What happened
On 19 April 1999, a Robinson R22 BETA, registration G-DEJL, was conducting a flight near South Elking. The flight was part of a series of checks following previous reports of engine instability. Prior to the accident, the pilot had noted irregular engine performance, specifically a 'knocking' sound and unstable manifold pressure. After an engineer performed hovering tests, a subsequent test flight was undertaken.
During this flight, the engine began knocking again at an altitude of approximately 200 feet. In response, the pilot reduced the manifold pressure and altitude. While flying at a low level back toward the base, the manifold pressure suddenly surged from 21 to 32 inches of mercury. This sudden change caused the helicopter to settle toward the ground. Due to the forward momentum of the aircraft and the slope of the terrain, the helicopter pitched forward onto its nose, causing the rotor blades to strike the ground. There were no fatalities and no injuries to the two occupants.
The investigation
Investigators examined the engine using a test cell run and inspected the aircraft's components. During the engine test, the investigators noted that while the engine showed slight rough running, it did not exhibit backfiring. When testing at reduced fuel inlet pressures, the engine exhibited RPM fluctuations, but the specific symptoms described by the pilot could not be replicated.
An inspection of the engine revealed burnt plug leads from the right-hand magneto to the bottom cylinders, though insulation checks were fine. The fuel system was also examined; it was noted that the main tank vent pipe was detached and a connecting vent pipe was missing at the time of recovery. However, fuel flow tests through the filter were found to be sufficient.
Findings
- The aircraft sustained extensive damage during the impact.
- Meteorological data indicated conditions that were within the range for potential carburetor icing.
- The investigation could not definitively reproduce the exact sequence of engine failure through engine test bed runs.
- The sudden surge in manifold pressure was the primary factor leading to the loss of altitude and subsequent impact.