What happened
On 9 January 2009, a Eurocopter AS350 B3 helicopter, registration SE-JKF, was performing aerial work near Stockholm/Arlanda airport when it experienced a catastrophic mechanical failure. The flight was part of a two-helicopter formation intended to transport passengers for a taxi flight.
As the pilot began reducing speed for landing, the chip warning system for the tail rotor gearbox activated. Shortly after, the pilot noted unusual vibrations and a persistent tendency for the aircraft to yaw to the left. Despite applying full right pedal deflection, the pilot could not maintain the heading and the aircraft began to deviate significantly from its path.
Unable to find a suitable landing site and facing an uncontrolled yaw, the pilot attempted an emergency landing in a small wooded glade. The helicopter struck trees and impacted the ground heavily, causing the aircraft to overturn. The pilot sustained minor injuries and was able to exit the wreckage independently.
The investigation
The Swedish Accident Investigation Board (SHK) examined the maintenance history and the mechanical state of the tail rotor gearbox. The investigation revealed that during a recent 100-hour inspection, the gearbox had been drained of oil, but the technician failed to refill it. This error was compounded by a breakdown in quality control; the technician recorded the oil change in the logbook after verifying covers were secure, rather than immediately after the fluid exchange, which prevented the logbook from serving as an effective real-time check.
Furthermore, the investigation looked into the pilot's pre-flight inspection. The pilot had observed the oil level through the sight glass and, despite noting it appeared low, deemed it acceptable for the flight. The SHK found that the visual measurement system on this model is imprecise and prone to misinterpretation, as residual oil on the glass can create a false impression of sufficient levels.
Findings
- The primary cause of the accident was that the tail rotor gearbox was drained of oil and not replenished following maintenance.
- A significant contributing factor was the deficiency in quality assurance management within the maintenance organization.
- The emergency instructions for the Eurocopter AS350 B3 provided insufficient guidance for managing a chip warning or a tail rotor drive failure.
- The design of the gearbox sight glass is imprecise, making it easy for a pilot to misinterpret the oil level during daily inspections.
- There were also noted deficiencies in the oversight provided by the national aviation authority regarding maintenance organization inspections.