Dalton's Law of Partial Pressure

 

 

 

Gas Laws and Flight Safety: Dalton’s Law of Partial Pressure

 

Abstract

In February of 2007 an aircraft suffered a windshield fracture at altitude. Due to lack of aircraft systems knowledge and flight physiology awareness, the pilot in command chose to depressurize the aircraft while the oxygen system was turned off. This led both pilots of the accident aircraft to lose consciousness for more than seven minutes while the aircraft descended out of control and suffered structural damage. This paper will outline how Dalton’s law of partial pressure pertains to the accident flight and how proper knowledge of this basic gas law could have prevented the accident.

Keywords: Dalton’s law, partial pressure, hypoxia, time of useful consciousness

 

Gas Laws and Flight Safety: Dalton’s Law of Partial Pressure

Knowledge of the basic gas laws and how they affect pilots and passengers is an essential part of every safe crew member’s awareness. I will first outline Dalton’s Law and how it correlates to altitude induced hypoxia as well as how ignorance for this gas law contributed to an aviation accident. I will then identify the error chain and provide corrective actions to clearly show how this accident could have been prevented.

Dalton’s Law and Hypoxia

The atmosphere that we live and breathe in is a mixture of several gases. The life giving ingredient that is required for almost all life on Earth is oxygen. Oxygen is a colorless, odorless and tasteless gas and is the most abundant element on Earth (Reinhart, 2008). Comprising approximately one fifth of the Earth’s atmosphere, oxygen deprivation can lead to several symptoms ranging from visual acuity impairment, slurred or incoherent speech, to total loss of consciousness.

Dalton’s Law

Dalton’s law states that the total pressure of a gas mixture is the sum of the individual pressure (also called partial pressure) that each gas would exert if it alone occupied the whole volume. This law can also be expressed mathematically: PT = P1 + P2 + Ps + Pn; PT is the total pressure of the gas mixture and P represents         the partial pressure value of each gas, which is determined by multiplying the percentage of the individual gases time the total pressure (Reinhart, 2008). Simply put, because each gas represents only a portion of the air that we breathe, as we climb in altitude the pressure of each individual gas decreases with the total decrease in pressure.

Partial Pressure

Each gas will exert its own pressure depending on the percentage of that gas in the mixture. As stated by Mortazavi, Eisenberg, Langleben, Ernst and Schiff (2003), “The proportion of atmospheric oxygen remains constant at 21% at altitudes below 100,000 m. Therefore, the partial pressure of oxygen (PO2 = barometric pressure X 0.21) falls substantially with lower barometric pressure at higher altitude. PO2 at sea level is 159 and decreases by 50% at 5496 m. For each additional 300 m, PO2 decreases a further 4–5 mm Hg.”

As the body ascends, even though the percentage of each gas in the atmosphere remains the same the available molecules of oxygen at a pressure required to pass to a blood cell decreases. This decrease in pressure leads to altitude induced hypoxia.

Hypoxia

Hypoxia is defined as an oxygen deficiency in the body and there are several different ways to get hypoxia. Dalton’s law can be used to explain hypoxic hypoxia caused by “high” altitude. As the body climbs in altitude, the partial pressure of oxygen decreases, making diffusion difficult or even impossible in the lungs. This leads to hypoxic symptoms such as euphoria, cyanosis, dizziness, visual impairment, loss of motor control, seizures, and eventually loss of consciousness.

Time Of Useful Consciousness

The time from when an oxygen deficiency begins until a pilot is no longer able to recognize and take action is called time of useful consciousness (or TUC). As altitude increases, TUC decreases, making recognition and action critical.

Accident and Analysis

According to an NTSB report from 2008, in February of 2007 an aircraft accident occurred following an in-flight depressurization. Operated as a 14 CFR part 91 flight, King Air N777AJ was a Raytheon Aircraft Company B200 which required only one pilot. On the accident flight a company employed pilot was the pilot in command and a non-company pilot was also present for the purpose of flight time accumulation. The non-company pilot was not trained nor checked out on the B200 aircraft.

Accident

While cruising at 27,000 feet mean sea level the aircraft experienced a windshield fracture. According to the CVR data, the pilot in command was not occupying his duty station but was in the cabin emptying a trash bin, leaving a non-trained pilot at the controls. After the fracture occurred, the pilot returned to his duty station, and made the decision to depressurize the aircraft because he was concerned about the integrity of the windshield. Using non-approved documents, non-approved procedures, and poor judgment, both pilots lost consciousness for more than 7 minutes due to altitude induced hypoxia. During this time the aircraft descended out of control and suffered structural damage and gravity-forces in excess of 4-g’s. Despite the out of control descent, both pilots regained consciousness and were able to successfully land the damaged aircraft.

Error Chain

Like most aviation accidents that occur, a chain of events known as the error chain can be pieced together to determine what eventually led to the accident. Rarely do accidents occur from a single event, but rather a series of errors that lead to a final event. As well as having a clearly defined error chain, this accident flight was also laced with poor decision making, lack of aircraft systems knowledge, failure to utilize manufacturer approved checklists, lack of physiological awareness, and improper pre-flight procedures.

The error chain for this flight began before the flight even started. The checklist found onboard the accident aircraft was not an approved checklist and it did not contain the recommended pre-flight items per the airplane flight manual (AFM). This unapproved checklist didn’t have a procedure for cracked or fractured windshields either. Proving just how inadequate and unprofessional this checklist was, the last item of the Shut Down checklist was “Pajamas…As Req.”

During pre-flight of the oxygen system, the pilot in command stated he successfully tested the oxygen mask and then turned the system off to “save” the oxygen. This was not in accordance with manufacturer recommended pre-flight procedures.

Once the windshield fractured, the error chain continued with the pilot in command’s decision to depressurize the aircraft. The AFM states that following an inflight windshield fracture, cabin pressure should be maintained and safe flight can be continued for up to 25 hours. Post-accident investigation of the windshield showed it to be structurally intact.

These events led to the precipice of the accident when the cabin was intentionally depressurized while the oxygen system was off. When the aircraft was depressurized at 27,000 feet mean sea level, the approximate time of useful consciousness was three to four minutes. Even though post-accident investigations revealed the oxygen system to be fully functional, it was simply never turned on.

Correlation To Dalton’s Law Of Partial Pressure

The pilot in command lacked sufficient awareness and knowledge of Dalton’s law as evidenced by his decision to depart with the oxygen system turned off, and further ignorance by intentionally depressurizing the aircraft. With appropriate working knowledge of flight physiology and the reduction of oxygen’s partial pressure at altitude, the pilot in command would never have decided to turn the oxygen system off prior to departure. Correlation of Dalton’s law with knowledge of decreased time of useful consciousness this accident could have been prevented completely.

Accident Prevention and Conclusion

Although there were several errors in the error chain that eventually led up to this accident, they are all preventable with proper procedures and aircraft systems knowledge. Regarding Dalton’s law, this accident could have been prevented with better knowledge of how altitude affects time of useful consciousness as well as better alertness for potential hypoxia situations. It should have been obvious to the pilot in command that prior to depressurizing the aircraft cabin that the oxygen system should be turned on. This point is over shadowed by the lack of adherence to manufacturer recommendations for aircraft pre-flight and configuration. There should never be a scenario at altitude where the crew would need to first activate the oxygen system prior to donning the oxygen masks.

Utilization of manufacturer recommended checklists, procedures, and operating practices isn’t just a really good idea, it’s required. There is a reason why human performance factors are on just about every pilot check-ride you can attempt, they’re important too. The last frontier of accident prevention that we must endeavor is that of human performance. With nearly every accident occurring because of human error, we must close the gap on preventable accidents like the one I have described.

 

 

 

References

Mortazavi, A., Eisenberg, M. J., Langleben, D., Ernst, P., & Schiff, R. L. (2003). Altitude-Related Hypoxia: Risk Assessment And Management For Passengers On Commercial Aircraft. (Vol. 74-9, pp. 922-927). Alexandria, VA: Aerospace Medical Association.

NTSB. National Transportation Safety Board, (2008). Full Narrative (CHI07LA063). Retrieved from website: http://www.ntsb.gov/aviationquery/brief2.aspx?ev_id=20070208X00156&ntsbno=CHI07LA063&akey=1

Reinhart, R. O. (2008). Basic Flight Physiology. (3rd ed.). New York: McGraw-Hill Professional.

Personal Fitness

Introduction

Personal physical fitness is a hot topic among pop culture but for many reasons other than the health benefits it yields. Even so, it could be said that next to sex, physical fitness sells second best. It’s the results that everyone likes and desires, never the work that is entails or requires. Having gone through personal transformations from hard work, exercise and strict dieting, everyone always asks, “What’s your secret?” or “What diet did you use?” It’s almost saddening for the asker to hear “Hard work, diet, exercise.” You can see the wind die from their sails and their hopes shatter. As with anything in life: if it were easy, everyone would be doing it.

My personal fitness plan is unique in that my lifestyle is unique. There aren’t very many diets or exercise plans out there that fit into a suitcase. One of the hardest parts of my diet is that I’m often in places with limited choices for healthy eating. This leads to the hard choice of eating unhealthily or not eating at all, not the best options when you’re trying to stay fit. This fitness plan is designed to maintain a healthy aerobic condition and body weight. It is not designed to make anyone (most importantly: me) into an Olympic athlete.

Warm up and Cool Down

To facilitate the slow warm up and cool down of the exercise period, I stretch for 10 minutes before and after the workout. In addition to stretching I begin my run with a short period of speed walking and usually end my run with the same.

Most important to me is the post workout stretching period. Whether scientific or not, I find that a post workout stretch helps prevent injuries and decreases recovery periods in relation to muscle strain.

Type of Exercise

Because it’s universally recognized and generally available anywhere, my primary form of exercise is running (most often on a treadmill). I will run a mile at approximately a 10 minute mile pace and then cool down for one minute. My goal is to reach a target heart rate of 190 beats per minute and sustain that throughout the workout. The cool down period should not bring the heart rate below 150 beats per minute. I run approximately 3 miles which should equate to around 30 minutes on the treadmill or on the trail.

In addition to running I perform resistance training with resistance bands that fit in my suitcase. I started carrying these bands with me everywhere I go when I realized that my excuse for not working out was that no place I stayed had free weight equipment. With the bands I do a variety of resistance training workouts. Bicep curls, tricep extensions, chest press, overhead press, lateral raises, butterfly press, in addition to sit ups and push-ups. I alternate working out my chest and back with legs. Using a combination of the resistance bands and body weight, I work out using: squats, lunges, leg-ups, and calf raises.

Frequency and Time

At a minimum I work out three times per week for just over one hour. This allows ample time for my body to recover from muscle strain while at the same time promoting progression and strengthening.

I schedule 10 minutes at the beginning of the workout for stretching and warm-up. I stretch the same way every time I work out, giving no preference to whether it is chest and back or legs emphasis day. This gives me another chance to help my body recover from previous workouts instead of only focusing on the workout of that day.

My primary method of raising my heart rate to my target is running either on a trail or on the treadmill. I like to walk at a faster than normal pace for 5 minutes and then slowly increase the pace until I’m running at approximately a 10 minute-mile pace.

After running about 3 miles, I will proceed to the resistance training. I schedule 15 minutes of resistance training in order to take advantage of my increased heart rate. I focus on repetitions instead of amount of “weight” or resistance. Counting backwards from 20, I will do 20 repetitions, then the second set I will do 10 repetitions, then finally 5 repetitions. This yields a total of 35 repetitions for each resistance exercise I complete.

Diet

One of the most important aspects of a healthy lifestyle is the diet; consequently it is also one of the harder plans to maintain. There is no fad or instant success story to use, it’s just plain math. I count calories to help prevent my intake exceeding my outtake. I budget myself for 1,800 calories a day and for three days a week I burn about 500-700 of those calories with my workout. Keeping this intake and outtake in balance (or less in than out) ensures that I can keep my weight at a healthy level or even aids in weight loss when desired.

I eat a balanced diet of fats, protein, and carbohydrates. Because my lifestyle tends to dictate what food is available to me (traveling, hotel bars and airports) I try to stick to low fat, high protein, and baked or grilled foods. It’s a funny thing about traveling, you can find almost every vegetable known to man, they’re just deep fried, or coated in butter, or in a crème sauce.

I focus less on what I specifically eat and more on the caloric content overall. Micromanaging my diet to the point of specific foods creates more headache than results I’ve found.

Conclusion

This diet and exercise plan has helped me maintain a healthy weight and a healthy lifestyle. It has changed the way I choose food while eating out at restaurants and when I go shopping. The workout regime has helped me find time in my busy schedule to spend time on myself and my well-being. Overall it has improved my quality of life as well as long term health outlook.

Eyes

Anatomically considered an extension of the brain, the eye is the biological camera for the human body. Focusing light energy and translating it into electrical energy then transmitting that data to the brain via the optic nerve is the complex process known more commonly as eye-sight.

Light first passes through the outer layer of the eyeball called the cornea. Working as a “lens cap” for the eyeball it also has refractive (bending) properties that help focus the light entering the eye. Next, light rays entering the eye are refracted by the lens and focused onto the retina. The retina is comparable to the film in a camera. Focus can be changed by ciliary muscles that surround the lens and with contraction or relaxation can change the shape of the lens which affects focus.

The colored membrane that determines eye color is the iris and the center portion of the iris is the pupil. By modulating the size of the pupil the eye can control how much light is allowed to enter and changes the depth of field. Allowing more light into the eye by increasing the size of the pupil, such as in low light situations, the depth of field is decreased. This means the nearest and farthest distances within which everything is in focus is shortened. This modulation is used primarily to make vision easier in high or low light situations.

Once allowed to enter the eye and focused, the light energy meets the retina, or the “film” of the eye. The retina contains cells that translate light energy into electrical energy. Using cone cells to sense bright lights and colors and rod cells for low light or night vision, this data is transmitted to the brain via the optic nerve. Rods are used primarily as peripheral vision and are considered to be 10,000 times more sensitive to light than the cones (which is the reason peripheral vision, or off-set viewing, is recommended for night time traffic scanning). Where the optic nerve is formed on the retina is devoid of cones and rods which results in a blind spot. Each eye compensates for the other eye’s blind spot.

There are several visual impairments that can either develop or be genetically inherited. Most common is myopia, or nearsightedness. This is caused by the focal point of the lens being in front of the retina. Conversely, presbyopia is a form of farsightedness caused by the stiffening of the lens makes accommodation (the ability to change the focal point) difficult.  This requires objects to be farther away from the eye to be in focus which could result in poor vision up close yet still yield good distant vision. Correction for this involves reading glasses which magnifies close up objects allowing them to be focused.

An additional impairment to visual acuity is called astigmatism, or an unequal and variable curvature of the lens and the cornea that prevents an equal focus at varying distances. This causes light rays to refract unequally through the lens creating varying levels of focus in either eye. This condition can be mediated or completely rectified with glasses, contact lenses or a vision correction procedure.

Along with refraction and focus irregularities, vision can be impaired by lens clarity. In some cases the lens can become opaque resulting in a cataract. Cataracts may form by age alone or extended exposure to ultra-violet rays absorbed while flying at high altitudes for long periods of time. In addition, less light may pass through the lens due to age-related yellowing which interferes with the depth of field. Prevention is the best method by wearing UV filtering sunglasses that block the full ultraviolet range. Once developed cataracts can be removed and replaced with surgery.

Finally, the eye can even be affected temporarily by factors that are known to affect the brain in the same manner. Hypoxia, fatigue, carbon monoxide, or toxins can severely impair the eyes ability to see clearly. Hypoxia increases time required for adaptation to night vision and can occur at altitudes as low as 5,000 feet. Carbon monoxide build up in the blood affects the pilot in the same way hypoxia does because carbon monoxide blocks the hemoglobin’s ability to attach to and carry oxygen. Fatigue can affect mental alertness and visual recognition, which could impair a pilot’s ability to scan, maintain focus, or even impair judgment. Toxins, such as alcohol, create a state of histotoxic hypoxia that can reduce visual acuity well past when the last drink was consumed.

These various components that work together, mostly unconsciously, allow us to operate visually in the cockpit and on a day-to-day basis. Maintaining a healthy lifestyle as well as taking precautions to protect and preserve your vision will ensure a long and safe career in the cockpit.

RVSM

REDUCED VERTICAL SEPARATION MINIMUMS (RVSM)

HOW TO HANG OUT IN CROWDED AIRSPACE

The flight levels these days are awfully crowded with corporate jet traffic, airliners, and now even the new very light jets. All this traffic means congestion and delays are more commonplace than anyone wishes them to be. One method used to reduce this congestion and inherent delays in some flight levels is called RVSM, or reduced vertical separation minimums.

Previously the flight levels had a vertical separation of 2,000 feet between aircraft. By reducing this to 1,000 feet, the capacity of the domestic airspace in North America essentially doubled. Beginning at FL290 and extending up
to and including FL410, RVSM covers the entire domestic United States, Canada, Mexico, and throughout Europe and Asia.

By reducing the vertical separation to 1,000 feet, the capacity of the domestic airspace in North America is essentially doubled.

A few of the proponents that allowed RVSM to become a reality were technological advancements in traffic avoidance and barometric altimeters. Higher sensitivity in altimeters proved to be reliable enough to reduce the separation safely to 1,000 feet.

Specifically the advent and broad use of air data computers—the proliferation of traffic collision avoidance equipment and vast improvements in the technology that drives it. In the near future, advances in air traffic control will provide even better display and control of aircraft that will likely further reduce separation minimums, such as next generation technology ADS-B (automatic dependent surveillance broadcast).

Certain equipment requirements must be met prior to entering RVSM airspace, unless a specific waiver is granted. Two primary altimeters, autopilot, altitude hold with an altitude alerter, and a mode C transponder are required. Depending on aircraft certification, an installed and operable TCAS system may also be required. Along with aircraft equipment, aircrew training is also required for operation inside of RVSM airspace.

While operating in RVSM airspace, it is important to not overshoot assigned altitudes because of the reduced separation. When approaching a cleared flight level, vertical speed should be restrained between 500 to 1,000 feet per minute and not exceed 1,500 feet per minute.

At no time should the aircraft be allowed to deviate more than 150 feet from an assigned flight level without manual intervention. These feet per minute (fpm) tolerances and strict level-off requirements stem not only from separation standards but also help prevent undesired TCAS resolution advisories.

For the purpose of precision altitude keeping, the autopilot should be used to capture assigned altitudes and during level flight unless turbulence or aircraft re-trimming require otherwise.

Coupled with the technology and the training required, RVSM is a safe and effective way to reduce congestion and increase capacity for the valuable airways of the sky.

Holding

TEARDROP, PARALLEL, OR DIRECT

HOLDING-PATTERN ENTRIES USING AN FMS

of your primary instrument training is learning the ins and outs of holding. Often it can be a confusing and difficult skill to attain; however, once you get it, holding becomes second nature. One of the luxuries of advanced aircraft systems is the ability to program and define holding points pretty much anywhere you want, allowing the aircraft to navigate on autopilot to the hold, and then enter the hold.

Rarely are you given a holding instruction that doesn’t coincide with a depicted hold on either an arrival procedure or instrument approach. However, when you are given a non-standard or non-defined holding instruction, it’s just as easy to program and execute.

Modern airliners and even general aviation aircraft are commonly equipped with a flight management system or a global positioning system to simplify most navigation tasks. These systems incorporate a working database of instrument approaches, arrival and departure procedures, and en route navigation aids.

Included in this database are the published holds scattered throughout the national airspace system. This makes entering a published hold as easy as a few button presses. Defining a non-published hold is just as easy as entering the key components of the hold: inbound leg, defining fix, and leg distance or time. Once built, you can arm a hold hundreds of miles down your flight plan or you can fly direct to the fix and enter the hold immediately.

The most common use of this tool is during instrument approaches, or more correctly during the missed from an instrument approach. The database contains the entire procedure from initial to missed approach fix and, when coupled to an appropriate autopilot, the aircraft can be directed to complete the entire approach, missed approach, and enter the hold. It is, in fact, so smart that it will even tell you what type of entry to make when arriving at the holding fix.

It’s easy to let yourself become complacent with holding when you have such advanced avionics handling all the hard work. Be sure to practice holding the old-fashioned way for that rainy day when the ILS glideslope is unusable and you have to hold without the autopilot or a fancy FMS/GPS.