As epitomized in DARPA's 'Augmented Cognition' program, next generation avionics suites are envisioned as sensing, inferring, responding to and ultimately enhancing the cognitive state and capabilities of the pilot. Inferring such complex behavioural states from imagery of the face is a challenging task and multimodal approaches have been favoured for robustness. We have developed and evaluated the feasibility of a system for estimation of cognitive workload levels based on analysis of facial skin temperature. The system is based on thermal infrared imaging of the face, head pose estimation, measurement of the temperature variation across regions of the face and an artificial neural network classifier. The technique was evaluated in a controlled laboratory experiment using subjective measures of workload across tasks as a standard. The system was capable of accurately classifying mental workload into high, medium and low workload levels 81% of the time. The suitability of facial thermography for integration into a multimodal augmented cognition sensor suite is discussed.