A method of controlling an air-cargo transport refrigeration unit comprising: an internal cargo space, and a refrigeration cycle comprising a compressor, a condenser for rejecting heat to ambient air and provided with a condenser fan, and an evaporator for cooling the internal cargo space; the method comprising performing a control loop including: a compressor speed regulation comprising: monitoring a temperature in the internal cargo space; regulating a speed of the compressor to maintain a rate of temperature decrease in the internal cargo space above a threshold rate during a temperature pulldown mode; the method characterised in that the control loop further includes: a first condenser fan regulation comprising: regulating a speed of the condenser fan based on monitoring a temperature of the ambient air; and a second condenser fan regulation comprising: regulating a speed of the condenser fan based on monitoring a discharge pressure of a refrigerant of the refrigeration cycle; wherein regulating the speed of the condenser fan based on monitoring a discharge pressure comprises: setting the speed of the condenser fan at a high condenser fan speed in response to a determination that the discharge pressure is in a high-pressure range; setting the speed of the condenser fan at a low condenser fan speed in response to a determination that the discharge pressure is in a low-pressure range; and setting the speed of the condenser fan at an intermediate condenser fan speed in response to a determination that the discharge pressure is in an intermediate-pressure range between the low-pressure range and the high-pressure range, and wherein within the control loop, the first condenser fan regulation is performed prior to the second condenser fan regulation, such that a result of the second condenser fan regulation is applied in preference to a result of the first condenser fan regulation. monitoring a temperature in the internal cargo space; regulating a speed of the compressor to maintain a rate of temperature decrease in the internal cargo space above a threshold rate during a temperature pulldown mode; a first condenser fan regulation comprising: regulating a speed of the condenser fan based on monitoring a temperature of the ambient air; and a second condenser fan regulation comprising: regulating a speed of the condenser fan based on monitoring a discharge pressure of a refrigerant of the refrigeration cycle; wherein regulating the speed of the condenser fan based on monitoring a discharge pressure comprises: setting the speed of the condenser fan at a high condenser fan speed in response to a determination that the discharge pressure is in a high-pressure range; setting the speed of the condenser fan at a low condenser fan speed in response to a determination that the discharge pressure is in a low-pressure range; and setting the speed of the condenser fan at an intermediate condenser fan speed in response to a determination that the discharge pressure is in an intermediate-pressure range between the low-pressure range and the high-pressure range, and regulating a speed of the condenser fan based on monitoring a discharge pressure of a refrigerant of the refrigeration cycle; wherein regulating the speed of the condenser fan based on monitoring a discharge pressure comprises: setting the speed of the condenser fan at a high condenser fan speed in response to a determination that the discharge pressure is in a high-pressure range; setting the speed of the condenser fan at a low condenser fan speed in response to a determination that the discharge pressure is in a low-pressure range; and setting the speed of the condenser fan at an intermediate condenser fan speed in response to a determination that the discharge pressure is in an intermediate-pressure range between the low-pressure range and the high-pressure range, and setting the speed of the condenser fan at a high condenser fan speed in response to a determination that the discharge pressure is in a high-pressure range; setting the speed of the condenser fan at a low condenser fan speed in response to a determination that the discharge pressure is in a low-pressure range; and setting the speed of the condenser fan at an intermediate condenser fan speed in response to a determination that the discharge pressure is in an intermediate-pressure range between the low-pressure range and the high-pressure range, and wherein within the control loop, the first condenser fan regulation is performed prior to the second condenser fan regulation, such that a result of the second condenser fan regulation is applied in preference to a result of the first condenser fan regulation.