What happened
On March 9, 2014, a Neiva EMB-721C, registration PT-EII, was conducting a private transport flight from Juína to Aripuanã, Mato Grosso, carrying one pilot and five passengers. While cruising at 4,500 feet, approximately 30 minutes into the flight, the aircraft experienced intense vibrations and a sudden loss of engine thrust.
As the engine RPM began to drop, the pilot performed the necessary emergency procedures, including feathering the propeller and shutting down the engine. During the subsequent emergency descent and landing in an unprepared field at Fazenda Barra Bonita, it was discovered that the tip of one propeller blade had broken off. The aircraft sustained substantial damage. While four passengers were uninjured, the pilot and one passenger sustained minor injuries.
The investigation
CENIPA investigators examined the engine, propeller hub, and governor. Tests conducted at an ANAC-certified workshop confirmed that the propeller hub and governor were functioning according to manufacturer specifications, with no internal or external leaks or operational abnormalities detected. The investigation focused on the structural integrity of the Hartzell propeller (model HC-C2YK-1BF).
A failure analysis performed by the Department of Aerospace Technology (DCTA) focused on the fractured blade. The analysis revealed that the propeller surface had previously undergone sanding, likely an attempt to remove corrosion pits and nicks caused by impacts from stones or tools. However, this sanding process failed to completely eliminate these surface defects.
Findings
- The propeller blade fracture was caused by the propagation of two fatigue cracks, which reduced the effective cross-section of the blade until it could no longer withstand operational stresses.
- The fatigue cracks originated at two specific surface nicks (mossas) that remained on the blade after previous maintenance sanding.
- Inadequate aircraft maintenance was identified as the contributing factor, specifically the failure to properly remediate surface defects during the sanding process.