2.I : High Altitude Operations

No person may operate a civil aircraft of US registry at cabin pressure altitudes above :

No person may operate a civil aircraft of US registry with a pressurized cabin at flight altitudes above :

The body functions normally from sea level to approximately 12,000 feet MSL. At these altitudes brain oxygen saturation is at a level for normal functioning, which is 96% saturation. As the body nears 12,000 feet MSL the saturation level has declined to about 87% which is close to a performance affecting level. Above 12,000 feet MSL oxygen saturation decreases further and peformance is affected. Note that these values and responses to altitude are very individualized, so each person may have somewhat different responses to altitude.

Hypoxia (as discussed in 2-A: Aeromedical Factors) is the main risk, and is specifically concerned with an inadequate supply of oxygen to the brain. To reprise briefly the different types of hypoxia, they are listed here :

With loss of oxygen there is a time of useful consciousness which decreases with altitude. This is the time in which you can make rational, lifesaving decisions and carry them out. This time starts to decrease at 10,000 feet and continues to decrease with altitude. Treatment is to supply the pilot with supplemental oxygen or to reduce altitude (potentially requiring an emergency descent).

However, it should be recognized that prolonged use of oxygen can also be harmful, and 100% aviation oxygen can create toxic symptoms if used too long. It is also the case that a sudden supply of pure oxygen after decompression can often aggravate hypoxia which suggests that oxygen should be taken gradually to build up in the system. Also, even if hypoxia symptoms are aggravated, it does not mean that the oxygen isn’t working and should be discarded.

Some symptoms of oxygen toxicity are bronchial cough, fever, vomiting, nervousness, irregular heartbeat, and lowered energy.

One approach to addressing the need for oxygen at high altitudes is to pressurize the aircraft. This practice of increasing the pressure inside the cabin to simulate a lower altitude inside the cabin. This reduces (or removes) the need for the use of supplemental oxygen and avoids rapid cabin pressure changes which otherwise might result in discomfort. It is common to maintain a cabin pressure of 8,000 feet.

The cabin is incorporated into a sealed unit capable of container air under a higher pressure than the outside ambient pressure. The pressure differential is the difference between the internal cabin pressure and the external ambient pressure. The maximum differential that can be maintained is aircraft-specific, and the higher the maximum differential, the higher you can go and maintain a comfortable cabin altitude.

Turbine aircraft use bleed air from the engine compressor section to pressurize the aircraft whereas piston aircraft use either turbocharger compressor air or an engine-driven pump to provide the pressurization. This is managed by a cabin control system which regulates the cabin pressure.

The control system normally consists of a pressure regulator, and outflow valve, and a safety valve. The regulator senses the cabin pressure and manages the outflow valve to achieve a preset pressure level. The safety valve is a combination of a pressure relief, vacuum relief, and emergency dump valves. The control system has instruments which show the differential pressure, the effective cabin altitude, and the current rate of climb/descent within the cabin.

There are various types of oxygen systems, generally categorized as : continuous flow, diluter demand, and pressure demand. Each has it’s own advantages.

Continuous flow oxygen systems are the most common found in GA aircraft. They are usually for passengers and have a reservoir bag which collects oxygen from the system when exhaling. Ambient air is added to the oxygen during inhalation after the reservoir supply is depleted. Exhaled air is released into the cabin.

Diluter demand oxygen systems supply oxygen only when the user inhales. Depending upon altitude the regulator can supply either 100% oxygen or a mix of cabin air and oxygen. The mask provides a tight seal and can be used safely up to 40,000 feet MSL.

Pressue demand oxygen systems provide oxygen to the mask under pressure at cabin altitudes above 34,000 feet MSL. This provides a positive pressure application of oxygen that allows the lungs to be pressurized. This system is safe above 40,000 feet MSL, and some systems include the regulator in the mask to eliminate purging a long hose of air.

For various reasons aviators oxygen is preferred for aviation use, and is specified to be 99.5% pure, with not more than .005mg of water per liter. It is generally recommended to not use industrial or medical oxygen as it is believed to not be as safe for this application. Medical oxygen has too much water which can freeze and block the flow of oxygen, and industrial oxygen can have impurities which may be unsafe.

Oxygen is usually stored in bottles at 1,800 to 2,200 PSI, and when the surrounding ambient temperature decreases pressure within the bottle decreases (and the opposite happens when ambient temperature increases, of course). Therefore a drop in pressure does not necessarily indicate a loss of oxygen if ambient temperatures have gone down.

Bottles should be clearly marked as to their PSI tolerance prior to filling. The filling operation should be performed in accordance with established practices as pure oxygen energetically supports combustion. Items which are normally fire proof may become susceptible to burning in the presence of a pure oxygen atmosphere, and oils/greases may spontaneously catch fire in the presence of pure oxygen. Before flight inspect all oxygen equipment. And to insure continued safe operations periodic inspections should be performed.

Rapid decompression is the inability of the pressurization system to maintain its designed pressure differential. This can be caused by a malfunction in the system or due to structural damage to the aircraft. Explosive decompression is a change in cabin pressure faster than the lungs can decompress (<0.5 seconds), whereas mere rapid decompression is where the lungs can keep up, thus reducing the chance of tissue damage.

During explosive decompression there may be noise and one may feel dazed for a second. During most decompressions the cabin will fill with fog, dust, and flying debris. The fog is the result of a rapid change in temperature and relative humidity. During the event air will rush from the mouth and nose, and the differential pressure on either side of the eardrum should clear automatically. Wind blast and exposure to cold temperatures may also occur.

The primary danger from any decompression is hypoxia. If proper use of oxygen equipment or a return to a lower altitude is not accomplished in time unconsciousness may occur. Recovery from all types of decompression entail the use of supplemental oxygen and a return to a lower flight altitude. Be aware that the time to make a recovery before loss of useful consciousness can be much shorter with an explosive decompression.