What happened
On 24 May 2011, a Dassault Falcon 7X, registered HB-JFN, was performing a repositioning flight toward Kuala Lumpur, Malaysia. While descending through 13,000 ft, the aircraft's horizontal stabilizer unexpectedly moved from a neutral position to a maximum nose-up setting of 12 degrees in just 15 seconds.
As the stabilizer moved, the flight control laws attempted to compensate with nose-down elevator inputs, but the autopilot eventually disconnected. The aircraft's pitch attitude and load factor increased sharply. The pilot flying (PF) applied maximum nose-down sidestick input and set the throttles to the takeoff position, but the nose-up movement continued until the stabilizer reached its limit. During the event, the aircraft's airspeed dropped from 300 kt to 125 kt, and the load factor reached a peak of 4.6g.
In an attempt to regain control, the PF executed a maneuver that resulted in a bank angle of 98 degrees to the right. During this period, the pilot non-flying (PNF) provided conflicting control inputs, triggering a "DUAL INPUT" alarm. After a period of intense maneuvering and conflicting inputs from both crew members, the stabilizer unexpectedly began to return toward a level position. The crew elected to continue the flight in manual mode, eventually landing without further incident.
The investigation
The investigation, conducted by the BEA, focused on the mechanical and electronic causes of the uncommanded stabilizer movement. Investigators examined the Horizontal Stabilizer Electronic Control Unit (HSECU) and analyzed the production and maintenance history of the aircraft. The inquiry also reviewed the safety assessment methods used during the certification of the flight control systems and the effectiveness of crew training regarding non-coupled control sticks.
Findings
Technical analysis revealed that a soldering defect in the HSECU was the primary cause of the incident. Specifically, a simple brazing error led to undetected failures in the system. This hardware failure allowed the horizontal stabilizer to move to an undesirable position without the monitoring systems being able to prevent the runaway.
Safety action
Following the incident, the Falcon 7X fleet was temporarily grounded by an EASA Airworthiness Directive. To return the fleet to service, Dassault Aviation implemented several modifications, including the introduction of an additional monitoring function and a cockpit push-button to allow pilots to manually force the flight control system to switch to the back-up system. Additionally, Rockwell-Collins updated the manufacturing process for the HSECU to include X-ray examinations of circuit boards to detect similar cold solder joints.