Incident Overview
While operating in clouds at flight level 290, in an area with thunderstorms and turbulence, the airplane experienced a loss of power in all four engines. Information from the flight data recorder indicated that an elevator oscillation occurred during the descent from flight level 290.
Engine Shutdowns and Restart
The #1 and #2 engines were shut down due to high turbine gas temperatures. The #2 engine was restarted before the crew made an emergency landing at the Des Moines Airport.
Icing Conditions and Flight Manual Compliance
The captain reported that the airplane had been flown into severe icing conditions. The aircraft flight manual (AFM) prohibited flight in icing conditions above 26,000 feet, except when necessary for descent. The AFM also stipulated the minimum engine speeds that needed to be maintained while operating in icing conditions and required that the thrust management system (TMS) be disconnected in such conditions. There were indications that the TMS was disconnected about 230 seconds after engine roll-back began.
Instrument Accuracy Issues
The AFM did not provide accurate information concerning the 'in cloud' correction factor for the outside temperature thermometer.