Aircraft Icing : Airborne Measurements for Certification Purposes

A cloud in the earth's atmosphere can be considered as the condensed portion of a fraction of the water vapour present. Condensation processes lead to the formation of hydrometeors of sufficiently small dimensions (from a few microns to 500 microns) for ambient molecular agitation to keep them in suspension.

In the first layer of the atmosphere (troposphere), which extends from the ground to an altitude of around ten kilometers, the air temperature profile decreases from +15 to – 60°C (case of a standard atmosphere).

The hydrometeors formed may therefore be located in a negative-temperature environment. In the case of liquid hydrometeors, i.e. droplets, freezing is not systematic, even if the internal temperature of the droplets is below the solidification point. This state, known as supercooling, is a metastable state that ceases very abruptly at the slightest disturbance.

Air purity conditions and cloud droplet formation processes mean that this phenomenon is relatively common in the atmosphere, while on the ground it is exceptional.

The passage of an aircraft through this type of cloud creates the disturbance required to freeze the droplets on the leading edges, causing accretion, firstly by modifying their geometry and consequently the aerodynamic qualities of the aircraft, and secondly by overloading the aircraft due to the frozen mass. Without an appropriate de-icing system, both phenomena can lead to the loss of the aircraft.

The European Aviation Safety Agency (EASA) is behind the current regulations governing civil aviation safety in Europe. These regulations stipulate that all aircraft manufacturers must demonstrate the ability of an aircraft to fly in natural icing conditions, in order to achieve the required level of safety.

Part of the demonstration may involve a flight test phase, in which icing conditions are sought, the severity of which is assessed on the basis of the cloud characteristics encountered. The values of these cloud characteristics are shown on the charts annexed to the regulations. The manufacturer must demonstrate that flight tests have made it possible to explore this regulatory area.

To achieve this, flight tests have to be organized so as to measure the cloud characteristics needed to assess the severity of icing. The test aircraft is therefore equipped with a set of specific measuring instruments.

The purpose of this article is to describe these instruments and explain their use.

First, the notion of icing conditions will be defined from a physical and aeronautical point of view. The specific instrumentation used to explore icing conditions will then be presented,...