What happened
On April 9, 2000, a Bell 407 helicopter, registration PT-YSL, departed from the Maresias helipad bound for Congonhas Airport. Approximately 25 minutes into the flight, the aircraft began experiencing severe vibrations. The pilot performed an emergency technical landing in a clearing to conduct a visual inspection, which revealed no evidence of bird strikes.
To avoid flying over the city of São Paulo under unstable conditions, the pilot elected to divert to Campo de Marte Airport, maintaining a lower airspeed and controlled altitude. During this transit, the vibrations persisted with intermittent peaks. Upon landing at Campo de Marte, a second visual inspection was performed, but no abnormalities were immediately visible. The following morning, a detailed inspection by maintenance technicians discovered a delaminated area on the lower surface of one of the main rotor blades.
The investigation
The investigation focused on the structural integrity of the main rotor blade, which had only 345 hours and 30 minutes of total time. The affected blade was sent to Bell Helicopter – Textron in the United States for forensic analysis.
Microscopic examination using Scanning Electron Microscopy (SEM) revealed that the separation had occurred between the composite skin and the honeycomb core. The analysis identified traces of adhesive being projected and thin walls where the honeycomb had made insufficient contact with the surface. Specifically, testing of a specimen from the leading edge of the delaminated area showed a failure strength of only 225 pounds, significantly below the required 400-pound threshold. This indicated that the delamination was expanding over time.
Investigators noted that while temperature and pressure logs for the curing process were available, they were not being attached to the blade's manufacturing records. The investigation also noted a significant delay in the reporting of the incident, which was not communicated to the aeronautical authorities until nine days after the occurrence.
Findings
- Manufacturing deficiency: The primary cause of the incident was a latent manufacturing defect. Inadequate control of temperature and pressure during the resin curing process led to improper adhesion between the lower surface skin and the internal honeycomb structure.
- Inadequate detection: Standard post-manufacturing tests, including hammer testing and bright light inspections, failed to detect the internal delamination.
- Reporting delay: The incident was not reported to the investigating authorities until nine days after the event, hindering the timely implementation of corrective measures.