An air cargo power drive unit has a motor, at least one driver roller element coupled to said motor, a light source, a light detector, and a processor having memory associated therewith, said memory storing instructions. The device is configured to emit light from the light source, receive reflected light from the light detector when an air cargo is overhead, and convert the detected light into a time series of a digital samples representing a time-varying intensity of the received light. The processor then performs calculations on the digital samples to determine whether the unit load device is moving. This determination may be based, for instance, on spikes among the digital samples, and/or on first, second, or even higher-order, statistics of the detected samples.