A high altitude long endurance aircraft comprising: one or more first heat-generating components and one or more second components, the one or more first heat-generating components and second components requiring thermal management; a thermal management apparatus comprising: a chassis, in the form of a hollow tube, in thermal contact with the one or more first heat-generating components and the one or more second components, the first heat-generating components and second components being thermally coupled with the chassis; characterised in that it further comprises a flow control unit for modifying a fluid flow path through the chassis; one or more first temperature sensors arranged to measure the temperature of the one or more first heat-generating components; one or more second temperature sensors arranged to measure the temperature of the one or more second components; and a controller configured to compare the measured temperature of the one of more first heat-generating components with a first threshold temperature and the measured temperature of the one of more second components with a second threshold temperature and, if the measured temperature of the one or more first heat-generating components is higher than the first threshold temperature and, if the measured temperature of the one or more second components is less than the second threshold temperature, operate the flow control unit to permit a fluid to flow through the chassis to transfer heat from the one or more first heat-generating components to the one or more second components. one or more first heat-generating components and one or more second components, the one or more first heat-generating components and second components requiring thermal management; a thermal management apparatus comprising: a chassis, in the form of a hollow tube, in thermal contact with the one or more first heat-generating components and the one or more second components, the first heat-generating components and second components being thermally coupled with the chassis; characterised in that it further comprises a flow control unit for modifying a fluid flow path through the chassis; one or more first temperature sensors arranged to measure the temperature of the one or more first heat-generating components; one or more second temperature sensors arranged to measure the temperature of the one or more second components; and a controller configured to compare the measured temperature of the one of more first heat-generating components with a first threshold temperature and the measured temperature of the one of more second components with a second threshold temperature and, if the measured temperature of the one or more first heat-generating components is higher than the first threshold temperature and, if the measured temperature of the one or more second components is less than the second threshold temperature, operate the flow control unit to permit a fluid to flow through the chassis to transfer heat from the one or more first heat-generating components to the one or more second components. a chassis, in the form of a hollow tube, in thermal contact with the one or more first heat-generating components and the one or more second components, the first heat-generating components and second components being thermally coupled with the chassis; characterised in that it further comprises a flow control unit for modifying a fluid flow path through the chassis; one or more first temperature sensors arranged to measure the temperature of the one or more first heat-generating components; one or more second temperature sensors arranged to measure the temperature of the one or more second components; and a controller configured to compare the measured temperature of the one of more first heat-generating components with a first threshold temperature and the measured temperature of the one of more second components with a second threshold temperature and, if the measured temperature of the one or more first heat-generating components is higher than the first threshold temperature and, if the measured temperature of the one or more second components is less than the second threshold temperature, operate the flow control unit to permit a fluid to flow through the chassis to transfer heat from the one or more first heat-generating components to the one or more second components.