Aerospace Physiology: Aeromedical and Human Performance Factors for Pilots

By Steven C. Martin

Aerospace physiology (sometimes called flight or aviation physiology, human factors, or aeromedical factors) is the scientific discipline studying the effects of flight conditions on human physiological and cognitive systems, teaching aviators to work and function at peak efficiency in the abnormal environment of flight. This information is introduced to pilots throughout their initial training including hypoxia, spatial disorientation, visual illusions, fatigue, trapped gases, and many others. The problem is all of these issues still create problems, as well as fatalities, for pilots on a regular basis even today. Why? Pilots may know about the information, but fail to completely understand it. This book will transform a pilot’s potential misinterpretation of this subject matter into definitive action on the flight deck.

The newest, most authoritative, and comprehensive resource on this critical subject is "Aerospace Physiology: Aeromedical and Human Performance Factors for Pilots," a pilot’s number one source for enhancing safety-of-flight for all pilot experience levels. As well as providing practical and realistic human performance information for private and professional pilots, this book has been specifically written for use in academic settings unlike other books on this subject matter. This book is currently the preferred text on flight physiology for the world-renowned University of North Dakota’s John D. Odegard School of Aerospace Sciences.

The book contains 22 chapters, discussing each topic thoroughly using the primacy of learning format and in an understandable manner, complete with chapter core competency questions. Each topic is covered in detail with environmental causes, potential physiological & cognitive responses, followed by effective and proven anticipation & mitigation strategies. The book uses the most current research and experience-based information combined with current incidents and accidents illustrating how these issues present themselves in real flight environments as well as how those accidents may have been prevented. The information in this book is based on Mr. Martin’s 30 years of military and civilian aviation experience, and is modeled after the US Air Force’s Physiological Training Program for pilots and the comprehensive European Union Aviation Safety Agency’s (EASA) flight physiology human performance standards.

Using Aerospace Physiology as your resource for aerospace physiology information will elevate the standard of training to its highest levels regarding this crucial knowledge.

• Language: English
• Publisher: Gatekeeper Press
• Release date: Aug 29, 2021
• ISBN: 9781662917660

Book preview

1. INTRODUCTION TO AEROSPACE PHYSIOLOGY

Almost every day somewhere in the world someone (or many people) will die as a result of aviation-related accidents. These accidents range from one person flying their own airplane, helicopter medical transport flights, single or dual pilot cargo flights, corporate jets, crop-dusters, any number of military aviation operations, to a commercial airline with hundreds of souls on board. Each and every fatal aviation accident is tragic, and the reality is a large percentage of aviation accidents may have been prevented had the pilot been given the proper tools to recognize and react properly to those hazardous situations.

Human Factors, as defined by the Federal Aviation Administration (FAA), is a multidisciplinary effort to generate and compile information about human capabilities and limitations and apply that information to equipment, systems, facilities, procedures, jobs, environments, training, staffing, and personnel management for safe, comfortable, and effective human performance (Role of Human Factors in the FAA, 2014).

Federal Aviation Administration (FAA) experts estimate the percentage of aviation incidents and accidents caused by human influence is approximately 60 – 80%. Those outside the aviation industry may assume pilots were to blame for all those incidents and accidents, but nothing could be further from the truth. Many other humans are involved in the aviation operations chain: aerospace and aviation engineers, software engineers and programmers, air traffic control, maintenance personnel, flight line workers, dispatchers, managers, and many others. Pilots, unfortunately, are the end-of-the-line recipients of whatever problems develop and are left trying to figure out how to correct complex issues in real time, usually while trying to fly the airplane. Some of those in-flight problems may be a result of pilot action (or inaction); however, in many cases the problem developed somewhere upstream before the pilots ever entered the flight deck. Regardless of the problem’s origin, the pilot is the one person in the chain that holds the key to life or death in their hands. For that reason, the pilot should be operating at their utmost peak efficiency, both cognitively and physiologically, to give themselves and their passengers the greatest chance for survival if something happens.

Human cardiovascular, respiratory, musculoskeletal, intellectual, and orientation systems developed to function with peak efficiency at sea level, where people operate at relatively slow speeds, maintain a ground level visual perspective, have their body in constant contact with the ground, and breathe copious amounts oxygen under a certain amount of atmospheric pressure.

Flight conditions cause faster speeds, elevated visual perspectives, misleading orientation cues, reduced oxygen and atmospheric pressure, and many other abnormal environmental factors. As part of the overall Human Factors spectrum, Aerospace Physiology (also known as Flight Physiology, Aviation Physiology, and Aeromedical Factors) is the scientific discipline studying the effects of flight on human physiological and cognitive systems. The goal of aerospace physiology is to teach aviators to optimize their performance in abnormal environments. In essence, training aviators to survive the worst days of their aviation career; the day they experience smoke or fumes on the flight deck, experience hypoxia inflight, become spatially disoriented, or experience any of the other myriad of physiologically or psychologically incapacitating events. Students of aerospace physiology learn to continuously analyze the environments they are exposed to, recognize physical or physiological symptomology potentially leading to impairment or incapacitation, and successfully react to prevent impairment and incapacitation instinctively and possibly instantaneously.

Aviators must ask themselves what the difference is between a fatal accident and non-event. Is it one checklist item? 0.4 seconds of reaction time? One quick glance at an instrument at the right time? During a mid-air crisis pilots need to take all of their training, synthesize critical and complex information, and develop an action plan possibly within a split-second timeframe. Aerospace physiology training provides significant information necessary for aviators to make the correct decision at critical times.

Although some form of aerospace physiology is taught throughout various phases of pilot training, historically APT is an under-trained facet of the aviation training curriculum. Training and learning to a standard is not enough. Pilots must learn what is truly necessary. Many pilots associate this training with simply high-altitude hypoxia training or hypoxia awareness training. The reality is this training encompasses much more than simply hypoxia.

Effective aerospace physiology training occurs when one intersects the domains of aviation with physiology. Aviators teaching the subject matter may miss critical nuances of physiology, and physiologists teaching the subject matter may miss crucial nuances of flight management. Aerospace physiologists merge the two domains with research and experience-based information, coupled with practical training, to provide the most effective aeromedical training available.

To effectively retain this information, military forces attend regularly scheduled APT refresher courses at intervals of three to five years. The same interval should apply to you, as much of the information presented in this book applies to all phases of an aviator’s career span and varying experience levels. For you to apply this knowledge most effectively over your flying career, retain and review this book every three to five years to ensure the information remains fresh in your mind.

1.1 History of Aerospace Physiology Training (APT)

According to USAF aerospace physiologist Irena Farlik, the first aircraft flight took place at Kitty Hawk, NC by the Wright Brothers on December 17th, 1903; surprisingly taking five years before the first fatal airplane accident occurred. The military recognized the strategic importance of flight and the mobility it provided. Less than 10 years following the first flight, the U.S. military invested in some aircraft to start training. The mishap rate in the first year of flight training was atrocious. Of the 100 aviators killed, it was determined that 90 of those deceased were killed by some type of pilot error. This led to the recognition that human error was a major factor in aviation-related deaths, and concept of human factors training was born. This epiphany led to the development of aerospace medicine and aviation physiology training.

U.S. Military Aviation Safety Initiatives

In 1916, U.S. involvement in World War I led to the formation of an air service in the armed forces. The U.S. War Department issued a special order directing the establishment of the field of Aerospace Medicine for the research of human factors issues and the training of such to pilots. This eventually led the development of the Air Service Medical Research Laboratory in 1918 at Hazelhurst Field, Mineola, Long Island, where the term Flight Surgeon was adopted describing those physicians devoted to the health and well-being of aviators. This laboratory and flight surgeon training center became known as the School of Aviation Medicine (SAM) in 1922, and in 1926 the school was relocated to Brooks Field, San Antonio, TX. The school was relocated around Texas a couple more times before settling back at Brooks in 1957, becoming the USAF School of Aerospace Medicine (USAFSAM) in 1961. In 2005, USAFSAM moved from Brooks to Wright-Patterson AFB, OH.

Through the 1920’s and 30’s, military necessity for speed, altitude, distance, and duration was growing. By 1930, the B-17 Flying Fortress could cruise at altitudes of FL300 without bombs and travel at 230 mph. With this increase in altitude, questions began to emerge regarding the physiological hazards of flight at such altitudes for extended periods of time.

By 1935, the Physiological Research Unit at Wright Field in Dayton, OH, received a new low-pressure chamber and also had a human centrifuge installed for high-G research. Interestingly, this new lab sparked some competition from the scientists at USAFSAM. Research at Wright Field sparked the publishing of over 30 papers and Principles and Practice of Aviation Medicine, which became the standard authority in aerospace medicine for decades.

Following the beginning of World War II in 1942, the U. S. Air Surgeon instituted a new Altitude Training Program. This program was Air Force-wide and mandatory for aircrew members to complete. During program expansion, doctors and instructors were assembled into 45 units to instruct military aviators on oxygen equipment and the physiological hazards of flying. Other training topics included night vision training, G-forces, and thermal stress. Following the ending of the war, training tempo slowed, and the doctors and instructors reentered private practice and teaching in the civilian sector. The Aerospace Physiology program was reactivated in 1949 by the Surgeon for the 8th Air Forces. The Surgeon expanded the program into the Aerospace Physiology Training Branch where he initiated written tests, altitude chamber technical orders, modified altitude chambers to allow rapid decompression training, night vision trainers, and the first training charts and slides. He opened 13 Aerospace Physiology Training Units in 1950 (Farlik, 2015).

The program accelerated through the 1950s, and by 1956 there were 51 Physiological Training Units (PTUs). Oxygen consoles, pressure suits, and ejection seat trainers were added to the training. The 1960s brought even more advancement, and the program was reorganized under the Aerospace Medicine Division. High-altitude airdrop support (HAAMS) began in the early 1960s, which became prominent during the Vietnam Conflict. NASA also included crucial aerospace physiology training to astronauts from the earliest days of space flight, and continues this training to this day. By the 1980’s, 22 USAF PTU existed world-wide supporting a wide variety of military training and support missions including physiology of flight, hypobaric chamber, oxygen equipment, ejection seat, emergency parachuting, survival and survival kits, spatial disorientation, vision and visual illusions, high-G centrifuge, full-pressure suit, hyperbaric chamber, and high-altitude airdrop/parachutist missions.

Civilian Aviation Safety Initiatives

The U.S. military was fully engaged in aviation safety training, but what about civilian aviators? The oldest (by far) organization established to investigate aviation safety and support & maintain high professional standards in aerospace disciplines was the Royal Aeronautical Society (RAeS), which became active in 1866. The RAeS today has an international network of 67 branches in virtually every corner of the world.

The International Civil Aviation Organization (ICAO) was established in 1944 at the Chicago Convention, funded and directed by 193 nations supporting cooperation in air transport.

In 1961, the recently-formed Federal Aviation Administration established the Civil Aeromedical Research Institute (CARI) in Oklahoma City, OK. In 1965, the scope of CARI’s mission was enlarged and renamed the FAA’s Civil Aeromedical Institute (CAMI) and initiated a Physiological Training Program for civilian pilots – a short course designed to give civil airmen basic aeromedical education on the concepts of APT.

The Civil Aviation Authority (CAA) was established in the United Kingdom in 1972, responsible for aviation safety standards. The European Union joined aviation safety in earnest as far back as 1996, legally establishing the European Union Aviation Safety Agency (EASA) in 2002. Others include the Air Transport Association (ATA), Experimental Aircraft Association (EAA), Flight Safety Foundation (FSF), International Air Transport Association (IATA), as well as many other organizations established in aviation’s long history as well.

In 1989, the University of North Dakota’s Center of Aerospace Sciences started the nation’s first non-government APT program as part of their Commercial Pilot degree program, including practical altitude chamber and spatial disorientation (SDO) training, which was ground-breaking at the time. UND’s immersive three-credit hour APT program now incorporates not only altitude chamber and SDO/visual illusions training in a full-motion trainer, but cockpit smoke and enhanced vision lab training as well, and is still the premier collegiate Aerospace Physiology Training program in the country. A shortened version of this training is also offered to professional corporate and commercial aviators as well.

Civilian APT refresher training should be accomplished on a five- to ten-year recurrent basis. Memory fades, technology changes, new research comes available, and new trends emerge. The Air Force required APT training for aviators every three years for decades and enjoyed a relatively low human factors-related mishap rate. In a wave of military budget-cutting under President Obama, the USAF moved the training to a five-year cycle and closed some the APT units. Interestingly enough, the USAF human factors mishap rate increased not long after the increased training cycle.

1.2 Human Response to Flight

Changes in atmospheric pressure, a reduction in the amount of oxygen we ingest, changing our visual perspective and typical orientation processes, increased radiation exposure, and a myriad of other environmental factors can have profound effects on the way we cognitively process information and physiologically function.

The reality of flying is a pilot may potentially be presented with physical, cognitive, or physiological pressures resulting in impairment or incapacitation of the pilot. For example, let us look at a young, relatively inexperienced visual flight rules (VFR) pilot who inadvertently flies into a cloud while working on an aircraft equipment issue. A possible response to this situation would be for the pilot to look up from their task-at-hand, erroneously believe the aircraft is turning or tumbling out-of-control, realize they are unable to orient themselves with the horizon, and panic. The resulting intellectual incapacitation causes the pilot to start instinctively over-controlling the airplane, truly losing control, and exiting the cloud in a vertical descent, ultimately impacting terrain.

While younger, less-experienced pilots may be somewhat more susceptible to many of these scenarios, more experienced pilots may never have been presented with many of the problems which may occur. Having never experienced these issues previously, their response could be just as catastrophic if the right issue happens at the wrong time. One way to view these potential problems is as low-probability, high-consequence events. Examples may include:

- Loss of runway visibility on short final due to weather

- Deviation from aircraft approach glide path due to automation failure

- Rapid loss of aircraft pressurization at high altitude

- Toxic fume intrusion onto the flight deck

- Debilitating sinus block on approach

- Failure to monitor instrumentation due to distracting personal problems

The list of potential problems regarding a pilot’s inability to cognitively or physiologically cope is almost endless. Because situations such as these rarely present themselves, any pilot may be unprepared to respond appropriately simply because of lack of exposure or awareness.

The mission of APT is to enable pilots and aircrew members to effectively mitigate physical, physiological, or psychological incapacitation, with the ability to turn potential incidents and/or accidents into non-events. By knowing the proper responses and steps to mitigate the above-listed issues (and many, many others), the aviator can improve their safety margin by a very large degree.

If we examine historical causes of aviation incidents and accidents, one disturbing trend is the same things which were killing pilots decades ago are still killing pilots today, despite the advancement of technology. Technology has caused overall numbers of incidents and accidents to fall greatly over the years, but has also created new sets of problems.

The most recent Worldwide Commercial Jet Fleet Aviation Occurrence Category statistics compiled by the International Civil Aviation Organization (ICAO) reveal the occurrence killing most crewmembers and passengers in commercial aviation from 2007 through 2016 is loss-of-control inflight (LOC-I) accidents with 1,345 fatalities during the timeframe (Accident Statistics, 2020).

One may wonder why LOC-I conditions even exist in a day and age where automated aircraft control is the norm for passenger-carrying aircraft. The National Aeronautics and Space Agency (NASA) has performed studies on LOC-I accidents and found 46% occurred as result of the cockpit crew’s inappropriate response or interaction with aircraft equipment (Jacobson, 2013). One may interpret this statement to mean the pilots failed to program the automated system correctly, failed to monitor the automated systems correctly, failed to respond to system failure correctly, failed to fly the airplane after system failure, or any number of other interpretations.

The simple fact remaining after analysis is flying still relies on human input, ability, cognition, and attention-to-detail. The phase of flight contributing to the highest overall number of incidents is the approach and landing phase, which involves human perception, judgement, calculation, rapid intellectual scene-building, and ability.

The common denominator is human. Humans still need to remain engaged with the aircraft, correctly program and monitor the aircraft, and actually fly it in cases of non-automated flight conditions. Of course, engineers will argue technology can solve many of the life-threatening problems associated with flight, and perhaps those systems can so long as the systems work as advertised. The only problem with this theory is anyone with even a simple understanding of mechanized or electrical systems knows those systems can break, fail, be improperly programmed, or otherwise be rendered inoperable. So long as those systems have a less-than-100% success rate, humans still need to be on the flight deck ensuring the aircraft remains safe.

Aerospace physiology training is about enhancing human performance in adverse aviation environments. A complete and thorough understanding of the concepts of APT will undoubtedly make ANY pilot safer, regardless of their experience level. This book should be used as a career-long resource for professional aviators for the prevention of physiological issues in the flight environment.

Chapter 1 Core Competency Questions

1. According to FAA experts, what percentage of aviation accidents are caused by human influence?

2. When the U.S. military first invested time and resources into aviation training, what number of pilots died from pilot error out of the first 100 fatalities?

3. What civilian aviation safety organization was the first, and when were they established?

4. According to NASA’s studies on loss of control inflight (LOC-I) accidents, what percentage occurred from the aircrew’s "inappropriate response or interaction with aircraft equipment?

2. SCIENCE OF THE ATMOSPHERE

The atmosphere (think environment) humans typically survive and thrive in has a profound effect on the way human’s function. As such, aviators need to have a basic understanding of the atmospheric function in general to better understand the overall influence it has on our human cognitive and physiological processes.

In this chapter we will discuss:

1. Definition of atmosphere

2. Atmospheric functions

3. Physical characteristics of the atmosphere

4. Climate change and aviation operations

5. Composition of the atmosphere

6. Physiological acclimatization

7. Physical divisions of the atmosphere

8. Physiological divisions of atmosphere

9. Physical gas laws

10. The partial pressure of oxygen

2.1 Definition of Atmosphere

The Earth’s atmosphere is a protective gaseous envelope surrounding the Earth consisting of gases, vapor, and solid particles held to the Earth by gravity.

Barometric pressure is measuring the weight, or pressure, of air molecules above the point the measurement is taken, which means barometric pressure decreases with altitude. As a result of air being compressible, the pressure decrease is not linear, as molecular density plays a role. This also means the closer one gets to the Earth’s surface, the denser air molecules become.

One may think of the surface of the Earth as being the bottom of an ocean of air; the closer to the bottom one gets, the exponentially denser the barometric pressure. With this in mind, the greatest amount of barometric pressure change aviators will be exposed to is below 10,000 feet MSL.

2.2 Atmospheric Functions

The basic atmospheric functions as far as humans are concerned are to provide life support, protect us from the harmful effects of radiation, thermal variations, and orbital material striking the Earth.

The life support function is provided through the proper amount of atmospheric pressure on our bodies for normal function, and to provide vital amounts of oxygen to allow us to operate normally. We will discuss those issues in depth later.

With regards to thermal protection, the atmosphere prevents extreme thermal loss. Think of the thermal extremes of the moon, for example. When the sun strikes the moon’s surface, temperatures can reach a robust 260 degrees Fahrenheit, whereas when sun goes down, temperatures can dip to a chilly -280 degrees F. Because of the insulating capability the Earth’s atmosphere provides, our temperature variations are not nearly as extreme.

Orbital material enters the Earth’s atmosphere, where the friction of the atmosphere heats the material up to the point of incineration. This material may be space exploration junk left in orbit from previous experiments, space pebbles, meteors, or any other material floating around in space.

The radiation protection the Earth’s atmosphere provides deserves somewhat more respect. Ultraviolet (UV) and/or galactic cosmic (ionizing) radiation could severely damage our human organs were it not for the protection afforded to us by ozone in the stratosphere and the mesosphere. Were it not for the atmosphere, acute and/or chronic symptoms of radiation overexposure would make survival of the species difficult.

Because radiation exposure increases with altitude, aviators do have an increased exposure to both UV and ionizing radiation; enough so that the Federal Aviation Administration classifies pilots as Occupational Radiation Workers. Based on FAA research, findings reveal a typical flight from New York, NY to Tokyo, Japan yields almost as much ionized radiation as one chest X-ray, the flight equating to .0754 millisieverts (mSv) of radiation exposure. The average U.S. citizen receives approximately 2.96 mSv radiation exposure per year from natural causes, while the average pilot adds another 2.2 mSv per year on top of that number. The FAA limits pilots to 20 mSv per year, so few pilots would be able to get close to that figure as 20 mSv would equal close to 225 New York to Tokyo flights per year. Even so, it is estimated flying at FL350 increases ionized radiation exposure by approximately 70% over ground level. Ionized radiation is known to promote free-radical production in the body, which is atoms losing orbital electrons, thus becoming cancerous. Taking all factors into consideration, the increase in fatal cancer risk in pilots has been observed to be an average of .5% over the general population. In addition to increased ionized radiation, UV radiation can reach up to 3,500% of Sea Level values near the top of the Troposphere (approximately 35,000 feet).

How do we protect ourselves from radiation? One method is flying where the protective elements of the atmosphere are at their greatest. The atmosphere is not a perfect sphere; in reality, it is more of an oblong shape. The atmosphere is thinnest at the polar regions of the Earth and thickest at the Equator. This occurs mainly because the angle of the sun striking the Earth is at its lowest angle at the poles, and at its highest angle at the equator. More heat from the Earth’s surface is radiated into the air, resulting in a higher troposphere in equatorial regions; thus, more atmospheric radioactive protection.

Another protection strategy is to protect our skin and eyes from UVA radiation. UVB and UVC ray are mitigated for the most part by the skin and windscreens of the aircraft, but UVA rays are still a concern. UVA rays can cause long-term cumulative damage to the eyes and skin, so wearing a high sun protection factor (SPF) sunscreen and UVA-protectant sunglasses is advised. The protective factor for sunscreen should be at least SPF 50, and sunglasses should provide 99% UV blockage below 400 nanometers.

Dr. Adrian Chorley of Aviation Vision Services conducted research from 2008-2015 studying the effects of ultraviolet radiation on a pilot’s eyesight. The study showed even though UVA is the least energetic of UV radiation, it causes the most harm to a pilot’s eyes and eyesight because a higher percentage of the radiation penetrates the flight deck and cabin of an aircraft. The study concluded there is good evidence long-term exposure to solar radiation, especially the ultraviolet and blue light components, is a risk factor for cataracts and, to a lesser extent, age-related degeneration of the retina."

2.3 Physical Characteristics of the Atmosphere

The Earth’s atmosphere shows relatively predictable characteristics in that barometric pressure, temperature, and water vapor all decrease as we ascend to higher altitudes.

According to the International Civil Aviation Organization (ICAO), the standard temperature at mean sea level (MSL) is 59 degrees F, or 15 degrees C. Temperature decreases as we ascend to altitude at a rate of 3.5 degrees F or 2.0 degrees C per 1,000 feet altitude. This temperature lapse rate continues until approximately the top of the troposphere, where the temperature stabilizes in the vicinity of -67 degrees F or -55 degrees C.

Gaseous atmosphere is influenced by Earth’s gravitational pull, and atmospheric (barometric) pressure is the combined gasses’ force on the environment at any given point. Barometric pressure is expressed in measurements of Pounds per Square Inch (PSI), Inches of Mercury (in/Hg) or Millimeters of Mercury (mm/Hg) as referenced in the chart below:

Figure 2.1 Barometric Pressure Table

Note that in terms of barometric pressure density, the half-way point is 18,000 feet. This means half of the Earth’s atmospheric density is below 18,000 feet, and the other half extends to the boundary of space, which is a complete vacuum. As pressure decreases, air becomes less dense due to the kinetic nature of atoms and molecules, which are in constant state of motion. As pressure around the molecules is reduced, the molecules travel further apart; therefore, air becomes less dense and gas expands. ¾ atmospheric pressure would equate to approximately 8,000 feet, which is a common cabin altitude for commercial aircraft, and ¼ atmospheric pressure would equate to approximately 34,000 feet, which is a common ambient cruise altitude for commercial aircraft.

As barometric pressure decreases, so does its ability to affect and assist bodily function. As mentioned, barometric pressure decrease is not a linear progression; rather, an exponential progression. Since pressure density increases the nearer we get to the Earth’s surface, the most likely place for human pressure-induced issues, such as ear blocks and sinus blocks, is below 10,000 feet.

The combination of barometric pressure and oxygen determines our human ability to perform. Another important question to answer is where, exactly, does reduced barometric pressure begin to seriously affect humans? The answer is some physiological and higher-level cognitive functions are already being affected as low as 5,000 feet for a sea level-equilibrated individual. One may note physical performance begins to degrade rapidly as altitude increases, as anyone who has skied, tried running, climbed mountains, or engaged in any physical activity at altitudes above 4,000 MSL.

At altitudes below 10,000 feet, humans can perform somewhat normally, even given the great amount of barometric pressure change between sea level and 10,000 feet which can be as much as 237 mm/Hg. As mentioned, barometric pressure decreases exponentially, so the same 10,000 feet altitude increase between 30,000 feet and 40,000 feet yields a decrease of 84.9 mm/Hg. Although the pressure change is less, the impact on human performance is greatly magnified at those altitudes.

Barometric pressure, temperature and water vapor variations interact and contribute to weather formation and climate. In the troposphere, air rises as it is heated by the sun, then falls towards the surface of the Earth as it cools. It will then intermix with evaporated water from the oceans, seas, lakes and rivers of the world to form clouds and precipitation. Uneven heating of the Earth’s surface due to sunlight, combined with the Earth’s rotation, cause rising, falling and horizontal air movements (also known as wind). These processes combined result in the development of snow and rain, and heat or freezing temperatures, and is called weather. Long-term trends with regards to weather patterns affecting the entirety of Earth is called climate, which is typically tracked in 30-year blocks of time. Variations in climate behavior over longer periods (100-year blocks or so) is referred to as climate change.

2.4 Climate Change and Aviation Operations

NASA scientists attribute the increasing global warming trend seen since the mid-20th century to the human contribution of the greenhouse effect, or warming which occurs when the atmosphere traps heat radiating from Earth toward space.

Certain gases in the atmosphere block radiated heat from escaping. Gases that remain semi-permanently in the atmosphere and do not respond physically or chemically to variations in temperature are described as forcing climate change. Gases, like water vapor, which respond physically or chemically to changes in temperature are seen as feedbacks.

According to NASA, gases that contribute to greenhouse effect include:

- Water vapor. The most abundant greenhouse gas. Crucially, it acts as a feedback to the climate. Water vapor increases as atmosphere warms, but also contributes to the possibility of clouds and precipitation. This effect makes these some of the most important feedback mechanisms to the greenhouse effect.

- Carbon dioxide (CO2). A relatively small but very important component of the atmosphere, carbon dioxide is released through natural processes such as respiration, volcanic eruptions and through human activities such as deforestation, land use changes, and burning fossil fuels. Humans have increased atmospheric CO2 concentration by more than a third since the Industrial Revolution began. This is the most prolific, long-lived forcing of climate change.

- Methane. A hydrocarbon gas produced both through natural sources and human activities, including the decomposition of wastes in landfills, agriculture, and especially rice cultivation, as well as digestive processes and manure management associated with domestic livestock. On a molecule-for-molecule basis, methane is a much more active greenhouse gas than carbon dioxide, but also much less abundant in the atmosphere.

- Nitrous oxide. A powerful greenhouse gas produced by soil cultivation practices, especially through the use of commercial and organic fertilizers, fossil fuel combustion, nitric acid production, and biomass burning.

- Chlorofluorocarbons (CFCs). Synthetic compounds designed entirely for industrial purposes and used in a number of applications, CFCs are now highly regulated regarding production and release into the atmosphere by international agreements for their ability to contribute to destruction of the ozone layer. They are also greenhouse gases.

Much research has gone into the development and effects of climate change in recent history, and climate change is destined to have a profound impact on aviation operations as we know it at the present time. The United States Environmental Protection Agency (US EPA) predicts a global climate shift that contains a decrease in extreme cold weather, and an increase in extreme hot weather. Based on NASA and EPA research findings, the following are some of the effects of climate change to consider for aviators:

- Ozone depletion in the stratosphere increases wind speeds in that region of the atmosphere, and can create shifts in the Intertropical Convergence Zone (ITCZ) located five degrees North and South of the Equator. These ITCZ shifts can cause global wind and rain belt fluctuation, causing more extreme weather in areas of the world that historically have not experienced such potentially hazardous conditions.

- Scientists predict a 50% increase in lightning strikes in the coming century. While most aircraft are arguably equipped to disburse lightning strikes somewhat effectively, having a highly electrical-dependent aircraft receive a 1,000,000,000-joule energy surge has the potential to create problems with aircraft electrical systems.

- Storm size and intensity is predicted to increase due to the increase in global temperatures. Flying into unpredictable weather that has the potential to blossom catastrophically and reach higher altitudes than the aircraft can fly, or increase the chances of icing conditions creating serious controllability issues. Research has shown the majority of business jet LOC-I accidents occur below 1,000 feet above ground level (AGL), and increasing the chances for turbulence, icing or flight control binding could possibly create an increase in those issues occurring.

- Warmer global temperatures may also impact typical arrival and departure patterns for airports in historically warmer regions, such as the American Southwest. Warm temperatures will affect atmospheric pressure density, making landing and/or taking off from the aircraft more difficult at certain times of the day, which could in turn demand a change in runway lengths, or even change the amount of weight a particular aircraft model could safety transport.

- A predicted increase in carbon dioxide (CO2) in the atmosphere has the potential to dramatically increase turbulence. Turbulence has the capacity to injure passengers and flight cabin crew members who are moving around the aircraft at the time of turbulence entry.

Figure 2.2 Two Hurricanes and the ITCZ

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With the above-mentioned information in mind, what does this mean to you? Unpredictable weather can potentially cause aviators to find themselves immersed in a psychologically challenging situation in a matter of seconds or even split-seconds.

2.5 Composition of the Atmosphere

The main atmospheric gases we as humans are concerned with are oxygen, nitrogen, and carbon dioxide. Oxygen is our primary life-support gas; it is essential for human and animal life and supports body metabolism (the catabolic breakdown of glucose for the production of heat energy). Nitrogen is inert to humans but still physiologically significant as we ascend to higher altitudes. Carbon dioxide is simply the by-product of human metabolic processes. The remaining atmospheric gases are considered noble gases, which have no bodily function. The table below lists the various atmospheric gases and the total percentage of that gas in the atmosphere.

Figure 2.3 Percentages of Gases in the Atmosphere

The percentages of these gases remain consistent to the outer edge of the atmosphere (which is anywhere from 50 to 1,000 miles, depending on which scientist is asked), but as already mentioned, the overall pressure decreases. In the lower reaches of the atmosphere, water vapor, dust and pollution make up the rest of atmospheric composition.

2.6 Physiological Acclimatization

Humans have the unique ability to adapt to their environment fairly effectively, with full physiological and cognitive function, up to an altitude of 10,000 feet. Based on research conducted by Dr. Zubieta-Calleja of the University of Copenhagen, full physiological adaption takes about 18.4 days per mile (5,280 feet) of altitude up to 10,000 feet.

Long-term physiological changes that occur to compensate for environmental pressure changes include:

- Increases in red blood cell (RBC) mass. Oxygen transport in the body is the responsibility of RBC’s binding to and carrying oxygen, so the body compensates by increasing the capacity of the transportation system.

- Increase in cellular mitochondria. Cellular mitochondria are responsible for oxygen metabolizing processes of the body, so the body increases production capacity.

- Increases in cardiovascular system capillaries. Capillaries are responsible for the exchange of oxygen between the blood stream and body tissue, so the body increases its ability to diffuse oxygen into tissue.

- Increases in arterial blood pressure. The body’s arteries are responsible for transportation of oxygen-rich RBC’s, so the body wants to increase its ability to transport decreasing amounts of oxygen around the system.

Individuals that do not fully physiologically acclimatize can expect the following symptomology:

- Hyperventilation: involuntary faster and/or deeper breathing rate.

- Shortness of breath, or air starvation, during exertion.

- Changing breathing patterns during periods of sleep.

- Awakening frequently at night.

- Increased urination.

What about people that live their lives at altitudes above 10,000 feet, or extreme mountain climbers who spend months at altitudes in excess of 10,000 feet? Indigenous mountain dwellers such as the famous Sherpa of the Himalayan Mountains of Asia have actually undergone an evolutionary physiological adaption over centuries to enable them to function normally at extreme altitudes. Their physiology has permanently adapted to be hyper-efficient with lower barometric pressures and lower oxygen-levels. Mountain climbers, or more appropriately sea-level adapted people, undergo the changes listed above over a period of time, but would never achieve peak physical or cognitive performance.

Knowing what we do so far about the lack of human efficiency at altitude, why do aviation operations risk flight at high altitudes? The answers are:

- Less air resistance equals more efficient fuel consumption, potentially saving an airline operator thousands of dollars per flight.

- Better routing as a result of the ability to fly over mountainous or hostile terrain.

- Potential for more favorable winds (again, better fuel efficiency) if a flight lines up correctly with the Jetstream.

- The opportunity to fly above hostile weather systems, ensuring a smoother, more comfortable and predictable customer experience.

Humidity

Water as a substance is unique, as it may exist as a liquid, solid, or gas. Water, or moisture, increases in the atmosphere primarily through evaporation from rivers, lakes, oceans, plants and the majority is typically contained below 10,000.

Absolute humidity is a measure of the actual amount of water vapor in the air, regardless of air temperature. Obviously, the higher the content of water vapor, the higher the absolute humidity.

Relative humidity is typically expressed as percent, and measures water vapor relative the air temperature. Typically, warm air may possess more water vapor than cold air so if both types of air possess the same absolute humidity, cold air would have higher relative humidity and warm air would have lower relative humidity. What humans feel outside in an unprotected environment is the actual amount of moisture, or relative humidity, in the air.

As mentioned, altitude and temperature play a role regarding moisture in air. The higher and colder the ambient altitude, the lower the relative humidity, and this dry air is being used to pressurize most commercial or corporate aircraft. As this ambient air is modified and warmed for use in the fuselage it gets even drier resulting in a very dry cabin environment. Most commercial aircraft operate with a very low internal relative humidity of 20% or less, especially on long flights. Aircraft are not equipped with humidifiers as the volume of water required to be stored on board would create significant operational weight penalties.

These low relative humidity levels may cause discomfort to passengers and crew such as sore eyes and less visual effectiveness, dry skin, and drying out of mucosal membrane. Dry mucosal membrane in nasal passages may crack and therefore become more susceptible to rhinovirus cold viruses penetration.

One may mitigate the effects of low humidity by:

- Inhaling steam from hot coffee or tea

- Using moisturizing creams

- Staying hydrated with bottled water

- Using saline solution for dry eyes

- Wearing a protective face mask.

2.7 Physical Divisions of the Atmosphere

As we know, atmospheric pressure decreases and temperature variances exist throughout the entire spectrum of the envelope of air surrounding the Earth. In the lowest division of the atmosphere, the Troposphere, this occurs as result of differential heating of air from the sun’s rays striking the Earth (radiant energy) at various angles, depending on where those rays strike. The sun’s rays will be at a very low angle at the poles, resulting in less heating and as such, less lift, so the Tropospheric layer at the poles can be relatively thin. The sun’s rays strike the Earth at a higher angle at the equator, resulting in more heating (higher lift) and a much thicker Tropospheric layer.

Each physical division of the atmosphere has specific characteristics associated with it, in that temperature, chemical composition and physical properties differentiates that division from the others. The list below describes these divisions:

Troposphere: Sea level – approximately 23,000 feet (poles) to 50,000 feet (equator). Variable temperature, water vapor, turbulence, storms, weather, temperature lapse rate. This is the common cruise altitude for most piston- and turbo-prop-powered aircraft.

Tropopause: While not officially a division of the atmosphere, the tropopause is a region of temperature stability that forms the boundary separating the Troposphere and Stratosphere.

It should be noted at this point that a large amount of ozone gas exists in the atmosphere exceeding tropospheric levels. Ozone is an important component of the atmosphere that filters out ultraviolet radiation. Heavier concentrations of ozone typically exist in the winter and spring seasons, and while flying northern routes.

Unfiltered, ozone gas ingestion in humans can cause a variety of symptoms, including headaches, fatigue, shortness of breath, chest pains, nausea, coughing, pulmonary distress, and exacerbation of asthma.

On aircraft capable of flying at those altitudes, catalytic ozone converters should be effective at filtering out 90-98% of ozone entering the occupied cabin. Issues that can arise is filter efficiency can degrade with age like any other mechanical device, and smoke events can lessen filter effectiveness.

Stratosphere: Approximately 36,000 feet - 165,000 feet. Relatively constant temperature of -55 degrees C or -67 degrees F at the lower levels, little water vapor, jet streams, little turbulence. Temperatures in the stratosphere may actually increase with altitude because of increasing amounts of ozone. This is a common cruise altitude for most commercial and corporate turbine-powered aircraft.

Mesosphere: Approximately 165,000 feet – 287,000 feet. The coldest regions of our atmosphere exist in this layer and reach -90 degrees C. Provides protection from UV rays, and gets its name from the ionized gas within this layer (UV rays strip electrons from gaseous molecules and creates ions). This layer also allows for effective radio communication.

Thermosphere: 287,000 feet – 1,650,000 feet (approximately 300 miles). The fourth layer of the Earth’s atmosphere is the region the International Space Station or space shuttles orbiting the Earth spend their time. The thermosphere is extremely sensitive to the sun’s activity, and can heat up to 1,500 degrees C as a result of solar influence.

Exosphere: 300,000 feet +. Gradually becomes the vacuum of space, and has so little pressure and density gaseous molecules rarely collide.

Space: Complete vacuum, average temperature between celestial bodies averages -270 degrees C or -458 degrees F.

Figure 2.4 Physical Divisions of the Atmosphere

2.8 Physiological Divisions of Atmosphere

In the scientific discipline of Aerospace Physiology, we classify the atmosphere in phases of how human function will be affected, and our life-support needs for peak performance. Modern aircraft quickly exceed human limitations with respect to common altitudes flown, so the following list describes the physiological divisions of the atmosphere:

Zone of Normal Adaptation. This zone extends from sea level to 10,000 feet. Humans can adapt and function relatively normally at these altitudes. Generally speaking, the human body has adapted to operate at peak efficiency in the lower regions of this zone and can experience minor physiological and cognitive problems resulting from a lack of oxygen in the upper reaches. Problems with the equalization of gas trapped in certain body cavities may occur as humans descend through this part of atmosphere.

Zone of Physiologic Deficiency. This zone extends from 10,000 feet to 50,000 feet. Most humans are unable to adapt and function with peak efficiency in this region of the atmosphere, so conventional aviation life-support equipment is necessary to support peak, or even normal, physiological and cognitive function. Life-support equipment efficiency needs to vary with increasing altitudes and conditions, as increasing amounts of oxygen and pressure ingestion are necessary to allow humans to continue to operate normally.

Space Equivalent Zone. This zone starts at 50,000 feet and extends until the outer edge of the atmosphere. Exposure to these altitudes would most certainly cause incapacitation, followed very shortly by death, in extremely short periods of time (seconds as opposed to minutes.) Survival at these altitudes would require the use of full-body pressure suits (space suits), sealed pressurization systems, etc. FL630 is known as Armstrong’s Line; the point in the atmosphere where liquid boils simply by exposure to the environment. Barometric pressure is so low and gas expansion so high that human blood would virtually boil at a temperature of 65 degrees F.

2.9 Physical Gas Laws

As discussed, the atmosphere is complex mixture of gases, and these gases are subject to the laws of physics like anything else. Since we must strive to survive in whatever environment we fly in, a basic understanding of the physical gas laws and how they interplay with the human body and aviation equipment you will use is necessary.

We will discuss the actions of gases when gases are subjected to pressure, subjected to thermal variation, and when gases are dissolved in a solution. We also explain the actions of gases when they diffuse, and their component structure.

Boyle’s Law. A volume of gas is inversely proportional to its subjected pressure, temperature remaining constant.

What Boyle is so eloquently saying is when a volume of gas exists, the environmental pressure surrounding it determines its size. Think of a balloon blown up half-way to its fullest volume; if one could reduce the atmospheric pressure around the balloon, its size would increase. If one could increase the atmospheric pressure on the balloon, its size would decrease.

The same phenomena occur in the human body when we ascend or descend in the atmosphere. There are areas of trapped gas inside the body whereas the gas expands as we ascend in altitude and contracts as descend in altitude. This may cause excruciating problems if that gas cannot be equalized as we ascend and descend, so Boyle’s Law explains how trapped gas issues occur in the body as we fly.

Charles’ Law. The pressure of a gas is directly proportional to its temperature.

What Charles’ Law explains is the propensity of gas volume to increase as it is heated, or contract as it is cooled. The significance of this in aviation has nothing to do with human physiology; rather, it mainly has to do with aircraft oxygen bottles.

Oxygen bottles must be filled to a certain pressure prior to flight, and the filling process takes place in a hanger or on the ramp where the temperature is 80 degrees F, for the purposes of this discussion. We know when we ascend to altitude, the temperature cools, so when we check the pressure of those oxygen bottles at altitude the pressure reading will be less than when we checked the pressure on the ground. Charles’ Law simply explains the fluctuation of oxygen bottle pressure at altitude vs. what they read on the ground.

Henry’s Law. The amount of gas dissolved in a solution is proportional to the partial pressure of that gas over the solution.

Henry’s Law is explaining what happens when opening a bottle or can of carbonated beverage. In a carbonated drink, the partial pressure of carbon dioxide over the liquid compresses carbon dioxide gas into the liquid itself. When we open the bottle or can, the liquid in the container fizzes as the partial pressure of carbon dioxide over the liquid is reduced, allowing carbon dioxide in the liquid to escape.

The physiological significance of Henry’s Law is because 78% of atmosphere consists of nitrogen, the resulting consequence is we have a lot of nitrogen molecules in body tissue and bodily fluids. When we ascend to altitude, especially under certain conditions (to be discussed later), those nitrogen (N2) molecules can supersaturate in the tissues and fluid and form bubbles. Depending on where those bubbles form and collect in the body, significant and profound issues to human function can arise. This bubble formation is known as Evolved Gas Disorders, or Decompression Sickness.

Graham’s Law (The Law of Gaseous Diffusion). Gases will diffuse from an area of high concentration to an area of low concentration.

Graham’s Law simply states the movement of smells and gases occur as a direct result of pressure differentials. If there is an area with a pressure of 100, for example, and an adjacent area of pressure of 75, the gas from the area of high pressure will naturally move to the area of low pressure until both areas’ pressure is equal. The same thing happens with high- and low-pressure weather systems, creating wind on the surface of the Earth.

The physiological significance of Graham’s Law is explaining how we can get oxygen from our environment into our lungs, blood stream and body tissues. The process simply depends on pressure gradients and varying concentrations to keep those gasses moving in the correct direction. Ultimately, Graham’s Law explains the respiration process of the human body.

Dalton’s Law. The total pressure of a mixture of gas is equal to the sum of the partial pressure of each gas in the mixture.

This isn’t a hard concept to comprehend; there are a number of fractions which equal a whole number. Each gas in the Earth’s atmosphere can be isolated, and that gas is referred to as a partial pressure, so when you add up all the partial pressures of each gas in the atmosphere (O2, N2, CO2, H2, etc.) you have the total atmospheric pressure.

The physiological impact to us is when we ascend, the total atmospheric pressure decreases, which means the partial pressure of oxygen we have grown accustomed to receiving from our environment is steadily decreasing along with the decrease in overall pressure. If we continue to ascend, we will reach a point in the atmosphere where we start to fail, both cognitively and physiologically, so Dalton’s Law explains how the phenomena of hypoxia occurs as we ascend.

2.10 The Partial Pressure of Oxygen

The environmental partial pressure of oxygen humans need is one of the main physiological issues of aviation, as oxygen is the currency by which we live and function. It is critical for aviators to have a thorough understanding of the relationship between the partial pressure of oxygen and human function.

To determine the partial pressure of oxygen in the atmosphere, we use a simple equation; PO2 = PB x %oxygen (the atmospheric partial pressure of oxygen equals the total atmospheric pressure multiplied by the percentage of oxygen in the atmosphere).

Let us figure out the partial pressure of oxygen at Sea Level: 760 mm/Hg (total SL atmospheric pressure) x .21 (percentage of oxygen in the air) = 159 mm/Hg (the partial pressure of oxygen).

159 mm/Hg is the amount of atmospheric oxygen humans need to be at our physiological and cognitive peak efficiency.

Let us see what the partial pressure of oxygen would be at 18,000 feet (FL180): 380 mm/Hg x .21 = 80 mm/Hg.

FL180 being the half-way point in terms of atmospheric pressure, it stands to reason the PO2 would be 50 percent that of sea level. Now that we know this, what can aviators do to maintain peak efficiency? The answer: use properly-calibrated oxygen equipment.

If we add an oxygen system that is properly calibrated, the system should be delivering adequarte partial pressure of oxygen at FL180 to keep us relatively close to sea level equivalency, as this is where we function at our peak. In order to achieve that goal, we need to figure out what the proper amount of oxygen would be. If we multiply 380 mm/Hg by .42, we end up with 159.6 mm/Hg as a PO2, so the oxygen system should be delivering somewhere between 40% - 42% PO2 to keep us fully functional.

This is the key to a properly functioning and calibrated oxygen system; either the aviator or the system itself has to continuously be aware and/or monitor the barometric pressure at any given altitude and provide the proper partial pressure of oxygen to keep the aviator safely flying the aircraft. This information will be covered in much greater detail later in this book.

Conclusion

Now we have a better understanding of the atmosphere and how we, as humans, fit into it.