A gas turbine engine comprising: a compressor providing a source of pressurized air; a secondary air system including a plurality of interconnected fluid passages (34,36,38,40) defining at least one flow path communicating with the source of pressurized air, some of the plurality of fluid passages (34,36,38,40) communicating with components of the gas turbine engine for delivery of the pressurized air thereto, the plurality of fluid passages including a common fluid passage (34, 36, 38), the common fluid passage (34, 36, 38) including a venturi configured for controlling a flow of the pressurized air from the source of pressurized air to the common outlet through the plurality of fluid passages (34,36,38,40); characterised in that the at least one flow path is defined between the source of pressurized air and a common outlet communicating with an environment of the gas turbine engine; all the circulation of the pressurized air to the common outlet passes through the common fluid passages (34, 36, 38); the venturi is configured to limit the flow through the plurality of fluid passages (34, 36, 38, 40) at high power but not at low power; and the flow through the fluid passages (34, 36, 38, 40) is from the same source of pressurized air throughout variations of power; and in that: a total pressure ratio (Pt u/s / Pt d/s) is defined between a total pressure (Pt u/s) upstream of the secondary air system and a total pressure (Pt d/s) downstream of the secondary air system, and wherein low power corresponds to a total pressure ratio (Pt u/s / Pt d/s) of less than 1.1, and high power corresponds to a total pressure ratio (Pt u/s / Pt d/s) of at least 2. a compressor providing a source of pressurized air; a secondary air system including a plurality of interconnected fluid passages (34,36,38,40) defining at least one flow path communicating with the source of pressurized air, some of the plurality of fluid passages (34,36,38,40) communicating with components of the gas turbine engine for delivery of the pressurized air thereto, the plurality of fluid passages including a common fluid passage (34, 36, 38), the common fluid passage (34, 36, 38) including a venturi configured for controlling a flow of the pressurized air from the source of pressurized air to the common outlet through the plurality of fluid passages (34,36,38,40); characterised in that the at least one flow path is defined between the source of pressurized air and a common outlet communicating with an environment of the gas turbine engine; all the circulation of the pressurized air to the common outlet passes through the common fluid passages (34, 36, 38); the venturi is configured to limit the flow through the plurality of fluid passages (34, 36, 38, 40) at high power but not at low power; and the flow through the fluid passages (34, 36, 38, 40) is from the same source of pressurized air throughout variations of power; and in that: a total pressure ratio (Pt u/s / Pt d/s) is defined between a total pressure (Pt u/s) upstream of the secondary air system and a total pressure (Pt d/s) downstream of the secondary air system, and wherein low power corresponds to a total pressure ratio (Pt u/s / Pt d/s) of less than 1.1, and high power corresponds to a total pressure ratio (Pt u/s / Pt d/s) of at least 2.