Turbulence – One of five influences in route optimization

In this post we continue to examine different influences on flight routing for commercial air transports. Our five categories are: (i) atmospheric winds and temperature, (ii) atmospheric turbulence, (iii) airspace closures, (iv) traffic loading of the national airspace system, and (v) tactical avoidance of neighboring traffic. In this post we look at the second category: atmospheric turbulence.

Atmospheric Turbulence, Storms, and Convective Weather

The definition of turbulence is complicated, but everyone knows what it feels like. A smooth, steady ride suddenly becomes bumpy and chaotic when an aircraft encounters atmospheric turbulence. Passengers dislike the experience, pilots work hard to avoid it, and extreme cases can damage aircraft and pose a safety risk.

The most obvious source of turbulence is stormy weather, or what meteorologists refer to as convection. Convective weather systems show up on weather radar as familiar yellow, orange and red regions. It often is an organized system, such as a rotating cell or line storm stretching for hundreds of miles. Other systems are less organized, such as smaller regions of convection weather scattered over a geographic area. Meteorologists refer to such systems using the less technical term: “popcorn.”

Detecting Turbulence

Regardless of how convection is organized, it can be detected with weather radar that reflects off the water in the atmosphere. Those familiar yellow, orange and red colors on the radar display simply represent the strength of the reflected radar signal. The stronger the reflection, the more water present in the atmosphere.

But water and turbulence are different things. In fact, atmospheric water, by itself, is not a problem for aircraft. You may have flown through rain and not felt a bit of turbulence. The reason why we sense atmospheric water in our weather radars is because it is a good indicator of turbulence. Turbulence, by its very nature, is rather difficult to sense and measure directly. Atmospheric water, on the other hand, is relatively straightforward to sense and measure. And where there’s water in the atmosphere (at least a lot of water), there is turbulence.

This is not to say that those orange cells are 100% full of turbulence. It simply means that turbulence is prevalent in such regions. It also is not to say that where there is no water, there is no turbulence. Turbulence can also occur in volumes of air not showing any appreciable water content. Often such turbulence is nearby a convective system and is considered to be associated with it. But the fact that turbulence can occur outside of a convective system, as defined by the water content, means that it is important for pilots and route planners to understand that while atmospheric water is a good indicator of turbulence, it is by no means perfect. Adding a safety buffer around convective systems is good practice. Beyond this, it also is possible for turbulence to occur in perfectly clear air, not even close to a convective system. We will look at that kind of turbulence next time.

Aircraft Routing

Turbulence encounters can cause more problems than merely some discomfort. If an airframe encounters sufficiently strong turbulence, it may be required to undergo an unscheduled maintenance and safety check. This can be a costly interruption to an otherwise smooth-running airline schedule.

So it is not surprising that flight routes are carefully tailored to avoid convective weather systems present that day. But since routes are designed hours before departure, they must use forecasts of the convection. This presents a problem because convective weather can be quite difficult to forecast accurately. This has been an active area of research, and meteorologists have improved such forecasts in recent years. 

Nonetheless, preflight routes are quite conservative, adding plenty of buffer, and maintaining a significant separation between the aircraft and any potential path the storm may take. This means that once airborne, there often is a rerouting opportunity to save flight time and fuel, as the location and path of the storm is known more accurately.

This problem of airborne rerouting around convection can be complicated. Even though the convective weather picture is more accurate, the geometry of the weather system, and flight trajectory, often is complex. And even though the aircraft is en route, its encounter with the weather nonetheless may be an hour or more in the future, so the forecast of the convection remains critical.

There are many details to this problem that are important to consider including how pilots tend to respond to weather, how airframes respond to turbulence, airline policy, the weather forecast, the business case and preferences of the flight, and of course the trajectory geometry and dynamics. It is a multi-dimensional problem involving many different factors. For instance, in any given scenario (and no two are alike), there are a multitude of different possible solutions, each with its own advantages and disadvantages. And oftentimes the best solution may be one that a planner might never have thought of. Furthermore, there can be substantial difference in the economic savings of different routes that might be selected. This is why SmartRoutes is such a powerful rerouting tool. It understands and solves this complicated problem, saving time, fuel, and money for operators and passengers.‍

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Next time we’ll look at the clear air turbulence problem.