A combustor for a gas turbine engine

A combustor in a gas turbine engine has a wall 18 which forms a gap 20 with an outer nozzle guide vane platform 14. A closure mechanism 30, 32 is operable to open or close the gap 20. Opening of the gap 20, for example during cruise conditions, reduces the mass flow rate of air through an open end 6 of the combustor 2, so enriching the air fuel ratio and improving flame stability and combustion efficiency. Under high power conditions, the gap 20 is closed, causing an increase in air flow rate through the open end 6 to achieve a lean burn air fuel ratio when the fuel flow rate is increased.