What happened
On April 6, 2016, a Robinson R22 Beta II helicopter, registration HB-ZGO, was conducting a training flight near Bern-Belp, Switzerland. Shortly after departing from Bern-Belp airport, the flight crew—a flight instructor and a student—noticed static-like interference in their headsets and observed the ammeter needle fluctuating wildly. Simultaneously, a faint smell of electrical burning was detected in the cockpit.
In response to the electrical instability, the crew deactivated the generator via the cockpit switch and pulled the circuit breaker. Despite these actions, the electrical fault persisted. After establishing contact with the tower, the crew decided to return directly to the airport. During the return leg, re-engaging the generator caused the ammeter to fluctuate again and the burning smell to return. Upon landing at Bern-Bel and approximately nine minutes after takeoff, the tower notified the crew that smoke was visible from the engine area. The student pilot immediately exited the aircraft and used a portable fire extinguisher to suppress an active fire located near the generator.
The investigation
The Swiss Transportation Safety Investigation Board (SUST) examined the electrical system and the installation of an aftermarket component. The investigation focused on a Lone Star Aviation Corp. LS03-01004 noise suppressor capacitor that had been retrofitted to the generator. This installation was not documented in the aircraft's technical records.
Investigators found that while such capacitors are approved for certain light aircraft (such as the Cessna 172) under FAA/PMA, they are not specifically approved for Robinson helicopters. The investigation also reviewed maintenance logs, noting that the aircraft had undergone a 50-hour inspection on the morning of the incident, which failed to detect the faulty installation.
Findings
- The fire was triggered by an electrical short circuit between the generator terminal and the housing of the retrofitted noise suppressor capacitor.
- The short circuit was caused by an improper installation of the capacitor, which resulted in mechanical damage to the insulating rubber nipple on the generator terminal, allowing contact with the grounded capacitor housing.
- The installation instructions for the capacitor did not provide specific guidance on the mounting method for the generator, leading to an inadequate setup.
- The crew's failure to fully follow manufacturer procedures for an in-flight electrical fire—specifically, not deactivating the battery—likely contributed to the fire's development, as the short circuit remained active even after the generator was disconnected.
Safety action
Following the investigation, SUST issued Safety Notice No. 18. This notice advises aircraft owners, operators, and maintenance organizations to ensure that any aircraft retrofitted with FAA/PMA noise suppressor capacitors are inspected to confirm that no electrical short circuit can occur between the grounded capacitor housing and the aircraft's electrical bus.