What happened
On March 21, 2017, an ATR-42-500, registration PR-TTH, operated by Total Airlines S/A, was performing a transfer flight from Manaus to Urucu Aerodrome. During the descent, the crew noted an increase in the propeller rotation of the number two engine (right side).
Upon landing on runway 09 at UruCT, the aircraft experienced an asymmetric power condition while the crew engaged the reverse thrust system. This asymmetry caused the aircraft to yaw strongly to the left, resulting in a runway excursion. The aircraft traveled approximately 180 meters through the grass, exiting the left side of the runway about 500 meters after the touchdown point. The aircraft sustained substantial damage, including significant impact to the nose, main landing gear, and fuselage, but all four crew members escaped without injury.
The investigation
CENIPA's investigation focused on the mechanical and electrical state of the propeller control systems. Investigators examined the Microswitch Box and identified discrepancies suggesting the unit had been opened in an unauthorized location, indicating potentially substandard maintenance.
Testing revealed that while the Microswitch Box passed continuity tests, there were irregularities in the activation angle of the 4KX Microswitch, which identifies the right engine's power lever position. Furthermore, investigators found evidence of an aircraft grounding condition that could have caused an intermittent electrical failure in the Low Pitch protection system. This condition likely prevented the Np Cancel function from working, which is designed to limit propeller rotation during a reverse failure to minimize yawing moments.
Findings
- Asymmetric reverse thrust was the primary cause of the directional control loss.
- A lack of attention to the Low Pitch warning lights contributed to the crew applying reverse thrust on both engines while only one light was illuminated.
- Potential unauthorized maintenance on the Microswitch Box may have introduced grounding issues affecting the electrical components and the Np Cancel function.
- The crew's performance may have been influenced by apprehension regarding the runway length during a night operation in rainy conditions.