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MLeake · July 17, 2011, 11:25 AM

Ok, more on pressurization, since we are having some, er, interesting numbers bandied about.

Per FARs, any flight above 12,500' must have supplemental oxygen, because that's the nominal pressure altitude where the partial pressure of oxygen is low enough to start causing hypoxia in the average person. So, no, passenger aircraft are NOT pressurized to 12,000 - 15,000'. If they were, infants and little old ladies would start turning blue (literally, it's called cyanosis, and is usually first noticed in the lips and the fingernail beds).

Airliners usually have a cabin altitude caution light and horn that will come on in the flight station, if cabin altitude reaches 10,000' plus or minus 500'. (So, if the cabin altitude reaches 9,500 in most cases, we get an alert, because this is not normal.)

If cabin altitude does go above 10,000', and cannot be brought back down; and if that condition is going to last for a while (EG, can't descend due to mountainous terrain, or due to being way out over the ocean and there won't be enough fuel if we descend too soon) then supplemental oxygen masks get dropped.

We normally pressurize aircraft depending on several factors. Initially, we set an altitude at least 500' above takeoff field elevation, because we don't want the aircraft pressurized on deck. (Weight on wheels switches provide backup, but we like redundancy.) Reason for this: If we need to do an emergency evacuation, and the aircraft is pressurized, we can't get out. One plane I used to fly had a warning: at just 1" PSI differential, there was 1500lbs of force holding the main cabin door closed.

In the climb, we will set cabin altitude based on two factors: 1) An altitude that will keep PSI differential within limits (on the plane I currently fly, that's 6.6PSID), or 500' above destination elevation, whichever is higher.

With my current aircraft, at 25,000', we can maintain 5,000 cabin altitude while staying within 6.6 PSID. But if I were flying into an airport in the Rockies, at 8,000' elevation, I'd program 8,500' cabin altitude.

July 17, 2011, 11:25 AM	#49
MLeake Senior Member Join Date: November 15, 2007 Location: Outside KC, MO Posts: 10,128	Ok, more on pressurization, since we are having some, er, interesting numbers bandied about. Per FARs, any flight above 12,500' must have supplemental oxygen, because that's the nominal pressure altitude where the partial pressure of oxygen is low enough to start causing hypoxia in the average person. So, no, passenger aircraft are NOT pressurized to 12,000 - 15,000'. If they were, infants and little old ladies would start turning blue (literally, it's called cyanosis, and is usually first noticed in the lips and the fingernail beds). Airliners usually have a cabin altitude caution light and horn that will come on in the flight station, if cabin altitude reaches 10,000' plus or minus 500'. (So, if the cabin altitude reaches 9,500 in most cases, we get an alert, because this is not normal.) If cabin altitude does go above 10,000', and cannot be brought back down; and if that condition is going to last for a while (EG, can't descend due to mountainous terrain, or due to being way out over the ocean and there won't be enough fuel if we descend too soon) then supplemental oxygen masks get dropped. We normally pressurize aircraft depending on several factors. Initially, we set an altitude at least 500' above takeoff field elevation, because we don't want the aircraft pressurized on deck. (Weight on wheels switches provide backup, but we like redundancy.) Reason for this: If we need to do an emergency evacuation, and the aircraft is pressurized, we can't get out. One plane I used to fly had a warning: at just 1" PSI differential, there was 1500lbs of force holding the main cabin door closed. In the climb, we will set cabin altitude based on two factors: 1) An altitude that will keep PSI differential within limits (on the plane I currently fly, that's 6.6PSID), or 500' above destination elevation, whichever is higher. With my current aircraft, at 25,000', we can maintain 5,000 cabin altitude while staying within 6.6 PSID. But if I were flying into an airport in the Rockies, at 8,000' elevation, I'd program 8,500' cabin altitude.