Incident Overview
On 15 July 2021 at approximately 0915Z, a Piper PA-28-140 Cherokee, registration ZS-LRO, departed Virginia Aerodrome (FAVG) on a private flight to Kruger Mpumalanga International Airport (FAKN). The aircraft carried two pilots and two passengers. The pilot in the left seat (pilot flying) held a Private Pilot Licence, while the pilot in the right seat (pilot monitoring) held a Commercial Pilot Licence.
Sequence of Events
Approximately 20 minutes into the flight, while cruising at 1,000 feet above ground level, the engine began running rough, accompanied by a slight reduction in revolutions per minute. The pilot monitoring assumed control and decided to turn left toward King Shaka International Airport (FALE) for a precautionary landing, as the aircraft was 16 nautical miles north of that airport. However, en route to FALE, the engine continued losing power and could not maintain sufficient airspeed, causing the aircraft to lose altitude rapidly. After an unsuccessful fault-finding attempt and inability to restore engine power, the pilot monitoring contacted FALE tower on 125.75 MHz to declare an emergency. Due to the rapid descent, she began scanning for a suitable landing site and spotted a dirt road on a private farm to the left of the aircraft's path. She executed a successful landing on the road; the aircraft sustained no damage, and no injuries occurred among the occupants.
Post-Incident Examination
Examination of the right-side magneto after removal from the engine revealed that 11 teeth of the nylon distributor gear were stripped. All tooth fragments were found inside the magneto housing. The magneto was cleaned, and a new nylon distributor gear was installed in accordance with the Continental Ignition System Service Manual. After fitting, the engine was test-run and operated satisfactorily.
Magneto Gear Failure Considerations
According to Airworthiness Bulletin 74-055 Issue 3, dated 20 October 2014, several factors can contribute to nylon distributor gear failure, including aging, hardening of gear material, fatigue cracking, prop strikes, high temperature, kick-back during starting, magneto locking devices left in place, lightning strikes, incorrectly installed gears, or shock loads on the gear train. However, the magneto had been in service for only 114.33 hours since its last overhaul (23% of the 500-hour TBO), and no previous incidents or technical defects were recorded in the aircraft documents that could indicate any of these possibilities. Consequently, the specific cause of the nylon gear failure was undetermined.
Probable Cause
The probable cause of the serious incident was power loss due to right-side magneto failure in flight, resulting from failure of the nylon gear, which led to failure of the engine ignition system.