A gas turbine engine system comprising: an engine core (70; 210; 310), a core housing containing the engine core (70; 210; 310), and an engine core driven fan forward of the core housing; a nacelle (220; 320) surrounding the fan and the core housing; a bypass duct (230; 330) defined between an outer diameter (234; 334) of the core housing and an inner diameter (232; 332) of the nacelle (220; 320); and a thermal management system including a coolant circuit including a second heat exchanger (260; 360) disposed on the inner diameter (232; 332) of the nacelle (220; 320); characterised by the coolant circuit further comprising: a third heat exchanger (280; 380) disposed on a leading edge of a bifurcation (BiFi) (270; 370) spanning said bypass duct (230; 330), wherein the second heat exchanger (260; 360) is in thermal communication with the third heat exchanger (280; 380); and a fluid containment component (240; 340) including a secondary heat exchanger (244; 344) operable to exchange heat from a second liquid coolant stored in the fluid containment component (240; 340) to a first liquid coolant in the secondary heat exchanger (244; 344), and having a first liquid coolant input in fluid communication with the second heat exchanger (260; 360), a first liquid coolant output, a second liquid coolant input and a second liquid coolant output. an engine core (70; 210; 310), a core housing containing the engine core (70; 210; 310), and an engine core driven fan forward of the core housing; a nacelle (220; 320) surrounding the fan and the core housing; a bypass duct (230; 330) defined between an outer diameter (234; 334) of the core housing and an inner diameter (232; 332) of the nacelle (220; 320); and a thermal management system including a coolant circuit including a second heat exchanger (260; 360) disposed on the inner diameter (232; 332) of the nacelle (220; 320); characterised by the coolant circuit further comprising: a third heat exchanger (280; 380) disposed on a leading edge of a bifurcation (BiFi) (270; 370) spanning said bypass duct (230; 330), wherein the second heat exchanger (260; 360) is in thermal communication with the third heat exchanger (280; 380); and a fluid containment component (240; 340) including a secondary heat exchanger (244; 344) operable to exchange heat from a second liquid coolant stored in the fluid containment component (240; 340) to a first liquid coolant in the secondary heat exchanger (244; 344), and having a first liquid coolant input in fluid communication with the second heat exchanger (260; 360), a first liquid coolant output, a second liquid coolant input and a second liquid coolant output. a third heat exchanger (280; 380) disposed on a leading edge of a bifurcation (BiFi) (270; 370) spanning said bypass duct (230; 330), wherein the second heat exchanger (260; 360) is in thermal communication with the third heat exchanger (280; 380); and a fluid containment component (240; 340) including a secondary heat exchanger (244; 344) operable to exchange heat from a second liquid coolant stored in the fluid containment component (240; 340) to a first liquid coolant in the secondary heat exchanger (244; 344), and having a first liquid coolant input in fluid communication with the second heat exchanger (260; 360), a first liquid coolant output, a second liquid coolant input and a second liquid coolant output.