What happened
On September 18, 2016, a Hoffman Aircraft H 36 Dimona, registration LN-GMY, was conducting a private flight departing from Sandane Airport. After performing initial maneuvers to gain altitude, the pilot proceeded east along the Nordfjord. While flying near the Åland farm in Randabygda, the engine suddenly ceased operation.
As the aircraft was over terrain with elevations of approximately 350 meters, the pilot was forced to select an emergency landing site. A sloping field west of the farm was chosen. The pilot performed two 3/60-degree turns to manage altitude for the approach, but the flare was executed too late due to the rising terrain. The resulting impact was severe; the wheels penetrated the turf, causing the main landing gear to break away. The aircraft bounced, striking the ground a second time before coming to rest.
The investigation
The Norwegian Safety Investigation Authority (NSIA) examined the engine's ignition system, specifically the single magneto used in the Limbach L 2000 EB 1 AC engine. Investigators discovered a fault in the wiring connecting the magneto to the ignition switch.
Upon inspection, it was found that the shielding on this wire had been improperly handled. During a previous, undocumented repair or replacement of a ring terminal, the wire's shielding was cut along with the insulation. This left the uninsulated shielding in close proximity to the center conductor. Testing revealed that the resistance between the shield and the conductor changed depending on how the wire was bent, creating a risk of intermittent grounding.
Findings
- The engine failure was caused by an intermittent electrical short circuit between the center conductor and the shielding of the ignition wire.
- The shielding was improperly terminated, leaving the conductor vulnerable to grounding.
- The pilot's late flare during the emergency landing contributed to the structural damage to the H 36 Dimona, including damage to the wings, tail, and propeller.
- The maintenance history did not record the specific repair that led to the improper wiring termination.
Safety action
- Maintenance personnel should be aware of the known risks regarding magnetos shorting to ground due to degraded wiring between the magneto and the ignition switch.