Russian investigators have indicated that an altimeter pressure-reference error contributed to the fatal Angara Airlines Antonov An-24 crash in the Amur region.

Preliminary findings from the inquiry into the 24 July accident state that the crew was conducting the non-precision BELIM 2A approach procedure to runway 06 at Tynda airport.

As the aircraft – arriving from Blagoveshchensk – descended, its crew set the altimeter pressure reference to the sea-level QNH figure of 751mmHg (1002mb).

The instrument approach chart for Tynda states that the airport’s NDB navigation aid should be passed at an altitude of 1,105m relative to the QFE pressure of the local airfield elevation. Tynda airport is 616m above sea level.

Although the crew informed the air traffic controller that they had passed the NDB at 1,100m, the altimeter’s reference to sea level rather than airfield elevation meant the An-24 was actually flying over 600m lower.

Terrain in the region is hilly and tree-covered, with terrain rising to 850m within a 5km radius of the airport.

The preliminary information, disclosed by Russian federal air transport regulator Rosaviatsia, states that the An-24’s tag on the air traffic controller’s display did not show the aircraft’s altitude.

As the flight proceeded southwest, preparing for a 180° right turn to the final approach path, it continued to descend.

The chart specifies that the turn should be carried out at 740m altitude, based on QFE pressure, and this was discussed by the crew.

But with a QNH setting for the altimeter the aircraft would have been barely 120m above the airfield elevation.

While still 15km from Tynda’s runway, the An-24 collided with trees at an altitude of 765m relative to sea level and 150m relative to the airport, says Rosaviatsia.

“Prior to the collision, audio signals from the [ground proximity warning system] and radio altimeter were recorded,” it adds.

None of the 42 passengers, four crew members and two maintenance personnel on board the aircraft survived.

Rosaviatsia says the descent was conducted in daylight and weather conditions which included visibility of 9km, light rain and scattered cloud with a base of 225m, as well as cumulonimbus cloud at a base of 600m.

It says it is classifying the accident as a controlled flight into terrain.

Russian authorities have been overseeing a gradual airspace transition from QFE references to QNH references – a measure which commenced at individual airports in 2017 but which became increasingly significant during a wide-ranging restructuring in 2020.

The captain of the aircraft – a 1976 airframe, RA-47315 – had accumulated nearly 6,000h on An-24s and An-26s, from a total of 11,200h, and the first officer had over 1,900h on type.

While the Interstate Aviation Committee is still probing the An-24 crash, Rosaviatsia has instructed regional authorities to ensure additional training for crews and air traffic controllers on procedures involving QNH and QFE pressure, with “special attention” to flights at airports which have recently switched to using QNH.

It adds that flight operations manuals should undergo revision to provide additional clarification, if necessary, regarding the use of QNH and QFE pressure during landing preparations.