Over the last few years, wireless sensor networks (WSN) have been considered for various aeronautical applications, including Structural Health Monitoring and flight tests. Isolated from the onboard electrical network, each sensor node needs to be self-powered. Environmental concerns, together with economical and safety-related issues, often prohibit the use of electrochemical batteries. However, energy may be harvested in the sensor's environment, providing a reliable and sustainable electrical power source. Nevertheless, storage is needed both to accommodate for transient power surges and to maintain the energetic autonomy in case of intermittent environmental source. Ultracapacitors (UCs) are good candidates for doing the job. Although being affected by extreme ambient temperatures similarly to batteries, they are safe, provide an infinite lifetime and are environment friendly. In this talk, we will present different case studies of energy harvesting coupled to ultracapacitors for autonomous battery-free wireless sensing in aeronautics applications. In this work carried out in cooperation with AIRBUS, we will highlight the main challenges that arise in such battery-free systems in two types of applications : flight-testing and aircraft health monitoring (AHM). Battery-free wireless system used for flight-testing implemented on an aircraft wing (left view). It is composed of a self-starting energy management system (right view), a thin-film photovoltaic energy source coupled to ultracapacitors (UCs) powering a matrix of pressure sensors and real-time data wireless communication. The main challenges of flight-testing application are that, on the one hand, the wireless sensing system should not disturb the aerodynamics of the aircraft and that, on the other hand, it will be submitted to an environment, which is not pressurized and not regulated in temperature. Moreover, data rate and then power consumption is relatively high since real-time sensing is required. Fortunately, flight-tests are generally performed during daylight and photovoltaic energy harvesting can be used. In AHM applications, the main issues is that the locations that need for monitoring are submitted to very severe conditions in terms of temperature, vibrations, lightning and corrosion. An example of energy harvesting based on thermogeneration will be presented for health monitoring of the aircraft engine. We will end this talk with some perspectives of such battery-free wireless sensing systems.