Fig.5 Example of A380 Multi-Function Probes

aircraft. For redundancy purposes, temperature sensors have also been added in the A350 fuselage.

Fig.5 Example of A380 Multi-Function Probes

2.5.1.4 Humidity

Humidity sensors offer real opportunities to improve weather forecast on specific phenomena (clear air turbulence, icing, convection) and provide in-situ measures to evaluate climate changes. Only few aircrafts are already equipped with humidity sensors in Europe but WMO initiative E-AMDAR promotes the integration of hygrometric sensors to improve weather forecast. There are few humidity sensors integrated in European commercial aviation yet but offer a real interest to improve weather forecast. This section will first specify the needs in term of performance and then detail two available humidity sensors integrated in American commercial aviation for AMDAR operating system.

2.5.1.5 Onboard Weather radar

Weather radar is designed to detect precipitation: it helps to identify that associated with the most active convective cells in order to avoid the dangers associated with them (turbulence, hail and lightning). Weather radar can detect water in liquid form, such as rain and wet hail. However, it hardly detects water in solid form such as dry snow and ice crystals. It can partly detect dry hail depending on the size of the hailstones. In a convective cell, in the part situated below freezing point (0 °C, that mean FL 75 in standard atmosphere), liquid precipitation constitutes the most reflective areas. Below -40°C (at FL 275 in standard atmosphere) water no longer exists in general in a liquid state. In the part of the cumulonimbus between freezing point and the altitude where the temperature

reaches -40 °C, liquid water and ice crystals produce areas where reflectivity decreases depending on the variation of the presence of liquid water. In the part above the altitude where the temperature reaches -40 °C, where there are only ice crystals, reflectivity is very low. Areas returning most of the radar signal may be harmless for flight, like melted snow showers for example, whereas hail showers which constitute a genuine threat to navigation may only return a weak radar echo. When cumulonimbus clouds swell swiftly, they may be overtaken by a zone of severe turbulence which could stretch several thousand feet above the visible peak. This turbulence zone is invisible to weather radar and the naked eye (The TURB function, which uses the principle of the Doppler effect, only helps detection of turbulence in wet zones).