What happened
On August 13, 2021, a Bombardier DHC8-400, registration SP-EQL, departed from Warsaw Chopin Airport (EPGD) for a flight to Katowice (EPKK). During the climb phase, at an altitude between 4,500 and 5,000 feet, the crew adjusted the propeller RPM to 850 and set both engines to Maximum Climb (MCL) power. Shortly after, the crew noticed a power discrepancy: engine number 1 remained at MCL, while engine number 2 remained at Maximum Cruise (MCR) power.
Despite the pilot's attempt to re-engage the MCL setting, the power on engine number 2 did not adjust, and a gradual decrease in propeller RPM was observed. Following the established engine failure checklist, the crew shut down engine number 2 at 16:14 UTC and declared a MAYDAY. The crew informed Air Traffic Control of their intention to perform an emergency landing at EPGD within approximately 15 minutes, providing essential details regarding passengers, fuel, and hazardous materials. The commander instructed the cabin crew to prepare passengers for an emergency landing. The aircraft landed safely at 16:36 UTC without further incident, and the passengers disembarked via stairs.
The investigation
Investigators analyzed data from the onboard flight parameter recorder, which revealed irregularities in the propeller control system. The investigation focused on the Propeller Electronic Control (PEC) unit and the Condition Lever (CL). Testing of the PEC unit at a facility in the United Kingdom identified error codes indicating malfunction upon receiving signals from the CL, though the specific errors could not be replicated during laboratory tests.
Further examination of the CL by a facility in France revealed some play in the lever, but investigators concluded this was insufficient to cause the significant propeller pitch change observed during the flight. Electrical wiring between the CL and the PEC was inspected and found to be in good condition. Notably, a similar incident involving a transient drop in the CL position had occurred on this same aircraft on July 31, 2021, which had been attributed to a faulty CL.
Findings
- The primary cause of the engine power discrepancy was a malfunction of the Condition Lever (CL) and/or the Propeller Electronic Control (PEC) unit within the propeller control system.
- Flight data indicated that after the lever was set to 55 degrees (850 RPM) and MCL power was selected, the CL sent a signal to the PEC indicating a position below that value, causing the propeller to move toward the feathered position.
Safety action
- The PEC and CL components were replaced.
- Following the replacement, the propeller control system was tested and confirmed to be operating correctly.