly. Bond discovered such a fault in its fleet during normal operations on Dec. 11, and took the precautionary measure of temporarily grounding its EC135 fleet to conduct checks of the aircraft. 
The AAIB report indicates that the helicopter’s warning unit recorded intermittent LOW FUEL 1 warnings for the left supply fuel tank, then a permanent LOW FUEL 2 warning for the right fuel supply tank, followed by a permanent LOW FUEL 1 warning. According to the report, these low fuel warnings indicate when there is approximately 32 kilograms and 28 kilograms in the left and right supply tanks, respectively. The flight manual instructs that the pilot should land within 10 minutes of these warnings.
The AAIB is also focusing on why Traill failed to achieve an autorotative descent and flare recovery following the double engine failure. According to the AAIB bulletin, the 51-year-old commercial pilot, who had 5,592 total flight hours, had 646 hours on the EC135 type. However, the bulletin does not provide details about what training Traill had received prior to the accident.
To read the complete report on the AAIB website, click here.

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