Page 28 - Dec18T

powered, engine failure can also cause a partial loss of pressurization in a twin (and a complete loss in a pressurized sin- gle) because the remaining engine simply does not put out enough air through the turbocharger to keep the cabin fully in- flated. But the failure of piston engines is quite rare especially at altitude.
So, there is a tendency to think that if the airplane is pressurized, then it must be safe for any pilot to personally fly
When a pressurized piston or turboprop aircraft operates in the high 20 ight levels,
the pilot’s body is only being supplied with
half of the oxygen available at sea level. This triggers the “Bohr Effect” (illustrated here), further decreasing
the amount of oxygen available to the brain and heart.
it for hours at a time near its maximal approved altitude. Though that may generally be true for the airframe and engines, it is a dangerous and false as- sumption for many pilots. This is because there is a series of physiologic events that accelerate and compound each other, making the risk of failure on the part of the pilot quite high relative to that of the aircraft. It is imperative both piston and turbine pilots know and understand the causes for this assumption.
In the mid-to-high 20 flight levels, many pressurized pistons, and even some turboprop aircraft, have cabin al- titudes at or above 10,000 feet. If the air- craft is piston-powered, the engines will have nearly sea level air pressure being supplied to them by the turbochargers and not even “know” they are up that high. The cabin altitude, however, is way up there at about 12,000 feet, which is often as high as the pilot has ever been in his entire life. And depending upon his age and underlying health status, this is already a potentially dangerous thing to do, even if sitting there. (By comparison, the pilots and passengers in a B787 at FL410 are sitting in a cabin at no more than 6,000 feet, and should that cabin altitude climb to 12,000 feet, it is considered an emergency).
The physiologic problem that needs to be understood is that of lowered blood oxygen levels – something dangerous for all pilots but more so as age and other health problems accumulate. Although the amount of oxygen in the air is a con- stant 21.9 percent of volume regardless of altitude, as one goes up, the air pressure itself decreases causing the available or partial pressure oxygen also to decrease. At 12,000 feet, the useful available oxygen is about half of that at sea level (13 per- cent vs. 21 percent), and this shortage of oxygen triggers a number of physiologic events which gradually accelerate and compound each other, particularly in pilots with a little grey hair and other medical issues.
Physiologically, the first thing that happens is the level of oxygen satura- tion in the blood goes down, a condition known as “hypoxia.” A medical term in which “hypo-” means “low” and “-oxia” means oxygen. The body attempts to compensate for hypoxia by increasing its respiratory rate, which helps im- prove the oxygen deficiency a bit, but also causes a lowering of carbon dioxide (CO2) in the blood, a condition known as “hypocapnia.” As hypocapnia develops, the hemoglobin molecules in each red blood cell which pick up oxygen as they pass through the alveoli of the lungs, paradoxically start to get very reluctant to release that oxygen when they arrive at areas of the body requiring it. This is called the “Bohr” effect, named after
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