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: P_T = P₁ + P₂ + P_s + P_n; P_T 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 (P_O2 = barometric pressure X 0.21) falls substantially with lower barometric pressure at higher altitude. P_O2 at sea level is 159 and decreases by 50% at 5496 m. For each additional 300 m, P_O2 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.