Gas turbine engine fuel control system with enhanced relight capability

Abstract

A relight fuel flow schedule controls fuel flow to the combustor of a turbofan aeroengine during in-flight relight procedures. A first fuel ramp, whose slope decreases with increasing altitude, enables the combustor to light with minimum fuel flow for the altitude and thus helps to prevent overfueling. The fuel flow rate undergoes a step reduction, after detection of satisfactory combustion, to prevent compressor stall, but cannot drop below a predetermined level to guard against flameout. The first fuel ramp is repeated if satisfactory combustion does not occur. After a successive relight a second fuel ramp is varied from flat to positive slope depending on the ratio of the rate of change of high pressure compressor speed to a datum level in order to prevent a hung start and accelerate the engine until normal acceleration control laws takes over.