Chemical Passenger oxygen generators disadvantages:
– Inability to control the flow -once started
– The generator goes for 15 minutes
– Non-reversible functioning (once used)
– Poor autonomy (they cannot be centrally operated)
– Type of passengers’ masks is not useable in smoke, fire condition
– One generator will supply more than one mask (typically 4 masks)
New Technologies for oxygen generations:
It’s time to change the minimum safety of oxygen supplementary in commercial flights especially for passengers. With a simple look to oxygen systems mention that with new technologies there have to be advantage in oxygen systems. For example PSA oxygen generation can be as a backup for other conventional systems. In this case can provide better oxygen for passengers than before and equal to flight crew members condition, and even can increase performance of flight crew members oxygen.
In flight up to 40000 feet if rapid decompression happen just only 12 seconds is maximum time to use supplemental oxygen, otherwise hypoxia and death will happen. As a general we pay more attention to Fuel and other systems than oxygen system, which is the most important. Surely this factor can be one of key advertising points for airplane manufactures and airlines!
Two separate oxygen systems:
Most civil transport aircraft are pressurized to maintain conditions inside the cabin equal to an altitude of approximately 8000 feet, regardless of actual aircraft altitude above this figure. Under these conditions Supplemental oxygen is not normally needed for passengers and crew, but oxygen equipment is installed for emergency use in the event of pressurization system failure or rapid decompression. Oxygen is typically carried as a gas or in a form of chemical generators. Gaseous oxygen is carried in high pressure steel bottles. There are three types of system: the continuous flow system, for crew and passengers; the diluter demand system ,for flight deck crew only; portable oxygen sets to supplement other systems and these can also be used for therapeutic use and for cabin crew to move around a depressurized aircraft. In today’s modern transport category airplanes the flight crew emergency oxygen system is gaseous (stored in a cylinder) and the passenger system is in the form of chemical generators, 2 separate systems.
Passenger oxygen system:
Passenger oxygen system on a large transport aircraft is in most cases supplied from chemical generators (survival oxygen) -a continuous flow oxygen system. The system is used after a cabin depressurization via automatically operated drop down masks, located above each seat. The system can be operated manually from the flight deck.
Supplementary/survival oxygen (built-in chemical generators) = supplied to the airplane occupants in case of accidental depressurization.
First aid oxygen (portable oxygen bottles) = provide some passengers with additional respiratory assistance after an emergency descent following a depressurization or to deal with medical conditions of certain passengers during normal flight (usually respiratory disorders).
Chemical oxygen generators:
The oxygen system most commonly fitted in today’s modern transport category airplanes for use by passengers and cabin crew in case of accidental depressurization is in a form of chemical oxygen generators. It is a cheap, light and effective way of providing passenger oxygen, which is produced from sodium and Iron powder. Quite simply, the chemical reaction is triggered by pulling on a dropout mask after it has been presented by automatic drop-out system (barometric ejection using an aneroid capsule that typically operates at 14 000 feet-releases the mask to a half-hung position) or by manual operation from the flight deck -by selecting the “PAX Oxygen System” to ON -this releases the masks, but the actual chemical reaction (flow of oxygen) can only be started by the passengers by pulling on the mask.
Pulling on the mask triggers the electrical firing mechanism (in some models the firing circuit is initiated mechanically) or electrical heater, which ignites a sodium chlorate and iron powder charge block. As the temperature of the block rises, a chemical reaction creates a flow of low pressure oxygen through a filter to the mask. Oxygen ,flow will normally be maintained for about 15 minutes and, despite the very high temperatures generated, the oxygen Itself is at a comfortable temperature .A relief valve is fitted to relieve excess pressure. The generator outside thermal paint changes color after use to show that it needs replacing. One generator will supply more than one mask (typically 4 masks).
Chemical oxygen generators performance:
Chemical oxygen generators have a shelf/service life of ten years are of a relatively simple and light construction (inexpensive to manufacture), and require no maintenance (unless it has been used). Also a risk of explosion is lower than in gaseous systems as there is no oxygen stored under pressure in the chem. generators and no leak can occur. Obvious disadvantages include: inability to control (modulate) the flow -once started, the generator go for 15 minutes; non-reversible functioning (they cannot be simply refilled -they have to be replaced with new generators once used); poor autonomy (they cannot be centrally operated the passenger has to actually pull on the mask to get the generator running).
The passenger masks are supplied with a continuous flow of oxygen from the chemical generators. Once the generator has been initiated, the flow of oxygen cannot be regulated or stopped -the generator will only stop after the chemicals are exhausted. The passenger is supplied a mixture of oxygen together with cabin air from outside of the mask -therefore the masks will not protect the passenger from a smoke environment as the smoke would be inhaled together with the oxygen flowing through the mask.
Passengers’ masks Disadvantage:
Passengers’ masks are not leak-proof. When you breathe oxygen with the mask, you also breathe ambient air. It becomes dangerous in case of severe smoke, because you inhale the smoke. There is often a big argument among the students saying “But it is better to breather at least a mix of oxygen and the smoke”. The masks will NOT protect the PAX from smoke. There is no need to argue about this fact .Second thing -if you have smoke in the cabin, you probably have a fire as well somewhere (or a fire about to start) in the cabin.
I’m sure that everybody knows the equation: Oxygen + Fire =. …. So do you really want to start a few dozen chemical generators in the enclosed confined space of the fuselage (absolutely no helpful reason at all, the masks will NOT protect against smoke) and start pumping huge amounts of oxygen to support the fire?
Flight crew oxygen:
The flight deck oxygen system is usually separated from the passenger system. Flight crew oxygen is stored in a gaseous form – pressurized cylinders supply the oxygen to a demand type mask and regulator. Gaseous oxygen is stored in high pressure cylinders at 1800 PSI and is reduced to about 300 PSI for use at the regulator. Oxygen storage cylinders are provided with an excess pressure rupture disc, fitted to the shut-off valve body and venting the cylinder contents to the outside of the aircraft in the event of a dangerous pressure rise in the cylinder (via a safety plug).
In most cases an indicator is fitted which will show that discharge has occurred due to excess cylinder pressure – most commonly this is in the form of a green discharge disc located on the outside of the airframe – if the disc is present, it means the oxygen bottle is OK. If a discharge has occurred, the green discharge disc will be missing.
The diluter-demand system
The diluter-demand system is for flight crew only and when switched on, will only supply oxygen when the recipient breathes in. Each crew member has an individual regulator. The system can be set to NORMAL in which case, the oxygen regulator, one for each crew, increases the amount of oxygen as the aircraft climbs; the recipient is supplied a mixture of oxygen and air supplied from the cabin. Above approximately FL320 100% oxygen is supplied with this setting. Alternatively, the system can be selected directly to 100% – in this case immediately from the time of this selection the recipient receives a 100% oxygen without mixing it with the cabin air. The regulator also has an EMERGENCY switch located on the front of the unit and when this is selected 100% oxygen is supplied at a slight positive pressure. This setting is used to keep the wearer from inhaling smoke and fumes. If a TEST MASK button is operated, oxygen is supplied at a slightly higher pressure and this tests the mask for fit and leakage.
Heavy and dense smoke in the cockpit
In case of heavy and dense smoke in the cockpit the pilots should use the EMERGENCY selector on the regulator. Providing %100 oxygen into the mask on a constant flow under a higher pressure than regulator demand type position. This will help clear the smoke from the smoke goggles and/or help to keep the heavy smoke positively out of the mask by the oxygen under pressure. This feature can also be used on a momentary basis for a few seconds to clear the smoke away from the smoke goggles once they are initially donned and then switching back to the NORMAL position.
Crew masks fit snugly to the user’s face (not the user’s head), with minimum leakage around the mask, and incorporate a microphone and jack-plug for connection to the aircraft communication system (pilots are able to radiotelephone even when using the quick donning oxygen masks) – usually the pilot has to change the position of a switch that activates the microphone in the mask. Do not confuse with a smoke hood (PBE) which covers the user’s entire head. Smoke masks are not necessarily a constant flow type as they are connected to an oxygen regulator (flow type depends on the setting of the regulator), whereas the smoke hoods are equipped with a small chemical oxygen generator that provides a continuous flow of oxygen.
PSA oxygen generation
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(Refer to OPS 1.770 Supplemental oxygen – Pressurized airplanes)