A Boeing 737-229, VH-OBN, was flying on a scheduled passenger flight from Brisbane to Norfolk Island.
VH-OBN 2 days before the incident flight ©Annette
The flight was uneventful and the crew became visual with the runway at Norfolk Island, 2 NM (3,6 km) from the runway, but were not aligned with the runway, A visual circling approach was flown over the ocean to better align the aircraft with the runway. On the second approach, visibility was too bad to continue the approach and a missed approach was executed.
During the missed approach while the crew reconfigured the aircraft a high-frequency vibration was felt through the aircraft and the control yoke deflected up to 40° left, at which point constant buffeting and uncommanded roll and yaw oscillations occurred. With the absence of any warnings the crew was unaware of what was going on, the only thing they knew, was that controlled flight was difficult to maintain. Boeing 737-200 wing flaps and slats
The cabin crew reported shaking and
vibration similar to flight in turbulence.
The Captain asked the purser to check the wings for anomalies through the cabin windows and report back to him. With the purser walking down the cabin a passenger reported to the purser that a slat on the leading edge of the right-wing was protruding in an abnormal way. The purser took a picture of the wing and slat to show to the flight crew and reported back to the flight crew that the number 4 slat on the right wing was at an abnormal angle.
By cycling the flaps and slats the #4 slat retracted a small amount, which improved the aircraft controllability. As the crew was not sure if they could reach their alternate due to the increased drag (Norfolk Island weather was below limits) or that they could land the aircraft in a controlled manner the captain ordered the Purser to prepare the cabin for a ditching. The crew had declared "PAN PAN" as they were unsure if they had fuel enough to reach their alternate Nouméa Airport. As they were in the cruise to the alternate the crew discussed what to do, and what checklist to use. Fuel burn was not as bad as expected and an approach to Nouméa could be flown. The crew decided to use the LEADING EDGE FLAPS TRANSIT non-normal checklist, According to this checklist, the recommended flap setting was 15. The crew performed a controllability check around 7000 feet after adding extra speed to the advised approach speed.
An uneventful landing at Nouméa completed the flight.
Inboard auxiliary track showing Main Slat Track fatigue failure failed bolt hole
An engineering inspection determined that the number-4 leading edge slat, the inboard main track had failed. An examination of the failed track identified fatigue cracking that originated at the intersection of diverging machining marks at the fracture site. Further inspection of the number-4 slat found corrosion damage on the outboard auxiliary track, with the inboard auxiliary track adjacent to the failed main track having failed in overload at the slat attachment The full report of the investigation by the ATSB can be found by clicking here.