Dream Of Flight Has A New Domain !

Thursday, February 23, 2012

Aviation Knowledge 1

Hi All,

Just thought that i have been sharing so much about my journey to becoming a pilot i may as well also share with you some general knowledge about aircraft which can be interesting to many. Today let’s discuss about Cabin Oxygen.

Ever been in a situation where, as a passenger on a commercial aircraft you suddenly sees a yellow mask dropping out from the ceiling? Guess not many of us will have such experience and if you do, please share it with us. Also, if this situation does happen, what should we do?

Cabin Oxygen Mask
Time for photo when mask drop?

I believe not many of us listens to the Safety Demonstration performed by Cabin Attendant before each flight. So before we really know what and how we should react, we should have a brief insight to how the system actually works.

Let’s first discuss when will the oxygen mask drop out from the ceiling. For most aircraft design, oxygen mask will drop down automatically if the cabin altitude exceed about 14000 feet. It will be followed by an auto seat belt sign on, a chime sound and then an automated message telling you to put on your mask.

So why 14000 feet? That is because 14000 feet is the average altitude a normal human can sustain normal breathing without being affected by hypoxia, which would make us unconscious followed by some symptoms before death.

In normal operation, a commercial aircraft flying at about 38000 feet will pressurize the cabin altitude to about 8000 feet so that we can function normally in the cabin. As you can see, if the oxygen mask does drop, something serious is happening, so time is very crucial because we could go into hypoxia if we do not put on the mask as soon as we can.

So with a cabin altitude of 8000 feet in normal operation, how can the cabin altitude ever exceed 14000 feet? There are a couple of scenarios.

We could have a sudden structural damage, giving a big hole in the cabin. So what happen if there is a big hole in the cabin? The pressurized air will rush out increasing the cabin altitude to almost the same as outside which is 38000 feet.

A Qantas with a cabin hole near the wing

Also an interesting fact to know is, how long does a normal human take, to go into unconsciousness with the change in altitude? As you can see below, given the scenario above if the cabin altitude was to shoot up to 38000 feet, we will have about maybe 15-20 seconds to stay conscious if we do not have the oxygen mask on.

Time of Useful Consciousness Chart

Of course the above scenario with a hole in the cabin will create loud noises and mist in the cabin which would alarm most passengers. What if there is another scenario whereby there is a failure in the air conditioning system. To make it simple, with no air coming into the aircraft, air will slowly escape through the toilet ventilation system as well as small leaks around the cabin. The cabin will appear normal and we call this a gradual depressurization. The cabin altitude will slowly increase and we might not even notice it. Only tell tale sign would be the oxygen mask.

So the next time if you have a mask drop down, do not be deceived by the fact that the cabin feels normal or thinking that it might be a malfunction or joke. Do put on the mask quickly.

Lastly just want to share with you on the duration of the oxygen mask. The oxygen is initiated when you pull the mask from the ceiling and wear it. A small oxygen canister is activated just above the mask and it will normally supply the row of 3 or 4 masks for about 15 minutes. Why 15 minutes? Isn’t that a little too short? Well, that is because pilots are trained to start their immediate descend if there is a situation to do with depressurization. Descending from 38000 feet to a safe altitude of 14000ft will take not more than 15 minutes. That’s how they derive that figure.

Hope we all learn something here and if there are any queries please email me. Thank you.

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