Incident Overview
On 9 January 1999, at 1600 UTC, a privately operated Yakovlev Yak-52 (registration RA44463) sustained damage during landing at RNAS Yeovilton, Somerset. The aircraft had been on the ground for approximately 10 minutes before departing for a practice forced landing. The pilot selected the landing gear down later than usual but believed the ‘three greens’ indication confirmed the gear was down and locked. However, upon touchdown, the nosewheel collapsed and the aircraft slid to a halt. No fire occurred, and the single pilot and single passenger evacuated without difficulty.
Aircraft Systems
The Yak-52 is equipped with a pneumatic system that powers engine starting, flap operation, landing gear extension and retraction, and wheel brakes. The engine can be started with the pneumatic system inadvertently switched off, using residual pressure stored in the system. Several previous incidents involving loss of brake function on the ground had been attributed to this design characteristic.
Previous Directives
In response to those incidents, the UK Civil Aviation Authority issued Mandatory Permit Directive MPD 1997-009 in 1997, requiring the embodiment of Aerostar Service Bulletin 7/95 on all such aircraft on the UK register. This modification isolates the pneumatic system to prevent engine start if the system is not switched on. The accident aircraft, however, was on the Russian Federation register and had not incorporated the service bulletin.
Investigation Findings
From the pilot’s report and discussions with the maintenance organisation, it was apparent that the engine had probably been started with the pneumatic system switched off. Sufficient residual pneumatic pressure remained in the system to retract the landing gear after takeoff. However, when the pilot later extended the landing gear, the main gear legs locked down under their own weight, but there was insufficient residual pneumatic pressure to lock the nosewheel down against the airflow, leading to its collapse on landing.
Damage
The propeller, spinner, engine, cowling, and nose landing gear were damaged. No injuries to occupants were reported.