What happened
On 28 March 2024, a Boeing 737-800, registration EC-OBO, was operating an international passenger flight from Zurich, Switzerland, to Madrid, Spain. While the aircraft was flying over the Santo Domingo housing estate in Algete, a component from the No. 2 main landing gear wheel—specifically a torque bar—detached from the assembly and fell onto the roof of a private house, causing material damage. The aircraft continued its flight and landed safely at Adolfo Suárez Madrid-Barajas Airport.
Following the incident, the operator discovered that the wheel was missing three torque bars in total. One part was later recovered at Palma de Mallorca airport, while a third remains unlocated. The crew and subsequent flights did not detect the loss of the component during the flight or immediately upon landing.
The investigation
The CIAIAC investigation examined the condition of the aircraft's main landing gear, specifically wheel No. 2 and its brake assembly. The investigation included a detailed inspection of the hardware by the manufacturer, Safran Landing Systems.
Investigators analyzed maintenance records, noting that the wheel had undergone a recent overhaul by SR Technics in February 2023 and several tyre changes. The investigation also looked into subsequent similar incidents involving another aircraft, EC-OBK, which experienced two separate events involving loose torque bars.
Technical examinations of the remaining hardware revealed circular friction marks on the bolt heads and matting areas on the bolt shanks. These findings suggested that the assembly had been rotating, likely due to the torque being applied to the bolt head rather than the nut during maintenance, or due to a loss of pretension.
Findings
- The loss of three torque bars from the No. 2 wheel of EC-OBO was confirmed.
- Loss of tightening torque on the No. 2 wheel torque bar fastenings was the most likely cause of the component detaching.
- It could not be definitively determined if the loss of torque resulted from improper tightening during overhaul/maintenance or from improper breakaway torque of the nut.
- Inspection of the remaining hardware showed that the nuts failed the required breakaway torque check.
- The investigation noted that the maintenance organization SEMASA had issued an internal notice to ensure mechanics apply torque to the nut rather than the bolt head.