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Emergency Oxygen

posted Apr 17, 2011, 2:47 PM by George Finlay   [ updated Oct 8, 2011, 11:21 AM by Nathaniel Cauldwell ]

Pilots of pressurized aircraft like the Piper Meridian practice switching smoothly to emergency oxygen in case cabin pressure is lost.

Captain Daniel (Boone) Gibson and his crew in the 79th Physiological Training Flight at Andrews Air Force Base in Maryland do a bang-up job educating civilian pilots on this topic. Ordinarily, they work with military pilots. But about once a month they open their altitude chamber and expertise up in full day classes that can be arranged through the FAA in Oklahoma City. Included are in-depth briefings on decompression sickness, hypoxia, trapped gasses, and other ill effects of sudden exposure to high altitude, along with proper use of supplementary oxygen. A closely-supervised ninety minute simulated flight in the chamber exposes trainees to conditions identical to what occurs at an altitude of 25,000 feet in actual flight. Each pilot has his or her own unique set of symptoms to contend with. The controlled environment in the chamber gives flight crews a chance to explore their symptoms safely. In flight, early recognition of symptoms can help crews detect problems with cabin pressure or oxygen systems.

The Piper Meridian, PA46-500TP, is certified for flight up to 30,000 feet. Behind the copilot's seat is a cabinet within reach of the pilot with a mask designed to be put on quickly with one hand – a “quick don”. It is fed from a green 4.25 cubic foot high pressure cylinder designed to withstand 1800 to 2000 lbs/sq. in. The Pilots Operating Handbook (POH) estimates that when fully charged the system has sufficient supply for one individual for 25 minutes at 30,000 feet when used with the mask supplied, a model MC-10 manufactured by In-Eros. A diluter demand regulator included with the mask normally mixes oxygen with cabin air to conserve supply. There is a switch on the mask that can be used to select higher pressure 100 percent oxygen when needed in an emergency. In this position, supply duration is reduced. There is a MIC SELECT switch in the lower left corner of the instrument panel to allow the pilot to select a built-in microphone in the mask. In-Eros also makes a mask that mates with airtight goggles. That is not standard equipment in the Meridian, but it would be a useful upgrade since the cabin might be decompressed to deal with a smoke condition, and that smoke could be so thick or acrid that it would interfer with vision.

The other five seats in the Meridian, including the copilot's position, are provided with less robust solid state oxygen generators, also referred to as sodium chlorate candles. Beneath the copilot's seat is a tray with two masks connected to an oxygen generator thru a clear plastic tube and a lanyard. The tray slides out into the aisle between the copilot's and pilot's seat. Pulling on either mask and the attached lanyards fires a percussion igniter cap to start a chemical reaction between sodium chlorate salt and iron wool, resulting in the release of heat, oxygen, sodium chloride, and ferrous oxide. Once started, the reaction cannot be stopped. The POH estimates the supply is sufficient for fifteen minutes with the use of the continuous flow masks attached to the generators, with rebreather bags that help conserve supply. In the event of failure or exhaustion of the pilot's emergency supply, the copilot's masks can be used by the pilot.

Solid state systems with rebreather bags are less robust because they do not allow for 100 percent oxygen flow, do not provide a tight seal between the mask and the user's face, do not include a microphone, last about half as long as the high pressure pilot's supply, cannot be turned off, and pose some fire risk due to the high temperatures generated by the chemical reaction.

At a typical cruise altitude of 25,000 feet in the Meridian, with a maximum cabin pressure differential of 5.5 psi, cabin pressure is the equivalent of about 8000 feet, which means oxygen partial pressure (pO2) is at about 135mm. Flatlanders in reasonably good health are comfortable under those conditions. A rapid decompression to 25,000 feet quickly reduces pO2 to about 50mm. Along with other possible stresses in a decompression like wind noise, fog, sudden temperature drop, expanding intestinal gasses, possible smoke or fire in the cabin, a decompression can be very distressing, especially for untrained passengers.

If it can be accomplished safely, the crew may wish to initiate an emergency descent to at least 10,000 ft, where the p02 is 100mm and most people will again be comfortable. Often announcing that a descent to an altitude where oxygen will not be needed will help reduce passenger stress.

To make a maximum rate descent in the Meridian, the autopilot is turned off, power is reduced to idle, the nose is pitched up to reduce airspeed to 168 KIAS or lower, the landing gear are extended to provide additional drag, and pitch is adjusted to keep airspeed below 168 KIAS in smooth air, or below 143 KIAS in rough air, to avoid overstressing the airframe. In the simulator, 3000 feet per minute descents were accomplished. In any case, the descent should ideally be completed within the 15 minute endurance of the solid state devices on board.

Dr. Wayne Isom, a heart surgeon and a pilot, suggests that flight crews advise passengers to try to remember to breathe slower and deeper than normal to avoid possible hyperventilation when first going on emergency oxygen. What apparently happens in some cases is that oxygen chemoreceptors in the carotid bodies command unnecessarily fast and deep respirations in response to the sudden decrease in oxygen blood concentration following a decompression. This in turn drives blood CO2 levels lower when they are often already at a proper level. Paradoxically, this negative feedback loop can lower blood oxygen concentration.

To check their own oxygen blood concentrations, flight crews could use battery-powered fingertip pulse oximeters such those made by Nonin, Med Choice, or Checkmate to confirm a blood saturation level of 90 percent or better. However, given the increased workload in a pressurization failure, it is usually more advisable to simply slow one's breathing to prevent hyperventilation and conserve a limited oxygen supply.