Why can't we use bleed air instead of oxygen cylinders?

Question

When cabin is depressurized due to some or the other reason then we usually have oxygen generators in airplane and they last for 12 minutes. It is very shot in usual terms but it is enough to descend a plane instead can we use bleed air so that it is long lasting?

Some expected counter questions and answers:

Q. What you will do for old people and small children?
A. For people aged above 60 and below 15 have to use oxygen generators however they are very less people in number as seen from normal aviation photos?

Q. What if they have problems like hypoxia?
A. Always while taking ticket they ask that do you have such kind of things and they will be treated as above category?

Q. If it fails how can we do it ?
A. What you will do if passenger oxygen generator is failed?? That is my counter question however I am not telling to remove that oxygen generators fully but decrease capacity and decrease costs (note that same mask has both bleed air and oxygen from oxygen generator and this lining is light and cheap you can save more lives)?

Q. This will reduce safety
A. No never. Only it only decreases a bit of cost and it even can last longer and save lives more it is more safer reason: old people and children use only oxygen from generator and others use only bleed air

Q. If the air quality is bad?
A. The people use both oxygen generators and this bleed air and we can monitor air quality it is very very light and very very cheap like it is not even 10% cost of normal economy class ticket it is just bad to ask this. Even you can install multiple of them to monitor more clearly and and that will not cost much

Q. What if bleed air system fails?
A. My counter question what if the oxygen generator fails and what if normal bleed air system fails? It is like breaking the whole aircraft into 4 pieces in air. Of course there is oxygen cylinders in normal system if they don't descend it will become like Helios Airways flight 522

Answer 1

In order to breathe normally, you need a minimum oxygen partial pressure in the air. Normally, this is provided by pressurizing the cabin to higher pressures than the outside air with bleed air, which contains the same fraction of oxygen as the outside air, but due to the higher total pressure it will have a higher oxygen partial pressure. See also Why is the time of useful consciousness only seconds at high altitudes, when I can hold my breath much longer at ground level?

In case of a pressurization failure, the total pressure of the air in the cabin is reduced and the oxygen partial pressure will be insufficient for normal breathing. The oxygen masks solve this problem by providing (almost) pure oxygen, which results in a higher oxygen partial pressure at the same total pressure. If you would supply the oxygen masks with bleed air, this does not work because it would immediately lose pressure when exiting through the mask and only contain a fraction of oxygen.

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What if the oxygen generators fail? Well, that can't really happen since that is not a single system. The bleed air supply is more likely to fail (although still very unlikely: you would need to have dual engine failure and APU failure (e.g. due to running out of fuel) or a dual pack failure). The oxygen system however uses one (or more) oxygen bottles for the flight crew (and their pressure is checked during the preflight) and chemical oxygen generators for the passengers. Each row has at least two of these generators, which are activated by pulling on one of the masks. They are all independent, so even if one of them fails, all others will still work and the cabin crew can get portable oxygen bottles to the row where one failed.

The Helios 522 accident report you linked (where the masks deployed automatically) actually has a good description of this system:

^{(AAIASB Final Report - Helios Flight 522)}

Answer 2

Because sometimes the cause of the depressurization is that there's a great big hole in the cabin. E.g Aloha Airlines flight 243: https://en.wikipedia.org/wiki/Aloha_Airlines_Flight_243 Try to design a bleed air system that will pressurize that!

Now if you mean running bleed air through tubing to masks, what if your engines have failed? Using an O2 system is redundant: it works even if the engines quit and there's a big hole.

Answer 3

why can't we use bleed air instead of oxygen cylinders?

Because if the cabin is depressurised, bleed air has already failed, and the oxigen tanks are there to provide a temporary help while the pilots bring the aircraft at a lower altitude.

Answer 4

Aircraft safety is built around the system of redundancy that has been established after many aviation accident case studies. In the case of cabin pressurization, bleed air is the primary system. A secondary system available to all passengers and crew must be available if bleed air is inoperable. You can not use bleed air as a backup to bleed air. And, you can not limit the backup to a limited amount of passengers.

As already stated, the chemical oxygen generators is a good compromise of reliability and weight. Their cost is about as negligible as designing a system to monitor he air quality and quantity received by each passenger. And, each row of passenger seats has two independent chemical oxygen generators to make their failure a statistical non-event.