Introduction to Turbulence: Hidden Threat in the Skies
High over the Pacific Ocean, about two-and-a-half hours southeast of Tokyo on the evening of Dec. 28, 1997, most of the 374 passengers on United Airlines Flight 826 had just finished their dinner. As the passengers tried to get comfortable for the rest of the nine-hour trip to Honolulu, Hawaii, flight attendants worked along the aisles of the Boeing 747, collecting meal trays. Suddenly and without warning, the jumbo jet lurched sideways, heaved sharply upward, then plunged toward the ocean. Anything that was not strapped down—including flight attendants and passengers not wearing their seat belts—was immediately thrown about the cabin. For several seconds, the plane was filled with tumbling debris and the screams of terrified passengers.
After the incident, the pilot advised ground controllers that the plane was heading back to Tokyo. A flight attendant and 13 passengers were seriously hurt, and 83 others had minor injuries. One passenger, a Japanese woman, later died in the hospital of severe head injuries. The 26-year-old plane was so badly damaged that United retired it from service six months ahead of schedule.
A Leading Cause of Injury and Damage In the Air
While they are rarely fatal, encounters with turbulence damage more aircraft and injure more passengers and flight personnel than any other aviation mishap except crashes. According to statistics kept by the Federal Aviation Administration (FAA), turbulence incidents involving U.S. airlines—including the Flight 826 mishap—were blamed for 3 deaths, 76 serious injuries, and nearly 1,000 minor injuries from 1981 to 1998. Each year, turbulence costs airlines hundreds of millions of dollars in payments for injuries, damage to aircraft, and revenue lost while planes are out of service for repairs. As widespread as the problem is, however, the only defense against turbulence, even after more than four decades of commercial jet travel, is to avoid it. But steering clear of turbulent air can be more difficult than it might seem. The most dangerous kind of turbulence—because it is the hardest to detect—occurs in clear air, where pilots have no indication of disturbed air in the aircraft's path until they fly into it.
Things are beginning to change, however. Growing concern about the harmful effects of turbulence on commercial aircraft since the 1980's, combined with a number of promising technological advances, has led to turbulence detection and avoidance being regarded as a problem that can be solved. Airlines, government agencies, and private companies have begun cooperating in international efforts aimed at finding ways to detect and avoid this chronic threat to safe air travel.
What Is Air Turbulence?
To understand why turbulence poses such a persistent problem to commercial air travel, it is important first to understand what turbulence is and what causes it. In the atmosphere, currents of air flow in all directions and at a variety of speeds. Turbulence is, essentially, any sort of erratic flow of air produced when an otherwise smooth-flowing air current becomes disrupted. A particular form of turbulence, called wind shear, also commonly occurs as the result of changes in the direction and speed of adjacent air masses. A sometimes extremely dangerous occurrence, wind shear plagued commercial aviation until the 1980's, when research and development efforts all but eliminated it as a threat to airline safety. Severe wind shear, which is associated with stormy weather, occurs in small patches of the sky, at the boundary between convection currents that are moving in different directions. It can be powerful enough to snap a plane's wings. Abrupt shifts in wind currents can also result from other weather conditions and even terrain.
Turbulence Associated With Storms and Cloud Masses
The most common type of atmospheric turbulence, called convective turbulence, develops with a rising mass of air. Energy from the sun is absorbed by the Earth's surface. The ground then gives up this energy in the form of infrared radiation, or heat. A mass of air near the surface absorbs this heat, which causes it to expand, forming a current of ascending air. As it rises, the air cools and the water vapor within it condenses, resulting in the formation of clouds. Eventually, the cooling air mass begins to drop, forming a descending current of cold air. This cycle of rising and falling air is known as convection, and the columns of air are called convection currents. These currents of air can ascend or descend at speeds up to 100 kilometers (60 miles) per hour, swirling the air between them into a highly turbulent flow. Strong convection currents are an essential ingredient in the development of a thunderstorm and can generate extremely powerful wind shear. For this reason, pilots are prohibited from flying into thunderstorms.
A thunderstorm juts up into the atmosphere like a boulder in a river, a situation that generates turbulence even beyond the immediate area of a storm. Winds are forced over and around the storm, and the disturbed air can roil for thousands of meters above or behind a thunderstorm. Pilots can often identify areas of convective turbulence ahead by looking for cumulus clouds—piled-up masses of white cloud. If the air in a particular region is too dry, however, there may not be enough water vapor present to form clouds, even though convection is at work.
As a rule, pilots avoid flying over the top of a thunderstorm. Flying below a storm, however, is sometimes unavoidable, as when an aircraft has to land. Cold air within a storm can plunge through the cloud base and continue all the way to the ground, producing an effect called a downburst. Occasionally, an extremely localized downburst—4 kilometers (2.5 miles) or less across—will form. This type of downburst, called a microburst, is narrow and brief but extremely strong. On Aug. 2, 1985, a Delta Air Lines Lockheed L-1011 bound for Dallas/Fort Worth International Airport attempted to land beneath a storm when a sudden microburst struck the plane. Because it was landing, the airliner was flying slowly and at a low altitude. The microburst slammed it into the ground short of the runway. Of the 166 people on the flight, only 30 survived.
An Invisible Danger: Clear-air Turbulence
Even though pilots are taught to avoid turbulent air by looking for cumulus clouds, turbulence can strike even in the absence of clouds. This type of turbulence—especially dangerous because of its invisibility—is known as clear-air turbulence. It accounts for most turbulence-related injuries, mainly because pilots have no time to warn passengers and flight attendants to get strapped into their seats. Nearly 7 out of 10 turbulence incidents are the result of encounters with the clear-air variety.
Clear-air turbulence most often occurs as the result of a surface cold front (a large, moving mass of cold air) encountering warmer air in its path. Colder air is denser than warm air and tends to remain closer to the ground. Therefore, as the leading edge of a cold front advances, it forces its way under the warmer, lighter air in its path. This effect can create a trail of turbulent air thousands of meters above and 80 to 160 kilometers (50 to 100 miles) behind the leading edge of the cold front.
Encounters with clear-air turbulence most often occur along the borders of the jet streams, enormous currents of air thousands of miles long flowing eastward at more than 100 kilometers per hour. The major jet streams are usually found at least 10 kilometers (6 miles) above the surface. There are six jet streams that meander around the Earth. Three are in the Northern Hemisphere and three in the Southern Hemisphere—one each near the North and South poles, two along each side of the equator, and two running roughly across the center of each hemisphere.
Airlines prefer to fly their eastbound aircraft in a jet stream, because the high-speed eastward tailwind allows a plane to fly faster while burning less fuel. But this benefit has a downside. As a jet stream races along, eddies of air swirl from its boundaries into the slower air around it, creating turbulence. Therefore, using a jet stream in this way often means risking an encounter with clear-air turbulence as an airplane enters and leaves the stream.
Clear-air turbulence also can swirl near a deep upper trough, an elongated area of low air pressure at an altitude of 3,000 meters (10,000 feet) or more. Like water running down a drain, air flows counter-clockwise around troughs in the Northern Hemisphere and clockwise in the Southern Hemisphere. This circulation pattern can create turbulence along the trough's edges. The most turbulent air is often found upwind of the trough's base and along its centerline. Upper-trough turbulence can develop in small areas of the sky. They may last only 30 minutes or persist for an entire day. Such an elusive culprit is as difficult for meteorologists (weather forecasters) to predict and study as it is for pilots to avoid.
Other Causes of Turbulence
Turbulent air can also be created by factors other than weather patterns. Features of the terrain like mountains and even tall buildings can create turbulence, disrupting smooth wind currents and causing them to burble in a turbulent flow. This phenomenon is called mechanical turbulence. Wind currents that become disrupted as they pass over mountainous areas can swirl chaotically for more than 800 kilometers (500 miles) beyond the peaks. Buildings disrupt wind currents on a smaller scale, creating turbulent eddies at lower altitudes, where they can pose a threat to airplanes that are taking off or landing.
Mechanical turbulence is sometimes a significant local problem. The international airport in Juneau, Alaska, for example, is surrounded by mountains that create turbulent wind conditions. Under certain weather conditions, aircraft taking off from Juneau International must make a sharp 180-degree turn during take-off to avoid turbulent air.
Mechanical turbulence can even be created by aircraft themselves. This phenomenon, called wake turbulence, is created as an aircraft's wings slice through the air. Some moving air along the top and bottom of the wings slips sideways toward the wing tips. When these two slips of air meet at the tip, they swirl off in invisible but powerful vortices, or spirals, that can trail off for miles behind and below the aircraft. The bigger the aircraft, the more powerful and persistent the wake turbulence it creates. An aircraft that inadvertently flies into the wake of another plane can be pummeled by these unseen swirls. In severe cases, the pilot can lose control of the aircraft.
Turbulence Becomes A Significant Threat
Turbulence has plagued commercial air travel since the beginning of the “Jet Age” in the late 1950's. Prior to that time, people did not worry much about turbulence because the propeller-driven airplanes of the prejet era rarely encountered it. For one thing, these planes cruised at an altitude of about 5,800 meters (19,000 feet), where turbulence is not a major threat, and flew at only about 325 kilometers (200 miles) per hour, which made recovery from wind-shear encounters easier. In addition, the lack of sophisticated navigation systems at that time prevented prop-driven aircraft from flying in stormy weather—conditions in which wind shear is likely to occur. Of course, wind-shear accidents probably did occur, but the less-sophisticated accident investigation techniques of that era made it difficult to identify wind shear as the cause.
By 1958, however, passenger jets had become commonplace, and commercial aircraft began flying higher and faster. Modern jets typically cruise at altitudes in the vicinity of 9,000 meters (30,000 feet), where turbulence can be a deadly threat. They also fly at speeds of more than 800 kilometers (500 miles) per hour, which greatly magnifies the force that turbulence can create on a plane and its occupants.
In 1958, a Soviet-built Tupolev Tu-104 jet airliner ran into severe turbulence while cruising at 11,000 meters (36,000 feet). The pilots lost control of the aircraft and it crashed, killing everyone on board. Since then, official records have logged more than 180 cases in which turbulence damaged an aircraft or injured its passengers. But that is only the tip of the iceberg. Safety experts agree that most turbulence encounters are never reported because they result in no damage or injuries.
Still, even the official figures are sobering. Turbulence accounts for more than 20 percent of airline accidents, an official record-keeping category that includes every incident in which an aircraft is substantially damaged or a person is seriously injured. And though modern airliners are built to withstand significant stresses in flight, turbulence severely damages at least one commercial jet per year.
Traditional Approaches to Dealing With Turbulence
At present, the most effective way to deal with turbulence is to avoid it, and the best means of avoiding turbulence is pilot reports. Pilots that encounter turbulent air radio their altitude and position to air traffic controllers, who warn other aircraft of the trouble spot if their workload permits. However, this does little to spare most passengers and flight attendants from a bumpy and often dangerous ride, and it does nothing to help scientists study the phenomenon. Furthermore, these reports are sparse (not all pilots bother to radio them in) and subjective (what one pilot calls "light" turbulence another might consider "moderate").
Turbulence-avoidance efforts today also depend on thorough preflight planning. The computer-aided flight-planning tools used by most airlines provide information, such as suspected areas of wind shear, that can provide clues to locations of possible turbulence. Most airlines also employ flight dispatchers, specialists trained in meteorology and aircraft performance. In addition to developing a plan for a flight's course, altitude, speed, and fuel consumption, dispatchers monitor a flight's progress and advise the pilots by radio of possible problems along the way, including areas of potential turbulence.
Airlines have also tried to educate pilots and flight crews about the need to anticipate turbulence and prepare for its effects. In 1997, the FAA produced a turbulence training kit in partnership with the aircraft maker McDonnell Douglas and the Air Transport Association, a trade group for the largest North American airlines. Distributed free to airlines, the kit includes a primer on turbulence, outlines of model training programs, and briefings advising airline executives and employees to take the problem seriously.
A Need For Better Communication
Studies have found that poor communication is the root cause of most turbulence-related injuries and aircraft damage. For instance, Delta Air Lines pilot Robert Massey recalls an incident in which the Airbus Industrie A310 he was piloting across the Atlantic from Europe to New York City suddenly plunged 300 meters (1,000 feet) in clear air near Greenland. The plane was then rocked by rough air for more than 10 seconds. After the mishap, Massey learned that the airline's dispatchers had expected turbulence in the area but issued no warning about it. The dispatchers had decided to wait until a pilot actually reported a turbulence encounter before issuing a warning.
Safety experts say that better communications among flight crews and between the crew and passengers can also help prevent turbulence-related injuries. They urge flight crews to tell passengers to keep their seat belts fastened whenever they are seated. Although airlines are not required to report injuries caused by turbulence, an estimated 90 percent of such injuries happen to people who were not using their seat belts when the turbulence struck—even, in some case, when the aircraft's “Fasten Seat Belt” signs were on. Moreover, many passengers interpret a turned-off sign as a signal to unbuckle, unaware that a plane can be buffeted by turbulence before the pilots have a chance to relight the sign.
Predicting Where Turbulence Will Strike
Experts agree that a greater use of seat belts, while helpful, can only do so much. In order to solve the problem of turbulence, they contend, a way must be found to detect and avoid it. With that goal in mind, some researchers are seeking to identify patterns in weather data that can be used to predict when and where turbulence might occur.
The huge amounts of of weather data routinely collected by federal agencies represent a tremendous resource for investigators studying turbulence. For several decades, meteorologists have used radar to monitor and help predict the weather. Radar (which stands for radio detection and ranging) detects approaching storms by bouncing radio signals off aerosols moving within currents of air. Aerosols are particles in the atmosphere, such as rain, snow, ice, or dust. The most significant advance in radar systems in recent years has been the development of a technology called Doppler radar, which employs a principle known as the Doppler shift in analyzing the reflected radar signal, or echo. An echo from a distant object that is in motion will have a slightly different frequency than the original signal, depending on the speed of the object and the direction it is moving in relation to the radar transmitter. This change in frequency is called the Doppler shift. In weather forecasting, computers connected to a Doppler radar system watch for Doppler shifts in air masses and use them to calculate the speed and direction at which the air is moving. By revealing areas of wind shear, Doppler radar can pinpoint areas of potentially turbulent airflow. This has made Doppler radar an extremely valuable tool in aviation safety.
Recent Advances In Ground-based Turbulence Detection
A growing volume of Doppler radar data is being supplied by NEXRAD (Next Generation Weather Radar), a nationwide network of advanced weather radars operated by the National Weather Service (NWS). The NWS began installing NEXRAD in the 1980's to help predict severe weather. Completed in 1999, NEXRAD consists of more than 150 ground stations that collect weather data on more than 90 percent of the skies over the United States at altitudes of 3,000 meters and up. The system is already proving its worth. Upon receiving the time and location of a aircraft turbulence report, NWS researchers can compare NEXRAD records from that region for patterns that might reveal recognizable “signatures” of turbulence.
Another NWS advance, called the Rapid Update Cycle 2 (RUC-2), also promises to help meteorologists identify areas of potential turbulence. This forecasting tool is an advanced computer model (simulation) of the atmosphere above the United States that analyzes weather data more quickly than the previous model and generates forecasts more frequently. The old model put out nationwide forecasts (broken down by subregions) every three hours. RUC-2 generates more precise three-hour forecasts and distributes them hourly. RUC-2 provides forecasts for 25-square-mile (65-square-kilometer) sectors, compared with the 37-square-mile (96-square-kilometer) sectors used in the previous system, and divides atmospheric data into 40 different altitude layers instead of the previous 25. This greater resolution should allow government and airline meteorologists to be more precise in forecasting the times and locations of probable turbulence.
Airborne Turbulence Detection Systems
To help researchers learn more about turbulence, an airborne data collection system was being tested in 1999. It involved loading a special computer program into the onboard computers of more than 200 aircraft operated by United Airlines. The program, developed by scientists at the National Center for Atmospheric Research (NCAR) in Boulder, Colorado, calculates precisely how much turbulence a plane encounters in flight. The software was designed for use with the existing computers and sensors that make up a modern airliner's flight management system.The system makes routine measurements of the airplane's speed, attitude, and direction of flight, as well as of atmospheric forces being exerted on the aircraft. The NCAR program creates a detailed description of turbulent air recorded by an aircrafts's sensors by analyzing how the turbulence affects the aircraft's motion. It then transmits that information to stations on the ground through the plane's Aircraft Condition and Reporting System (ACARS), a data link that airlines use in everyday operations.
Even the best predictions can be inaccurate, however. More must still be learned about the nature of turbulence before it can be accurately forecast. Therefore, many researchers are more interested in fitting aircraft with equipment that detects turbulence ahead of a plane and warns the pilots. Engineers had yet to perfect such a device in 1999, but they were working on several different systems.
Given the success of ground-based Doppler radar in helping aircraft avoid wind shear, engineers have been trying to develop an onboard Doppler radar system to detect turbulence ahead of an aircraft. By 1999, two electronics firms, AlliedSignal Aerospace and Rockwell Collins, had developed an airborne system that could detect wind shear. However, these devices can only be used during landing approaches because the radar transmitter is located beneath the airplane's nose and points slightly downward. To make the system useful throughout a flight, engineers were working on a way to point the beam straight ahead of the aircraft while it is at cruising altitude.
Using Laser Light to Detect Rough Air
Another manufacturer has been working with NASA and NCAR to develop an airborne turbulence detector that is similar in principle to Doppler radar but employs light waves. The system, called Airborne Coherent Lidar for Advanced In-flight Measurement (ACLAIM), was developed for NASA's Dryden Flight Research Center in California, by Coherent Technologies Incorporated, of Lafayette, Colorado. (Lidar stands for “light intensity detection and ranging.”) The system transmits a laser beam that reflects off dust and other tiny particles in the air and uses the Doppler shift to measure the speed and direction of the particles. Any significant changes in the motion of particles in the skies ahead of the aircraft is a clear indication of turbulence.
A ground-based version of the ACLAIM system was ground-tested in mid-1997 and was able to detect turbulence in the path of airplanes that were coming in for a landing. However, the test unit employed a fixed beam. For use on aircraft, the beam must be able sweep across the sky ahead of the plane in search of turbulent conditions.
Another system under evaluation in 1999 used a laser beam to detect wake turbulence by measuring not the motion of particles in the air but the acoustic (sound) energy generated by their movement under different weather conditions. The system, dubbed SOCRATES (Sensor for Optically Characterizing Ring-Eddy Atmospheric Turbulence Emanating Sound), was developed by Flight Safety Technologies of New London, Connecticut. SOCRATES is based on the idea that turbulence causes density changes in air that can be detected as sound. A two-week test in May 1998 at New York City's Kennedy International Airport confirmed this theory, but further testing was needed.
So far, the best turbulence-detection systems under evaluation would give pilots reliable warnings no more than one minute before the plane hits rough air. For any such system to be useful, however, experts agree that it would need to provide a warning at least two minutes ahead of time. This is considered to be the minimum amount of time needed to give people time to sit down and strap in. Such a system must also be extremely reliable. Too many false alarms would cause passengers to begin ignoring the warnings, making the system almost useless.
Integrated Systems—tracking Turbulence In the Future
In the future, some experts predict, a variety of turbulence prediction and detection methods and technologies will be combined in a comprehensive detection and warning system. Researchers at NCAR are working toward the goal of equipping commercial airliners with an integrated system that would merge onboard sensor data, radar and satellite data, and short-term computer weather models in a unified cockpit display. Such a “Forecast/Nowcast” system would give pilots comprehensive data on the present situation as well as projected flight conditions up to an hour in the future. For the rare occasions when turbulent air cannot be avoided, these systems might also incorporate the capability to minimize the effects of turbulence by using the plane's automatic pilot to compensate for any sudden movements.
However, even after turbulence forecasting techniques are made more accurate and detection systems are refined, airline passengers will still be best served by taking one simple precaution—always keeping their seat belts fastened. A seat belt may be uncomfortable, but the alternative is to risk a possibly serious injury. Even in the best flying conditions, it pays to be cautious, because—as history and experience prove—you never know when turbulence will strike.