While some specialized aircraft can and do fly above 40,000 feet, commercial airliners generally avoid exceeding this altitude for a combination of physiological and engineering reasons. One key factor is the decreasing air pressure and oxygen availability as altitude increases.
Physiological Limitations
Oxygen Requirements
A critical reason planes generally don't fly significantly above 40,000 feet relates to the physiological limits of pilots and passengers.
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Pilot Performance: Even when breathing pure oxygen, there is a limit to how much oxygen a pilot can effectively absorb at high altitudes.
- At 40,000 feet, the air pressure is approximately 5 inHg (inches of mercury). This means that even breathing 100% oxygen at this altitude, a pilot is only receiving about 4 to 5 inHg of oxygen. According to the reference, this is considered the limit for acceptable pilot performance. Beyond this, cognitive function and alertness can be significantly impaired.
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Passenger Safety: While aircraft are pressurized, a sudden loss of cabin pressure at extreme altitudes poses a severe risk. Even with oxygen masks, the time available to descend to a survivable altitude is limited, especially for passengers with pre-existing health conditions.
Other Considerations
While the pilot oxygen limit at 40,000 feet is a primary factor, other reasons also contribute to the decision to generally cap commercial flight altitudes:
- Engine Efficiency: Jet engines have optimal performance ranges. While higher altitudes offer less air resistance, the decreased air density can also reduce engine efficiency beyond a certain point.
- Aircraft Design: The design of commercial aircraft is optimized for specific altitude ranges. Flying significantly higher might require structural changes to withstand the different aerodynamic forces and pressures.
- Emergency Descent: In the event of a pressurization failure, aircraft need to descend quickly to an altitude where passengers can breathe safely without supplemental oxygen. Extremely high altitudes increase the risk during this critical descent phase.
In summary, while 40,000 feet is not an absolute ceiling, it represents a practical balance between fuel efficiency, passenger safety, and pilot performance capabilities. This balance keeps the air travel as safe and efficient as possible within the technological limitations.
