A method of monitoring an air operated heat exchanger (100, 200), said air operated heat exchanger comprising: a plurality of process tubes for process fluid, a plurality of rotating fans to move ambient air along one or more air stream paths past the plurality of process tubes, and characterized by at least one optical fibre configured within the one or more air stream paths, the optical fibre comprising a first optical fibre located in a first layer in the air stream path downstream of the process tubes to provide downstream locations (17a-i), the optical fibre comprising a second optical fibre located in a second layer in the air stream path upstream of the process tubes 25 to provide upstream locations (37a-i), the first layer and the second layer being within the air operated heat exchanger (100, 200), the method comprising the steps of: - passing a light pulse into the optical fibre; - detecting an optical signal from the optical fibre in response to the light pulse, to provide a signal profile; - determining from the signal profile air temperatures of the air as it is being moved along in the one or more air stream paths, at pairs of locations (17a-i) and (37a-i) along the optical fibre; - correlating said air temperatures to monitor the operation of a particular rotating fan and the cooling provided to the corresponding segment of the process tubes; and - evaluating at least one of the air temperatures from at least one of the plurality of locations against one or more comparison operational conditions. a plurality of process tubes for process fluid, a plurality of rotating fans to move ambient air along one or more air stream paths past the plurality of process tubes, and characterized by at least one optical fibre configured within the one or more air stream paths, the optical fibre comprising a first optical fibre located in a first layer in the air stream path downstream of the process tubes to provide downstream locations (17a-i), the optical fibre comprising a second optical fibre located in a second layer in the air stream path upstream of the process tubes 25 to provide upstream locations (37a-i), the first layer and the second layer being within the air operated heat exchanger (100, 200), the method comprising the steps of: - passing a light pulse into the optical fibre; - detecting an optical signal from the optical fibre in response to the light pulse, to provide a signal profile; - determining from the signal profile air temperatures of the air as it is being moved along in the one or more air stream paths, at pairs of locations (17a-i) and (37a-i) along the optical fibre; - correlating said air temperatures to monitor the operation of a particular rotating fan and the cooling provided to the corresponding segment of the process tubes; and - evaluating at least one of the air temperatures from at least one of the plurality of locations against one or more comparison operational conditions. - passing a light pulse into the optical fibre; - detecting an optical signal from the optical fibre in response to the light pulse, to provide a signal profile; - determining from the signal profile air temperatures of the air as it is being moved along in the one or more air stream paths, at pairs of locations (17a-i) and (37a-i) along the optical fibre; - correlating said air temperatures to monitor the operation of a particular rotating fan and the cooling provided to the corresponding segment of the process tubes; and - evaluating at least one of the air temperatures from at least one of the plurality of locations against one or more comparison operational conditions.