Abstract: In a fluid system for a gas turbine engine, oil is supplied from an oil tank (10) by a constant displacement pump (12) to lubricate engine components (18). The oil is supplied to the components (18) via a heat exchanger (16) in which oil and fuel are placed in direct heat exchange relationship. The flow of oil to the components (18) is controlled by recirculating a proportion of the oil flow through a bypass (20). The bypass (20) regulates the flow of oil to the components (18) so that the amount of heat transferred to the fuel in the heat exchanger (16) is controlled. At low engine powers a greater proportion of the oil flows through the bypass (20) to reduce the oil flow to the components (18). This reduces the heat transferred to the fuel in the heat exchanger (16) and so prevents overheating of the fuel.