What happened
On 6 January 2024, a Boeing 747-433 cargo aircraft, registration G-UNET, landed at London Heathrow Airport. Following touchdown, a large metal panel from the upper left wing detached from the aircraft. The flight crew was unaware of the separation during the flight, and the loss was only identified during the subsequent turnaround process at the stand.
The detached component, measuring approximately 1.1 m by 0.9 m, was not discovered until 7 March 2024, when it was found in a grass area south of Runway 27L, roughly 600 m west of the runway threshold. The delay in discovery was attributed to less frequent grass mowing during the winter months.
The investigation
The AAIB investigation focused on the structural failure of the panel and the circumstances surrounding its detachment. Examination of the panel revealed a structural failure at its forward edge, exposing the internal honeycomb, and a failure at its inboard rib, which showed evidence of previous repairs.
Investigators reviewed a Service Letter (SL 747-SL-57-101-B) issued by the aircraft manufacturer. This document addressed known issues with fatigue cracking in the fixed trailing edge ribs, which could lead to the departure of wing panels. While the manufacturer's analysis suggested that such a detachment poses a low risk to the continued safe flight or occupants of the aircraft, the primary concern is the creation of runway foreign object debris (FOD).
At Heathrow, while a FOD radar system was active, it is designed to scan runway surfaces and did not detect the panel because it came to rest in the grass. Furthermore, the airport's Surface Movement Radar lacked the sensitivity required to identify a component of this size.
Findings
- The left wing panel detached from G-UNET after touchdown, likely triggered by the deployment of spoilers.
- The primary cause of the detachment was fatigue cracking of the supporting rib.
- Although the manufacturer had provided guidance via a Service Letter to mitigate such cracking, there was no evidence that the specific recommended actions had been performed on this aircraft.
- The detachment of such a panel is considered a low-probability event for causing in-flight emergencies or injuries, but it presents a significant risk of creating runway FOD, which could damage other aircraft.