Energy-autonomous wireless sensor nodes (WSNs) in aircraft, acting as health monitoring systems (HMS), have the potential to reduce aircraft maintenance costs. Thermoelectric energy harvesting is a solution for self-powered systems, since it captures enough energy to power up a WSN. The energy harvesting device used in this work consists of a thermoelectric generator (TEG) attached to the inner part of the fuselage and to a thermal storage device, in order to artificially enhance the temperature difference between the bottom and the top surface of the TEG during take-off and landing. In this study, the results of 28 flight tests during a 6-month flight campaign of two identical energy harvesting devices are presented. The results are clustered into two different classes, each having its own characteristics. The two classes comprise typical, similar to standard European short/mid-range flights, as well as atypical flight profiles, where specific flight tests have been performed. In addition, for each class, different parameters such as flight altitudes, flight duration, and temperature profiles are investigated. Moreover, a detailed comparison between a typical and an atypical flight profile is given. In general, for a typical flight profile, the experimental results are in good agreement with simulations predicting the energy output. The average energy output is sufficient to power up a wireless sensor.