What happened
On October 27, 2016, an Air Tractor AT-401B, registration PT-WEB, was attempting to depart from Aeródromo Fazenda São João in Nova Granada, São Paulo, for an agricultural spraying mission. During the takeoff roll, the pilot noted that the engine was not performing normally. Believing there was insufficient runway remaining to abort the takeoff, the pilot continued the rotation. Shortly after liftoff, the aircraft lost altitude and struck an orange plantation approximately 150 meters beyond the end of the runway. The impact caused substantial damage to the aircraft and triggered a fire that partially consumed the airframe. The pilot sustained minor injuries and was the sole occupant of the aircraft.
The investigation
CENIPA investigators examined the engine, a Pratt & Whitney R-1340 radial piston engine, and found that the fuel quality was within legal specifications and showed no signs of contamination. However, technical analysis of the engine revealed a significant mechanical discrepancy: the external intake valve spring in cylinder number 4 had lost tension and length, a condition often referred to as a "tired spring." This defect prevented the valve from closing completely, leading to a loss of compression in that cylinder and a subsequent reduction in engine power. The investigation also noted that while the aircraft's maintenance records showed several extensions to the engine's overhaul interval, the engine's maintenance logs for the cell, engine, and propeller were not up to date.
Findings
- The primary cause of the power loss was a defective intake valve spring in cylinder number 4, which caused a loss of compression.
- The pilot elected to continue the takeoff despite detecting abnormal engine performance due to insufficient remaining runway for an abort.
- The engine had exceeded its recommended Time Between Overhaul (TBO) through authorized extensions, and the failure to perform a timely overhaul prevented the detection of the worn valve spring.
- There were deficiencies in the management supervision regarding the monitoring of engine performance and maintenance documentation.