“….The bag will not inflate…”

[reptile]

When referring to the drop-down passenger oxygen system, safety information leaflets often contain the following; “Please note that the bag will not inflate”.

What is the purpose of the plastic “bag” attached to the mask, and why does it not inflate?

[mbga9pgf]

When referring to the drop-down passenger oxygen system, safety information leaflets often contain the following; “Please note that the bag will not inflate”.
What is the purpose of the plastic “bag” attached to the mask, and why does it not inflate?


The Oxy Generators do not start functioning (hence do ont fill the bag) unless the user pulls on the mask. This way, if:

1) the masks are dropped accidentally, they dont waste the one-shot oxy generator (expensive as rubber jungles are not that uncommon, especially if your landing is on the harder side of "Firm"!)

2) Only masks with a user attached are used, thus not pumping excess oxy into a cabin where smoke/fumes even fire are an issue... (although in such cases the crew may elect not to drop the masks, as the mask mixes ambient air with the oxygen coming out of the generator.

As to the purpose of the bag, I dont actually know. At a guess, I would say its some form of basic oxygen pressure regulation.

Not sure it that answers your question!?

[john_tullamarine]

Flightcrew oxygen is protective and can be selected to suit various circumstances.

Passenger oxygen is a supplemental arrangement intended to cover the depressurisation case and provide a bit extra partial pressure of oxygen during the resulting descent to lower levels. As such the system utilises rebreathing. The comparatively low oxygen flow rate is supplemented by cabin air to provide an oxygen enriched mixture but certainly not straight oxygen.

The bag provides the means to introduce cabin air into the breathing activity and a mixing volume for the cabin air, supplemental oxygen, and exhaled lung air. The extent to which the bag does/doesn't inflate/deflate during use is not a critical consideration .. however, by gripping the bag, one can force a small degree of inflation to check that the oxygen supply actually is functioning.

The cabin air component is the reason that the cabin oxygen system is not a good idea in a smoke filled environment .. the passengers just get a lung full of smoke ....

[Captain Calamity]

Similar bags are used in systems to deliver high flow oxygen in a healthcare setting.

During inspiration air flows into the lungs very quickly, usually at several hundred litres/minute. When wearing an oxygen mask one breathes in a mixture of ambient air plus supplemental oxygen. As the highest flow achievable from the regulator on the oxygen cylinder is quite low (typically fifteen litres/minute) the contribution made to the inspired gas mix would only be very small if it were fed into the mask. Increasing the oxygen flow is impractical as the cylinder size would have to be enourmous, so to get round this a reservoir bag is used - this is constantly filling by the trickle of oxygen from the supply (even when breathing out). When one breathes in the bag collapses as the oxygen is drawn into the mask all in one go, enabling a higher percentage of oxygen than otherwise available (roughly 80-90% vs. 50% or so for a direct fed mask).

I guess the flow rate from an oxygen generator is also quite low, hence the reservoir bag.

Andy

[reptile]

Thanx !:ok:

[Dan Winterland]

In aviation terms, sometimes known as an 'economiser'. Some aircrew systems of old had them. I seem to remeber the Jet Provost Mk3 had an economiser although it was in a box behind the ejector seat rather than a bag hanging down form the mask, and if you opeoend the box it looked more like a set of bellows rather than a bag.

[chksix]

Question about the passenger masks, is the clip that's clamping the hose coming off easily when pulling on the hose or is there a risk that someone is sitting there just rebreathing his/her own air?

The clip is never shown on the safety demo masks so pax are unaware of why one has to pull on the mask.

[Golden Rivet]

masks used for continuous flow oxygen systems use the rebreather bag principle. In this type of mask, oxygen flows into the bottom of the bag. When the person inhales, he takes the air from the bag into his lungs. He then exhales into the rebreather bag. That air which entered last still has a good portion of its oxygen content and is the first to enter the bag. Oxygen is continuously flowing into the bag, and this oxygen plus the air being exhaled will fill the bag. The air that was in the lungs longest is the last to be exhaled, and by this time the bag is full. This air then escapes through the valve in the face piece. The first air to enter the lungs on the next inhaling is that which was first exhaled, along with the supply of oxygen.

In essence, the rebreather bag is just a means of conserving oxygen.

[Dolley]

The clip is never shown on the safety demo masks so pax are unaware of why one has to pull on the mask.

Yes, they are. At least with some companies:

'Masks like this will drop from the panels above you. Remain seated and pull a mask towards you. This will start the oxygen supply. Place the mask over your nose and mouth...' and so on.

I'm doing it up to four times a day so I know at least my pax have been told about it. If they actually listened is obviously a different story...sigh!

[bfisk]

The purpose of the bag it selv is to dilute the oxygen with normal air, to improve system efficiency. When breathing 100% pure oxygen some O2 will be exhaled; it gets mixed with fresh oxygen in the bag, thus it's called a rebreather bag. When in use it will inflate to a certain extent after a while; just don't panic when it doesn't inflate right away. You inflate it - not the oxygen.

As far as I know anyhow...

[Volume]

The clip is never shown on the safety demo masks so pax are unaware of why one has to pull on the mask.

Depends on the operator. Some explicitly mention that you have to pull the mask towards you to start the oxygen flow, some don´t. Even for those who tell you, I believe that 80% of the passengers don´t understand, what they have to do exactly.

The better safety briefing would be the following (found somewhere on the web years ago) :
In case of a decompression, oxygen masks will drop from the ceiling. In that case, stop screaming hysterically, pull a mask towards you and place it firmly over mouth and nose, and breathe normally. If you travel with a child, assist him with the mask. If you travel with more than one child, you should decide now which one you like best.
;)

I noted in some aircraft, that the oxygen masks are more or less located straight above your head. I fear, that a lot of pax would grab the masks above the head of the passenger in front of them, as this is the one you can actually see. In that case, the rearmost set of masks will not be used, and the front row passengers will have a problem.

The safety briefing for emergency exits is also depending on the operator. British Airways does a detailled briefing, the FA removes the handle cover to show you the opening handle, remarks that the door is quite heavy, and tells you that you have to throw it out of the opening.
Lufthansa just have pictures on the seat back close to the exit, no briefing is done.
Checking in at JAL you get a leaflet, that tells you you are seated at an emergency exit, and you should ask the FA for a briefing. When asking, the answer is "don´t touch anything".

[screwdriver]

I suppose that the cabin crew could spend an hour giving a comprehensive safety briefing. But lets face it, most people can't be bothered to watch the 3 to 4 minutes thats on offer anyway.