Avoiding the bumps

You’ve enjoyed your meal and a glass of wine, and you’re sitting back feeling a bit dozy, watching a favourite film. Suddenly there’s an announcement to buckle up because it’s going to get bumpy.

We’ve all experienced turbulence and wished it would stop. Yet long-haul aircraft are built like battleships, to standards that exceed by many times the extra loading that may be placed on them by heavy turbulence.

Atmospheric scientists have warned that climate change will affect the paths of the midlatitude jetstreams, those highaltitude snaking corridors of extreme wind, up to 300kmh, that pilots like to use when they provide a tailwind to speed up flights and save on fuel.

Their studies show jetstreams in the northern and southern hemispheres are also expected to strengthen at the height where aircraft generally cruise – between 10,000 and 12,000 metres up – due to the effect climate change will have on temperatures at different heights.

As a result, sharper vertical wind shears – the change of wind direction and speed with altitude – are likely to increase the frequency and intensity of clearair turbulence (CAT) along the edges of the jetstreams. That would make flights through that region bumpier and lower aircraft efficiency, especially if pilots are forced to change route or fly lower or higher to avoid the turbulence.

In a May 2017 paper in

Advances in Atmospheric Sciences, University of Reading meteorologist Professor Paul Williams concluded that on trans-Atlantic flights in winter under climate change, the area with light CAT would increase by 59 per cent, light-to-moderate CAT by 75 per cent, moderate CAT by 94 per cent, moderate-tosevere CAT by 127 per cent and severe CAT by 149 per cent.

Some had estimated there were more than 63,000 encounters with moderate or greater turbulence globally a year, and 5000 with severe or worse turbulence, the paper said.

Aviation meteorology defines moderate turbulence as causing changes in an aircraft’s altitude or attitude, along with variations in airspeed, but a pilot remains in control of the plane. In severe turbulence, however, a pilot can lose control of the plane for several seconds.

Turbulence associated with jetstreams may last for minutes, or even hours if the route of the flight is parallel with the edges of the jetstream.

Williams told Stuff the increase in winter jetstream turbulence with climate change was worse because that was the time of year when the biggest temperature gradients were driving the winds.

The modelling also showed a rise in CAT turbulence in summer but it was not as significant.

For pilots flying long-haul routes from New Zealand, the main regions of significant highaltitude turbulence are the Inter Tropical Convergence Zone (ITCZ), within about 10 degrees of latitude on either side of the equator, and on the margins of jetstreams.

The ITCZ is the region where the trade winds of the northern and southern hemispheres meet. The uplift from those converging winds forms severe thunderstorms along both sides of the equator. Aircraft cruising at 12,000m-13,000m can bounce along through the icy tops of those cumulonimbus clouds for hours, depending on the angle of their route across the equator.

Air New Zealand’s chief operational integrity and standards officer, Captain David Morgan, operates the Boeing 787-9 Dreamliner on long-haul routes.

He told Stuff a combination of modern aircraft technology and radar, accurate en-route weather forecasts and communications from other pilots allowed flight crew to minimise the exposure of passengers to significant turbulence.

The flight deck radar units also indicated shafts of hail and areas of lightning, both associated with thunderclouds and likely heavy turbulence.

‘‘The problem with the air is it is transparent and we can’t see the rivers of air flowing around. But most turbulence is wellpredicted and measured.

‘‘We like to get into the jetstream but if you have to come out of the jetstream, it’s best to do it quickly.’’

Radar units in the flight deck showed what was ‘‘a threat to aircraft’’. ‘‘We wouldn’t cross the ITCZ without the radar. If you have it, you can pick your way around the cells quite easily.’’

Most turbulence is well-predicted and measured.