What happened
On 22 August 2003, a Yak-50, registration G-YAKK, was undergoing engine start and taxi preparations at North Weald Airfield, Essex. The aircraft was being prepared for a scheduled aerobatic display. While the pilot was taxiing slowly toward a fuel bowser, a pneumatic system reservoir, which was pressurized to approximately 711 psi, suddenly burst.
The force of the rupture caused the engine throttle to accelerate unexpectedly. As the aircraft surged forward, it pitched nose-down, causing the propeller to strike the concrete surface. The impact shattered the propeller blades and caused extensive damage to the forward fuselage, the engine mount, and the fuel tank. The pilot was unhurt and managed to shut off the engine, though bystanders noted a small fuel leak following the event.
The investigation
Investigators examined the wreckage and found that the main pneumatic reservoir had split into large fragments. The energy from the burst was sufficient to punch holes in the fuselage sidewall and deform the aircraft's fuel tank. The investigation focused on the cause of the reservoir's structural failure and the maintenance history of the pneumatic system.
Analysis of the recovered reservoir fragments revealed severe internal corrosion. Deep pitting was present throughout the interior, with some pits extending nearly to the outer wall. The investigation also looked into the maintenance procedures for the aircraft's pneumatic system, noting that while water traps were drained, the reservoirs themselves were subject to a five-year inspection interval.
Findings
- The primary cause of the accident was the fracturing of the pneumatic reservoir due to severe internal corrosion.
- Water had pooled at the bottom of the reservoir, leading to significant thinning of the steel wall.
- The failure occurred at normal operating pressure because the structural integrity of the vessel had been compromised by deep pitting.
- Existing maintenance procedures were inadequate, as draining the water trap did not remove moisture trapped within the reservoirs.
- There were no established industry standards for the inspection or corrosion protection of these specific types of bottles on the UK register.
- Previous failures of similar components had been relatively minor (pinhole leaks), which may have led to an underestimation of the risk of a catastrophic burst.
Safety action
Following the investigation, the CAA issued a Letter to Operators to alert owners of similar aircraft types to the risk of undetected internal corrosion. The CAA also worked to specify more rigorous maintenance schedules, including more frequent inspections and proof-pressure testing, to prevent similar reservoir failures.