History of the Flight
The aircraft, a Diamond DA 42 NG Twin Star registered G-SELC, was conducting an Instrument Rating training flight on 28 September 2012. The flight included general handling and circuit practice. The crew consisted of an instructor in the right front seat, a student in the left front seat, and an additional student in the rear as an observer. After three acceptable touch-and-go exercises on Runway 22L, the aircraft performed a landing. Pre-landing checks were completed, with both the instructor and student verifying that three green lights illuminated, indicating all landing gears were down and locked. The green lights were rechecked as landing flap was selected on final approach. Touchdown occurred on the mainwheels just before the runway identification markings, and the control column was held back before lowering the nosewheel onto the runway. Approximately three seconds after the nosewheel contacted the runway, the nose landing gear collapsed, causing the propellers to contact the runway surface. The aircraft came to a halt in a straight line near the runway centreline. After informing the control tower, all systems were shut down, and the three occupants vacated the aircraft normally. The instructor reported that everything had appeared normal and all three green lights had been illuminated and checked twice, leaving him unable to explain the collapse.
Aircraft Information
The Diamond DA 42 Twin Star is a twin-engine aircraft with fully retractable tricycle landing gear. The nose gear retracts forward, and the main gears retract inward. The landing gear is operated by hydraulic actuators powered by an electro-hydraulic pump. A manual 'free-fall' emergency system is provided for lowering the gear if the normal system fails. In addition to the three green indication lights, a red GEAR UNSAFE indication illuminates when the gear is selected down but one or more legs are not locked. A warning horn sounds if any gear is not locked down and either landing flap is selected or the engines are at very low power. The aircraft was manufactured in 2005 with serial number 42.032.
Post-Incident Examination
After the incident, staff from the maintenance organisation attended to recover the aircraft. Several people pulled down on the rear fuselage to raise the nose off the ground. It was then observed that the nose gear leg dropped down freely and went into lock. The aircraft was towed back to the hangar and placed on jacks. Approximately thirty retraction/extension cycles were performed with no malfunctions of the system or the associated warning. Maintenance company staff noted that had there been pressure in the down hydraulic line to the nose gear and landing loads reacted by the retraction/extension system instead of the downlock mechanism, damage to the mechanism would have been expected; however, no such damage was found. The investigation could find no fault with the retraction/extension or gear warning systems.