What happened
During a local instrument flight rules (IFR) training flight from Roskilde (EKRK) on April 8, 2021, a Piper PA28-161 Cadet, registration OY-JEC, experienced an electrical malfunction. While performing an ILS approach to runway 11 and subsequently transitioning to a circling maneuver for runway 29, the flight instructor and student pilot noticed the "Low volt" indicator light flickering at low engine RPM.
As the aircraft joined the left-hand downwind leg for runway 2 and 9, the low voltage light remained illuminated, and a significant amount of white smoke began emanating from the left side of the instrument panel. The flight instructor immediately took control of the aircraft, declared an emergency, and executed the Electrical Fire emergency checklist. Following the checklist, the instructor also performed the Forced Landing procedures. The aircraft was successfully landed on runway 03, and the crew evacuated the aircraft without any injuries.
The investigation
The Danish Accident Investigation Board conducted a technical examination of the aircraft's electrical components. The investigation focused on the alternator and the associated switching mechanism. Investigators examined the manufacturer's service history and the regulatory status of existing safety recommendations regarding the aircraft's electrical design.
Findings
- The investigation established that the B+ terminal stud on the alternator broke off during flight.
- This mechanical failure likely caused the voltage regulator to detect a voltage drop, prompting an increase in field voltage.
- The resulting increase in voltage and current at the alternator aux terminal exceeded the design limits of the alternator out switch.
- This electrical overload caused the alternator out switch to overheat and burn, producing the smoke observed in the cockpit.
- The execution of the Electrical Fire emergency checklist acted as an effective safety barrier by removing electrical power from the alternator, which halted the overheating process.
- While a manufacturer Service Bulletin (SB 991A) exists to install a fuse on the alternator aux terminal to prevent such issues, it was not mandatory via an Airworthiness Directive (AD) at the time of the incident.
Safety action
Following the incident, the FAA indicated it would re-evaluate the necessity of mandating Service Bulletin 991A through an Airworthiness Directive to address potential smoke formation in the fleet.