CONTROLLING SOOT

A gas turbine engine (105) comprising a variable geometry combustor (206, 206', 206") having fuel injectors (302), a rich-burn zone (1101), a quick-quench zone (1102), and a lean-burn zone (1104), and further comprising quench ports (1103) for admitting quench air to the quick-quench zone; a variable geometry airflow arrangement (303) for the variable geometry combustor, which is configured to vary an airflow through the fuel injectors and/or the quench ports; and a control system configured to control the variable geometry airflow arrangement in dependence upon an airflow delivered to the combustor, a fuel flow to the fuel injectors, and a target index of soot emissions to control the quantity of soot produced by combustion.