For an “all or nothing” solution, the option of “nothing” is an unnecessary risk. Yet this option is the preferred choice in the recent action by the Federal Aviation Administration (FAA) to eliminate emergency oxygen from aircraft lavatories. (See Aviation Safety Journal, “Emergency Oxygen Ordered Removed From Lavatories”)
On 10 February 2011 the FAA ordered airlines to render nonfunctional in 6,000 planes’ lavatories the oxygen supply to the “little yellow cups” that ordinarily drop from the overhead in the event of loss of pressurization. The emergency oxygen supply to regular passenger seats will be unaffected.
There is an apparent fear on the part of federal authorities that terrorists could utilize the oxygen generators above the lavatories as a means to accelerate a fire. The oxygen generators stowed above passengers’ heads utilize burning sodium chlorate to produce oxygen, which then travels down a plastic tube to the yellow oxygen mask. The oxygen canister gets hot, so the fear is that an ignited canister could not only ignite nearby flammable materials but also the oxygen produced could worsen the flames.
Exactly how a terrorist in a locked lavatory could get access to a canister stowed in the overhead is not clear. The fact is that rendering the canister “safe” from terrorist use also means the lavatory has no emergency oxygen. A passenger in the “loo” responding to a call of nature has approximately a minute or two to don an oxygen mask before losing consciousness. This is the reason why lavatories were outfitted with emergency oxygen; there is not time to complete one’s “business”, unlock the door, and dash to the little yellow cup dangling over the passenger’s seat. Elderly and infirm passengers with slow mobility may not win the race from the lavatory to their seat, losing consciousness in the lavatory or in the aisle.
A plaintiff’s lawyer will have a straightforward case to plead: my client is brain damaged/dead because there was no emergency oxygen. It will cost the airlines about $1 million to deactivate all lavatory emergency oxygen canisters. The settlement from one deceased passenger could total $10 million or more. The airlines are not being exempted from liability at the same time they have been directed by the FAA to nullify emergency oxygen in the lavatories.
One gets the impression that the FAA has not considered an option which avoids the heat generated by the present canisters while providing a supply of oxygen-enriched air to a passenger caught by a surprise decompression in the lavatory. The steel oxygen canister stuffed with sodium chlorate and other chemicals dates back to at least World War II; the canisters were used for emergency oxygen aboard submarines.
Advances in technology have made possible a new means of providing emergency oxygen: pressure swing adsorption (PSA). Special adsorptive materials, notably zeolites, are used to preferentially adsorb target gasses at high pressure. When the pressure is reduced, the trapped gas is released, or desorbed. Thus, ambient air passed through a PSA device will capture the nitrogen and the output gas will be enriched in oxygen.
Zeolite, whose chemical properties are at the heart of PSA gas separation
When the capacity to adsorb nitrogen is reached, pressure is reduced and nitrogen is released. Two adsorbent vessels allow for near-continuous production of a target gas.
Schematic of the PSA process. Zeolite is used as a molecular sieve, preferentially adsorbing the target gas at high pressure. When the pressure is reduced, the target gas is released.
This is exactly the process featured in portable oxygen concentrators used by emphysema patients and others who require oxygen enriched air to breathe.
The system operates at near-ambient temperature; in other words the danger of fire is greatly reduced, and the PSA technology is not a flame accelerant. Since the system is electrically powered, it can be turned off in the cockpit, if not by a designated switch then by a circuit breaker. For any person in a locked lavatory suspected of trying to gain access to the PSA oxygen generator, the malign intentions can be foiled by switching the PSA off. In fact, the terrorist could be restrained in the locked lavatory until landing.
Innocent passengers caught in the lavatory during a cabin decompression event would have emergency oxygen, and terrorists would be denied a hot generator emitting oxygen.
This situation is a “best of two” solutions. Replacing the chemical canister with a PSA device is much preferable to the “nothing” adopted by the FAA in a dubious scenario involving terrorists.