Russian investigators have described as “unreasonable” a Ural Airlines Airbus A320 crew’s decision to divert to Novosibirsk, having incorrectly estimated the fuel required after failing to take into account the aircraft’s extended landing-gear.

The aircraft, originally bound for Omsk on 12 September, was forced to land in a field 186km short of Novosibirsk after running short on fuel.

According to federal air transport regulator Rosaviatsia’s investigation commission, the green hydraulic system failed 10s after the landing-gear was deployed during descent to Omsk.

The hydraulic failure was triggered by a flexible hose fracture, caused by natural wear, at a connection point in the right-hand main landing-gear assembly.

After the failure, says the inquiry, the crew did not determine the position of the landing-gear and flaps.

Ural A320 emergency landing 2-c-MChS

Source: Russian emergency situations ministry

All those on board the jet survived the landing in a field 186km short of Novosibirsk

The A320 had departed Sochi with 14.2t of fuel, and burned about 9.1t during the cruise at 37,000ft to Omsk. Diversion fuel to Novosibirsk was calculated at 2.6t.

It was cleared by Omsk air traffic control to land on runway 07 and the approach was normal, says the inquiry.

When the crew deployed the landing-gear, at an altitude of 2,170ft, the flight-data recorder captured a signal that the gear had extended, and the inquiry says this signal was maintained until the end of the recording.

Four seconds after the landing-gear locked in position, an alert warned of low pressure in the green hydraulic circuit. Loss of this circuit means the landing-gear can only be released through gravity extension.

Other effects, says the inquiry, include loss of certain systems, inability to retract the landing-gear or close the gear doors, and unreliability of flight-management system calculations because it does not account for increased fuel consumption.

The crew aborted the approach to Omsk. At this point the jet had 4.4t of fuel remaining.

Ural A320 in field-c-Ural Airlines

Source: Ural Airlines

Investigators found the A320 had 184kg of fuel on board after the emergency landing

Cockpit-voice recorder information shows that the crew “was aware” of the inability to retract the landing-gear as well as the small quantity of fuel on board.

But the crew opted to divert to Novosibirsk because the airport had a longer runway and the wind conditions were favourable.

Analysis of the situation, says the inquiry, shows that the available runway distance of 2,500m at Omsk would have been sufficient to allow the crew to land.

The diversion to Novosibirsk involved a gradual increase in altitude but the aircraft did not climb above 18,000ft.

While the inquiry says the fuel calculation for the flight to Omsk, with Novosibirsk as an alternate, was correctly carried out, the failure of the hydraulic system – preventing landing-gear retraction – meant at least 7t of fuel was needed to reach Novosibirsk, far more than the 4.4t on board.

There were no other airports along the flightpath which could have accommodated an emergency landing. But a return to Omsk could still have been made 18min after the diversion decision.

Cockpit-voice recorder information indicates that, during the diversion, the crew was uncertain of the landing-gear situation, believing the gear was raised but the doors were open. The crew also used the flight-management system to estimate the fuel balance – despite its information being unreliable.

About 50min into the diversion the remaining fuel had depleted to just 1.3t, insufficient to reach Novosibirsk or attempt a return, and the crew opted to carry out an emergency landing after selecting a suitable site from the air. The A320 landed in a field about 8km southeast of a village, Ubinskoye, with just 184kg of fuel left in the tanks.

None of the passengers or crew members on board was injured but the aircraft sustained damage to its landing-gear and engine fan-blades.