What happened
On August 17, 2017, a Twin Commander 50/B (registration PT-KUK) was conducting a marine wildlife observation flight over the Atlantic Ocean, departing from Aeródromo Bartolomeu Lisandro in Campos, Rio de Janeiro. The aircraft was carrying one pilot and five passengers.
During the cruise phase of the flight, the entire propeller assembly from the left engine detached from the flange and fell into the ocean. The propeller did not collide with any other part of the aircraft during its separation. Following the incident, the pilot successfully operated the aircraft in a single-engine configuration and returned to the departure airfield to land safely. All six occupants remained uninjured, though the aircraft sustained light damage.
The investigation
CENIPA investigators examined the remains of the mounting bolts from the left engine. Laboratory analysis conducted by the Department of Science and Technology of the Air Force (DCTCA) revealed that all analyzed bolts had failed due to fatigue. Specifically, in five of the bolts, the fatigue cracks originated at stress concentrators located at the radii between the bolt heads and the bodies.
To assess the quality of previous maintenance, investigators also inspected the right engine's propeller assembly. This inspection revealed that the required shim (part number B-1322) was missing between the engine flange and the propeller hub, and the mounting bolts were torqued below the required specifications. Because both propellers had been installed during the same maintenance event in August 2013, investigators concluded that the left engine likely suffered from the same installation error.
Findings
- The primary cause of the propeller separation was the fatigue failure of the mounting bolts.
- The failure was likely triggered by the improper installation of a shim (PN B-1322) between the flange and the propeller hub, a condition specifically warned against in the Hartzell propeller maintenance manual.
- Maintenance of the aircraft contributed to the incident.
- The maintenance company responsible for the installation had its certification revoked by ANAC in 2014, preventing a full audit of the specific work performed on this aircraft.