What happened
While in cruise flight, the crew of a four-engine aircraft experienced a partial power loss affecting three of the four engines. At the time of the incident, the airplane was operating in icing conditions with all anti-ice systems activated.
As the flight passed through an area of rapidly warming air, the engine bleed bands became open and could not be closed using normal throttle movement. To prevent overtemperature conditions, the crew shut down two engines and reduced one additional engine to idle. Following these actions, the crew declared an emergency and diverted to a nearby airport.
The investigation
Subsequent testing performed on a similar aircraft revealed a specific design characteristic regarding the engine's operation. It was determined that when the aircraft is operating at high altitude with all anti-ice systems turned on, the bleed air valves cannot be closed using normal procedures.