A thermal management system for a gas turbine engine having a principal rotational axis, an engine core, and a nacelle, the thermal management system comprising: a bypass inner wall circumferentially surrounding the engine core around the principal rotational axis and extending along the principal rotational axis, wherein the bypass inner wall is radially disposed between the engine core and the nacelle relative to the principal rotational axis, such that a main bypass duct is defined between the bypass inner wall and the nacelle; an outer cowl circumferentially surrounding and radially spaced apart from the bypass inner wall relative to the principal rotational axis, wherein the outer cowl extends along the principal rotational axis between an inlet end and an outlet end, and wherein the outer cowl is disposed within the main bypass duct; a plurality of inlet vanes extending from the bypass inner wall towards the outer cowl and disposed proximal to the inlet end of the outer cowl, wherein the plurality of inlet vanes are circumferentially spaced apart from each other relative to the principal rotational axis; a plurality of outlet vanes extending from the bypass inner wall towards the outer cowl and disposed proximal to the outlet end of the outer cowl, wherein the plurality of outlet vanes are circumferentially spaced apart from each other relative to the principal rotational axis; a heat exchanger extending from the bypass inner wall towards the outer cowl and axially disposed between the inlet end and the outlet end relative to the principal rotational axis; a plurality of first removable shims corresponding to the plurality of inlet vanes, wherein each first removable shim from the plurality of first removable shims extends between and engages the outer cowl and a corresponding inlet vane from the plurality of inlet vanes; a plurality of second removable shims corresponding to the plurality of outlet vanes, wherein each second removable shim from the plurality of second removable shims extends between and engages the outer cowl and a corresponding outlet vane from the plurality of outlet vanes; and a third removable shim extending between and engaging the heat exchanger and the outer cowl, the third removable shim circumferentially extending around the principal rotational axis; wherein, for positionally adjusting the outer cowl relative to the bypass inner wall: each of the plurality of first removable shims is removable from between the corresponding inlet vane and the outer cowl; each of the plurality of second removable shims is removable from between the corresponding outlet vane and the outer cowl; and the third removable shim is removable from between the heat exchanger and the outer cowl. a bypass inner wall circumferentially surrounding the engine core around the principal rotational axis and extending along the principal rotational axis, wherein the bypass inner wall is radially disposed between the engine core and the nacelle relative to the principal rotational axis, such that a main bypass duct is defined between the bypass inner wall and the nacelle; an outer cowl circumferentially surrounding and radially spaced apart from the bypass inner wall relative to the principal rotational axis, wherein the outer cowl extends along the principal rotational axis between an inlet end and an outlet end, and wherein the outer cowl is disposed within the main bypass duct; a plurality of inlet vanes extending from the bypass inner wall towards the outer cowl and disposed proximal to the inlet end of the outer cowl, wherein the plurality of inlet vanes are circumferentially spaced apart from each other relative to the principal rotational axis; a plurality of outlet vanes extending from the bypass inner wall towards the outer cowl and disposed proximal to the outlet end of the outer cowl, wherein the plurality of outlet vanes are circumferentially spaced apart from each other relative to the principal rotational axis; a heat exchanger extending from the bypass inner wall towards the outer cowl and axially disposed between the inlet end and the outlet end relative to the principal rotational axis; a plurality of first removable shims corresponding to the plurality of inlet vanes, wherein each first removable shim from the plurality of first removable shims extends between and engages the outer cowl and a corresponding inlet vane from the plurality of inlet vanes; a plurality of second removable shims corresponding to the plurality of outlet vanes, wherein each second removable shim from the plurality of second removable shims extends between and engages the outer cowl and a corresponding outlet vane from the plurality of outlet vanes; and a third removable shim extending between and engaging the heat exchanger and the outer cowl, the third removable shim circumferentially extending around the principal rotational axis; wherein, for positionally adjusting the outer cowl relative to the bypass inner wall: each of the plurality of first removable shims is removable from between the corresponding inlet vane and the outer cowl; each of the plurality of second removable shims is removable from between the corresponding outlet vane and the outer cowl; and the third removable shim is removable from between the heat exchanger and the outer cowl. each of the plurality of first removable shims is removable from between the corresponding inlet vane and the outer cowl; each of the plurality of second removable shims is removable from between the corresponding outlet vane and the outer cowl; and the third removable shim is removable from between the heat exchanger and the outer cowl.