Cirrus clouds may give clues to risks of clear air turbulence

One of the worst things about flying before the jet age began on a large scale in the late 50s was turbulence, causing air sickness.

But there are warnings, as well as some statistical claims, that things are getting rougher than before for the smoothness of jet airliners that has long been taken for granted by generations of air travellers.

This week’s ‘return to your seat and buckle up’ contribution to the genre comes from America’s ABC. It is based on this article in Geophysical Research Letters, and as is often the case, the general news story has a high tease to factual content ratio that makes it perhaps less than useful to the curious who want to know what it is really about and how it might affect them.

It’s an analysis as to how increased upper atmosphere turbulence attributable to climate change is potentially lifting the risk of clear air turbulence, or CAT, which first came to the attention of the mass of air travellers with the shift to faster, higher flying jets because they flew where the propeller driven aircraft of old couldn’t venture.

The days of a Lockheed Electra or Vickers Viscount being full of passengers chundering into airsick bags while bouncing around for more than an hour below or right in the middle of clouds between Sydney and Melbourne were replaced by the velvet like ride of higher reaching 727s.

However at infrequent intervals, something like the CAT’s ‘claws’, as headline writers liked describe them, would strike with zero warning where the 707s and DC-8s and Comet IVs went, far above the bumpy flight levels below at around 26,000 feet. CAT can strike with enough violence to punch unrestrained passenger heads through ceiling panels, and break arms and legs. The ‘attack’ was often over in seconds, but the results could be devastating, and the certification strength and resilience of airframe components were raised to levels in later designs to where the risk of severe structural damage was much less likely.

But these events continue to happen, and according to the study, with rising frequency, and come with do-not-exceed rules that mean compulsory component inspections and any necessary repairs or replacements are required when the load or gust data recorded during a flight crosses the limits imposed by the regulations.

Recent generations of regular flyers have withstood these events with less damage to themselves by taking to heart the admonition to keep seat belts loosely fastened even when the warning sign is off.

Which means this is an emerging issue of concern. It is not like the other more common forms of in-flight turbulence that have been ameliorated by the active gust alleviation systems that all the latest Airbuses and Boeings use to smooth out the type of bumps that can be detected or predicted by on-board sensors and massaged away by rapid changes to ailerons, elevators and rudders.

Those fast and frequent systems driven adjustments to these control surfaces soften, confine or even eliminate the up, down and sideways jerkiness that can sometimes intrude on a flight. But if this study highlighted in the general media recently is correct, the CAT’s claws will become much more of a problem, not just because more people are flying more often, but because of more pent up heat energy destabilising the higher flight levels their jets are using.

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