Incident Description
On 14 September 2015, an Embraer 170 was on approach to landing at LFPG when, at approximately 1,400 feet, the autopilot disconnected uncommanded. The flight crew encountered difficulty manually controlling the aircraft's pitch. The approach was continued and landing was completed. Following the flight, a technical log entry was made and an ASR report was filed.
Investigation Findings
The investigation revealed that on 7 September 2015, maintenance work had been performed on the horizontal stabilizer. On 10 September 2015, two autopilot disconnections had occurred during a different flight on the same aircraft. The flight crew reported that after autopilot disconnection, the elevator trim operated normally, but control forces on both control wheels were approximately three times greater than normal. During a walk-around inspection, abnormal crackling sounds were heard from the right side when the elevator was moved.
Analysis of flight data recorder parameters indicated that during the approach, opposing forces were applied to the control wheels. Such behavior is typical when one control is obstructed, for example by a seat cushion. The investigation identified that the day before the incident, the captain's seat cushion had been replaced due to poor attachment. No other causes were identified.
Probable Cause
The most probable cause of the autopilot disconnection was increased control wheel forces resulting from improper attachment of the pilot's seat cushion.
Preventive Measures
The operator's technical services took several actions: they directed a search for maintenance records to verify if other work had been performed on the aircraft; they checked that the maintenance organization had correctly performed all tasks according to documentation; they reviewed fault codes stored in the Fault History Data Base (FHDB); they checked available service bulletins related to the issue; they reported the event to the aircraft manufacturer via the E-Track system; and they determined whether additional fleet-wide inspections or information bulletins were needed. The event was recorded in the operator's Safety Management System (SMS) database and classified according to established criteria for inclusion in safety performance indicators.
Safety Recommendations
No safety recommendations were formulated by the investigation commission.