Investigators probing the Air France Airbus A330 crash in June are advising studying the composition of high-altitude clouds with a view to amending icing certification criteria.
French investigation agency Bureau d'Enquetes et d'Analyses' main lead in the inquiry has been evidence of unreliable airspeed readings during the final moments of flight AF447.
BEA's recommendation follows an examination of other unreliable airspeed events in cruise. It says it analysed 13 "significant" events involving five carriers using Airbus A330 and A340 aircraft, and BEA says it also had incomplete data on some 40 other instances.
These events showed certain similarities, it states. They occurred in highly-unstable air masses in the vicinity of deep convective weather phenomena.
In each case the autopilot disengaged. The aircraft experienced uncommanded altitude variations within a range of 1,000ft - although they stayed within their flight envelope - and the unreliable airspeed information persisted to a maximum of 3min 20s.
BEA says the analysis shows that most of the reported events occurred outside of European certification criteria which define the supercooled water-icing conditions against which pitot tubes must be protected.
These criteria include two icing envelopes: continuous cloud with low water concentration, rising to 22,000ft with temperatures down to minus 30°C, and intermittent cloud rising to 30,000ft with temperatures down to minus 40°C.
"The certification criteria are not representative of the conditions that are really encountered at high altitude - for example, with regard to temperatures," says BEA.
"In addition it appears that some elements, such as the size of the ice crystals within cloud masses, are little-known and that it is consequently difficult to evaluate the effect that they may have on some equipment, in particular the pitot probes."
BEA is recommending that the European Aviation Safety Agency conducts studies to determine high-altitude cloud composition more precisely and, with these results, modify the certification criteria.