What happened
On June 10, 2016, a SAAB SF-340A operated by SprintAir SA was performing a final approach to runway 33 at Warsaw Chopin Airport (EPWA). At approximately 19:58 LMT, the crew encountered a series of critical system failures. A "MASTER CAUTION" alert was triggered, the autopilot and Flight Management System (FMS) disconnected, and the right-side EFIS displays lost power. Simultaneously, warnings for the "R MAIN BUS" and "R ESS BUS" electrical installations appeared, resulting in the loss of all instruments powered by the right-side electrical bus, including the landing gear position indicator.
Located roughly 7.5 NM from the runway threshold, the commander decided to execute a missed approach to perform a low pass over runway 15/33 to visually verify the landing gear configuration. During this low pass at 20:00 LMT, the tower confirmed that the landing gear was extended. Due to the nature of the electrical instability, an uncertainty state was declared at the airport at 20:03 LMT, and emergency services, including airport fire and medical teams, were placed on standby. The commander did not declare a formal emergency. The aircraft subsequently completed a second ILS approach to runway 33 and landed safely at 20:22 LMT. The uncertainty state was canceled at 20:24 LMT.
The investigation
An investigation conducted by the operator examined the electrical systems of the aircraft following the incident. The technical inspection focused on the power distribution architecture to identify why multiple buses and flight instruments had lost power simultaneously.
Findings
- The investigation established that the primary cause of the incident was the malfunctioning of the Power Distribution Unit (PDU).
- The faulty PDU caused the failure of the right-side electrical buses and the subsequent loss of critical flight displays and landing gear indication.
- The malfunction was resolved by replacing the defective unit, and subsequent engine runs confirmed that the electrical system was functioning correctly.