What is it? Where is it?
Each year, rough-and-tumble encounters regularly take place between high-altitude aircraft and an invisible but insidious danger - clear air turbulence. By definition, CAT occurs away from visible moisture, and so is impossible to avoid by visual means alone. It often appears without warning. Unrestrained persons and objects can be thrown violently about the cabin, and serious injuries, including broken bones, have been caused by it. Last December, a passenger aboard a Boeing 747 died from head injuries suffered when the aircraft flew into an area of severe CAT over the Pacific Ocean. Besides wreaking mayhem on occupants, severe CAT can inflict damage to the aircraft itself, and can sometimes precipitate the aircraft's departure from controlled flight.
One skirmish between a jet and CAT, which took place several years ago, illustrates the seriousness of the problem. A Boeing 727 en route from the island of Nassau, Bahamas, to Atlanta, Georgia, experienced sudden and severe CAT while cruising at FL370. The jet rolled steeply right, then left. Before the crew regained control, the ainflict damage to the aircraft itself, and can sometimes precipitate the aircraft's departure from controlled flight.
One skirmish between a jet and CAT, which took place several years ago, illustrates the seriousness of the problem. A Boeing 727 en route from Nassau, Bahamas, to Atlanta, experienced sudden and severe CAT while cruising at FL370. The jet rolled steeply right, then left. Before the crew regained control, the aircraft had lost 1,500 feet of altitude and a number of passengers were hurt. Like many reported instances of CAT, the crew had no warning that conditions would change so abruptly. During the climb, the flight had experienced only light chop. Weather forecasts did not predict significant turbulence, and the crew was operating in clear and smooth meteorological conditions just prior to the encounter.
Aboard domestic airline flights, an average of 58 persons are injured each year due to CAT. By far, most of these injuries happen to persons not wearing seatbelts. Experience shows that unrestrained passengers in rear sections of large aircraft are more likely to be injured than their counterparts located closer to the center or forward sections. Pitching and yawing moments caused by turbulence occur around the aircraft's CG. Since the tail is located farther from the CG than other sections of the aircraft, it has a longer arm, and thus potentially greater leverage, when an aircraft moves suddenly about these axis. The result can be higher G loads in aft sections of the aircraft than center or forward sections. Pilots need to be aware that what feels like only light chop in the cockpit may be felt as moderate turbulence in row 32.
Fortunately, the vast majority of CAT reported by pilots is the light to moderate variety. However, flight data recorder readings taken from aircraft that experienced severe CAT show that G forces ranging from -2.0 G to +3.0 G are common. Such forces are strong enough to launch people and objects about a cabin. Aboard one Douglas DC-8, a peak force of +4.5 Gs was recorded. Actual structural damage is rarely caused directly by CAT. However, pilot attempts to recover from unusual attitudes following a CAT-induced upset can place additional loads on an aircraft, and have resulted in such damage.
Although CAT can be present at almost any altitude, the most attention-getting jousts with jet aircraft tend to happen at the higher flight levels. Statistics from the National Transportation Safety Board, which defines an aircraft accident as an occurrence in which any person suffers death or serious injury - or in which the aircraft receives substantial damage - indicate that about two-thirds of turbulence-related accidents take place at FL300 and above.
What causes CAT? Interaction between a fast-moving jet stream and some impediment to its smooth flow are the basic ingredients. These obstacles may be an air mass of a different temperature and density than the jet stream itself, a thunderstorm standing in its way, or a tall mountain range literally poking into the flow. Frontal activity and sharp bends in the jet stream can disturb it too. Regardless of the source of the disturbance, CAT is the byproduct.
The level of the tropopause often has a direct bearing on the altitudes where CAT may be found. The tropo-pause is the boundary separating the troposphere from the higher stratosphere. It is the altitude at which air temperature stops getting colder with increasing altitude. When a jet stream encounters this boundary layer, the result is drag on the jet stream flow, which can produce turbulence.
The height of the tropopause varies with both latitude and season. It ranges from an average of FL240 near the equator to FL550 near the poles. During the winter in the United States, the tropopause descends and is found closer to the typical cruise altitudes of jet aircraft than it is in summer. Combined with the fact that jet streams themselves tend to be at their strongest in winter, the potential for CAT increases. This relationship is borne out by the increased number of encounters between aircraft and CAT reported in winter months. By one estimate, they take place three to four times more frequently in the winter than in the summer.
Pilots of high-altitude aircraft can look for certain clues during the flight-planning process to help them avoid the most likely areas of this type of CAT. Experience has shown that CAT associated with the tropopause frequently occurs several thousand feet below it. If the aircraft is capable of climbing above the tropopause, smoother conditions can sometimes be found just above it.
Vertical windshear - rapid changes in wind speed between closely spaced altitudes - can also be an indicator of bumps in the road. Similarly, rapid changes in wind velocity at a given flight level over relatively short distances should raise the red flag as well. Perhaps the most predictable form of CAT is that associated with the interaction of the jet stream with tall thunderstorms. Pilots can generally expect the downwind side of a thunderstorm to be turbulent at high altitudes, and should remain 20 to 30 miles from the cell at all times. Certain mountainous regions regularly produce turbulent mountain wave activity when strong jet streams exist. Jeppesen depicts these areas with a brown diamond symbol on some high-altitude charts, which can be useful preflight planning information as well.
Computer-generated flight plans that display forecast winds, vertical shear values, and tropopause levels for en route waypoints make it a fairly simple matter for flight-level-bound pilots to get a quick overview of the expected CAT picture for a flight. Lacking such niceties, pilots can derive the same information from a variety of weather products, including upper level winds aloft and tropopause charts. Sigmets and area forecasts often contain information useful in gauging the likelihood of turbulence.
While such planning tools are helpful, they can suffer from being overly broad. CAT itself is often highly localized. Thus, the best preflight and in-flight CAT avoidance tool is often the pirep, which can detail the specific location and severity of CAT. Generally, pilots are quick to report bouts with turbulence to ATC. Pireps are real-time indicators of what actually exists, as opposed to what is forecast. While ATC often volunteers this information, pilots should proactively seek it out. Keeping on top of pireps allows a pilot to request a route or altitude change at the earliest possible time. Waiting for ATC to offer a smoother flight level may mean being put at the end of a long list of other aircraft vying for the same airspace.
Sometimes it just isn't possible to find a smooth flight level, and pilots of jet aircraft must employ procedures designed to minimize some of the dangers posed by CAT. The first step is to set thrust so as to attain the turbulence penetration airspeed or Mach (VB) appropriate to the aircraft and altitude. This is a compromise speed, intended to protect against structural damage to the aircraft due to gust overloads, while at the same time helping the crew avoid temporary forays into either the high-speed or low-speed buffet regions. Such excursions could result in the aircraft stalling and departing from controlled flight. In cases of severe turbulence, pilots should focus primarily on maintaining a wings-level attitude. They should generally not adjust power or stabilizer trim settings further in an attempt to correct for temporary gust-induced altitude and airspeed deviations. Doing so could result in greater stresses being placed upon the aircraft than would otherwise happen. Minor pitch oscillations caused by the turbulence should be allowed to occur. If large pitch corrections are needed, they should be made using elevator. The autopilot should be turned off, unless it has a specific turbulence mode, and in any case altitude hold should not be engaged. Engine igniters should be turned on to help prevent engine flameout, which could occur as air flow to the engines is disrupted by turbulence.
While not much has changed over the decades that jet aircraft and their pilots have dealt with high-altitude CAT, that may be about to change. This past spring, NASA flight tested an airborne turbulence detection system designed to give pilots several minutes' advance warning of CAT. The Doppler lidar (a radar-like laser device) sensor uses a laser beam to detect ahead of an aircraft the motion of miniscule dust particles and aerosols as small as a millionth of an inch. It then converts these particle movements into a turbulence reading. A commercially available version is probably still several years in the future. Another approach being weighed involves data linking real-time turbulence measurements from aircraft to ground stations, which will then disseminate the data to other aircraft in the area. Until such technology becomes widely available, however, diligent flight planning and use of the old-fashioned seat belt remain a pilot's best friends.
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