Accident Overview
On 28 August 2015, a Boeing 737-300 Freighter registered PK-BBY, operated by PT. Cardig Air, was conducting a scheduled cargo flight from Sentani Airport (WAJJ) in Jayapura to Wamena Airport (WAVV) in Papua, Indonesia. The aircraft departed at 1234 local time (0334 UTC) with two pilots and 14,610 kg of cargo. The Pilot in Command (PIC) acted as Pilot Flying (PF) and the Second in Command (SIC), under line training, acted as Pilot Monitoring (PM). No aircraft system abnormalities were reported or recorded during the flight.
Flight History
At 0637 UTC, as the aircraft approached PASS VALLEY, Wamena Tower controller advised that runway 15 was in use with wind 150°/18 knots, QNH 1,003 mbs, and temperature 23 °C. At 0639 UTC, the pilot reported position over PASS VALLEY and descending through FL135. The controller instructed the pilot to report over JIWIKA. At 0645 UTC, the pilot reported over JIWIKA and continued to final approach for runway 15. At 0646 UTC, the pilot reported on final, and the controller issued landing clearance with wind 150°/15 knots and QNH 1,003 mbs.
Accident Sequence
At 0647 UTC, the aircraft touched down approximately 35 meters before the runway 15 threshold, recording a vertical acceleration of 3.68 G. The left main landing gear collapsed, and the left engine contacted the runway surface. The aircraft stopped about 1,500 meters from the runway threshold. No injuries were reported.
Investigation Findings
The Komite Nasional Keselamatan Transportasi (KNKT) conducted the investigation and issued a list of findings. The pilots held valid licenses and medical certificates. The aircraft had a valid Certificate of Airworthiness and Certificate of Registration and was operated within the weight and balance envelope. No system abnormalities were noted. After passing JIWIKA at 10,000 feet, the Flight Data Recorder (FDR) recorded engines at idle with an average descent rate of approximately 2,000 feet per minute. At about 8,000 feet, flaps were selected to 40 and reached 39.9° one minute 25 seconds later.
The BMKG weather report indicated wind 150°/14-19 knots, but the Wamena Tower controller reported only wind 150°/15 knots; gust wind information, indicating possible windshear, was not passed to the pilot. The Enhanced Ground Proximity Warning System (EGPWS) activated a "CAUTION WINDSHEAR" at 5,520 feet. At 06:45:43 UTC, engine power increased when the aircraft was at 5,920 feet, before the EGPWS called "ONE HUNDRED". Starting at 06:45:45 UTC, the FDR recorded calibrated airspeed (CAS) increasing from 148 knots to 154 knots, while N1 decreased from 73% to 38%. Three seconds before touchdown, the descent rate was constant at 1,320 feet per minute, and the EGPWS issued a "SINK RATE" warning. The trunnion link of the left main landing gear assembly was found broken. The FDR data, covering 107 flight hours and 170 sectors, recorded five instances of vertical acceleration exceeding 2 G during landings at Wamena, suggesting possible landing gear strength degradation. The Visual Approach Slope Indicator (VASI) for runway 15 was not operational after the runway extension. Several touchdown marks were found before runway 15, and excessive rubber deposit was noted on the runway surface starting about 600 meters from the threshold.
The absence of speed correction following the reported 15-knot headwind and the lack of a crew briefing after the EGPWS windshear caution indicated that the pilots were unaware of existing windshear, possibly due to the lack of gust wind information. The large thrust reduction was not in accordance with the Flight Crew Operating Manual (FCOM) for windshear precautions, resulting in a rapid descent. The accident flight's recorded vertical acceleration of 3.683 G was within the landing gear design limit, but the gear had strength degradation from previous hard landings.
Probable Cause
Contributing Factor: The large thrust reduction during the windshear resulted in rapid descent, and the aircraft touched down with 3.683 G, causing the landing gear to collapse due to strength degradation.