What happened
On December 17, 2015, a SOCATA TBM 700 A was performing a short cross-country flight from Liège Airport to Genk/Zwartberg. During the approach, the pilot extended the landing gear and confirmed via the cockpit indicators that all three gears were down and locked, noting three green lights and no red warning lights.
Upon touchdown at the Zwartberg airfield, the aircraft experienced an abnormal vibration and noise. The nose landing gear immediately collapsed as the aircraft made contact with the runway. The pilot was uninjured, but the aircraft sustained damage to the propeller blades, the engine air intake cowling, and the nose landing gear doors. The aircraft came to a stop after a 400-meter run.
The investigation
The AAIU(Be) investigation focused on why the nose landing gear failed to remain locked despite the cockpit indications suggesting a successful extension. Investigators examined the landing gear control and indication system, specifically the interaction between the dual switches and the actuator.
Technical analysis revealed that a spurious triggering of the nose landing gear "extend" dual switch occurred. This malfunction caused the system to provide a false indication of "extended and locked" on the Landing Gear Control Panel (LGCP) while simultaneously halting the extension process of the gear leg before it had actually reached the locked position.
Findings
- The primary cause was the failure of the nose landing gear actuator to lock down, paired with a control system that incorrectly indicated the gear was properly secured.
- A mechanical improvement involving differential plungers (MOD70-0334-32), which could have prevented the issue, had not been installed on this specific aircraft.
- The availability of this safety improvement was not effectively communicated or mandated to all end-users.
Safety action
- The investigation identified a safety issue regarding the lack of a mandate for the installation of improved actuators with differential plungers.
- A recommendation was made to EASA to mandate the improvement of switch kinematics using hydraulic pressure to ensure more reliable gear locking.
- A recommendation was also issued to the manufacturer, Daher-Socata, to explore further design alternatives, such as a time-delay relay, to ensure the electro-hydraulic generator remains operational long enough to guarantee all gear legs are fully locked even in the event of a switch malfunction.