Penghu County (澎湖縣)

- 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.

The following findings were identified:
1. The accident flight encountered severe icing conditions. The liquid water content and maximum droplet size were beyond the icing certification envelope of FAR/JAR 25 appendix C.
2. TNA's training and rating of aircraft severe icing for this pilots has not been effective and the pilots have not developed a familiarity with the Note, CAUTION and WARNING set forth in Flight Crew Operating Manual and Airplane Flight Manual to adequately perform their duties.
3. After the flight crew detected icing condition and the airframe de-icing system was activated twice, the flight crew did not read the relative Handbook, thereby the procedure was not able to inform the flight crew and to remind them of "be alert to severe icing detection".
4. The "unexpected decrease in speed" indicated by the airspeed indicator is an indication of severe icing.
5. The flight crew did not respond to the severe Icing conditions with pertinent alertness and situation awareness that the aircraft might have encountered conditions which was "outside that for which the aircraft was certificated and might seriously degrade the performance and controllability of the aircraft".
6. The flight crew was too late in detecting the severe icing conditions. After detection, they did not change altitude immediately, nor take other steps required in the Severe Icing Emergency Procedures.
7. The aircraft was in an "unusual or uncontrolled rolling and pitching" state, and a stall occurred thereafter.
8. After the aircraft had developed a stall and an abnormal attitude, the recovery maneuvering did not comply with the operating procedures and techniques for Recovery of Unusual Attitudes. The performance and controllability of the aircraft may have been seriously degraded by then. It cannot be confirmed whether the unusual attitudes of the aircraft could have been recovered if the crew's operation had complied with the relevant procedures and techniques.
9. During the first 25 minutes, the extra drag increased about 100 counts, inducing a speed diminishing about 10 knots.
10. During the airframe de-icing system was intermittently switched off, it is highly probable that residual ice covered on the wings of the aircraft.
11. Four minutes prior to autopilot disengaged, the extra drag increased about 500 counts, and airspeed decayed to 158 knots, and lift-drag ratio loss about 64% rapidly.
12. During the 10s before the roll upset, the longitudinal and lateral stability has been modified by the severe ice accumulated on the wings producing the flow separation. Before autopilot disengaged, the aerodynamic of the aircraft (lift/drag) was degraded of about 40%.

Findings related to probable causes:
1. Based on the recordings of CVR and FDR, radar data, the dado panel open-close positions, the wreckage distribution, and the wreckage examinations, the in-flight breakup of CI611, as it approached its cruising altitude, was highly likely due to the structural failure in the aft lower lobe section of the fuselage.
2. In February 7 1980, the accident aircraft suffered a tail strike occurrence in Hong Kong. The aircraft was ferried back to Taiwan on the same day un-pressurized and a temporary repair was conducted the day after. A permanent repair was conducted on May 23 through 26, 1980.
3. The permanent repair of the tail strike was not accomplished in accordance with the Boeing SRM, in that the area of damaged skin in Section 46 was not removed (trimmed) and the repair doubler did not extend sufficiently beyond the entire damaged area to restore the structural strength.
4. Evidence of fatigue damage was found in the lower aft fuselage centered about STA 2100, between stringers S-48L and S-49L, under the repair doubler near its edge and outside the outer row of securing rivets. Multiple Site Damage (MSD), including a 15.1-inch through thickness main fatigue crack and some small fatigue cracks were confirmed. The 15.1-inch crack and most of the MSD cracks initiated from the scratching damage associated with the 1980 tail strike incident.
5. Residual strength analysis indicated that the main fatigue crack in combination with the Multiple Site Damage (MSD) were of sufficient magnitude and distribution to facilitate the local linking of the fatigue cracks so as to produce a continuous crack within a two-bay region (40 inches). Analysis further indicated that during the application of normal operational loads the residual strength of the fuselage would be compromised with a continuous crack of 58 inches or longer length. Although the ASC could not determine the length of cracking prior to the accident flight, the ASC believes that the extent of hoop-wise fretting marks found on the doubler, and the regularly spaced marks and deformed cladding found on the fracture surface suggest that a continuous crack of at least 71 inches in length, a crack length considered long enough to cause structural separation of the fuselage, was present before the in-flight breakup of the aircraft.
6. Maintenance inspection of B-18255 did not detect the ineffective 1980 structural repair and the fatigue cracks that were developing under the repair doubler. However, the time that the fatigue cracks propagated through the skin thickness could not be determined.

The exact cause of the accident remains unknown but it was reported that the crew apparently encountered technical issues with the undercarriage.

It was determined that the accident was the consequence of an in-flight disintegration of the airplane after a bomb exploded on board. The author and the reason of this act of terrorism remains unknown.