Incident Overview
During cruise flight, the flight crew experienced a partial power loss of three of the aircraft's four engines. The aircraft was operating in icing conditions with all anti-ice systems activated. The flight passed through an area of rapidly warming air, which caused the engine bleed bands to open. The crew was unable to close these bleed bands using throttle movement.
Crew Actions
To prevent overtemperature conditions, the crew shut down two of the affected engines and reduced a third engine to idle power. They declared an emergency and diverted to a nearby airport, where the aircraft landed without further incident.
Investigation Findings
Subsequent testing of a similar aircraft revealed that the engine design had a specific limitation: when operating at high altitude with all anti-ice systems on, the bleed air valves could not be closed using normal procedures. This design characteristic contributed to the inability to manage the engine bleed bands during the warming air encounter. In response, the manufacturer developed a modified checklist to address this situation in future operations.