What happened
On August 1, 2021, a Twin Commander 681, registration PT-IEC, was performing a ferry flight from Porto Velho to Apuí, Brazil. The aircraft, operated by RIMA – Rio Madeira Aerotáxi Ltda, was carrying a single pilot.
During the landing roll on runway 09 at Pousada Pirá-Açu Aerodrome (SNLM), the aircraft experienced a sudden loss of directional control. The plane veered to the right, exiting the runway and coming to a stop in dense vegetation. The impact with the surrounding vegetation caused substantial damage to the fuselage, engines, propellers, and landing gear. The pilot was uninjured.
The investigation
CENIPA investigators examined the aircraft's engines and maintenance records to determine the cause of the loss of control. While the right engine functioned normally, the investigation of the left engine revealed that it was operating with an insufficient oil level.
Technical analysis of the left powerplant identified a deformed flange on the air/oil carbon seal assembly, which had caused an ongoing oil leak. Because the propeller-pitch control system relies on oil pressure to adjust blade angles, the low oil pressure prevented the left engine's propeller from entering the reverse pitch position. Consequently, while the right engine entered reverse to assist in braking, the left engine continued to produce forward thrust, creating a severe asymmetrical thrust condition that pulled the aircraft off the runway.
Findings
- The primary cause of the excursion was asymmetrical thrust resulting from the left engine's inability to enter reverse pitch.
- A deformed flange in the air/oil carbon seal assembly caused an oil leak that depleted the system's fluid levels.
- Maintenance oversight failed to identify or record the progressive oil loss/consumption prior to the accident.
- The pilot's reaction to the asymmetry may have been hindered by a lack of specific training or procedures in the aircraft's manual regarding reverse pitch failure during landing.