History of Flight
On 22 August 2003 at approximately 1430 UTC, a Yak-50 (registration G-YAKK) was being prepared for departure from North Weald Airfield, Essex, for a formation aerobatic display. The pilot started the engine and noted normal pneumatic system pressures. About eight minutes after engine start, he reduced throttle to idle and released the brakes to taxi forward. As the aircraft moved slowly, a loud bang was heard, the engine accelerated to high speed, the aircraft pitched nose down, and the propeller contacted the concrete, shattering. The pilot shut off the magnetos; bystanders saw a small fuel leak, which stopped when the pilot used the fuel cut-off. The aircraft was tipped back onto its tailwheel, and the pilot vacated unhurt. He later participated in the display in another Yak-50.
Aircraft Description
The Yak-50 is a low-wing, single-seat monoplane with tailwheel landing gear, powered by a radial engine and wooden variable-pitch propeller. A pneumatic system provides engine starting, landing gear retraction, and wheelbrakes, with a nominal operating pressure of 50 kg/sq cm (711 psi). The main reservoir is a spherical steel bottle (9.1 inch diameter, 6.4 litre capacity) mounted on the aft side of the engine bay firewall. A pressure relief valve (PRV) is set nominally at 55 kg/sq cm (782 psi) according to a UK overhauler.
Aircraft Examination
Examination showed the main reservoir had burst, with parts of the bottle causing structural and systems damage. A portion struck the right fuel tank, severing pneumatic and fuel lines and punching holes in the fuselage. Another portion tore a hole in the lower left fuselage. The throttle cable sheath was deformed, opening the throttle fully. Rust-like discoloration was noted on the apron, and the left main tyre showed signs of rust-contaminated water. The two major bottle portions were recovered; internal surfaces were covered with rust and deep corrosion pitting, with some pits extending nearly through the wall. The PRV was tested and released at 55 kg/sq cm.
Maintenance System
G-YAKK, imported in 1999 after overhaul by Smolensk Air Services, was maintained under the CAA Light Aircraft Maintenance Schedule (LAMS). Manufacturer's requirements included draining the water trap after each flight, removing reservoirs every 50 hours for water drainage, and every five years for pressure testing and inspection. The pressure check was at 1.5 times operating pressure (75 kg/sq cm). Internal inspection was limited to visual examination through ports, with no published rejection criteria. Some maintainers used lubricating oil for the pressure check, expecting a corrosion-inhibiting layer.
Conclusion
The pneumatic reservoir burst due to severe internal corrosion resulting from the presence of water and the absence of effective surface protection. The accident highlighted issues with water draining procedures, lack of inspection standards, and inappropriate five-year intervals for internal inspection.