A fuel system for a gas turbine engine of an aircraft comprising: a main inlet feed conduit fluidly connected to a primary manifold feed conduit and to feed a secondary manifold feed conduit; a primary manifold fluidly connecting the primary manifold feed conduit to a plurality of primary fuel injectors; a secondary manifold fluidly connecting the secondary manifold feed conduit to a plurality of secondary fuel injectors; a primary fuel flow valve disposed in the primary manifold feed conduit wherein the primary fuel flow valve has an open condition operative to permit flow of fuel through the primary manifold feed conduit and a closed condition operative to inhibit flow of fuel through the primary manifold feed conduit; a secondary fuel flow valve disposed in the secondary manifold feed conduit wherein the secondary fuel flow valve has an open condition operative to permit flow of fuel through the secondary manifold feed conduit and a closed condition operative to inhibit flow of fuel through the secondary manifold feed conduit; and a controller operatively connected to control the primary fuel flow valve, wherein the controller is operable to control the primary fuel flow valve to pulse a flow of fuel passing therethrough in pulse width modulation pulses at a low end of a fuel flow range for the primary manifold; and characterised in that the fuel system further comprises a gaseous fuel source defining an inlet end of the main inlet feed conduit to supply gaseous fuel to the primary fuel manifold via the main inlet feed conduit, and a fuel metering mechanism disposed in the main inlet feed conduit operable to regulate flow through both the main inlet feed conduit and the primary fuel manifold. a main inlet feed conduit fluidly connected to a primary manifold feed conduit and to feed a secondary manifold feed conduit; a primary manifold fluidly connecting the primary manifold feed conduit to a plurality of primary fuel injectors; a secondary manifold fluidly connecting the secondary manifold feed conduit to a plurality of secondary fuel injectors; a primary fuel flow valve disposed in the primary manifold feed conduit wherein the primary fuel flow valve has an open condition operative to permit flow of fuel through the primary manifold feed conduit and a closed condition operative to inhibit flow of fuel through the primary manifold feed conduit; a secondary fuel flow valve disposed in the secondary manifold feed conduit wherein the secondary fuel flow valve has an open condition operative to permit flow of fuel through the secondary manifold feed conduit and a closed condition operative to inhibit flow of fuel through the secondary manifold feed conduit; and a controller operatively connected to control the primary fuel flow valve, wherein the controller is operable to control the primary fuel flow valve to pulse a flow of fuel passing therethrough in pulse width modulation pulses at a low end of a fuel flow range for the primary manifold; and characterised in that the fuel system further comprises a gaseous fuel source defining an inlet end of the main inlet feed conduit to supply gaseous fuel to the primary fuel manifold via the main inlet feed conduit, and a fuel metering mechanism disposed in the main inlet feed conduit operable to regulate flow through both the main inlet feed conduit and the primary fuel manifold.