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Passengers & SLF (Self Loading Freight) If you are regularly a passenger on any airline then why not post your questions here?

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Old 27th Jul 2010, 04:02   #1 (permalink)
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Cabin oxygen level on take-off

Am I just a very relaxed flyer or is there some reason why I ALWAYS fall asleep on take-off? No matter what time of day I fly I make it to the safety briefing but am always spark-o by take off - to awaken when the food arrives.

My better half in convinced that it's because of lower oxygen levels in the cabin at take-off? Is there any truth to this?

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Old 27th Jul 2010, 05:00   #2 (permalink)
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As long term SLF but no expert - I doubt it. Those that know will tell you the details shortly but my first guess is that it triggers in you:
  • The knowledge that you cannot do anything about your surroundings and so you might as well relax.
  • A protective mechanism, if you are sub-consciously worried about flying, your brain might think it easier to go to sleep.
  • Getting to the airport on time is stressful, getting through check-in and 'security' is stressful, finding the remote gate is stressful ... time to snooze until the grub arrives!
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Old 27th Jul 2010, 08:10   #3 (permalink)
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Oxygen levels are the same throughout the flight - irrespective of the cabin altitude. It remains at 21% of the air, what does change is the air pressure. As you climb up through the atmosphere, the pressure drops such that by the time you reach c18,000 feet the pressure has roughly halved, but at this altitude there is still 21% oxygen in the atmosphere.

Oxygen makes up 21% of the air and therefore makes up 21% of the air pressure. Air pressure at sea level can be measured in many ways but at sea level is usually 760mm of mercury - 21% of that is 159mm of mercury - this can be called the partial pressure of oxygen.

Your lungs work on a pressure gradient. There needs to be a greater partial pressure of oxygen in the air than in your blood for them to work. As you climb the air pressure (and with it the oxygen pressure) drops and the pressure gradient lessens. As you get to approx 10,000 ft most normal people need some assistance in breathing and above this it starts to become important. By 18,000ft or so you need oxygen adding to the air to boost this partial pressure, above 33000ft this oxygen needs to be given to you under pressure.

So as you can see, at sea level before take off there is no reduction in the oxygen in the cabin. Indeed there is not really a reduction in the oxygen level as you climb but rather a reduction in the pressure gradient between the air and you blood.

You sleep because you are warm, comfortable, have a lot of sound around you and your senses are being overstimulated and you are immobile. If you want to make a baby sleep, put it under a lamp, turn on the hoover and rock the cradle - bingo! sleeping baby.
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Old 30th Jul 2010, 04:41   #4 (permalink)
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I think it's the child in you ...

Whenever I used to take my young children for a flight in my bugsmasher ... as soon as we lifted off, bang, they were asleep. As the pilot, I had to resist the urge.
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Old 30th Jul 2010, 21:28   #5 (permalink)
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It's not just you. I get into that seat and bang, sleepy times. I've slept through take off in the past. Yet to figure out how I slept through the sound of two CFM56s roaring away when I can't sleep at home without complete silence.

I think it's a combination of relaxing as I'm on my way and the early starts that flying seems to demand. Up at 4am for a 9:30 flight... 2 hour drives are fun at that time.
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Old 31st Jul 2010, 14:57   #6 (permalink)
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I don't turn the oxygen off until we pass through 10000 feet. Otherwise you'd still be able to find enough to remain conscious....Passengers are much more pliable if you take all of their oxygen away for a few hours. Very slow to unload though.

The cabin oxygen levels at sea level are exactly the same inside, and outside the cabin. Perhaps its just that last drink sending you to sleep.
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Old 31st Jul 2010, 17:58   #7 (permalink)
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Good question smiley41.
I love aviation and I am a passionate flyer, but! .. in every climb and descent I yawn... feels like there is no oxygen... and its not only me who yawn, I have spoken to other flyers and they report the same. I even have seen it with my own eyes. So there is something going on.. I believe it has something to do with the pressure changes not as much with the oxy.

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Old 1st Aug 2010, 18:55   #8 (permalink)

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I even have seen it with my own eyes. So there is something going on..
Your eyes don't deceive you - you are absolutely correct. A friend of mine's ex-girlfriend once went out with a Pilot, and she said that he said that they turn off the air for take-off and landing to save fuel and so that the cabin crew don't get asked for all those drinks. I flew to Majorca two years ago and feel asleep almost as soon as the door was closed on my flight home which proves it - I've heard that the Russian and Turkish airlines are the worst, and that the crew steal stuff from you when you are asleep!
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Old 1st Aug 2010, 20:10   #9 (permalink)
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Now you have let the cat out of the bag Tightslot. We will have to resort to chloroform from now on!

Without delving too much into what has already been said about the composition of air, recycling and the partial pressure reduction effects, in fact when an aircraft takes off and power is applied to the engines, the cabin pressure actually increases above ambient pressure. It does this to ensure a smooth transistion in the pressurization process and also to help ensure the door seals are more effective than they might otherwise be, during the take off and early climb.

So albeit rather academic, you could actually say that the reverse was true in that for a given volume, there are actually slightly more oxygen (and other gas) molecules during take off, inside the cabin than there are outside.
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Old 1st Aug 2010, 20:14   #10 (permalink)
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For goodness sake

....now exactly how would you reduce the oxygen fraction in the air (well without a nasty oxidising event like a fire)??? It seems on the verge of too difficult keeping the in-flight entertainment systems going, let alone such novel chemical engineering, and it's too quick to be caused by the recirculating respiratory viruses the air-con drenches us with.

My bet is that the seats are too comfortable..........

Only wish I had that problem.

The FD already think we are dumb (dreadfully, even the CC think they are smarter than us pax), can we stop providing the evidence they crave

Last edited by Mr Optimistic; 1st Aug 2010 at 20:45.
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Old 3rd Aug 2010, 09:02   #11 (permalink)
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It is the mass of air we breath that matters

The mass of air that is available in the cabin, as soon as the doors are closed, is mostly at the contol of the crew. (FD).

I agree 21% proportion does not change dramatically at sea level, but taking an extreme example there is no O2 in space, or even at the top of a mountain there is very little O2.

The aircraft cabin is designed to be at 8000 ft or similar, and the breathable O2 is less than at sea level.

The partial pressure is less, and thus the effort needed to actually get sufficient O2 from the air is greater. Thus slightly self defeating, less O2, and more effort, to get the same mass of air into the lungs.

Also the air in the cabin is drier, and has a slighly higher percentage of Ozone. Less mass air flow, and usually warmer that one would like.
When driving to the airport in your drop head E Type, cold wind blowing thru your hair: never felt tired. NO, never.

All these go to make the feeling of sleepyness, thats why we turn the air off at night, to save fuel, as to make fresh air the burners are on all the time, and secondly the lights are off to save electricity.

Yes the cabin pressure CAN go up after the doors close, and CAN go up as power applied, depending on aircraft type, and more importantly TO Config bleeds.

Anything to save a few cents.


Tightslot: it was SOP on all return fligfhts, in my day, did not know it was the full rotation.
The French are leading the way forward in in fliight theft, and its not just the charge for the use of the head rest. I think MOL should install seat back safes.
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Old 3rd Aug 2010, 15:50   #12 (permalink)
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Cabin oxygen level on take-off
Not enough when I'm flying with a hangover that's for sure.
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Old 4th Aug 2010, 14:05   #13 (permalink)
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It won't be long now before Ryan Air start charging extra for oxygen. Pay the 20 quid O2 supplement or breathe nitrogen.
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Old 6th Aug 2010, 20:37   #14 (permalink)
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I'm definately not an expert here, but I learnt a school that the level of CO2 in the atmosphere is important. The level of 02 is unlikely to drop much below 21% but even if CO2 concentration rises to 1% (from well below 0.1% in the atmosphere) it wil be very uncomfortable. (The scene in Das Boot where the crew have trouble breathing is more realistic than the likes of Stingray where subs run out of air like fuel.) Cabin air is recirculated. It is filtered to remove bugs, etc, but CO2 levels may well rise. Perhaps someone in the know could comment on this and correct me if neccesary.

Previous posts mention low levels of humidity in flight which can certainly affect nasal membranes. However I find high levels of humidity before the engines are started more uncomfortable. Most US airports and LHR T5 have external air conditioned air supplies, which are generally OK, but APUs can't always cope and some airlines save money but limiting APU use on the ground. I find that you are so more comfortable once you start getting engine bleed air into the cabin that I can nod off.
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Old 7th Aug 2010, 18:36   #15 (permalink)
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I know airlines where they switch off the airconditioning system when they enter the runway, after take off they switch it back on. They do that to safe the system.

That means that from the moment we line up there is no fresh air flowing into the cabin and air is just recirculated. But all the passengers are using the oxigen and are breathing out the co2 in a small space...

Also during taxi they use only one airco system instead of two, but during flight we use the full airco system.

Also they tend not to use the APU on the ground when the engines are switched off until just before push back. Therefore during boarding you may find there is not that much fresh air in the cabin. Something that I personaly find very unconfortable when I travel as a passenger.

As far as I know this is all common practice in many airlines.

Last edited by bobwi; 8th Aug 2010 at 16:04.
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