There appears to be some difficulty in recognizing what has previously been theory and is now accepted as repeatable evidence, or that the established facts might apply to range of aircraft and engine types as circumstances change.
An earlier ‘peer reviewed’ article at
http://www.specinc.com/publications/...craft_1998.pdf provides an interesting ‘operational’ view in the summary; vis:-
“
Based on the available meteorological data from research aircraft, it appears that only the most vigorous thunderstorms. i.e. super cells and complexes of thunderstorms, are associated with anvils that contain IWC (Ice Water Content) > 1 gm m^3 in regions outside of the main core of the storm.
There is currently insufficient data to determine the threshold concentration of IWC that will induce rollback. Based on the fact that literally billions of hours have been logged by commercial turbofan engines in common cirrus and anvils, with IWC typically <-0.2 gm m^3 it would appear that these clouds do not contain sufficient IWC to cause rollback problems.
Also, it appears that rollback may be more likely to occur the longer the engines are exposed to relatively high mass concentrations of ice particles and snow at very cold temperatures.
If indeed the TAT temperature rise and associated power loss reported in 1974 was an incipient rollback, it should be noted that this is the only reported incident of this kind associated with a Boeing 747, and this could possibly be a result of the large extent of high concentrations of IWC in the anvil remnants of Tropical Storm Kerry.
While we are not in a position to make recommendations to pilots, we can concur that the standard practice (see FAA Circular 00-24B) of avoiding the regions of high radar reflectivity by at least 20 nm is advisable. In terms of rollback avoidance, the main reason to remain as far as possible from the region of high radar reflectivity is that measurements show that the IWC typically drops off sharply as a function of distance from the storm center. Also, any regions containing convection, i.e. rising cloud parcels or overshooting tops could contain super cooled liquid water, which may exacerbate the rollback problem.
Lastly, the probability of rollback incidents appears to increase with duration in areas of relatively high IWC, so that minimizing the time of encounter in these thunderstorm anvil regions may be advisable.”
The more recent
Boeing position, particularly slide 6, links the 100 (plus) Large Transport Engine events to ice crystals.
IMHO this presentation answers the originating questions.
Note that engine manufacturers do not sit back without action; -
General Electric Company (GE) CF6- 80C2A5F Turbofan A300, but also 747/767 if applicable.
Whilst there may not be specific advice in airframe manuals because defensive measures are already in place, this should not be taken to mean that a problem cannot be encountered.
We all require continued professional enquiry into atmospheric threats, particularly where we might not know how close to the edge of a safe boundary we are currently operating, e.g. a small change in operating procedures for air bleed during the cruise – driven by economics, might cross an ‘icing’ boundary.
In many BAe146 rollback incidents, the FDR showed that crews were unaware of other incipient events; a single engine event was often a multiple event. So in the larger engines with more tolerance to such conditions crews might encounter ice crystals and accrete ice, but due to the different concentration or duration of encounter they do not suffer visible symptoms.
The latter might be theory – possibly just speculation, but it is something that I wish to bear in mind when flying in such conditions.