What happened
On December 5, 2008, an AS-300B2 helicopter, registration PR-EBQ, operated by the Department of Transit of the Federal District (DETRAN-DF), was performing a traffic monitoring flight over the Guará I area in Brasília, Brazil. While in flight, the crew heard a loud noise and discovered that the left rear sliding door had detached from the aircraft structure and fallen.
The crew was unable to locate where the door landed. Following the event, the crew performed a check of the flight parameters, found them to be normal, and successfully returned the aircraft to the Fire Department's "Resgate" Base, where it landed without further incident. There were no fatalities and no injuries among the two crew members and two passengers. The aircraft sustained minor damage to the door and the surrounding structure.
The investigation
CENIPA's investigation focused on the mechanical integrity of the door attachment mechanism. The investigation examined the maintenance history of the aircraft, specifically the recent 100-hour inspection performed in November 2008. Investigators analyzed the procedures for the removal and installation of the sliding doors as outlined in the manufacturer's maintenance manual (MET).
Technical analysis revealed that the door's lower shoe is secured via a bolt and a specific self-locking nut (PN ASN52320BH060N). The investigation found that grease had been applied to the bolt's spherical end. While the grease was not on the threads themselves, the presence of any lubricant in this area was considered an improper maintenance practice. Furthermore, the investigation looked into the reuse of self-locking nuts, noting that the manufacturer's manual prohibits the reuse of certain types of self-locking nuts with integrated locking elements.
Findings
- The primary cause of the door detachment was likely inadequate maintenance involving the failure to follow proper criteria for the reuse of the self-locking nut.
- The investigation identified a high probability that the self-locking nut was improperly reused, similar to an occurrence previously noted on the aircraft's right-side door.
- Inadequate maintenance supervision likely allowed the failure to occur.
- The application of grease to the component, while not directly on the threads, represented an improper maintenance process that could lead to premature wear or facilitate loosening.
- The co-pilot was operating with an expired helicopter rating (CHT) since May 2008.