If an aircraft loses all its engines, does the oxygen inside the cabin escape?
If so, after the air pressure is gone, can the doors be opened at a lower altitude to let oxygen in?
Tan Jen Xen
Hi Jen Xen,
Before answering your question, I think I should keep you highlighted about the composition of oxygen in the air.
Many people share your belief that oxygen reduces as the plane increases in altitude. That is a fallacy. The percentage of oxygen at higher altitude and at sea level is about the SAME at around 21 per cent.
What is different is that air pressure decreases as altitude increases. At sea level the air pressure is 14.7 pounds per square inch. This makes it easy for the air that we breathe in to pass through the lung membrane into our blood.
For example, the oxygen at 10,000 feet is still the same at 21 per cent but the air pressure is 30 per cent lower. This is because at this altitude, the air is less dense and the air molecule is farther apart and hence the oxygen absorption would be reduced.
Because the air molecules are more dispersed at higher altitude, each breath that we make delivers less oxygen to the body. When you take a breath at 10,000 feet you're breathing in 30 percent less oxygen than at sea level.
Hence air pressurization is very important in a plane. It helps to maintain the same amount of pressure at higher altitude as that of the lower level.
When a plane loses all the engines, the pressurization is lost. Pilots must immediately descend to between 8000 to 10,000 feet where a human can stay alive without oxygen.
If you understand this, I will now answer your question.
If an aircraft loses all the engines, the oxygen will not ‘escape’ as the percentage is still the same. What will change will be the loss of pressurization. Depending on the level when the engines are lost, air pressure gradually reduces as altitude increases.
At 40.000 feet, air will be very sparse; the partial pressure (lack of oxygen available) will be extremely low. As such oxygen will have great difficulty of being absorbed into the blood stream and a person can lose consciousness between 15-20 seconds at this height.
When a plane is fully depressurized, say at 10,000, it is possible to open an aircraft door (normally not possible due to the airflow) and the pilots can also slide open their cockpit windows.
Anyway, this no longer necessary or as you said, ‘to let the oxygen in’. The percentage of oxygen is the same - outside or inside the plane!
The pilots or passengers survive because at lower attitude, it has the effect of regaining the pressure necessary for the oxygen to be ‘squeezed back’ into our blood stream!
Hopefully all the engines could be restarted (if due to volcanic activity - see BA 009 ) at lower altitude too!
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