What happened
On 4 September 2005, a Schweizer 269C helicopter, registration G-OGOB, was conducting a private flight near Dunkeswell, Devon. The pilot and one passenger were returning to Dunkeswell from the Sidmouth area when, at an altitude of 650 ft, the pilot noticed the aircraft would not climb despite collective inputs.
To avoid an approaching ridge line, the pilot attempted a precautionary landing in a nearby field. During the descent, the aircraft's rate of descent increased rapidly, and the helicopter landed heavily, resulting in a rollover onto its left side. The pilot exited the wreckage unaided and assisted the passenger, who sustained one injury.
The investigation
The AAIB examined the engine, flight controls, and transmission systems, finding no evidence of catastrophic failure, mechanical disconnection, or engine icing. The engine had previously shown signs of magneto issues during pre-flight checks, but subsequent testing showed the spark plugs were clean and the engine was functioning normally.
Investigators focused on the fuel tank vent system, which had been modified by a Schweizer kit. Laboratory tests simulated a rollover and revealed that at roll angles greater than 91º, the fuel vent valve could "shuttle," allowing fuel to escape from the vent tube at a rate of approximately 0.5 litres per minute. The investigation also considered whether the pilot had inadvertently entered a vortex ring state or experienced rotor speed droop during the descent.
Findings
- The helicopter was operating at a weight that could have reduced its climb performance at speeds exceeding 60 kt.
- The pilot may have entered a vortex ring state or experienced rotor speed droop while attempting to avoid obstacles, leading to an uncontrollable descent.
- The fuel tank vent system modification allowed for the potential release of fuel during a rollover.
- No technical defects were identified that could directly explain the initial loss of engine power, though a transient defect in the fuel or ignition systems remains a possibility.
Safety action
- Safety Recommendation 2006-064: It is recommended that the Federal Aviation Administration require the Schweizer Aircraft Corporation to review modification SA269K-101-1 to further reduce the risk of fuel escaping the vent system during a rollover.