Time Of Useful Consciousness Table

Cabin Decompression and Hypoxia

by Mark Wolff -- Source: PIA Air Safety Publication

At the hypobaric sleeping room at the RAAF base in Edinburgh several hundred air force pilots each year get to check out their reactions to depressurization and the effects of hypoxia.

The sleeping accommodation is fix to an altitude of 25,000 feet, which gives a time of useful consciousness of around three to five minutes.

Up to x pilots at a fourth dimension sit in the chamber tensely-waiting for the depressurization, which starts at eight,000 feet and moves to 25,000 feet in simply x seconds. Each clutches a checklist of tasks they are to perform. Each is determined to remain conscious and capable for equally long as possible.

After most two minutes one of the subjects is asked to echo dorsum a number. Inevitably the subject field is unable to do so. In fact, well-nigh don't recall beingness asked.

Trying to go through the checklist the pilots tend to exhibit one of two kinds of behaviour; they are either "folio flickers" or "fixators". The page flickers will simply sit there mindlessly flipping through the checklists while the fixators will just stare at one page. They are "passengers in their own bodies". These ii quite different behavioural responses to rapid depressurization hint at the variation in individual responses to lack of oxygen, or hypoxia.

Hypoxia is a threat to safety for all pilots operating pressurised shipping and for unpressurised aircraft that fly at an altitude of ten,000 anxiety or above --- the legal ceiling above which oxygen must be used by flying crew members in unpressurised aircraft.

Some individuals with reduced lung function will become hypoxic well below this level. This includes people with emphysema, industrial lung disease, sure forms of anaemia, ischaemic heart affliction and even mild degrees of center failure.

If you fume, you may have already reduced your oxygen intake past a significant factor. Avoid smoking before and during flight.

The Nature of Hypoxia:

The term hypoxia translates from the Latin to mean below normal (hypo) oxygen (oxia). It is a physiological state in which tissues are deprived of adequate oxygen, and organs such as the brain, eyes, ears, lungs and heart are adversely afflicted.

When an shipping undergoes rapid decompression higher up effectually 35,000 feet, the fourth dimension of useful consciousness for coiffure may be 30 seconds or less, depending on the distance (encounter table).

Time of Useful Consciousness
Distance (feet)		Consciousness
15,000 eighteen,000 22,000 25,000 28,000 30,000 35,000 xl,000 45,000 50,000		30 minutes or more twenty-30 minutes 5-x minutes 3-5 minutes 2.5-3 minutes i-iii minutes thirty-lx seconds 15-xx seconds 9-15 seconds 6-9 seconds
Pressure and Altitude
Altitude (feet)	Pressure hpa          lb/in2		Temperature oC
0 5,000 10,000 fifteen,000 twenty,000 25,000 xxx,000 35,000 40,000	1013.25 843.1 696.8 571.8 465.6 376.0 300.9 238.4 147.5	14.70 12.23 10.11 8.29 6.75 5.45 4.36 iii.46 2.72	+xv.0 +5.1 -iv.8 -14.seven -24.6 -34.5 -44.4 -54.ii -56.v

At lower altitudes the fourth dimension of useful consciousness maybe longer, but subtle effects may still impair your functioning. The more than rapid the decompression, the faster the symptoms of hypoxia will appear.

Coiffure surprise and perhaps lack of familiarity with decompression can contribute to dangerous delays in appropriate response. Enquiry by the US Air Strength shows lxxx per cent of pilots with no feel of decompression wait equally long as xv seconds to respond correctly to a loss of cabin pressure.

Because of the insidious effects of hypoxia on judgement and reasoning, the right response to loss of motel pressure is always to don the oxygen mask - immediately. That's the only way you can exist sure that you lot will make the right choices.

The decease of US golfer Payne Stewart in October 1999 and two recent Australian incidents - one in a RAAF King Air and the other in a civilian Male monarch Air - have put the spotlight on the issue of hypoxia. While the RAAF incident is known to have involved hypoxia, we may never know the contributing factors to the civilian accidents.

In the RAAF incident, shortly after the shipping was levelled at the planned cruising level of FL250, the right-paw seat passenger noticed that the pilot was acting erratically while manipulating the Global Positioning Arrangement (GPS). Before long after, the airplane pilot slumped over the controls and turned a curious shade of blue.

Fortunately the passenger (who was a pilot but was not endorsed on type) was able to have command of the aircraft. He descended and, after having some trouble locating the communications console, alleged an emergency. He was extremely lucky - he should first accept donned his own oxygen mask to ensure he was able to office correctly.

Preliminary reports from US investigators reveal the cockpit vocalisation recorder on Payne Stewards aircraft contained no voices, but that there were sounds consistent with various alarms (cabin altitude / low pressure level, stall warnings). Speculation is that the blow may have been related to decompression early on in the flying and that the pilots and passengers may have been incapacitated by the low level of oxygen. Pilots of military shipping assigned to follow the Learjet after it failed to respond to ATC transmissions and climbed above its assigned altitude said the windows were covered with ice and that there was no sign of flying control movement.

The flight concluded when the shipping dived into the ground at the fourth dimension that the Learjet'south fuel supply would have been exhausted.

Symptoms and Signs:

The symptoms of hypoxia are similar to alcohol. Similar alcohol, at that place can be a personality change.

The offset signs include both mental and physical furnishings. Mentally there can be a loss of judgement, self-criticism and brusque-term retentivity. This can be accompanied by an increase in reaction time and a kind of mental "tunnel vision" similar to the fixation on the GPS unit experienced by the RAAF King Air pilot. You may even get euphoric.

The physical furnishings include muscular incoordination and an impairment of color, night and peripheral vision. Hearing as well deteriorates. You may experience hot flushes and turn blueish at the extremities (cyanosis). Rapid breathing or hyperventilation is one of the early physical signs. Simply because hypoxia impairs judgement, you may non notice loss of vision and hearing or other concrete or mental signs. Information technology's the contrary of "You don't know what you've got until you lot lose it". More like, "You lot don't know what you've lost until you get it back".

Uncomplicated tasks become extraordinarily difficult and functioning fails. As hypoxia continues, you become semiconscious. Subsequently you lot lose consciousness entirely, y'all have only minutes to live, depending on the altitude. At that place are many factors which affect the tolerance to hypoxia. The faster the rate of rising, the quicker the onset.

Apart from smoking and lung disease, you lot maybe more susceptible if you are sick, stressed, unfit, drawn, under the influence of drugs or are suffering from a hangover. There is a high degree of individual variation in the response to hypoxia. Some people may hyperventilate and plough blueish immediately; others may not. In a small number of individuals, unconsciousness may occur before whatsoever other symptom. (See footnotes for variability of symptoms)

Recovery Procedures:

Don your oxygen mask immediately, select 100 percent oxygen if y'all accept differential settings, then descend to x,000 feet or beneath, terrain permitting. Yous may find that you experience worse immediately after putting your oxygen mask on. Practice non accept it off. This is called the oxygen paradox and you will feel better after about 1 minute.

Breathe at a normal charge per unit and depth. Declare an emergency, and land as before long as possible. After recovery from an episode of hypoxia, some symptoms may persist. These include headache, fatigue and lethargy.

Note that if you accept been in an aircraft which has been decompressed, you should not wing once more the same day because you will increase your chance of decompression affliction. Decompression affliction (or the bends) can be incapacitating, particularly if nitrogen bubbles enter the brain.

You should also utilise oxygen if you detect fumes or smoke. Once more, set the oxygen at 100 per cent in order to prevent any toxins from the fumes or fume inbound the system.

Prevention:

The key to prevention is twofold. First, y'all demand to follow your flight manual prompts to accurately set and monitor motel pressure. As soon as the cabin pressure drops beneath recommended levels y'all should take preventative activeness.

If warning systems signal problems with cabin pressure, you must immediately don your oxygen mask and descend if terrain permits. It pays to know your equipment, because information technology can take some time to put the oxygen mask on. Brand sure you know how to use the masks, exercise using them, and fourth dimension yourself in putting them on.

In that location are traps with checklists which you must guard against. You should understand your pressurization and oxygen systems, and fill in the gaps in your checklists and so that you are sure of what to do in an emergency. For case, if you are over sea and have an uncontained engine failure which leads to depressurization, y'all will want twenty,000 feet or and then to retain range. But do you have enough oxygen? Do you know how to calculate that?

Sixty per cent of corporate jet depressurizations are acquired by uncontained engine failures. Well-nigh others are caused past doors or windows parting the aeroplane. Fifty-fifty if you lot prepare and utilise checklists properly, things can notwithstanding go wrong.

A checklist is a skill based action which means it is a stored design of preprogrammed action. The greater the skill level, the greater the gamble of "strong, just incorrect" error. All it takes is a change to a well practised routine and a missed attentional check. The intention may exist correct, simply the action may be wrong.

You could get it wrong through inattention, jisti action or preoccupation. That'southward why checklists need to be monitored advisedly as they are the final line of defence.

If you are halfway through a checklist, and are interrupted, go back to the beginning and start again. Be sure, however, not to introduce new errors by operating things like switches which have already been activated correctly.

The one thing that you should not exercise get-go is to start working out what'southward gone wrong. That is, you should not exist problem solving and planning on line. This is difficult to do. If you know the aircraft well and you have a pressurization warning going off, but the cabin pressure level indicator seems OK, you should not be trying to work out which is correct. You must immediately put on the oxygen mask, descend and then expect to problem solving.

The reason yous should not work the problem early is that hypoxia interferes with your ability to solve problems and limits your fourth dimension of useful consciousness. Become the oxygen right first, then ponder your situation. If you go into problem solving manner y'all volition lose valuable time. Yous should take the course of least regret.

From a human factors signal of view, in one case yous find a pressurization warning, you need to quickly don your oxygen mask. And so your well-practised rule should e'er be:

• Pressurization warning.
• Don mask.
• Descend (terrain permitting).
• Solve the problem.

---

Footnotes:

Variability of Symptoms: Larger aircraft are not immune. Cabin alerts are fallible and hypoxia symptoms are insidious and variable.

Even if cabin altitude alerts function correctly, time of useful consciousness may be less than expected for a given altitude. In 1995, a US Navy P3C departing Japan suffered a rapid decompression to motel distance of 24,000 anxiety in 10 seconds.

Despite the captain's immediate directions, it took the flying pilot some time to don oxygen equipment and initiate descent. He then had difficulty remembering the emergency descent procedure. The non-flying pilot fabricated several radio calls without response, before others realised she had non replaced her headset after donning her fume mask.

The flight engineer became fixated with the uncorrectable pressurization problem and the helm placed his mask on him. The helm, sitting behind the flight pilot, noticed that his fingernails were bluish and decided to check the coiffure aft. They had differing symptoms:

• One member felt giddy, experienced tingling, started to walk aft but had to sit down.

• A spare pilot noticed the pressure change, discussed rapid decompression and hypoxia with some other member, felt light headed and lost colour vision. He felt nervous as he had no oxygen mask.

• The navigator felt tingling and was disoriented. He noticed misting in the cabin, became claustrophobic, hyperventilated and fixated on his station.

• Some other sailor felt his ears pop, felt cold, dizzy, confused, disoriented and sabbatum down until another member administered oxygen.

• Another felt cold, short of jiff and suffered an upset stomach.

• Another attempted to catch a walkaround oxygen canteen but became confused and remembers searching for a toolbox primal for a ratchet to unfasten the wall bracket.

• 1 rider became nervous, laid down, began shaking and had blurred vision.

• Another rider saw the misting, idea there was a fire, turned bluish, had ear pain and noticed everything in irksome move.

• The spare flight engineer was fascinated when the java pot hat exploded and java sprayed everywhere. He walked to the flight station, became exhausted, disoriented, saw vivid flashing lights and had to be administered oxygen.

• Another rider started cleaning the spilled coffee, felt ill, couldn't effigy out how to open the toilet door and vomited on himself.

Following this incident, the United states of america Navy recommended "Don Oxygen Mask" as the first item in the Rapid Decompression and Cabin Pressure Low-cal On emergency checklists.

Time Of Useful Consciousness Table,

Source: https://www.theairlinepilots.com/forumarchive/aeromedical/decompressionandhypoxia.php

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