Abstract: An air operated heat exchanger has a plurality of process tubes for process fluid, a plurality of rotating fans to move ambient air along an air stream path past the plurality of process tubes. At least one optical fibre is configured within the one or more air stream paths. At least one light pulse is passed into the at least one optical fibre, and at least one optical signal is detected from the at least one optical fibre in response to the at least one light pulse, to provide at least one signal profile. One or more air temperatures at a plurality of locations along the at least one optical fibre are determined from the at least one signal profile and evaluated against one or more comparison operational conditions.