Bell 407 N1197 incident near Eugene Island 182A

Probable cause

The improper repair of the outer combustion case, which led to its ultimate failure due to the continued progression of a fatigue crack and resulted in a partial loss of engine power during takeoff.

NTSB analysis

The commercial pilot was conducting an on-demand air taxi commuter flight in the helicopter. The pilot reported that, after the helicopter lifted off of the oil platform and entered a hover, the engine started to lose power. He nosed the helicopter forward to clear the platform, and the low-rotor speed horn sounded and the warning light illuminated. The pilot subsequently inflated the floats, leveled the helicopter, and landed it in the water. After shutting down the engine and securing the main rotor, the passengers and pilot safely exited the helicopter.

A postincident engine examination revealed a rupture in the brazed patch area of the outer combustion case (OCC). Metallurgical examination revealed a fatigue crack in the outer skin beneath and extending beyond the perimeter of the patch. The portion of the patch adjacent to the rupture had braze material nearly covering the mesh screen. The braze alloy had also flowed into the crack and covered portions of the fracture surface. The evidence is consistent with the patch being rebrazed after the crack had formed, likely in an attempt to repair the crack. In addition, the braze alloy composition in this area did not conform to the engineering drawing requirement.

The engine maintenance manual permitted welding to repair OCC cracking; however, it did not authorize brazing to repair OCC cracking. The maintenance records provided by the operator indicated that cracks had been repaired by welding about 3 years before the incident. The records did not include any subsequent entries related to the OCC, which precluded a determination of when or by whom the unauthorized braze repair had been attempted. Although the OCC was last inspected about 100 hours before the accident, it could not be determined whether the cracking would have been visible or concealed under the repair patch at that time.

Text from the NTSB. Open the NTSB docket for CEN14IA270 for the full record.