What happened
On November 22, 2002, an S-76A helicopter, registration PT-HUI, was performing a local maintenance verification flight departing from Macaé, RJ. Approximately 15 minutes after takeoff, the crew heard a loud bang, followed by severe vibrations and a sudden left-hand nose pitch. The aircraft entered an uncontrolled descent and the pilots performed a forced landing on uneven terrain approximately six miles from the runway.
Following the impact, the aircraft caught fire and was completely destroyed. The two crew members sustained minor injuries and were able to abandon the aircraft before the fire consumed it.
The investigation
CENIPA's investigation focused on the mechanical failure and the operational circumstances leading to the flight. Investigators examined the maintenance history, noting that the aircraft had recently undergone oil and filter changes following the activation of an engine chip light. The investigation also reviewed the cockpit voice recorder (CVR) and maintenance logs.
Technical analysis revealed that the incident was triggered by a manufacturing defect in the PTO gear. This defect allowed a crack to propagate, leading to the rupture of the gear's central region. This failure subsequently caused the collapse of turbine wheel 4 and subsequent blade shedding.
Findings
- Manufacturing defect: A production flaw in the PTO gear caused a structural failure that led to the engine's disintegration.
- Operational deviations: The maintenance flight was not required by the manufacturer's manual; the standard procedure called for a 30-minute ground run rather than a one-hour flight.
- Reduced situational awareness: The crew was reportedly focused on external references to take photographs during the flight, which may have delayed their response to the initial symptoms of the failure.
- Inadequate supervision: Maintenance procedures performed by the operator deviated from the manufacturer's recommended practices.
Safety action
Following the investigation, several safety recommendations were issued:
- To the operator, to establish better supervision of maintenance services to ensure compliance with manufacturer manuals.
- To helicopter operators, to implement mandatory briefings between pilots and mechanics before and after maintenance flights to standardize procedures and identify risks.
- To ANAC, to disseminate the lessons learned from this investigation to offshore helicopter operators, specifically regarding the risks of deviating from maintenance manuals.