Scheduled Revenue Flight

Findings as to causes and contributing factors:
1. During the extension of the landing gear, a wire bundle became entangled around the landing-gear rotating torque shaft, preventing full extension of the landing gear.
2. The entanglement by the wire bundle also prevented the alternate landing-gear extension system from working. The crew was required to conduct a landing with only the nose gear partially extended.
Other findings:
1. The wire bundle consisted of wiring for the generator control circuits, and when damaged, disabled both generators. The battery became the only source of electrical power until the aircraft landed.

The following contributing factors were identified:
- The flight crew continued the descent in instrument meteorological conditions without confirming their position.
- The flight crew’s assessment of their position was incorrect and they had lost situational awareness
- The flight crew deprived themselves of the “Caution” and “Warning” alerts that would have sounded about 20 sec and about 10 sec respectively before the collision, by not deactivating the EGPWS Terrain Inhibit prior to departure from Woitape.

The aircraft’s left main landing gear failed as a result of stress corrosion cracking in the forward pintle housing, at the top of the left landing gear cylinder. The landing gear material is known to be susceptible to stress corrosion cracking. The investigation determined that a design solution implemented by the aircraft manufacturer following the 2012 accident, which was intended to prevent stress corrosion cracking, had not met its original design intent.

The accident investigation team determined that the main cause of this accident was combination of:
1. Electronic engine control (SAY-2000) failure simultaneously with engine No: 2 shutdown, just about 2 seconds before aircraft lift-off.
2. AFM Confusing performance chart resulted the pilots relying on performance calculation that, significantly over-estimate the aircraft MTOM.
Contributing Factors to the accident were:
1. Aircraft flight manual unclear procedure, including the procedure for calculating maximum allowable take-off weight, VR and V2 and ambiguity in the climb segment definition and applications.
2. Crew performance, including:
- PIC rotated the aircraft at the speed of about 219 km/h (whereas 224 km/h is the speed recommended by the AFM table 4.2.3),
- The crew failed to perform the manual propeller feathering procedure for the failed engine,
- The PIC's decision to fly with the aircraft, notwithstanding, had about 190 kgf overweight,
- Aircraft fuel was about 500 kg more than required fuel for the accident flight.
3. The appearance of negative thrust from the unfeathered propeller blades at takeoff were not considered during the aircraft certification tests, as it was considered improbable. However, in
the accident flight the negative thrust did appear and affected the flight performance.

About two minutes after leveling off at an altitude of 31,000 ft, calculations performed by the manufacturer and validated by the investigation team indicate that the recorded EPR, the main parameter for engine power management, became erroneous on the right engine and then about 55 seconds later on the left engine. This was likely due to icing of the pressure sensors located on the engine nose cones. If the engine anti-ice protection system is activated, these pressure sensors are heated by hot air. Analysis of the available data indicates that the crew likely did not activate the system during climb and cruise. As a result of the icing of the pressure sensors, the erroneous information transmitted to the auto throttle meant that the latter limited the thrust delivered by the engines. Under these conditions, the thrust was insufficient to maintain cruise speed and the aeroplane slowed down. The autopilot then commanded an increase in the airplane's pitch attitude in order to maintain the altitude in spite of this loss of speed. This explains how, from the beginning of the error in measuring the EPR values, the airplane’s speed dropped from 290 kt to 200 kt in about 5 minutes and 35 seconds and the angle of attack increased until the aeroplane stalled. About 20 seconds after the beginning of the aeroplane stall, the autopilot was disengaged. The aeroplane rolled suddenly to the left until it reached a bank angle of 140°, and a nose-down pitch of 80°. The recorded parameters indicate that there were no stall recovery maneuvers by the crew. However, in the moments following the aeroplane stall, the flight control surfaces remained deflected nose-up and in a right roll. It was concluded that the accident was caused by the combination of several factors, among them the fact that the engine anti-icing systems were not activated by the crew. The final report is not available in English yet.

- The flight crew did not comply with the published runway 20 VOR non-precision instrument approach procedures at Magong Airport with respect to the minimum descent altitude (MDA). The captain, as the pilot flying, intentionally descended the aircraft below the published MDA of 330 feet in the instrument meteorological conditions (IMC) without obtaining the required visual references.
- The aircraft maintained an altitude between 168 and 192 feet before and just after overflying the missed approach point (MAPt). Both pilots spent about 13 seconds attempting to visually locate the runway environment, rather than commencing a missed approach at or prior to the MAPt as required by the published procedures.
- As the aircraft descended below the minimum descent altitude (MDA), it diverted to the left of the inbound instrument approach track and its rate of descent increased as a result of the flying pilot’s control inputs and meteorological conditions. The aircraft’s hazardous flight path was not detected and corrected by the crew in due time to avoid the collision with the terrain, suggesting that the crew lost situational awareness about the aircraft’s position during the latter stages of the approach.
- During the final approach, the heavy rain and associated thunderstorm activity intensified producing a maximum rainfall of 1.8 mm per minute. The runway visual range (RVR) subsequently reduced to approximately 500 meters. The degraded visibility significantly reduced the likelihood that the flight crew could have acquired the visual references to the runway environment during the approach.
- Flight crew coordination, communication, and threat and error management were less than effective. That compromised the safety of the flight. The first officer did not comment about or challenge the fact that the captain had intentionally descended the aircraft below the published minimum descent altitude (MDA). Rather, the first officer collaborated with the captain’s intentional descent below the MDA. In addition, the first officer did not detect the aircraft had deviated from the published inbound instrument approach track or identify that those factors increased the risk of a controlled flight into terrain (CFIT) event.
- None of the flight crew recognized the need for a missed approach until the aircraft reached the point (72 feet, 0.5 nautical mile beyond the missed approach point) where collision with the terrain became unavoidable.
- The aircraft was under the control of the flight crew when it collided with foliage 850 meters northeast of the runway 20 threshold, two seconds after the go around decision had been made. The aircraft sustained significant damage and subsequently collided with buildings in a residential area. Due to the high impact forces and post-impact fire, the crew and most passengers perished.
- According to the flight recorders data, non-compliance with standard operating procedures (SOP's) was a repeated practice during the occurrence flight. The crew’s recurring non-compliance with SOP's constituted an operating culture in which high risk practices were routine and considered normal.
- The non-compliance with standard operating procedures (SOP's) breached the obstacle clearances of the published procedure, bypassed the safety criteria and risk controls considered in the design of the published procedures, and increased the risk of a controlled flight into terrain (CFIT) event.