When bleed air and pressurization systems are properly functioning, most modern light jets flying at their maximum certified ceilings can create cabin altitudes at or below 8,000 feet. But those ceilings are as high as 45,000 feet, where ambient pressure is only one-seventh that at sea level. At such low pressures, pilots and passengers subjected to a rapid depressurization can expect to have a maximum time of useful consciousness of as little as five to 10 seconds. Time of useful consciousness, also called effective performance time, is defined as the time during which, after exposure to oxygen-deficient air, the pilot can perform flying duties efficiently.
Fumbling with the oxygen mask and removing sunglasses and headsets can eat up this tiny window of useful consciousness, making a pilot completely unable to finish the seemingly simple task of donning an oxygen mask before complete unconsciousness ensues.
Pilots of turboprop airplanes, which are most efficient flying at altitudes in the range of 25,000 to 28,000 feet, also face diminished capability when pressurization is lost. At 25,000 feet, time of useful consciousness is three to five minutes; at 28,000 feet, it’s from 2.5 to three minutes. If the depressurization is rapid or explosive, time of useful consciousness can be half those values, depending on the pilot’s physical condition.
Pilots sometimes have difficulty understanding this concept, figuring they may be able to hold their breath for a minute or more. But here’s where it’s critical to understand how oxygen is transported from the lungs to tissue. Lining the 300 million alveoli—air sacs—within our lungs are walls of such thinness that gas can be forced through them and into the circulatory system, and vice-versa. At sea level the ambient pressure of oxygen in the lungs is significantly higher than the pressure of the oxygen present in the blood surrounding the air sacs. Physics dictates that oxygen will travel from high to low pressure, ensuring adequate oxygenation of the blood.
At very high altitude, however, the ambient pressure of atmospheric oxygen is lower than the pressure of the oxygen in the blood, and the flow of oxygen is reversed. Now oxygen is pulled out of the bloodstream and into the lungs, depriving body tissues of a continuous supply. Holding your breath won’t work because blood-oxygen saturation still declines. Without a fast, fresh supply of oxygen, a pilot exposed to the ambient air at very high altitudes is in a dire situation.
Enter the oxygen system—typically one of the simpler systems in turbine aircraft. Gaseous oxygen is stored under pressure (commonly around 1,800 psi when full) in a metal bottle that is generally located in a nose compartment. This high-pressure oxygen travels into a combined shutoff valve and pressure regulator—which, when open, reduces bottle pressure to a much lower value (commonly 70 psi) before the supply line enters the cabin. On some aircraft this shutoff valve is controlled from the cockpit, and an annunciator gives an alert when it’s closed. For many jets, however, the valve is only accessible from the aircraft exterior, and the pilots may not be given a direct indication of shutoff state.
Departure with a closed shutoff valve could prove catastrophic in these aircraft should a depressurization occur, so a diligent preflight test of the oxygen system is vital.
The lower-pressure oxygen entering the cabin’s pressure vessel typically splits into two paths—one to the cockpit, and one to the cabin. The pilots’ masks are directly connected to the oxygen supply with no intermediate control. Instead, the mask itself typically allows the pilot to select from three modes determining how the oxygen is released into the mask.
It can be easy to forget oxygen systems in the course of your day-to-day flying. After all, they’re seldom used beyond the preflight checks. Just remember that many memory items have “don oxygen mask” as first steps.In the default setting of 100 percent, the mask releases 100 percent oxygen “on demand,” or when inhalation is sensed. Because of the very tight fit of the mask, the flow of 100-percent oxygen will ensure adequate oxygen delivery to the body at cabin altitudes up to 40,000 feet. Some oxygen systems will deliver the oxygen in this setting under pressure higher than ambient, allowing for sufficient oxygen delivery even above 40,000 feet.
The second common setting, often titled “normal,” allows for conservation of the limited oxygen supply. In normal mode, the mask will dilute the oxygen supply with ambient cabin air, varying the proportion to ensure that physiological needs are met. At or above an approximate 35,000-foot cabin altitude, only oxygen will be supplied, and the mask will function the same as when in the 100 percent setting.
The third mask mode—emergency—is used when oxygen is needed for smoke hazards. In emergency mode, 100 percent oxygen is delivered to the mask under constant, rather than on-demand, flow. This creates higher pressure inside the mask than outside, preventing fire-generated smoke and toxic gases from entering the mask. Some of the high-pressure oxygen is diverted to an outlet at the top of the mask, which fits into entry ports at the base of a pilot’s smoke goggles. Oxygen from the mask is forced through the ports, keeping the goggles clear of smoke.
Aft of the cockpit, and before flowing to the cabin, oxygen passes through a control valve connected to a three-position switch located in the cockpit. Two of the positions are mechanically controlled Open/Closed settings. The Open setting allows oxygen to flow past the control valve to the overhead cabin oxygen masks. Because the masks are usually deployed by oxygen pressure opening the mask cover, selecting this position with pressure in the oxygen line will cause all the masks to fall, allowing oxygen to flow to any masks in use. The Closed switch position does the opposite. It restricts the flow of oxygen past the valve, so masks will not drop. If the masks have already deployed, any mask in use will now be deprived of oxygen.
The third switch setting is the default position, often labeled Normal or Auto. In this position, the valve is controlled electronically by a cabin altitude transducer. When a defined cabin altitude is reached—typically between 14,500 feet and 15,000 feet—the control valve automatically opens, allowing cabin masks to fall and providing oxygen to the passengers. However, the passengers’ masks are of a lower performance than those provided to the pilots. They have looser face seals and lower oxygen flow rates, so some ambient air is mixed in when passengers inhale. For these reasons, cabin masks typically are able to meet physiological needs only up to a maximum cabin altitude of 25,000 feet.
It can be easy to forget oxygen systems in the course of your day-to-day flying. After all, they’re seldom used beyond the preflight checks. Just remember that many memory items have “don oxygen mask” as first steps. The more you know, and the more you practice that critical first step, the safer you’ll be. And know this: Most depressurizations are not sudden or explosive. They happen slowly, and sometimes begin at takeoff. By the time you’ve spent the 20 minutes or so to climb to cruise altitude, hypoxia already may have set in. Here’s where having a pulse oximeter nearby can provide an early warning to put on the mask.
Neil Singer is a pilot examiner and jet mentor with more than 10,000 hours of flight time.