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Slow-Onset Hypoxia: An insidious killer<\/b><\/p>\n Captain Shem Malmquist and Dr. Paul Buza, Florida Institute of Technology<\/b><\/p>\n Have you ever ran a checklist, and then looked up a few minutes later and realized that something that you thought you had done was not <\/i>done?\u00a0 Is there a professional pilot with any experience who has not <\/i>had this experience?\u00a0 The point is that as pilots we often read reports and dismiss them with the thought that \u201cit won\u2019t happen to me\u201d\u00a0 If you could honestly say that you have never missed a checklist item, then you can skip this article as you are likely not a pilot!<\/p>\n Hypoxia.\u00a0 That\u2019s one of those things that you have to worry about flying little airplanes.\u00a0 In our modern transport aircraft, that\u2019s not a real risk, is it?<\/p>\n We all learned about hypoxia early in our flying careers.\u00a0 Part of that discussion likely involved a chart that showed the time of useful consciousness at various altitudes in the event you lost pressurization.\u00a0 Like most pilots, I ran through the scenarios in my head.\u00a0 I had climbed mountains over 14,000 feet, so it did not seem that bad, so how bad could it be if we lost the cabin?<\/p>\n Time of Useful Consciousness Chart from Pilot\u2019s Handbook of Aeronautical Knowledge<\/strong><\/p>\n The truth is that a sudden, rapid loss of pressurization is not likely to lead to a problem of hypoxia.\u00a0 The reason being that the crew is going to have no question as to what is happening and the need to don oxygen masks.\u00a0 While this is the type of event that seems like the \u201cworst case\u201d scenario, like many risks, the one with the most salience is not the one that is the most likely to kill you.<\/p>\n On August 14, 2005, the crew of a Helios airlines B-737 were preparing for departure[i]<\/a>.\u00a0 Things moved quickly during the preflight and departure stage, as we have all experienced.\u00a0 Flows were ran, checklists completed, and the flight departed.<\/p>\n A few minutes after takeoff, the Captain called back to their dispatcher to discuss a problem that appeared to be the air-ground shift.\u00a0 The crew had an intermittent warning horn, a master caution and an equipment cooling alert.\u00a0 The Captain inquired as to where the circuit breakers were for the equipment cooling, not surprising as the aircraft maintenance history showed that it had a history of equipment cooling problems.<\/p>\n \u00a0 Helios B-737<\/p>\n Expectation bias is the tendency to believe or see results that agree with those details that agree with our expectation, and to disbelieve, discard or downgrade<\/i> the cor\u00adre\u00adspond\u00ading weight\u00adings for the data that appear in con\u00adflict with those expec\u00adta\u00adtions<\/em>[ii]<\/b><\/i><\/a>.<\/p>\n The crew continued to trouble-shoot the issue while they climbed.\u00a0 13 minutes after takeoff there were no further communications from the flight crew.\u00a0\u00a0 The actual problem turned out to be that the pressurization was not properly set;\u00a0 it was part of the checklist, but it was not accomplished.\u00a0 The Captain was known for being meticulous on procedures and quiet.\u00a0 According to the report, the first officer had a history of rushing through checklists.\u00a0 This is not uncommon, although what is even more common (and likely something every professional pilot has done at one time or another), is \u201clooking\u201d at an item without really \u201cseeing\u201d it.<\/p>\n Unlike a rapid decompression, the cues for \u201cslow onset\u201d hypoxia are not as clear.\u00a0 Think of it as the \u201cboiled frog\u201d effect.\u00a0 While the cabin slowly ascends there are the usual cues.\u00a0 Ears need to be cleared, but for anyone that flies often, that is pretty much an unconscious act.\u00a0 Other cues are more subtle, such as gradual decrease in the ability to discern color and a slow degradation in the ability to think clearly.\u00a0 The crew of the Helios B-737 never sorted out what was happening.\u00a0 The aircraft continued to fly on autopilot, finally running out of fuel and crashing about three hours later, killing all 121 aboard.\u00a0 The flight crew never regained consciousness.\u00a0 It appears that a flight attendant either maintained or regained consciousness and attempted to regain control of the aircraft just prior to the aircraft running out of fuel.<\/p>\n All humans are subject to cognitive biases of various sorts.\u00a0 These biases are heuristic \u201cshort cuts\u201d that we use to make sense of the world around us.\u00a0 They are the basis of \u201cintuition[iii]<\/a>\u201d, which allow us to make more rapid decisions in familiar situations.\u00a0 The more careful thinking required to logically analyze a situation or concept requires considerably more energy.\u00a0 Humans do not like to do this under the best of circumstances (preferring the much faster and easier intuitive method).\u00a0 If our mental ability is compromised in any way, such as being fatigued, ill, impaired by medicines or alcohol, or even with low blood sugar, we will be even more inclined to think the \u201ceasy\u201d way.\u00a0 Hypoxia will have similar effects.<\/p>\n The effects of hypoxia are subtle, while the effects of biases are very strong.\u00a0 Pilots have often continued flying without seeing or hearing warnings;\u00a0 accident, ASRS and ASAP databases are full of these types of events.\u00a0 Given a combination of circumstances, it is not hard to see how an event such as this happened to the Helios crew could happen to any of us.\u00a0\u00a0 Missing a checklist item, a warning or alert is common.\u00a0 This has been a factor in so many different types of accidents, that it cannot be overemphasized. Regarding this issue, the Helios accident report, in discussing missed procedures or checklist items, states:<\/p>\n The Board was also sensitive to the fact that automatic execution of actions was very much affected by assumptions \u2013 in the case of performing a large number of verification steps, the assumption that all switches and indications were in the usual, normal for this phase of flight position. A superfluous green indication on the pressurization panel could be easily (inadvertently) overlooked when perception was biased by the expectation that it should not be present.<\/p>\n Exacerbating this tendency (expectation bias) is the rarity with which switches (especially, and directly relevant to this case, the pressurization mode selector) are in other-than-their-normal position. A pilot automatically performing lengthy verification steps, such as those during preflight, is vulnerable to inadvertently falsely verifying the position of a switch to its expected, usual position (i.e. the pressurization mode selector to the expected AUTO position) \u2013 especially when the mode selector is rarely positioned to settings other than AUTO\u2026<\/p>\n \u2026because they are performed repeatedly on the line, they are also performed by memory, typically in time-pressured circumstances (i.e. indirect pressure to maintain on-time\u00a0 departures). For these two reasons, checklists are often performed in a hurried, automatic fashion. From a human factors standpoint, rushing is known to lead to the inadequate allocation of attention to the task at hand \u2013 and thus to errors. Furthermore, like procedures, checklists are also vulnerable to \u201clooking without seeing\u201d because they are biased by the assumption that since each item verified an action performed only moments ago, then it must be already in the desired position\/set\u00a0 (P.118-120)<\/p>\n These lessons alone can save your life.\u00a0 The only way to break this tendency is to make a conscious, direct effort to focus on each item and ensure it is accomplished.\u00a0 The failure to pressurize while on ascent, while the crew is attending to other issues, represents one of the most dangerous hypoxia situations.<\/p>\n Having now established how easily a crew might find themselves in a scenario where the aircraft is not properly pressurized, let us turn our attention back to the effects of hypoxia.\u00a0 Unlike the rapid decompression situation, the \u201cslow boil\u201d hypoxia is not obvious, and there is nothing that \u201cjumps out at you;\u201d this is the situation that killed Paine Stewart.\u00a0 Any sort of depressurization is rare, and one that is very subtle is even more dangerous.\u00a0 The following chart outlines the effects:<\/p>\n Sequence of Events \u2013 Slow Onset Hypoxia<\/p>\n Notice how subtle the effects are.\u00a0 The items up through 18,000 feet could be easily ignored, and the loss of cognitive functions from 18,000 through 24,000 feet are also things that we might not notice or might attribute to other things.\u00a0 Tunnel vision, like loss of color vision, is much less obvious while a person is experiencing it, particularly when focused on trouble-shooting a task.\u00a0 Once past that phase, things happen rapidly.\u00a0 Unfortunately, the symptoms are impairing our ability to see the symptoms and recognize them!<\/p>\n Hypoxia can be a factor in other scenarios.\u00a0 Cargo aircraft that have been depressurized as part of a smoke procedure could put crews at risk if they elect to go back and investigate without an activated walk-around oxygen mask.\u00a0 The canular-type supplemental oxygen used in general aviation can put a person at risk at higher altitudes as well due to the insidiousness of the slow degradation.\u00a0 A similar effect could occur due to a leaking mask or a diluted oxygen flow.<\/p>\n Hypoxia is one of those threats which most professional pilots seldom consider.\u00a0 With modern aircraft, the chances of a rapid decompression are very low.\u00a0 Many pilots do not worry about it and approach the entire issue casually.\u00a0\u00a0 Slow-onset hypoxia has received little attention over the years.\u00a0 The thought of the pressurization not working on climb out is not one that strikes fear in our hearts or grabs our imagination like the thought of a door blowing off an aircraft leading to explosive decompression.\u00a0 Despite that, it is the slow one which is more likely to kill you.<\/p>\n The degradation of our cognitive functioning which occurs with hypoxia can lead to a fixation that is so strong that the last thing you will ever know is \u201cwhat a great pilot you are\u201d.<\/p>\n Mitigations<\/b><\/p>\n Awareness is the key.\u00a0 Each persons\u2019 hypoxia experience will be unique to them. Crew communication (i.e., good CRM) becomes a vital component in identifying the problem prior to a loss of situational awareness and subsequent incapacitation.\u00a0 Furthermore, flight attendants should be trained to have a fundamental understanding of the sequence of events once the \u201crubber jungle\u201d drops and should proactively participate in the CRM with the flight crew.<\/p>\n![\"cabin](\"https:\/\/airlinesafety.blog\/wp-content\/uploads\/2013\/05\/cabin-pressure.jpg\")
<\/a>\u00a0(Image from\u00a0http:\/\/www.wisegeek.com\/what-is-cabin-pressurization.htm)<\/p>\n<\/a><\/p>\n<\/a><\/p>\n<\/a><\/p>\n
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