Mechanical & Engine Failure · NTSB ERA14FA123
WILLIAMS CHRISTOPHER T SONEX — Wellington, FL
| Date | February 17, 2014 |
| Location | Wellington, FL |
| Aircraft | WILLIAMS CHRISTOPHER T SONEX (amateur-built) |
| Purpose of flight | Personal |
| Conditions | Day · Visual Meteorological Cond |
| Phase / occurrence | Prior to flight Aircraft maintenance event |
| Pilot age | 58 |
| Pilot total time | 13,300 hrs · High time |
| Time in type | 400 hrs |
| Fatalities | 1 |
Probable cause
NTSB findings
- Aircraft-Aircraft oper/perf/capability-Performance/control parameters-Airspeed-Not attained/maintained - C
- Aircraft-Aircraft oper/perf/capability-Performance/control parameters-Angle of attack-Not attained/maintained - C
- Personnel issues-Task performance-Use of equip/info-Aircraft control-Pilot - C
- Personnel issues-Task performance-Maintenance-Repair-Pilot - F
- Aircraft-Aircraft power plant-Ignition system-Spark plugs/igniters-Incorrect service/maintenance - F
What happened
The airplane had just departed the airport; one witness reported that during the initial climb the engine "sputtered," and another reported that it "backfired." The pilot then made a steep turn back toward the airport, but the airplane stalled and spiraled to the ground. The airplane was equipped with an electronic flight instrument system that recorded numerous engine and flight parameters. Review of the downloaded data revealed that, initially, the engine was operating normally and within design parameters. However, toward the end of the recorded data, the No. 1 cylinder head and exhaust gas temperatures had begun to decrease while the other cylinder temperature parameters remained fairly constant. The engine data then recorded a decrease in engine rpm followed by a steep 180-degree turn toward the airport.
A witness who assisted the pilot with the airplane's oil change 2 days earlier stated that the pilot had cross-threaded a spark plug in the No. 1 cylinder and attempted a helicoil repair. During examination after the accident, the No. 1 sparkplug was easily removed by hand. This was likely the cause of the power loss that preceded the pilot's attempt to return to the airport. The pilot's steep, 180-degree turn exceeded the airplane's critical angle of attack, which resulted in a stall at low altitude and collision with terrain. A review of the pilot's toxicology revealed that even though he tested positive for antidepressants, they were not a factor in the accident.