Fatal fire, what is the flashover effect

At the origin of the tragedy at Crans-Montana is a phenomenon that marks the transition from a controllable fire to an out-of-control one: the so-called flashover, in the face of which even firefighters often have to retreat. Despite the devastation, fires of this type nevertheless leave valuable clues for reconstructing what happened, even though technical and scientific analyses can take weeks or months, explains Valeria Di Sarli, from the Institute of Science and Technology for Sustainable Energy and Mobility of the National Research Council (CNR-Stems).

A flashover 'is not an explosion, although the visual effect may be similar, and above all it is not generated by chance', notes the expert. This phenomenon, also known as a 'generalised fire', is the culmination of a precise combination of factors, on which thermal energy and the presence of combustible gases in an enclosed environment depend, resulting in the heating of all materials. 'The time in which it develops also depends on the conditions of the environment,' he adds. During a flashover, "the fire may start in a limited way, involving for example a single object such as a piece of furniture or a curtain, while hot fumes containing combustible gases, derived either from the combustion itself or from the pyrolysis of materials heated by the fire, rise towards the ceiling, forming a layer that gradually thickens and reduces visibility

The heat radiated by the smoke layer and the surfaces already involved causes the surrounding materials to heat up. When the temperature of the fumes and surfaces reaches around 500-600 °C, the gases and materials self-ignite and numerous objects, even far from the initial focus, catch fire almost simultaneously. Under these conditions, opening doors and windows encourages the entry of oxygen, which in an already critical phase further fuels the fire. Now the long investigation work begins, looking for clues as to why such a violent fire developed.

"First the initial focus has to be identified by identifying obvious traces such as deep or directional burns on surfaces. Then the path of the fire is analysed'. In reconstruction, "all elements are closely interrelated". This is an integrated analysis, in which 'it is also essential to consider the type and arrangement of combustible materials, such as fabrics and synthetics, and to assess possible ignition sources'. In the case of the Crans-Montana venue, initial suspicions focused on the pyrotechnic fountains and the suspended ceilings, which were often made of flammable materials.