Preliminary findings from the Sukhoi Superjet 100 crash inquiry show that the aircraft had been cleared to descend below the minimum safe altitude for the area.
Sukhoi had intended to conduct a 30min demonstration flight from Halim to Pelabuhan Ratu at an altitude of 10,000ft.
Indonesia's National Transportation Safety Committee states that air traffic control cleared the aircraft to depart runway 06 then, upon reaching 2,000ft, turn right to intercept the 200 radial from the Halim VOR beacon before climbing to 10,000ft.
After transferring to Jakarta approach control the crew was instructed to maintain this altitude. But two minutes later - and just 5min into the flight - the pilot requested descent to 6,000ft.
Indonesia's aeronautical information publication showed, however, that the minimum safe altitude was 6,900ft within 25nm of the Halim beacon.
The controller asked the pilot to repeat the descent request and, after he did so, acknowledged it. The pilot responded: "Descend to 6,000ft."
Two minutes later the pilot contacted Jakarta approach again, asking to make a right orbit, which was also approved at 6,000ft. The controller told investigators that, at the moment of this request, radar showed the aircraft was over a training zone at Atang Sanjaya about 17nm southwest of the Halim beacon.
But the aircraft was later found to have crashed into high terrain 28nm from the beacon, on the 198 radial. The impact site, on Mount Salak, was at a height of about 6,000ft.
The route selected for the flight was not a published airway, the inquiry points out, adding that the minimum off-route altitude was 13,200ft.
Weather information for the time of the accident - taken from a meteorological station about 7nm from the crash site - indicated a cloud base of about 2,000ft, with substantial cloud cover and cumulonimbus activity in the vicinity.
Investigators state that the flight-data recorder contains 22min of information from the flight, from engine start. But their preliminary findings have not detailed the operational status of the terrain-awareness and warning system or the various avionics and navigation systems on board the Superjet.
Neither pilot was found to have any traces of alcohol or drugs in their system. No distress signal was received by emergency services. The aircraft's emergency locator transmitter was found with its antenna detached.
None of the 45 occupants - comprising two pilots, a navigator, test-flight engineer and 41 passengers - survived the 9 May crash.
Source: Air Transport Intelligence news