Boeing 2707 SST Problems

The SST was very sensitive to fuel consumption variations due to ambient temperatures and fuel reserves. A 1% increase in fuel would reduce payload by approximately 5% on long flights. The SST behaved operationally like a short range airplane and it was expected that the achieved utilization will be about 85% of subsonic aircraft. The high volume of services, fuel and payload handling at station stops would increase ground time and further decrease utilization.

With all of the improved productivity, technological advance and overall appeal af the SSTs, the fact remained that the airlines of the world faced the greatest challenge in their history to ensure their SST fleets were flown efficiently and profitably on a consistent basis.

There are a number of factors, associated with transportation, which had not changed. The geographical distances between major cities of the world remained the same. Weather patterns around the world remained the same. People do not walk any faster than before. The speed of ground transportation had not improved much.

Because of the inherent constancy of such factors, there were a number of severe problems facing the operators of SSTs which necessitated an exceptionally high degree of forecasting, planning and monitoring, if the airlines were to succeed economically.

The SST project faced fierce opposition from environmentalists, who had concerns about possible depletion of the ozone layer and about noise at airports and from sonic booms. The project also suffered political opposition from the right, who disliked the government subsidizing the development of a commercial aircraft to be used by private enterprise.

The potentially adverse effects of commercial supersonic flight on the environment were the subject of considerable controversy and, at times, heated debate. The principal issues are noise, the sonic boom, pollution from engine emissions, and, to a lesser extent, radiation effects on passengers and crew. During the debate, both fact and conjecture have been used to support opposing points of view, clouding the issues in the minds of most Americans.

A burgeoning environmental movement was rising to new heights of influence. This movement drew strength from a surge in public outrage against air and water pollution. As early as 1965, the Opinion Research Corp., a polling organization, found that up to one-third of the American people viewed such issues as serious. Here was a level of concern that no political leader could ignore. By 1970, nearly three-fourths of the public shared this attitude, representing a power capable of sweeping everything before it.

Matching this rise was a dramatic increase in the prominence and clout of leading environmental organizations. In 1967, the Sierra Club, then with only 55,000 members, was already one of the largest and most active of these groups. Though its emphasis was on protecting wilderness areas, its focus at the time was on a regional issue, fighting the construction of Marble Canyon Dam on the Colorado River. To win political support, it had to bend to the needs of such powerful senators as Henry Jackson, chairman of the Senate Interior Committee and a strong SST supporter. By 1971, its membership was at 200,000 and rising, and its leaders were taking pivotal roles in the fight against the SST.

In July 1969 David Brower, who had been executive director of the Sierra Club, founded Friends of the Earth. It took a strong stand against the SST. The following March, a wealthy Baltimore man, Kenneth Greif, took the lead in organizing a nationwide coalition of SST opponents. The Sierra Club now signed on. So did the National Wildlife Federation, the Wilderness Society, and the Consumer Federation of America. In this coalition, opponents now had an instrument suited for work in the political arena.

On 22 April 1970 the first annual Earth Day observance throughout the United States included protests indicating environmentalists? rising opposition to the supersonic transport (SST) program. Concerns about the SST included such issues as sonic booms and the aircraft's effect on the ozone layer of the earth's upper atmosphere.

In the struggle over the SST, the environmental movement came of age and took its place as a major and powerful political force. In defeating the SST, the nation's environmentalists showed that they had the clout to block such a program even when it held support from the AFL-CIO, the Administration, and the aerospace industry with its well-funded lobby.

Boeing 2707 SST Sonic Boom

Engine noise was a critical factor in the cancellation of the SST program [and also the focus of controversy about the Concorde operating at Washington and New York airports]. The acoustic impact of the SST was, in large part, responsible for its demise. Anticipated high levels of side line noise and restriction to subsonic operation over land compromised the economic viability of the Boeing SST.

One of the more objectionable of the problems facing any supersonic transport is commonly referred to as the "sonic boom." To explain sonic boom, one must return to a description of the shock-wave formation about an airplane flying supersonically. A typical airplane generates two main shock waves, one at the nose (bow shock) and one off the tail (tail shock). Shock waves coming off the canopy, wing leading edges, engine nacelles, etc. tend to merge with the main shocks some distance from the airplane. The resulting pressure pulse changes appear to be "N" shaped as shown. To an observer on the ground, this pulse is felt as an abrupt compression above atmospheric pressure followed by a rapid decompression below atmospheric pressure and a final recompression to atmospheric pressure. The total change takes place in one-tenth of a second or less and is felt and heard as a double jolt or boom.

The sonic boom, or the overpressures that cause them, are controlled by factors such as airplane angle of attack, altitude, cross-sectional area, Mach number, atmospheric turbulence, atmospheric conditions, and terrain. The overpressures will increase with increasing airplane angle of attack and cross-sectional area, will decrease with increasing altitude, and first increase and then decrease with increasing Mach number. The strongest sonic boom is felt directly beneath the airplane and decreases to nothing on either side of the flight path. The shock produces a moving wall of compressed air that trails along the ground, sweeping out a swath up to 50 miles wide and the full length of the supersonic flightpath. Within this swath, every person feels the boom when the shock passes. It is interesting to note that a turning supersonic airplane may concentrate the set of shock waves locally where they intersect the ground and produce a superboom.

Perhaps the greatest concern expressed about the sonic boom is its effect on the public. The effects run from structural damage (cracked building plaster and broken windows) down to heightened tensions and annoyance of the citizenry. The pressure rise is not large, rarely more than a thousandth of atmospheric pressure. It is, however, both sharp and sudden; hence it can startle people and crack plaster. The strength of a sonic boom is measured as an overpressure; designers expected that an SST would produce values of around two pounds per square foot during cruise. By contrast, loud noises have their intensity measured in decibels, a completely different unit. Hence the FAA wanted to know how boomy an SST could be and still produce no more annoyance than conventional subsonic jets.

An initial exercise, Operation Bongo, took place around Oklahoma City during 1964. It was a joint FAA-Air Force experiment that sought to determine whether people could learn to accept sonic booms as just another type of noise, akin to that of railroad trains or trucks on a highway. For six months the Air Force sent supersonic F-104 fighters over the city, day after day and at specified times. Observers found reason to believe that there might indeed not be much of a problem, for a number of people put the booms to their advantage.

A secretary used the recurring booms as an alarm clock. She got out of bed at the window-rattling crack of the seven a.m. boom, then took a shower. She shut off the water when she heard the next boom, for this meant it was 7:20, time to start her day. Other people also treated the eight daily booms as if they were blasts from a factory whistle. One group of construction workers used the eleven a.m. boom as their signal for a coffee break. Animals as well went undisturbed. In El Reno, a nearby town, a farmer saw a tom turkey chasing a hen. Though a boom rattled the barn, the tom never broke stride.

In several respects, these tests were biased toward minimizing citizen complaints. Oklahoma City was strongly aviation-minded, with a major FAA center and an Air Force base. The booms came by day, never at night, and people knew when to expect them. They also knew that the test would run for only a few months. The booms themselves were weaker than those of an SST and carried less energy, though they did increase in strength over the months.

On 27 January 1965 the National Academy of Sciences' Committee on Supersonic Transport Sonic Boom concluded that prototype development of a supersonic transport was "clearly warranted" by evidence from research, tests, and studies of sonic boom phenomena. This finding was largely based on data collected by FAA in the Oklahoma City area.

On 25 April 1965 the FAA made public a summary of its Oklahoma City sonic boom study, in which U.S. Air Force jets had subjected residents to 1,253 booms during daylight hours. Most boom intensities ranged between 1.0 and 2.0 pounds of overpressure per square foot, but adverse atmospheric influences caused approximately 11 percent to exceed the intended limit of 2.0 pounds of overpressure.

Nevertheless, the results were enough to give pause, as some 4,900 people filed claims for damages. Though most involved little more than cracked plaster, one man did receive a payment of $10,000. Two high-rise office towers sustained a total of 147 cracked windows. During the first three months of the tests, polls indicated that 90 percent of the people felt they could live with the booms. After six months, this number was down to 73 percent. This meant that some one-fourth of these citizens believed they could not live with them and would regard them as unacceptable. Fully 27 percent of the people polled in the Oklahoma City area during the closing weeks of testing declared they could not live with sonic boom; additionally, 40 percent of those polled were unconvinced that booms did not cause damage to buildings. The subjective reaction of individuals to sonic boom would be the area of greatest concern for the US SST program.

This was bad news at the FAA in Washington. The news soon grew worse, as a second series of tests, at Edwards Air Force Base, introduced the use of larger supersonic aircraft. These included the XB-70, the only plane in the world with the size and speed of an SST. The workhorse of the new studies, the B-58 bomber, was only slightly smaller. Already it had shown its uses in sonic-boom tests, flying from Los Angeles to New York in two hours. Unfortunately, it had shattered windows as well as speed records, showering offices and living rooms alike with broken glass. Police switchboards from coast to coast had lit up with calls as frightened people reported they had heard a terrible explosion.

FAA released an interim report on the related test at White Sands, NM, in which Air Force jets subjected 16 representative structures to 1,494 booms varying in intensity from 2.0 to 20.0 pounds of overpressure. The tests at Edwards took place during 1966.

The two tests found that sonic booms of less than 5 pounds of overpressure caused no discernible damage to structurally sound buildings; however, booms of this intensity probably triggered cracks in faultily constructed walls, breaks in cracked windows, and other damage in structurally unsound buildings. Booms of the order of those expected to be generated by the U.S. supersonic transport (SST) had no measurable physiological effect on humans.

In releasing the information, Administrator Halaby stated his conclusion that a supersonic transport could be designed in terms of configuration, operating attitude, and flight paths so as to achieve public acceptance in the early 1970s.

Karl Kryter, a sonic-boom specialist at Stanford Research Institute, summarized the findings in the journal Science: "When both European and American SSTs were fully operational, late in the 1970s, it is expected that about 65 million people in the United States could be exposed to an average of about ten sonic booms per day.... A boom will initially be equivalent in acceptability to the noise from a present-day four-engined turbofan jet at an altitude of about 200 feet during approach to landing, or at 500 feet with takeoff power, or the noise from a truck at maximum highway speed at a distance of about 30 feet."

The historian Mel Horwitch would note that when these results reached an SST coordinating committee, "an almost instant consensus developed that the American SST could never fly overland."

This did not rule out going ahead with the program. Boeing and the FAA estimated that even if the SST was restricted to overwater flights, it could still sell 500 airplanes. That would suffice to ensure commercial success. With no restrictions, Boeing's managers believed they could sell as many as twelve hundred. Business Week noted that "at $40 million per SST, a ban would mean a sales penalty of $28 billion-greater than Boeing's total sales for the last fifteen years."

Similar warnings came from Senator William Proxmire, an eventual opponent of the Space Shuttle who was already taking the lead as a strong opponent of the SST: "The SST will start by flying the ocean routes. Soon the economic pressures of flying these high-cost planes on limited routes will force admission of the planes to a few scattered land routes. And ultimately they will be flying everywhere."

Also during 1966, design studies and analyses reached a level that allowed the FAA to select contractors through a design competition. Boeing won, with a proposal that called for engines from General Electric. This contract award came through on the last day of that year; a four-year program now lay ahead, aimed at building two prototype aircraft. This selection of contractors was crucial. The program now was in a new phase, no longer one of endless study but rather of mainstream airliner development.

This shift in status brought a quick response from SST critics, as the beginnings of organized opposition took form. The man who did the organizing was William Shurcliff, a physics professor at Harvard. Early in 1967, he set up the Citizens League Against the Sonic Boom. His son and sister were founding members; its office was in his home. He did not set out to arrange protest demonstrations. Instead, he proceeded to run a clearinghouse for critics, taking out newspaper ads, writing letters, raising questions, and generally working to argue that the emperor had no clothes. His organization was never large, its peak membership running to only a few thousand. The rudder of a ship is also quite small. Like that rudder, Shurcliff would prove to be highly influential in steering the SST to its fate.

On 08 March 1969, the Federal government lost its appeal in a class action suit involving claims for property damage allegedly caused by the Oklahoma City tests.

Boeing 2707 SST Ozone Depletion

In the early 1970's, concern was aroused that the engine emissions from a fleet of supersonic transports would deplete the ozone in the upper atmosphere, reduce the shielding from the Sun's ultraviolet rays. Concerns were raised that the exhaust emissions might chemically react to thin the stratospheric ozone layer, which helps protect from sunburns and skin cancer. This concern, originally directed only at anticipated supersonic aircraft, spread to the potential impact of the growing fleet of subsonic aircraft. At the time the issue was raised, there was simply not enough knowledge from which to draw the needed scientific conclusions.

About 90 percent of the ozone resides in a layer between 10 and 40 kilometers (6 and 25 miles) above the Earth's surface in a region of the atmosphere called the stratosphere. Ozone there plays a beneficial role by absorbing dangerous ultraviolet radiation from the sun.

No one dreamed human activity would threaten the ozone layer until the early 1970s, when scientists discovered two potential problems: ultrafast passenger planes and spray cans. The plane threat surfaced first, after the invention of a new breed of commercial aircraft called the supersonic transport (SST). These planes could fly faster than the speed of sound and promised to trim hours off long journeys. In the 1970s, the United States and other nations began considering whether to build large fleets of such ultrafast jets.

At a congressional hearing held in May 1970, Russell Train, a member of Nixon's Council on Environmental Quality, described noise as the SST's "most significant unresolved environmental problem." Train also opened a new attack by introducing the issue of whether a fleet of SSTs might damage the ozone layer in the upper atmosphere. The air at its cruising altitude, some 65,000 feet, is very dry and low in humidity. It also is rich in ozone, which forms a layer that protects the earth from the sun's dangerous ultraviolet rays. The atmospheric scientist Conway Leovy, writing in the Journal of Geophysical Research, had set forth a "wet photolysis" theory whereby water vapor in the stratosphere could speed the destruction of ozone.

Train stated in his testimony that the SST would discharge "large quantities of water vapor, carbon dioxide, nitrogen oxides and particulate matter." He added that "500 American SSTs and Concordes flying in this region of the atmosphere could, over a period of years, increase the water content by as much as 50 to 100 percent." This water vapor, formed copiously from the burning of jet fuel, could destroy some of the ozone, putting the world at greater risk from the ultraviolet. Proxmire welcomed Train's statement as a "blockbuster."

Another round of hearings preceded votes in mid-1970, and again the opponents had new ammunition. James McDonald of the University of Arizona, a member of a National Academy of Sciences panel on climate modification, asserted that 500 SSTs could deplete enough ozone to produce 10,000 cases per year of skin cancer within the U.S. This would result from the increased power of the solar ultraviolet. His statement caused a sensation. McDonald had based his conclusions on the threat to ozone from water vapor. Ironically, this wet-photolysis theory was overturned within months, as new research in atmospheric science showed that the effects of water vapor on ozone were all but nil. Because he was also a researcher of unidentified flying objects (UFOs) and extraterrestrial visitation, his credibility was easily attacked.

Another scientist, Harold Johnston of the University of California at Berkeley, rode to the rescue by asserting that nitrogen oxides would also damage the ozone layer. SST engines would produce such oxides in large quantities. Johnston calculated that 500 SSTs would destroy up to half the ozone in the air over the United States.

There also was countering testimony on the atmosphere, as William Kellogg, associate director of the National Center for Atmospheric Research, stated that effects due to SSTs would be imperceptible amid those due to natural causes.

Johnston developed a photochemical model for ozone that showed that the oxides of nitrogen (NO*) emitted by the SST's may reduce the total amount of ozone. His model assumed that the NO* emitted by a proposed fleet would be distributed throughout various arbitrary layers in the stratosphere with an average residence time ranging from six months at 15 km to two years above 20 km. His calculations showed that the total ozone would dimmish by as much as a factor of two.

Johnston's report was met with criticism which fell in two basic areas: the uncertainty of his photochemical model and his neglect of the details of the atmospheric transport processes. The chemical model contained many reactions with unknown rate constants, in particular the reactions that convert the NO* into less harmful molecules. The transport processes, on the other hand, were felt to be important because the time constants for the reactions are very long for altitudes of 20 km or less. Since this is the region in which the first generation SST's, the Anglo-French Concorde and the Soviet Tupblev 144, flew, transport processes could dominate the proposed NO* photochemistry and prevent the destruction of ozone.

As scientists such as Harold Johnston and Paul Crutzen looked at the SST issue, they grew concerned about the effects such planes might have on the stratosphere. SSTs are unusual because they must fly high up in the atmosphere -- where the air is thin -- to achieve their fast speeds. Several researchers suspected that the reactive nitrogen compounds from SST exhaust might accelerate the natural chemical destruction of ozone, causing ozone levels to drop.

During the congressional debate over the future of the SST program in 1970, the Department of Transportation (DOT) was directed to mount a Federal scientific program to obtain the knowledge needed to judge how serious the conjectured ozone-depletion effects might be and report the results to Congress by the end of calendar year 1974. This directive led to the establishment of DOT's climatic impact assessment program (CIAP), which drew on 9 other Federal departments and agencies, 7 foreign agencies, and the individual talents of 1,000 investigators in numerous universities and other organizations in the United States and abroad. At the same time, a special committee of the National Academy of Sciences (NAS) was organized to review the work of CIAP and to form an independent judgment of the results.

The principal findings of the CIAP study was that operations of supersonic aircraft and those SSTs scheduled to enter service (about 30 Concordes and TU-144s) cause climatic effects which are much smaller than minimally detectable. Future harmful effects to the environment can be avoided if proper measures are taken in a timely manner to develop low emission engines and fuels. If stratospheric vehicles (including subsonic aircraft) beyond the year 1980 increased greatly in number, improvements over 1974 propulsion technology would be necessary to assure that emissions do not significantly disturb the stratospheric environment. The cost of developing low-emission engines and fuels would be small compared to the potential economic and social costs of not doing so.

Congress supported a NASA program to develop the technology for low-emission jet engines. This program was successful in defining and testing a conceptual design for a burner which might solve potential future highaltitude emission problems as well as reduce low-altitude emissions.

Livermore's model of stratospheric ozone was one of the world's first to examine ozone interactions with the SST's nitrogen oxide emissions. In addition, the model made clear that the use of a large number of megaton-size explosions in a nuclear war would seriously deplete stratospheric ozone. This finding played a central role in a 1974 National Academy of Sciences study on the potential long-term worldwide effects of multiple nuclear weapons detonations, adding impetus for the two superpowers to reduce weapon yield and the size of their nuclear arsenals.

A later Livermore model indicated that continued use of chlorofluorocarbons (CFCs) would severely deplete stratospheric ozone, prompting international negotiations on limiting CFCs and U.S. prohibition of CFCs as spray-can propellants.

In 1975, WMO convened a group of experts to prepare an authoritative statement entitled "Modification of the ozone layer due to human activities and some possible geophysical consequences". The statement focused on the effects of both supersonic transport and CFCs. It signalled the first international warning of the danger of substantial ozone decrease and recommended international action to provide better understanding of the issue.

Concerns about NO and NO2 (i.e., NOx) emissions from present-generation subsonic and supersonic aircraft operating in the upper troposphere (UT) and lower stratosphere (LS) were raised in 1977 by Hidalgo and Crutzen because these emissions could change ozone levels locally by several percent or so. Despite extensive research and evaluation during the intervening years, in 1995 WMO-UNEP concluded that assessments of ozone changes related to aviation remained uncertain and depended critically on NOx chemistry and its representation in complex models.

Also, on the CIAP recommendations, FAA initiated a high-altitude pollution program (HAPP) to monitor continuously the upper atmosphere and conduct systematic research to address the uncertainties regarding ozone depletion attributable to future subsonic and supersonic aircraft. The ongoing HAPP studies indicated that the earlier CIAP and NAS studies substantially exaggerated the extent to which future aircraft will reduce the ozone layer. Present understanding of the phenomena indicates much smaller impacts and perhaps no net impact at all.

A United Nations Environment Program to protect the ozone layer was signed in Vienna in 1985, and a protocol outlining proposed protective actions followed. The Vienna convention of 1985 embodied an international environmental consensus that ozone depletion was a serious environmental problem. However, there was no consensus on the specific steps that each nation should take. The Montreal Protocol, signed in September 1987, stated that there would be a 50 % cut back in CFC production by 2000. The United States ratified the Montreal Protocol in 1988.

Scientists are now confident that stratospheric ozone is being depleted worldwide. However, how much of the loss is the result of human activity, and how much is the result of fluctuations in natural cycles, still need to be determined.

Boeing 2707 SST Contrails

Contrails are visible line clouds that form behind aircraft flying in sufficiently cold air as a result of water vapor emissions. Contrail formation can be accurately predicted for given atmospheric temperature and humidity conditions. In the exhaust, water droplets form on soot and sulfuric acid particles, then freeze to form contrail particles.

Contrails cause a positive mean radiative forcing at the top of the atmosphere. They reduce both the solar radiation reaching the surface and the amount of longwave radiation leaving the Earth to space. Contrails reduce the daily temperature range at the surface and cause a heating of the troposphere, especially over warm and bright surfaces. The radiative effects of contrails depend mainly on their coverage and optical depth.

The combustion of hydrocarbon fuels, such as gasoline and kerosene, results in the addition of water vapor, carbon dioxide, and heat to the wake of the aircraft. Under the proper enviromental conditions of pressure, temperature, and relative humidity, the wake becomes saturated and condensation trails composed of ice crystals result. If the environment is dry, the contrails will evaporate --- slowly in the case of a stable atmosphere, alriost instantaneously in a turbulent atmosphere. However, if the environment is saturated with respect to ice, the trails may spread out to form a persistent cloud deck over a large area.

Subsonic jets operating near 35,000 feet frequently produce persistent cirrostratus contrails. Persistent contrails often develop into more extensive contrail cirrus in ice-supersaturated air masses. Ice particles in such persistent contrails grow by uptake of water vapor from the surrounding air.

Significant increases in stratospheric aerosol were expected for the operation of a large fleet of supersonic aircraft. Several eminent physical scientists raised the question whether the proposed supersonic transports, with their tremendously high fuel consumption and operating in the stable stratosphere, might produce such widespread and persistent cirrus as to cause substantial changes in the radiation balance and eventually in the earth's climate.

Quite early in the formative stages of this effort , it was acknowlcdged by most scientists that SSTs would only rarely form contrails in the stratosphere, owing to that region's extremely low (typically only a few percent) relative humidity.

In August 1970, a group at MIT, conducting the Study of Critical Environmental Problems, gave further support to concerns about the upper atmosphere. It stated that a fleet of SSTs could produce effects similar to those of the 1963 eruption of the volcano Mt. Agung, which had increased stratospheric temperatures by as much as 12 degrees.

Simulations in the 1990s suggested that contrails would form even without any soot and sulfur emissions by activation and freezing of background particles. Hence, the formation of contrails cannot be avoided completely by reducing exhaust aerosol emissions. Contrails formed in plumes with fewer exhaust particles are likely to be composed of fewer and larger particles, have smaller optical depths, hence cause less radiative forcing. Reduced soot and sulfate particle emissions may also lead to the formation of cirrus clouds with fewer but larger particles and less radiative forcing.

Reducing the frequency of contrails for a given amount of air traffic could otherwise be effected by reducing the number of flights in the humid and cold regions of the upper and middle troposphere. Numerical weather prediction schemes may be used to predict and circumvent such regions on long-distance flights. Contrail-forming regions could also be avoided by flying at generally higher altitudes, but the climatic impact of contrails may not be reduced because of counteracting effects. For example, higher flight altitudes at low latitudes could increase contrails, possibly causing a net increase instead of a decrease in global radiative forcing by contrails. In addition, more flights in the lower stratosphere could result in enhanced aerosol and chemical impacts not related to contrails.

Boeing 2707 SST Cosmic Radiation Exposure

The Supersonic Transport (SST) program, proposed in 1961, first raised concern for the exposure of pregnant occupants by solar energetic particles (SEP), and neutrons were suspected to have a main role in particle propagation deep into the atmosphere. An eight-year flight program confirmed the role of SEP as a significant hazard and of the neutrons as contributing over half of the galactic cosmic ray exposures, with the largest contribution from neutrons above 10 MeV. The FAA Advisory Committee on the Radiobiological Aspects of the SST provided operational requirements.

Ionizing radiation particles (mostly protons and alpha particles) enter Earth's atmosphere, where they collide with nitrogen, oxygen, and other atoms, breaking apart their nuclei. Both the charged particles entering the solar system and the secondary radiation they produce in the atmosphere are referred to collectively as galactic cosmic radiation.

At every latitude, the altitude at which the dose rate is highest is different. The initial interaction of galactic cosmic radiation with the Earth's atmosphere can be so intense that a unique phenomenon is observed at high altitudes above the equator: The intensity of the radiation is lower at 80,000 feet than at 60,000 feet, where particle interactions reach their peak.

Dose rates from cosmic radiation at commercial aviation altitudes are such that crews working on present-day jet aircraft are an occupationally exposed group with a relatively high average effective dose. Crews of high speed commercial aircraft flying at higher altitudes would be even more exposed. At the higher cruise altitudes expected of supersonic transports, cosmic rays are filtered by the atmosphere less than at subsonic cruise altitudes or on the ground. This factor has gave rise to some concern that crew personnel will undergo excessive exposure to cosmic rays.

The intensity of the different particles making up atmospheric cosmic radiation, their energy distribution, and their potential biological effect on aircraft occupants vary with altitude, geomagnetic latitude, and time in the sun's magnetic activity cycle. The galactic cosmic radiation field at aircraft operating altitudes is complex, with a large energy range and the presence of all particle types. The calculation of the complex radiation fields is difficult, as is the measurement. British Airways cooperated with the U.K. National Radiological Protection Board in measuring cosmic radiation doses on supersonic Concorde and subsonic aircraft using a range of devices.

Galactic cosmic radiation exposure approximately doubles with every 6000 feet of increased altitude. While cosmic radiation poses little or no risk to the "pleasure" traveler, business travelers who log as many hours as air crew themselves could be labeled occupationally exposed. The increased intensity of radiation will be somewhat compensated for by the decrease in exposure time resulting from the air craft's supersonic speed. The best evidence to date is that such radiation exposure will not exceed permitted occupational levels.

The 1990 lowering of ICRP-recommended exposure limits (1990) with the classification of aircrew as "radiation workers" renewed interest in GCR background exposures at commercial flight altitudes and stimulated epidemiological studies in Europe, Japan, Canada and the USA. The proposed development of a High Speed Civil Transport (HSCT) required validation of the role of high-energy neutrons, and this resulted in ER-2 flights at solar minimum (June 1997) and studies on effects of aircraft materials on interior exposures. Recent evaluation of health outcomes of DOE nuclear workers resulted in legislation for health compensation in year 2000 and recent European aircrew epidemiological studies of health outcomes bring renewed interest in aircraft radiation exposures. As improved radiation models become available, it is imperative that a corresponding epidemiological program of US aircrew be implemented.