What happened
On 9 January 1999, a Yak-52, registration RA44463, was conducting a private flight at RNAS Yeovilton, Somerset. The aircraft had been stationary on the ground for approximately ten minutes prior to departure, as the pilot intended to perform a practice forced landing. During the approach, the pilot extended the landing gear later than usual but believed the three green lights indicating the gear was down and locked were visible. Upon touchdown, the nosewheel collapsed, causing the aircraft to slide to a halt. There were no injuries to the pilot or the passenger, though the aircraft sustained damage to the propeller, spinner, engine, cowling, and nose landing gear.
The investigation
The investigation focused on the operation of the aircraft's pneumatic system, which powers the engine starting mechanism, flaps, landing gear, and wheel brakes. It was determined that the engine had likely been started while the pneumatic system was switched OFF. Because of the pressure stored in the pneumatic reservoir, the engine could still be started despite the system being inactive.
While the main landing gear legs locked into position due to their own weight, there was insufficient residual pressure remaining in the system to lock the nosewheel against the force of the airflow. The investigation also noted that while a mandatory directive existed to modify the system to prevent engine starts without the pneumatic system being active, this specific aircraft was registered in the Russian Federation and had not undergone the required modification.