Russian investigators indicate that the crew of a Turkish-operated Boeing 747-400F knew the aircraft was too high on its approach before it overflew Bishkek airport and crashed far beyond the end of the runway.

But the low-visibility approach was belatedly aborted, and a go-around only executed after the aircraft had descended below a 100ft decision height – too late to clear terrain.

Russia’s Interstate Aviation Committee has confirmed that the ACT Airlines aircraft – operated under the company’s MyCargo Airlines brand – was too high on its approach as it descended towards runway 26 in darkness on 16 January.

It states that, 25min before the accident, the jet left its cruise altitude of 34,000ft some 130nm from Manas airport.

The pilots had conducted a low-visibility landing briefing, and had tuned to the runway 26 ILS frequency as well as the separate frequency of the VOR/DME situated 0.8nm before the threshold.

Investigators state that the 747 was proceeding along the TOKPA 1 standard arrival pattern from the south-east, requiring overflight of waypoints RAXAT and TOKPA.

It correctly overflew RAXAT at its cleared height of 18,000ft before its handover to approach control, which instructed a further descent to 6,000ft.

While the TOKPA waypoint should typically be overflown at 6,000ft, the inquiry states that it was crossed at 9,200ft.

The crew set the QNH – the reference to sea-level pressure – and was cleared to descend to 3,400ft. Investigators state that, according to the approach chart, this altitude should be reached at 5.4nm distance from the VOR and maintained to enable glideslope capture at 3.2nm.

“At that stage of flight the crew was monitoring the flight altitude and was aware they were higher than the [chart height],” says the inquiry, citing cockpit-voice recorder information.

The 747 captured the localiser at a distance of 6nm from the VOR when the jet was still at 5,700ft. Investigators state that three autopilots were engaged on the aircraft and that it was manoeuvring vertically in ‘flight level change’ mode.

It did not reach the required 3,400ft until just 1.7nm from the VOR. Although the glideslope mode was armed the aircraft was too high and missed the glideslope intercept, failing to capture it, and instead held its altitude, maintaining level flight at 3,400ft rather than descending towards the airport.

“The flight was performed at constant altitude along the runway 26 landing course,” says the inquiry.

But just ahead of the VOR the aircraft did intercept and capture a false glideslope signal – the reflection at 9° rather than the genuine 3° slope – and it automatically commenced a descent, at rates of up to 1,425ft/min, as the airport and runway passed beneath.

Visibility was severely limited owing to the presence of freezing fog.

Category II approaches establish a decision height of no less than 100ft. The aircraft crossed the far end of the runway at a height of 110ft and, after a ground-proximity warning system call-out at 100ft, the first officer declared: “Minimums.”

Owing to the absence of visual references, the captain ordered a go-around, although go-around thrust was not initiated until the aircraft was just 58ft above the ground. The inquiry says that 3.5s after the go-around switch was pressed, the aircraft collided with upwardly-inclined terrain and obstacles at a speed of 165kt, with a 6g vertical impact.

This impact, some 930m beyond the far end of runway 26, destroyed the aircraft and demolished 31 houses and outbuildings, damaging a further seven. None of the four crew members survived and the fatalities included 35 people on the ground.

Investigators have issued recommendations in a preliminary report on the accident, advising crews to “pay attention” to charts and monitor distance and altitude at reference points, particularly during low-visibility approaches. Any go-around, the inquiry adds, should be initiated no lower than the decision height.