Incident Overview
On an unspecified date, Express Net Airlines Flight 137 (XNA137), an Airbus A300 B4-203 (registration N372PC, serial number 0196), departed Dayton, Ohio, United States, on a cargo flight to Toronto, Ontario, Canada, with three crew members on board. At approximately 2130 eastern daylight time, while approaching Runway 05 at Toronto/Lester B. Pearson International Airport, the flight crew disconnected the autopilot and the aircraft began to roll right. The roll was corrected with left rudder input, and the aircraft landed without further incident. After parking, ground crew informed the flight crew that the left centre inboard flap tab was missing. The separated flap tab was found in a retail parking lot at 2900 Argentia Road in Mississauga, Ontario, approximately 13 kilometres (7 nautical miles) from the airport on the Runway 05 approach path. It had struck and significantly damaged an unoccupied parked vehicle.
Flight Crew Information
The pilot-in-command (PIC), seated in the left seat, held a valid United States airline transport pilot licence and a Class 1 medical certificate issued on 16 February 2006. As of 01 May 2006, he had accumulated approximately 4681 total flying hours, with 381 hours on type. The second-in-command (SIC), seated in the right seat, held a valid United States airline transport pilot licence and a Class 1 medical certificate issued on 16 September 2005. As of 01 May 2006, she had accumulated approximately 6000 total flying hours, with 2655 hours on type.
Weather Conditions
At the time of the incident, the weather was reported as visibility miles, wind 100° magnetic at 9 knots, temperature 18°C, dew point 15°C, and atmospheric pressure 30.20 inches of mercury.
Sequence of Events
The flight data recorder (FDR) and cockpit voice recorder (CVR) were analyzed by the Transportation Safety Board of Canada (TSB) Engineering Laboratory. No anomalies were noted during takeoff, climb, and cruise. At 2107:42 eastern daylight time, the aircraft began descent from flight level 290. At 2125:12, the flap lever was moved to the flap 10 position at 5400 feet above sea level (asl) and 183 knots. At 2125:48, flaps were selected to 15° at 4850 feet asl and 182 knots. At 2129:15, flaps were selected to 25° at 2550 feet asl and 170 knots. At 2129:19, aileron and rudder movement increased significantly, estimated as the time of flap tab separation, at approximately 2450 feet asl, 170 knots, and 7 nautical miles from the runway threshold. Less than one minute later, at 2130:06.5, the autopilot was disengaged, heading began to increase right, and left rudder was applied to correct. The approach remained stable, and the aircraft touched down at 2132:15.25 with a maximum of 1.8 g.
Flap Tab and Hinge Bracket Examination
The left centre inboard flap tab, part of the trailing edge flap system, is approximately 2.8 m (112 inches) long, 0.6 m (27 inches) wide, and weighs about 16 kg (35 pounds). The main structural attachment, the flap tab hinge bracket, was located mid-span on the bottom of the flap tab near the leading edge. Attachment used PRC-type sealant, rivets, and bolts, with a doubler plate covering the bottom skin around the hinge bracket. The hinge bracket failed just below the bottom level of the doubler plate in line with the flap tab skin. The inboard rod link was bent and failed in overload; the outboard lever assembly was bent and its attachment ball bearing pulled out.
Cause of Failure
The hinge bracket failed due to low cycle fatigue originating at impact damage that occurred either before or during installation. This damage created a stress riser that developed into a fatigue crack over the operating life of the bracket. The damage was in a protected area after installation, making pre-installation damage likely unnoticed and post-installation detection very difficult. No abnormalities were noted in previous recorded flights or service history indicating problems before the incident.
Related Occurrences and Risk
Airbus identified one additional failure of this nature in a fleet of 240 aircraft incorporating this type of hinge bracket. Of the 240 aircraft, only 17 were reported to have the improved hinge bracket, and only 7 remain in service worldwide as of May 2006. The investigation concluded that although another failure is possible, there were no adverse effects on flying characteristics; however, debris poses a risk to populated areas.
Safety Action
Airbus is developing a repetitive inspection program (planned Inspection Service Bulletin 57-0250) for the hinge bracket and will consider further modifications based on inspection results. The TSB released this report on 23 October 2007.