On 26 August 2015, contact was lost with an ATR 42-300 (PK-YRN) being operated by Trigana Air Service on a scheduled domestic passenger flight from Jayapura to Oksibil as IL267 shortly before it reached its destination. The wreckage of the aircraft, with evidence of a post crash fire, was subsequently located approximately 10 nm west northwest of its destination at an altitude of 8,300 feet amsl. Terrain impact had occurred in day IMC. There were no survivors amongst its 54 occupants.
An Investigation was carried out by the Indonesian National Transportation Safety Committee (NTSC) - the Komite Nasional Keselamatan Transportasi (KNKT). Both the FDR and the 2 hour CVR were recovered and downloaded but the data on the FDR was found not to include the accident flight and more generally such earlier data as were readable “were not consistent with the previous flights recorded in the aircraft log”. It was found that the FDR had a 3 year history of unreliability and had been sent for attention at the same repair station but no cause of the problems could be identified. The data on the CVR was complete and covered the previous three flights as well as the accident flight. Because of the unavailability of any FDR data, the Investigation had a sound spectrum analysis of the CVR made in order to determine engine torque. There was no evidence that engine power had significantly increased prior to impact. It was also noted that although the aircraft was fitted with an EGPWS, there was no CVR record of the automatic “500” callout which this equipment should have generated on both the previous flight into Oksibil and the subsequent one from Oksibil back into Jayapura. There were also no EGPWS alerts or warnings prior to collision with terrain on the accident flight. . It was found that the 60 year-old Captain had 25,287 hours total flying experience including 7,340 on the ATR42/72 and the 44 year-old First Officer, who had been PF for the accident flight, had 3,818 hours total flying experience including 2,640 hours on the ATR42/72. The accident flight was the fifth flight of the duty day for the crew and their second flight on the same Jayapura-Oksibil sector.
It was established that an IFR Flight Plan had been filed to cruise at FL 155 on airway W66 as far as the waypoint MELAM. It was noted that the MORA for W66 was promulgated as 18,500 feet QNH. From MELAM, the Operator-issued ‘Visual Guidance Chart’ specific to the ATR42 should have been followed. This routing (see the first illustration below) would have taken the aircraft to the Oksibil overhead at 13,500 feet QNH from where another Operator-issued chart (see the second illustration below) depicted a ‘Visual Circling Approach’ to the 1,350 metre long runway 11 (aerodrome elevation 4,000 amsl). An associated Company ‘chart page’ marked ‘Oksibil Area Instruction’ provided detailed instructions on how to fly this approach.
ATS at Oksibil was provided by a FIS. Whilst the Oksibil NDB ‘ZX’ was not mentioned on the Company visual guidance charts and there was no instrument approach procedure based upon it, it was found that on the day of the accident it had been inoperative but this status had not been the subject of a NOTAM.
The Company Initial Visual Approach Chart for Oksibil beginning at MELAM. [Reproduced from the Official Report]
The aircraft departed Jayapura and the 40 minute flight was uneventful until on the visual sector after MELAM, the end of the IFR part of the route. Nine minutes before the destination ETA, the crew made their first contact with the aerodrome FISO and reported being in the descent from an altitude of 11,500 feet at waypoint ABMISIBIL. After acknowledging the call, the AFISO “suggested the pilot should report when overhead the airport” but when the pilot replied that they “intended to fly direct to a left base leg for runway 11” the AFISO replied suggesting that the pilot could continue to approach and call when positioned on final for runway 11.
Five minutes later, when he would have expected that the aircraft would be on final approach, the AFISO had attempted to establish contact but there was no reply. The wreckage of the aircraft was found on a ridge of the Tanggo Mountain in the Okbape District of Oksibil at approximately 8,300 feet amsl approximately 10 nm from the aerodrome on a bearing of 306°. From the CVR data it could be established that during the accident and previous flights there had been no crew briefing or Checklist reading from cruise until landing / impact, contrary to Operations Manual requirements. This violation in flight crew behaviour had not been identified by the aircraft operator and the Investigation was unable to establish whether this practice was specific to this crew or more widely prevalent.
The unofficial weather observation made by the FISO to pass to the inbound flight gave a visibility of 4,000-5,000 metres and BKN cloud at 4,000 feet aal with cloud covering the area of the final approach path. Satellite imagery indicated that the Oksibil area was “covered by stratocumulus clouds”.
The Company Visual Approach Chart for Oksibil runway 11. [Reproduced from the Official Report]
It was noted that the airline averaged five flights per day on the Jayapura-Oksibil route using its three ATR 42-300 aircraft. It was noted that several ATR pilots reported sometimes finding the EGPWS CB tripped when conducting pre-flight Checks and when reset, that the system functioned normally. Trigana Air Service management also stated that several ATR pilots “including the pilot in command of the accident flight” had a habit of tripping the EGPWS CB. A witness statement provided to the Investigation stated that most of the time, the accident aircraft Captain “did not follow the visual approach guidance while conducting approaches at Oksibil” and it was noted from the CVR that the previous flight to Oksibil had also been made direct to left base runway 11. Several pilots stated that the reason for tripping the EGPWS CB was that they considered that its warnings were sometimes not appropriate.
An estimate of the aircraft’s descent profile for the 9 minute period between MELAM and the impact position was made. Almost as soon as ABMISIBIL was passed 5½ minutes later, at an airspeed of about 200 knots, engine power and airspeed were reduced to something near to idle and 160 knots respectively and one minute later the flaps were set to 15, the engine power slightly increased and the landing gear was selected down. Impact occurred one minute after this with an estimated airspeed of 160 knots. The corresponding predicted flight track after MELAM is shown in the Illustration below.
The annotated estimated flight track from MELAM to impact. [Reproduced from the Official Report]
This estimate of the ground track was plotted against the terrain below and the result, showing the EGPWS activations which would have occurred had it been functioning, is shown below.
An approximation of the flight vertical profile immediately prior to impact. [Reproduced from the Official Report]
The Investigation formally identified two Contributory Factors in respect of the occurrence as follows:
- The deviation from the visual approach guidance under visual flight rules without considering the weather and terrain condition, with no or limited visual reference to the terrain resulted in the aircraft flying into terrain.
- The absence of an EGPWS warning to alert the crew of the immediate hazardous situation led to the crew not being alerted to their situation.
A total of 9 Safety Recommendations were made as a result of the Investigation as follows:
- that Trigana Air Services should review the approach guidance for Oksibil to ensure that it contains correct information and is easy to fly so as to minimise pilot workload. [04.O-2015-17.1]
- that Trigana Air Services should ensure that any modification to the aircraft, especially if related to aircraft safety, is communicated to the aircraft manufacturer and/or DGCA. [04.O-2015-17.2]
- that AirNav Indonesia should ensure that the current status of navigation aids is disseminated to air navigation users in a timely manner. [04.A-2015-17.3]
- that AirNav Indonesia should ensure that the filing of flight plans and the execution of flight plans is in accordance with the regulation. [04.A-2015-17.4]
- that the Directorate General of Civil Aviation (DGCA) should develop regulatory requirement for training on any additional or modified equipment that affects the safety of aircraft operations. [04.R-2015-17.5]
- that the Directorate General of Civil Aviation (DGCA) should publish visual route guidance for airports without instrument approach procedures. [04.R-2015-17.6]
- that the Directorate General of Civil Aviation (DGCA) should consider the application of Performance Based Navigation (PBN) approaches for compatible aircraft to fly in areas where ground-based navigation system implementation is limited. [04.R-2015-17.7]
- that the Directorate General of Civil Aviation (DGCA) should ensure that aeronautical information for air navigation is updated in accordance with the current conditions, including the serviceability of navigation aids. [04.R-2015-17.8]
- that the Directorate General of Civil Aviation (DGCA) should coordinate with the manufacturer to provide several airports in Indonesia (including Oksibil) with an EGPWS high resolution terrain database. [04.R-2015-17.9]
The Final Report of the Investigation was released on 29 December 2017.