Russian investigators believe the crew of a Ural Airlines Airbus A320 did not realise the undercarriage had failed to retract during a diversion to Novosibirsk, forcing the aircraft to land in a field as it ran short on fuel.
The A320 had aborted an approach to its original destination, Omsk, after arriving from Sochi on 12 September.
Russian federal air transport regulator Rosaviatsia says the crew executed the missed approach because the aircraft suffered a failure of its green hydraulic circuit as its landing-gear was extended.
Rosaviatsia attributes the failure to a damaged hydraulic control line in the right-hand main-gear door.
As the aircraft climbed away from Omsk, the flaps and slats – which are linked to the blue and yellow circuits, as well as the green – retracted.
But the landing-gear uses the green circuit and, although the crew selected the landing-gear lever ‘up’, the undercarriage did not retract.
“The crew did not notice any failure to retract the landing-gear,” says Rosaviatsia.
Flight-data recorder information revealed that a signal, confirming that the landing-gear remained extended, was generated throughout the rest of the flight.
After entering a holding pattern over Omsk, the pilots accessed the quick-reference handbook and, after recalculating landing distances to account for the hydraulic failure, opted to divert to Novosibirsk which had a longer runway.
Rosaviatsia says the pilots determined the fuel required for the diversion but, while en route, discovered that the aircraft was burning more fuel than expected and had an “insufficient” quantity to reach Novosibirsk.
The aircraft’s extended landing-gear had generated additional drag, causing higher fuel consumption.
Airbus puts the fuel-penalty factor for extended gear on an A320 at 180%, meaning the fuel-burn rises to 2.8 times its normal rate.
With the Ural aircraft unable to reach Novosibirsk, the crew selected an emergency landing site from the air and managed to land the twinjet (RA-73805) in a field without injury to the 161 passengers and six crew members.
Inspection of the A320 found that its tanks contained just 216 litres of fuel.
Airbus modified the hydraulic system of A320s from MSN1939, switching the nose-wheel steering from the green circuit to the yellow. The Ural A320, MSN2166, is one of these later models.
While the inquiry is still analysing the event, and has yet to draw conclusions, it points out that statistics on low-fuel landings reveal various causes, including repeated missed approaches, unexpected headwinds or other weather conditions, and “untimely” decisions by pilots to divert, owing to “self-confidence and negligence”.
Investigators have highlighted to operators the importance of taking all factors into account when calculating fuel requirements, including the effects of in-flight failure of engines or other systems, and to adhere to basic crew resource management principles.