What happened
On August 28, 2014, at approximately 18:30 UTC, an Xair-F ultralight, registration TF-142, was conducting a private flight from the Úlfarsfell airstrip. Less than 15 minutes into the flight, the aircraft's 70 HP Hirth 3503E18H piston engine lost power while flying at a low altitude near Helgafell.
While attempting to execute an emergency landing, the pilot identified a gravel road branching off from Road 35 as a potential landing site. During the glide toward this site, the aircraft's left wing struck a light pole as it crossed Road 35. The aircraft subsequently performed a 180-degree turn and came to rest in the middle of the road. There were no injuries to the two occupants, though the aircraft sustained moderate damage.
The investigation
The Icelandic Transportation Safety Board (RNSA) examined the engine components, specifically focusing on the spark plugs, which the pilot had replaced prior to the flight. The investigation found that the spark plugs installed were "NGK BR 8 ES" models. The pilot had selected these parts by referencing Table 4.1 in the engine's Operator's Manual.
Upon inspection, investigators found damaged spark plugs and burned engine heads. The investigation compared the installed plugs against several manufacturer references, including the Hirth Engine 3502/3503 spare-parts list and Hirth-Information 0046/1N. The inquiry revealed that the installed "NGK BR 8 ES" plugs were too long and were not authorized for use in the Hirth 3503 engine.
Findings
- The engine failure was caused by the installation of incorrect spark plugs based on erroneous information found in the engine's Operator's Manual.
- A discrepancy exists between the engine manufacturer's various technical documents regarding approved spark plug types.
- The pilot followed the instructions provided in the Operator's Manual, but those instructions led to the use of incompatible components.