China Airlines Flight 611

Cause

The final investigation report found that the accident was the result of metal fatigue cracking caused by an unapproved repair procedure that was performed on the aircraft after a much earlier tailstrike incident. On 7 February 1980, the aircraft was flying from Stockholm Arlanda Airport to Taoyuan International Airport via Jeddah International Airport and Kai Tak Airport as China Airlines Flight 009 (Callsign CAL009, pronounced Dynasty 009). While landing in Hong Kong, part of the plane's tail had scraped along the runway. The aircraft was depressurised, ferried back to Taiwan on the same day, and a temporary repair done the day after. A more permanent repair was conducted by a team from China Airlines from 23 to 26 May 1980.

The permanent repair of the tailstrike was not carried out in accordance with the Boeing Structural Repair Manual (SRM). According to the SRM, repairs could be made by replacing the entire affected skin or by cutting out the damaged portion and installing a reinforcing doubler plate to restore the structural strength. Rather than following the SRM, the China Airlines team installed a doubler over the damaged skin.

Though the kind of damage inflicted on the tail was far beyond the damage that a doubler plate is meant to fix, this accident would have probably not occurred had the doubler been installed properly. This would mean that all of the scratches would be completely contained by the innermost row of fasteners, and the fasteners themselves would be strong enough to stop the propagation of any new and existing fatigue cracks. However, the doubler that was installed on the aircraft was too small, so it failed to completely and effectively cover the damaged area, as scratches were found at, and outside, the outermost row of fasteners securing the doubler. Installing the doubler with scratches remaining outside the rivets provided no protection against the propagation of any concealed cracks beneath the doubler, or worse, in the area between its perimeter and the rows of rivets.

Consequently, after repeated cycles of pressurisation and depressurisation during flight, cracks began to form around the exposed scratches until finally, on 25 May 2002, coincidentally 22 years to the day after the faulty repair was made on the damaged tail, the hull broke open in midair. A rapid decompression occurred once the crack opened up, causing the separation of the aircraft's fuselage at section 46 (aft of the main wingbox). The remainder of the aircraft forward of section 46 entered an abrupt descent, causing all four engines to separate from the wings near-simultaneously, as the engine fuse pins failed at about 29,000 ft. After this point, the wings and fuselage forward of the initial breakpoint remained connected until impact with the sea.

This was not the first time that a 747 had crashed because of a faulty repair following a tailstrike. On 12 August 1985, 17 years before Flight 611's crash and 7 years after the accident aircraft's repair, Japan Air Lines Flight 123 from Tokyo to Osaka with 524 people on board had crashed when the vertical stabiliser was torn off and the hydraulic systems severed by explosive decompression, leaving only four survivors. That crash had been attributed to a faulty repair to the rear pressure bulkhead, which had been damaged in 1978 in a tail strike incident. In both crashes, a doubler plate was not installed according to Boeing standards at the time.

China Airlines disputed much of the report, stating that investigators did not find the pieces of the aircraft that would prove the contents of the investigation report. One piece of evidence during the investigation that came up was the discolouration on the damaged section of the tail from photographs from maintenance records. The discolouration was determined to be tar stains from cigarette and cigar smoke. Smoking was permitted onboard China Airlines' flights until 1995. Had the stains been investigated, the cracks in the fuselage would have been discovered, and the plane might have avoided the fateful flight.