Understanding clear air turbulence
Like death and taxes, clear air turbulence (CAT) is seemingly inevitable and certainly something most of us would rather avoid. While it isn't likely that Uncle Sam or the Grim Reaper will change their longstanding policies anytime soon, progress has been made in helping pilots avoid CAT. The latest advances reflect a variety of approaches to the problem. Results are best described as incremental improvements that over time should yield significant benefits to pilots just looking for a smooth ride.
The term CAT is self explanatory, but as with most blanket labels, the devil is in the details. It can occur at any altitude, but according to the Aeronautical Information Manual, CAT is of particular concern to aircraft flying in excess of 15,000 feet. At lower altitudes, turbulence is most often the result of convection from solar heating of the Earth or the collision of moving air masses with rough terrain and other obstacles. Although these phenomena can cause CAT at high altitudes as well, jet stream-induced turbulence is unique to high-altitude CAT.
Jet streams, often described as high-altitude rivers of fast-moving air that encircle the Earth, don't truly behave as rivers do. Unlike the Mississippi, for instance, they change position regularly. Meteorologists have identified several jet streams that exist on a more or less permanent basis. Two in particular, the polar front jet stream and the subtropical jet stream, have the greatest effects on high-altitude air traffic over the United States. In the winter months the polar front jet stream tends to migrate southward and the subtropical jet stream moves northward, potentially placing either or both over portions of the country.
Jet streams move fast, sometimes very fast. Winds in the core of the polar front jet stream often exceed 100 knots, and in the winter months over the United States 180- to 200-knot speeds are not unheard of. The velocity of the wind itself does not cause turbulence, and in fact the ride inside the core of a high-speed jet stream can be deliciously smooth. It is the rapid change in speed or direction of the wind — in other words wind shear — that causes a rough ride.
This can happen for various reasons. When a jet stream collides with a thunderstorm standing in its way, turbulent airflow on the downwind side of the storm is the usual result. This is perhaps the easiest kind of CAT for pilots to avoid, since the storm itself, especially one topped with a telltale anvil shape, serves as a billboard of sorts advertising the likely presence of turbulence. It's good practice to give such a storm a wide berth of 20 to 30 miles, preferably on the upwind side. Jet streams also can collide with other jet streams, and this too can make life miserable for pilots and passengers. Similarly, sharp bends in a jet stream can result in localized regions of CAT.
Interaction of jet streams with the tropopause layer is another common recipe for CAT. The tropopause is the boundary between the troposphere and the stratosphere, and its height varies by latitude and season. In terms of flight levels, the tropopause is often found around the mid-20s over the poles and in the low- to mid-50s at the equator. During the winter months over the United States, it tends to sit pretty much smack in the middle of typical cruise flight levels for jet aircraft, around the mid-30s. Combine this with the presence of the intensified polar front jet stream dipping well into the lower 48 states, and it is easy to see why the majority of pilot reports of CAT are received during wintertime.
While the general cause-and-effect relationship between jet streams and CAT has been understood for many years, more recent research undertaken by the National Center for Atmospheric Research (NCAR) has uncovered a new twist — literally — on the problem. Under certain conditions, violently twisting tornadolike funnels of air "peel off" from jet streams and induce strong CAT. Called horizontal vortex tubes (because they tend to rotate on their side, unlike a vertically oriented tornado), they can be visualized as a sort of wingtip vortex on steroids. This phenomenon was detected a decade ago and is the focus of weather researchers who would like to be able to predict when and where it might form.
Like any other weather forecasting, predicting the location and intensity of CAT is only as good as the available data. One program developed by NCAR involves using airliners to vastly increase the amount of real-time turbulence data available to forecasters. Using the aircraft's existing flight management system (FMS) sensors and avionics, various atmospheric parameters are measured continuously during the flight. This data is then massaged by special software programmed into the FMS that translates (he data into a measure of turbulence intensity. Once a minute the information is datalinked to the National Weather Service and other users through the aircraft's ACARS — Aircraft Communications Addressing and Reporting System.
Imagine many hundreds of aircraft equipped with this capability and it's easy to see why some weather researchers believe their ability to accurately forecast CAT should increase markedly as more aircraft participate. Think of it as a means to automatically generate large numbers of pireps without actual pilot input. For now, though, the program involves a relatively small number of aircraft. The eventual goal is to get this real-time information to pilots well before they fly into an area of CAT.
Another CAT detection effort is being mounted by the major avionics manufacturers. Weather radar does a pretty good job of detecting moisture, and not such a good job of detecting ice crystals. Generally, the more moisture it detects (a huge thunderstorm, for instance), the more likely it is that turbulence lurks within. However, specific turbulence modes available on some radar units infer the presence of turbulence by analyzing the relative motion of even tiny amounts of moisture. The magic is in the mathematical modeling contained within the unit's software. Thus, in certain instances, a radar's turbulence mode can spot turbulence that would qualify as CAT. But these modes are only available at the radar's shorter-range settings (generally 40 nm or less), and thus don't solve the problem of long-range detection and avoidance. Nor can they help much when there is no "wet" moisture to speak of, as is frequently the case at high altitudes where any water is likely to be in the form of ice crystals.
Both Honeywell and Collins are also spending research-and-development dollars on systems that use either passive infrared or LIDAR (light-detection and ranging radar) sensors to detect CAT. These not-quite-ready-for-prime-time technologies are still a year or more away from product launch. But they may one day provide at least several minutes' real-time warning of imminent CAT encounters to pilots.
If avoidance isn't possible, pilots should slow to the appropriate maneuvering speed (V A) for the aircraft. In light aircraft, this is the maximum speed at which full abrupt control deflection can be made without exceeding the design load factor. The wing may momentarily stall during turbulence-induced G loading, but it won't break. Jet aircraft generally use so-called turbulence penetration or rough air speeds and Mach numbers that can vary by altitude. These are compromise speeds that provide a margin above stall speed while still providing some protection from structural damage. It's important to remember that G forces caused by aircraft maneuvering can be compounded by turbulence-induced G forces, so a run-in with high-altitude CAT isn't the time to practice steep turns.
A review of the NTSB accident database shows that CAT can and does cause physical injuries to persons on a regular basis. The FAA estimates nearly 60 such injuries happen in an average year. Most tend to be minor, although serious injuries, and on rare occasions deaths, have resulted. Often the injuries occur to passengers or flight attendants standing near the back of the aircraft, where yawing and pitching moments induced by turbulence around the aircraft's CG are greater than over or forward of the wing.
The experience of the crew of a Dassault Falcon 900B that encountered moderate CAT during descent from Flight Level 310 is perhaps typical of those flights where injuries have occurred. During the bout with rough air, a standing flight attendant fell, breaking her ankle. The aircraft itself was undamaged.
CAT is nothing to be toyed with and aircraft have been lost because of it. The pilot of a Rockwell Commander 690C turboprop paid the ultimate price for exceeding V A in an area of known severe turbulence. The right outer wing separated from the aircraft during descent, along with the horizontal and vertical stabilizers. Investigation revealed that the aircraft had not only exceeded V A8 but also had been at or near its never exceed speed (V NE) at the time it broke apart.
Vincent Czaplyski is a Boeing 737 captain for a major U.S. airline.
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