Indonesian investigators have determined that the captain of a Sukhoi Superjet 100 inhibited the terrain-collision system, believing its alerts to be erroneous.
The terrain-awareness system initially sounded 38s before the aircraft struck the slope of Mount Salak on 9 May this year, killing all 45 occupants who were participating in a demonstration flight.
Investigators from the National Transportation Safety Committee found the crew had requested a descent to 6,000ft and to fly a right-hand orbit. This was "approved" by Jakarta air traffic control.
But the crew was distracted by "prolonged conversation" which was unrelated to the flight, says the NTSC, and the captain - who was flying - did not continue to change the Superjet's heading during the orbit.
"Consequently, the aircraft unintentionally exited the orbit," it adds.
The aircraft's terrain-awareness system started called "terrain ahead, pull up" about 38s before the crash, while the alert "avoid terrain" sounded six times.
But the NTSC says: "The [captain] inhibited the [terrain-warning] system assuming that the warning was a problem on the database."
Seven seconds before the impact with the mountain the aircraft's warning systems also alerted the pilot to the landing-gear not being deployed.
Analysis by the NTSC suggests the aircraft might have been able to avoid the impact up to 24s after the initial terrain warning.
The Superjet had been taking part in an Asian tour for potential customers, and a customer representative had been seated in the cockpit with the two pilots.
The aircraft departed Jakarta's Halim Perdanakusuma airport at 14:20 local time, turning right to intercept radial 200 from the airport's VOR beacon.
It climbed to 10,000ft en route for a demonstration flight planned over Bogor but the NTSC points out that available charts on board the aircraft "did not contain" information relating to the Bogor area and the surrounding terrain.
NTSC investigators says the descent request to 6,000ft was transmitted about 6min after departure and the aircraft struck Mount Salak, at a height of 6,000ft, some 6min later.
Jakarta's radar service has not established a minimum altitude for vectoring aircraft in certain areas, says the inquiry, and its system was not equipped with functioning minimum safe altitude warnings for the area surrounding Mount Salak.
It adds: "The crew were not aware of the mountainous area surrounding the flight path due to various factors, resulting in disregarding the [terrain] warning."
Source: Air Transport Intelligence news