What happened
On March 23, 2017, an ATR-72-212A, registration PP-PTQ, operated by Passaredo Transportes Aéreos, was performing an Instrument Landing System (ILS) approach to runway 09R at André Franco Montoro International Airport (SBGR) in Guarulhos, Brazil. While intercepting the locator with the autopilot engaged, the aircraft exhibited a persistent tendency to continue a left turn.
The flight crew disconnected the autopilot to regain control, but the leftward tendency continued, causing a lateral deviation from the localizer axis. This prompted the crew to execute a go-around procedure. The crew subsequently performed a visual traffic circuit for landing, utilizing right-hand turns without the assistance of the autopilot. The aircraft landed successfully on runway 09R with minor damage to the aircraft, and all 59 occupants—including 55 passengers and 4 crewmembers—remained unharmed.
The investigation
CENIPA investigators examined the aircraft's aileron command system and discovered that the movement of the control wheel was restricted. The investigation focused on the servo drive of the aileron command through the autopilot and its associated cabling.
Technical analysis revealed that a specific control cable (PN S2211001100000) had broken. The broken strands of the cable became entangled in the drum of the autopilot servo actuator. This entanglement prevented the capstan from rotating, which in turn restricted the movement of the aileron control surfaces. Further examination using scanning electron microscopy (SEM) and stereoscopic equipment determined that the failure was caused by an ongoing fatigue process within the cable.
Findings
- The primary cause of the flight command malfunction was the fatigue failure of the control cable connecting the autopilot servo actuator to the aileron command bellcrank.
- The broken cable strands became entangled in the servo actuator drum, which physically restricted aileron movement and triggered the AILERON MISTRIM alarm.
- While the aircraft's maintenance records were up to date and all scheduled inspections had been performed, the manufacturer's existing inspection criteria were insufficient to detect the internal deterioration of the cable.
- There were no established flight hour or cycle limits for the replacement of this specific control cable component.