"The phenomenon of uncommanded thrust reduction (rollback) is known to occur at altitudes above 26,000 feet in temperature conditions of ISA +9 degrees C. or greater and in the vicinity of thunderstorm activity. . .Except for descent, flight in icing conditions above 26,000 feet and at an outside air temperature above -40 degrees C. SAT is prohibited within thirty nautical miles of thunderstorms.
X-XXXX was equipped with four Honeywell (formerly Allied Signal) ALF502R engines, each rated at 6,970 pounds of thrust. According to Honeywell, when operating at or above FL280 and in certain temperature and moisture conditions, the engines become susceptible to accumulating ice on the supercharger exit guide vanes (EGV). This reduces core engine airflow, resulting in a loss of power."