Paul Lewis/Singapore David Learmount/London

AN UNUSUALLY low altitude-clearance by Medan airport air-traffic control (ATC) appears to have played a crucial part in the Garuda Indonesian Airlines Airbus A300B4 fatal accident in Sumatra, Indonesia. The crash on 26 September, in poor visibility among the foothills of a mountain range to the south-west of Medan, killed all 234 people on board.

The let-down was under radar vectors from Medan approach. Exactly 2min before impact, the Garuda aircraft, heading towards the mountains, was given clearance to descend to 2,000ft (610m). This is 500ft below the initial-approach altitude for the VHF navigation-beacon published let-down procedure, which keeps the aircraft much further from the hills.

Meanwhile, physical evidence from the crash site is assuming heightened importance as the flight-data recorders have not been found. According to surveyors at the site, the aircraft hit left-wing-low on a heading of about 220 degrees in a ravine. This tends to confirm that the pilot of Garuda flight GA152, having acknowledged an ATC instruction to turn right to intercept the instrument-landing system (ILS), had scarcely deviated from his previously cleared heading of 215¹ (see diagram). The crash site is about 1,150ft above sea level.

Within 20s of ATC's instruction to turn right to intercept the ILS, acknowledged correctly by the crew, the pilot queried the instruction, asking: "GIA 152, confirm turning left or turning right heading 046 degrees?" ATC replied: "Turning right, sir." The response was: "Roger, 152." Ten seconds later the controller, concerned that the aircraft had begun to turn left, asked ambiguously: "152, confirm you are making a left turn now?" The pilot responded that he was turning right, to which the worried controller replied: "OK, you turning left now." The pilot stated: "Confirm turning left? We are starting turn right heading 046." The crash occurred 10s later.

Source: Flight International