Scheduled Revenue Flight

The energy accumulated in the set of tires deflated the tire n°2, reducing the braking effectiveness by 50% due to the loss of the antiskid of that set in the same proportion, and subsequently the loss of 100% of the effectiveness of braking when tires n°3 and 4 burst successively, causing the aircraft to exceed the remaining runway length leading to the accident situation.
The following contributing factors were identified:
- The increase in speed of the VRF by the crew that increased the landing length,
- An unstabilized approach completed by the the crew,
- The pressure exerted on the crew due to the public order situation in the city of Florence for the purpose of a possible overnight stay of the aircraft and consequently the crew.

Engine failure due to poor maintenance. Deficiencies were reported during investigations regarding the operator's maintenance procedures. The documentation related to aircraft maintenance was incomplete. Also, several mechanics and engineers working on the AN-2 fleet were not qualified and authorised to do so.

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.

Controlled flight into terrain.

It was determined that the in-flight fire was the consequence of an act of sabotage. A passenger contracted seven life insurances from different companies for a total amount of 170,000 US$ and was able to board at Beijing-Capital Airport with a bottle filled with gasoline that he ignited during flight.

The accident was due to the crew's failure to perform and control the final approach. This was materialized by the failure to respect the approach fix associated with the decision to start the final approach prematurely.
The following contributing factors were identified:
- The poor weather conditions that prevailed at the time of the accident, particularly the limited visibility,
- The shortcomings noted in the training of both pilots, particularly those relating to conventional VOR/DME approaches,
- The relative weakness of the flight experience of both crew members on the type of aircraft operated.

3.1 Findings Related to Probable Causes:
1. The flight crew of flight 129 performed the circling approach, not being aware of the weather minima of widebody aircraft (B767-200) for landing, and in the approach briefing, did not include the missed approach, etc., among the items specified in Air China’s operations and training manuals.
2. The flight crew exercised poor crew resource management and lost situational awareness during the circling approach to runway 18R, which led them to fly outside of the circling approach area, delaying the base turn, contrary to the captain’s intention to make a timely base turn.
3. The flight crew did not execute a missed approach when they lost sight of the runway during the circling approach to runway 18R, which led them to strike high terrain (mountain) near the airport.
4. When the first officer advised the captain to execute a missed approach about 5 seconds before impact, the captain did not react, nor did the first officer initiate the missed approach himself.

Controlled flight into terrain after the crew failed to follow the approach procedures and the company SOP's. This caused the aircraft to deviate from the approach path by 3 nm to the north when it struck the mountain that was shrouded in clouds. The lack of visibility due to poor weather conditions was a contributing factor.