A three-stage lean burn combustion chamber (28) comprises a primary combustion
zone (36), a secondary combustion zone (40) and a tertiary combustion
zone (44). Each of the combustion zones (36, 40, 44) is supplied
with premixed fuel and air by respective fuel and air mixing ducts (54, 70,
92). The fuel and air mixing ducts (54, 70, 92) have a plurality of
air injection slots (62, 64, 76, 98) spaced apart transversely to the direction
of flow through the fuel and air mixing ducts (54, 70, 92). The air injection
slots (62, 64, 76, 98) extend in the direction of flow through the fuel
and air mixing ducts (54, 70, 92) to the reduce the magnitude of the fluctuations
in the fuel to air ratio of the fuel and air mixture supplied into the at least
one combustion zone (36, 40, 44). This reduces the generation of harmful
vibrations in the combustion chamber (28).