HYPOXIA – THE REASON WE HAVE EMERGENCY OXYGEN SYSTEM ON BOARD AIRLINERS?

 

“Pull the mask and breathe your oxygen before you help others – sounds rude?”

  • If the pressurisation on board an airliner fails at 41000 feet, aircrew and passengers have just 15 seconds before they receive Supplement Oxygen ……. to be able to function usefully and be capable of taking normal corrective or protective action”

When we breath air, we breath in oxygen. The lungs extract the  oxygen from the inhaled air and supply it to the brain and tissues. The breathing cycle keeps us going and we continue to work seamlessly.

Are things different when we are on board aeroplane at 41000 feet?. Why do Flight Attendants brief us about oxygen supplement each time we are on the plane? Let us review the topic from the ground up!

Atmosphere contains approx 21 % Oxygen at all heights – right from the ground to tens of thousands of feet above. If that be the case, why does a mountaineer atop Mount Everest (approx 29000 feet) require supplement oxygen? In fact, it is the pressure of air (the weight of the column of air above us) which plays the vital role in regulating the amount of oxygen reaching our body parts. On ground, the partial pressure or in simple words, the atmospheric pressure is high enough to force the required quantity of air into our lungs as we breath. As the partial pressure drops when we climb higher, we breath harder and deeper to get the same quantity of air. However, beyond a certain point it reduces to a level when breathing more air becomes difficult causing lack of required oxygen in the inhaled air.

From aviation perspective, human beings can breath normally till 10000 feet….. even though the symptoms of High Altitude discomfort can start around 9000 ft. At higher altitudes, when we breath in lesser air, we are breathing in lesser Oxygen – The lack of Oxygen in the body parts is an aero-medical concern called – Hypoxia. The key symptoms of Hypoxia affecting an  aviator include –

  • Euphoria or sensation of dissociation from self
  • Confusion, memory loss and cognitive problems
  • Disorientation and uncoordinated movement
  • Severe hypoxia can lead to loss of consciousness, seizures or convulsions, coma and even death.

The time available between onset of Hypoxia and loss of self control is called “Time of Useful Consciousness” (TUC). It decreases with increase in the altitude ….. at 40000 ft, an aircrew or passengers have just 15 seconds of TUC before they becomes incapacitated. A ready reckoner TUC chart is as follows –

 

Commercial Airliner typically cruise upto 41000 ft. So, how can we built a reasonable pressure of air inside an aeroplane at those heights? In order to address the issues, the cabin and cockpit of a plane is made air tight and pressurised forcibly till the air pressure inside equals to that seen at 8000 ft or lesser. To emphasise this arrangement, take an example of a plane in cruise at 41000 feet. Air from the atmosphere is compressed by the engine or other means and forced into the aeroplane’s cabin & Cockpit until the air pressure inside the aeroplane increases gradually and simulates an altitude of 8000 feet or less in cruise. This makes us breath normally. The pressure is regulated which makes our air journey possible and comfortable.

 

However, this process of pressurisation of aeroplane cabin is an artificial mechanism and can fail for many reasons. Whatever be the reason, as soon as the level of pressurisation drops (usually called decompression), the cabin altitude will rise correspondingly. In case of a total failure, say at 41000 ft, the cabin altitude will rise to 41000 feet. It is like a person climbing from 8000 ft to 41000 ft in no time!! How fast can this happen in an aeroplane? It depends on how fast the cabin looses pressure. Decompression of cabin is classified as slow, rapid and explosive. In case of a small leak, the “rise in cabin altitude” is gradual. In case of a explosive decompression, the cabin will rise to 41000 feet almost instantly. Onset of Hypoxia will occur almost simultaneously, leaving the crew and passengers with just about 15 seconds before Hypoxia could incapacitate them.

To overcome this situation, all commercial aeroplanes have emergency / supplement oxygen system. The system for the passengers and cabin crew include many sets of ‘Chemical Oxygen Generators’ and the masks which are usually in the overhead panel above passenger seat. The panel opens and the mask automatically drops when the cabin altitude rises above approx 14000 ft. The oxygen system is armed with a safety pin (ready for use) and activates by removal of the pin when the mask is pulled. There are usually more masks per row than the number of seats to cater for infants or walking passengers in the isle. The seats for flight attendants, lavatories and galley area (where passengers can be expected) also have oxygen outlets. Additional oxygen cylinders with mask are available to Cabin crew to carry out critical safety related tasks.

 

The pilots have a separate Oxygen system with ‘Quick Donning Masks’ next to their seats. Since supplement oxygen is critical to safe operations of flight, the functionality of Oxygen system is tested by the pilots prior to flights.

 

 

 

 

 

As a safeguard, whenever a cabin altitude rises beyond normal the pilots are notified by way of aural warning and caution lights. All persons on board are expected to put on their mask ASAP before the Time of Useful Consciousness (TUC) elapses. Across all fleets of airliners, the immediate actions include the following-

(a) The pilots put on their Quick Donning Oxygen Mask immediately and breath supplement oxygen to be able to fly the plane.

(b) The pilots ensures that the oxygen mask in the cabin have dropped automatically. If not, the pilot activates it manually.

(c) The passengers and Flight Attendants in their seats or in lavatories don their mask.

(d) The passengers in the isle or the galley area must take the nearest available mask without bothering to return to their seats.

(e) The supplement oxygen system caters for 15 minutes typically. Therefore, without exception, the pilot will make an emergency descend within this time to 10000 ft where supplement oxygen is not required any more.

(f) Although the situation may be under control, the pilot will divert and land at the nearest airport.

 

In light of our understanding of the subject let us review a typical in flight Cabin briefing in this regard and its significance –

WHAT FLIGHT ATTENDANT BRIEFS WHAT THEY MEAN
In case the cabin pressure reduces …. Due to fault in the pressurisation system
…. the oxygen masks in the overhead cabin will drop …. As the cabin height goes greater than 14000 ft
…. Pull the mask …. to release the pin and activate the Oxygen Generator
… and place is on your face ….. as demonstrated by the Flight Attendant so that the nose and mouth are in the mask
…Put on your mask first …. before your Time of Useful Consciousness is over !!!
…before offering to help your fellow passengers. slight delay in offering help to co passenger or your infant is not a safety issue.

 

Pilots and cabin crew are fully trained to deal with cabin decompression. However, dropping of oxygen masks are known to cause panic in the cabin. Trouble starts when the controlled steep descend to 10000 ft is misunderstood  with  fears of the plane dropping down. My guess is that in times to come, a word or two regarding emergency descent after decompression may become a part of the standard briefing by Flight Attendants.


Recommended viewing –

Aviation & Hypoxia by FAA –  https://www.youtube.com/watch?v=gAuXV2XtCnU


About the Author

Wing Commander K Dinesh is a former Indian Air Force officer and the founder of Cockpit Vista, an innovative Aircraft Simulator based set up in Mumbai (India). He is an  Aviation Safety Consultant with emphasis on Flight Anxiety Removal amongst air travelers using a full size Aircraft Simulator. The Simulator is also utilised by pilots and corporates. 
For details / booking please visit  www.cockptvista.com or call +91 9833892747 (India)
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