What happened
On January 17, 2018, an EC 135 P3 helicopter was conducting a scheduled test flight at Szczecin Airport (EPSC) following routine hangar maintenance. During a period of hovering, a mechanic present in the passenger cabin observed a red "FAIL" warning light on the main electrical box for the number 1 bus of the left engine generator.
Although the pilot observed no immediate abnormalities on the cockpit displays (CAD and VEMD), the flight was aborted to allow for troubleshooting. Ground technicians performed a functional test according to the Aircraft Maintenance Manual (AMM), which initially yielded a positive result (green light). Based on this successful test, the decision was made to proceed with the test flight.
However, upon takeoff, the red "FAIL" warning light recurred. The flight was again terminated, and subsequent ground tests failed to resolve the issue. Following consultation with the manufacturer and a detailed analysis of the ARINC 429 data bus (specifically labels 37, 370, 371, and 372), three errors were identified on label 37. Following the manufacturer's recommendation, the Electrical Master Box 1 for the left engine generator bus was replaced. After successful testing, the aircraft was returned to service.
The investigation
The investigation focused on the intermittent failure of the electrical signaling system. The investigation team analyzed the maintenance logs, the results of the ground tests performed by SOT Szczecin, and the digital data retrieved from the aircraft's data bus. The technical analysis of the ARINC 429 labels provided the necessary evidence to pinpoint the hardware malfunction.
Findings
- The primary cause of the incident was a malfunction of the main electrical box 1 for the left generator bus.
- The intermittent nature of the failure was not initially detectable through standard ground testing procedures until the component was replaced.