What happened
On March 18, 2002, a Sikorsky S76B helicopter, registration PH-NZU, was conducting a passenger flight toward an offshore drilling platform near Den Helder Airport. This flight was the first operation following a scheduled maintenance interval. After a standard walkaround inspection and a routine takeoff, the aircraft reached a cruising altitude of 2,000 feet. While flying approximately 1.5 nautical miles offshore, the crew noticed a sudden and significant increase in vibration levels. Despite the vibration, the aircraft's flight controls remained responsive, no warning lights illuminated, and all engine and system indicators remained within normal parameters.
Suspecting a mechanical issue, the crew decided to return to Den Helder. The return was conducted via a standard visual approach without an emergency declaration. During the hover phase while approaching the platform, the captain briefly took control to test the stability of the aircraft but felt no abnormal control forces. Once the rotors were shut down, a physical inspection revealed significant damage to one of the four rotor blades. The blade featured a hole in the leading edge and the underside, torn skin plating, and a broken trailing edge.
The investigation
The investigation focused on identifying the source of the impact that caused the rotor blade damage. The damaged blade was sent to the manufacturer, Sikorsky, for detailed analysis. Investigators ruled out external factors such as bird strikes, ballistic impacts, meteorological conditions, or debris from other aircraft.
Following the incident, the operator discovered that two wrenches used during a recent 500-hour inspection had gone missing. The investigation established that these tools were used during the maintenance of the rotor blades. It was noted that the mechanic had a habit of temporarily placing tools on the rotor head. During the maintenance process, a misunderstanding occurred between the mechanic and the supervisor; the supervisor signed off on the inspection believing the work was complete, while the mechanic believed the supervisor would finish the task. This led to the suspicion that the wrenches were left on the rotor head, where they were struck by the blade during engine start-up, causing the structural damage and subsequent vibration.
Findings
- The primary cause of the vibration was the structural damage to the rotor blade, specifically the tearing of the skin plating and the hole in the leading edge.
- The wrenches left on the rotor head during engine start-up struck the blade, causing the initial damage.
- A breakdown in communication and oversight during the maintenance sign-off process allowed the tools to remain on the aircraft.
- The pre-flight inspection performed before this specific flight was limited to a walkaround and a logbook review, which failed to detect the internal or localized damage caused by the tools.