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Narcosis and Hypoxia Kill

by Jeremy Cox 1. February 2007 00:00
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Recently many of us were surprised to follow the story of the two crewmembers in a King Air B200 bound for Virginia from Arkansas who experienced what appears to have been a case of Hypoxia and possibly even a mild case of Narcosis as a result of some kind of cabin depressurization, probably due to the failure of the outer pane of the King Air's Windscreen. From what I have learned about this eventful flight, the crew were approaching the Cape Girardeau area of South-eastern Missouri at FL290 when it appears (on radar and flight-tracker) that the aircraft performed a 360 degree turn while varying its speed and altitude, and then it plunged down to somewhere around 7,000 feet where the crew appeared to have recovered the aircraft having lost pitch control of the aircraft (it was later discovered that both elevators had fluttered right off the horizontal stabilizer while both the entire empennage and both wings had deflected so severely that they were creased and bent!) Fortunately with the prodigious use of airspeed and engine power, the crew were able to make a fast, but safe landing into Cape Girardeau. These chaps were lucky, but there have been countless instances where Hypoxia as a direct result of a cabin depressurization event has caused the loss of aircraft and their occupants. A recent and most horrific example is Helios Airways flight 522, a Boeing 737 that eerily cruised over the Mediterranean skies with no response from either the cockpit or any of the almost 120 passengers in the main cabin. The world's television networks watched in horror as the aircraft eventually crashed into a Greek mountainside. The Payne Stewart, Learjet 35A crash is also a carbon-copy of this accident. So how can these types of 'near-accidents' and tragedies happen?, what is Hypoxia? and how can it be prevented?

According to a medical report authored by various aero-medical physicians of the United States Air Force (USAF), people flying in un-pressurised aircraft at high altitude after a rapid decompression can experience Altitude Decompression Sickness (ADCS; a.k.a Nitrogen Narcosis as it is known outside of the USAF) which is the same as the 'Bends' that deep-sea divers suffer from when they ascend to the surface too quickly. ADCS is an phenomenon governed by Henry's Gas Law, whereby when the pressure of a gas over a liquid is decreased, the amount of gas dissolved in that liquid will also decrease; in simple terms open up a can or bottle of carbonated beverage and the hiss and subsequent bubbling is a direct result of Henry's Gas Law. This phenomena applies to we humans, especially to pilots and divers, because we all store nitrogen, an inert gas, all-through-out our bodies as a result of respiration. When we experience a rapid reduction in pressure, either water or air, the nitrogen stored in our tissues starts to bubble and fizz just like a beverage being uncorked and when we have gas bubbling within our bodies we will experience an 'Air Embolisim' which results in paralysis and subsequent death unless it is quickly treated by re-compression. Unfortunately the 'Bends' (pain may be reduced by bending the affected joint like an elbow, shoulder, hip, wrist, knee or ankle to a more comfortable position, therefore the term 'Bends' was applied) can creep up on you and therefore you should always keep an eye on your Cabin Differential Gauge and the Cabin Vertical Speed Indicator, while looking for the tell-tale medical signs that you are: Localized deep pain from mild to excruciating in any of your joints; confusion, memory loss, headache, tunnel vision, double vision, blurry vision, unexplained fatigue, vertigo, nausea; tingling or burning sensations around the lower chest or back, abdominal or chest pain; incontinence, muscle weakness or twitching; shortness of breath, dry constant cough; itchy ears, face, neck, arms and upper torso, mottled or marbled skin around the upper body, and or welling of the skin accompanied by tiny scar-like skin depressions. According to the USAF, 'recompression' is the only effective treatment for severe ADCS, although rest and oxygen (delivered from a tight fitting mask) when applied to milder cases may bring about recovery without damage. For specialist aviators like astronauts, record breaking balloonists and/or parachutists, etc. or high altitude reconnaissance military pilots, the pre-breathing of 100% pure oxygen for 30 minutes before the commencement of their flight, and then the continuation of the breathing of 100% pure oxygen will 'drive-out' the nitrogen that has already formed in their tissues and then it will continue to prevent any new nitrogen entering the body. Having oxygen in your tissue instead of nitrogen does not eliminate ADCS, but it does make the recipient less susceptible to an early onset.

In addition to the massively debilitating effects of ACDS as a result of a rapid decompression, flight at high altitude in a reduced air pressure environment without the availability and use of supplemental breathing oxygen will result in Hypoxia (a.k.a Altitude Sickness), a medical state that can quickly lead to incapacitation and/or death. Hypoxia is basically a shortage of oxygen in the blood. As soon as we start to suffer from a shortage of oxygen at altitude, we will start a very dangerous 'clock' ticking which will determine how long you will remain conscious. This 'clock' can be renamed the Time of Useful Consciousness (TUC.) According to Doctor Paul W. Fisher, Ph.D., '....TUC is the period of time from the interruption of the oxygen supply or exposure to an oxygen poor environment, to the time when useful function is lost. The individual is no longer capable of corrective and protective action. It is not the time to total unconsciousness.'

For an individual of average health sitting at rest in a cockpit or cabin may see their TUC to correspond to the following table:



FL500 and above

<9 seconds


9 to 12 seconds


15 to 20 seconds


0.5 to 1 minute


1 to 2 minutes


2.5 to 3 minutes


3 to 5 minutes


10 minutes


20 to 30 minutes

A rapid decompression can reduce the TUC by up to 50% caused by the forced exhalation of the lungs during decompression and the rapid rate of ascent in cabin altitude. The symptoms of Hypoxia include: a feeling of apprehension, headache, dizziness, fatigue, nausea, hot/cold flashes, blurred vision, tunnel vision, tingling, numbness, an increase in both the depth and rate of your breathing, mental confusion, poor judgement, loss of muscle coordination and unconsciousness. People who are out of condition and people who smoke will be affected by Hypoxia much faster than their healthy, non-smoking colleagues.

So how can you prevent you and your fellow crew members and passenger's injury or demise at the hands of Hypoxia or Narcosis? Well first it is extremely important to make certain that your pressurization system is functioning properly. Follow all of the operating and maintenance procedures specified for your system and in addition to keeping a parameters 'trend' log for the cabin differential, like you do for your engine parameters, which you provide to your maintenance personnel, make sure that your aircraft is periodically 'blown-up' (ground pressurized by a 'Huff' Cart) and physically inspected for leaks using either soap and water, 'owls fluid' or other means. Don't just rely on a ground check using engine bleed air or the engine gearbox driven compressors to verify the integrity of your aircraft's cabin pressure vessel. Additionally have your technical people keep a close eye on the condition of your windows to make sure that they catch a potentially catastrophic delamination or failure before it occurs. Obviously it is also vital that you regularly check your oxygen delivery system as well. Fortunately this is often incorporated into your aircraft's maintenance schedule; however it is believed that a 'turned-off' oxygen bottle main valve was a contributing factor in the Payne Stuart accident. If you can't don (put on) your oxygen mask and start sucking a lungful of O2 in less than 5 seconds, you had either better practice this procedure with a friend who has a stopwatch, or it is time to buy new masks and hoses that allow 'quick donning.' Now that you have done everything in your power to ensure that your aircraft and its equipment are up to the rigours of high altitude flight, you will serve yourself well to arrange a ride in a Hypobaric Chamber (Altitude Chamber) so you can experience first hand, under close professional supervision, the effects of Hypoxia on your own body so that you are better equipped to recognize your own symptoms thus allowing you to possibly react quicker to making the necessary environmental changes necessary to save your life. Your local FSDO or AME can recommend a location where you can experience this.

So keep your cabin altitude below 10,000 feet, keep your wits about you, and most importantly keep the shiny side up!

As this is a very important issue with any stage of piloting skills please share any experiences you may have had with either Narcosis or Hypoxia.  Your learning curve with experience will help all others without.  Please click on the link below which states "Reply to this Article", your thoughts and comments would be very much appreciated.  Until next month dear readers.


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