Pilots of an executive jet failed to set their altimeter to the correct pressure level before the aircraft descended low enough to collide with trees, investigators in Russia have determined.
The crew allowed the BAe 125-800 to continue descending despite automated warnings and the aircraft suffered substantial damage from the collision – some 18km from the airport – before the pilots aborted the approach.
Russia's Interstate Aviation Committee says the jet had departed Tyumen bound for Neryungri Chulman airport on 5 June 2016. It had been conducting the approach at night and had been following the RUGIL2 pattern, which involved flying south-east before turning left onto the 083° heading for runway 08.
The crew initially had difficulty contacting the local Chulman air traffic centre on 129.7MHz and instead reached the regional Neryungri centre on 121.7MHz.
This centre, upon the crew's request, cleared the flight to descend to flight level 90 (2,750m) before transferring the aircraft to Chulman tower control.
The tower confirmed the transition level as 2,450m and the airfield pressure (QFE) as 685mmHg, and allowed the aircraft to descend to 500m ahead of its turn towards the runway approach heading.
As the jet descended the tower asked the crew to confirm the QFE setting. While the crew replied with the correct number, 685, they needed to convert this to millibars – which would have resulted in a QFE of 913mb.
Instead the crew asked for confirmation of the sea-level pressure (QNH) to which the tower gave the figure of 1012mb.
The crew "did not recalculate" the airfield pressure of 685mmHg to give a QFE figure in millibars, says the inquiry, and instead set the altimeter to the QNH while continuing to descend.
This effectively meant that the altimeter was falsely showing the aircraft to be more than 800m above its actual height. Upon reaching the cleared height of 500m the aircraft would have still been indicating a height of more than 1,300m.
The enhanced ground-proximity warning system began to issue sink-rate and terrain alerts and, within a few seconds, ordered the crew to "pull up".
But the inquiry says the crew "did not follow the requirements of the flight manual" and instead continued with the descent. By the time the crew responded to the height warnings, the aircraft was flying so low that it collided with trees.
It climbed away despite sustaining damage to the leading edge of the wings, ailerons, horizontal stabiliser, and the engine inlets and fan blades. The collision partly jammed the elevator and the jet's left-hand winglet was torn off, says the inquiry, causing "considerable difficulty" for the crew as they tried to control the aircraft.
Inspection of the Aerolimousine aircraft (RA-02773), which managed to land without further incident, found that it had suffered impact damage to several other structures and system including its air brakes, radio altimeter antenna, and angle-of-attack sensor.
Investigators state that the crew's "delayed response" to the ground-proximity warning led to the collision with the trees. But it adds that there was "carelessness" in the treatment of the altimeter settings and that the crew had demonstrated that they had received "insufficient" training to carry out the flight.
Five passengers and a crew of three had been on board the jet, none of whom was injured.