Cabin Altitude
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Cabin Altitude
Question on cabin altitude for those who fly bigger aircraft than I (ie most of you!).
I've read that heavy metal typically flies with a cabin altitude in the region of 7000 to 8000ft when at cruising altitude.
We all know that aircraft normal and emergency exits can't be opened at this altitude due to the pressure differential.
Question is "is cabin altitude kept at ambient until passing this approx 7000 ft threshold or is it pressurized slightly above ambient the whole time so that a panicked pax couldn't remove an emergency door at - say - 3000 feet"?
Many thanks.
I've read that heavy metal typically flies with a cabin altitude in the region of 7000 to 8000ft when at cruising altitude.
We all know that aircraft normal and emergency exits can't be opened at this altitude due to the pressure differential.
Question is "is cabin altitude kept at ambient until passing this approx 7000 ft threshold or is it pressurized slightly above ambient the whole time so that a panicked pax couldn't remove an emergency door at - say - 3000 feet"?
Many thanks.
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Cabin altitude
Normally pressurised to about 250 ft below field altitude during taxi to prevent pressure "bumps" on T/O then pressurised further at a pre- determined rate based on selected cruise altitude.
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Many thanks.
So in other words cabin pressure is always - to some degree - above ambient at all times?
Just wondering if that would be sufficient to prevent the removal of an emergency exit until the differential increased?
So in other words cabin pressure is always - to some degree - above ambient at all times?
Just wondering if that would be sufficient to prevent the removal of an emergency exit until the differential increased?
Different types do it differently and they do it at different rates, I.e. Cabin rises at 2-300 fpm from takeoff whilst aircraft is climbing at 2-3000 fpm, but the principle is that there is some pressurisation all the time so I doubt that anyone could open a door
Many emergency exits are plug type exits as well so the pressurisation effectively holds them in place even if the latches are released, so again it would be a physical challenge to get an emergency exit open.
So all the terrified, my life flashed before my eyes, reports from punters when someone pissed or on drugs leaps up and starts fiddling with the door handle are a bit on the melodramatic side, though in fairness they aren't to know.
Many emergency exits are plug type exits as well so the pressurisation effectively holds them in place even if the latches are released, so again it would be a physical challenge to get an emergency exit open.
So all the terrified, my life flashed before my eyes, reports from punters when someone pissed or on drugs leaps up and starts fiddling with the door handle are a bit on the melodramatic side, though in fairness they aren't to know.
BugSmasher1960,
Indeed as the Snakecharma said, you haven't a hope in hell of moving a plug type door, be it an overwing escape door or a main entry door due to pressure differential.
On the 737NG's the overwing exit doors open outwards and I'm led to believe that these doors have flight locks installed to prevent "accidental" opening. The requirements to activate these flight locks are..... 3 of 4 entry doors closed AND 1 engine running (or simulated) AND both thrust levers above 53 degrees OR flight mode.
McHale.
Indeed as the Snakecharma said, you haven't a hope in hell of moving a plug type door, be it an overwing escape door or a main entry door due to pressure differential.
On the 737NG's the overwing exit doors open outwards and I'm led to believe that these doors have flight locks installed to prevent "accidental" opening. The requirements to activate these flight locks are..... 3 of 4 entry doors closed AND 1 engine running (or simulated) AND both thrust levers above 53 degrees OR flight mode.
McHale.
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Thanks folks,
@McHale - thanks. Yep - fully understand the physics @ cruising altitude - just wondered if cabin stayed at ambient until passing some kind of threshold (thus wondering if the "can't open in flight due to pressure differential" that's always mentioned didn't really kick in until approx 7000 -> 8000ft).
So by all accounts - in summary - "already been thought of and - although the differential is lower until passing 7 to 8 thousand - it's still high enough to prevent any door being opened"?
@McHale - thanks. Yep - fully understand the physics @ cruising altitude - just wondered if cabin stayed at ambient until passing some kind of threshold (thus wondering if the "can't open in flight due to pressure differential" that's always mentioned didn't really kick in until approx 7000 -> 8000ft).
So by all accounts - in summary - "already been thought of and - although the differential is lower until passing 7 to 8 thousand - it's still high enough to prevent any door being opened"?
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Cabin Altitude
As pointed out by previous posters a very difficult proposition. Consider this: A B737-400 Type III o/wing emergency exit which is a plug type, has an area of roughly 0.5 sq M. or 775 sq in. At a 2psi differential (less than 1/4 of max diff on the -400) the force on the exit is over 1500 lbs or 680 kg. The Pax entry door has an area of over 1.5 sq M and even with the mechanical advantage of the handle and mechanism requires around 20kg to "unlock" when unpressurised. Ever watched the struggle some of the cabin crew have opening a "classic '37" pax door? ps. I think I have the maths right. Cheers,30/30
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Correct for the 737NG
and they're regularly tested in maint visits.
From what I understand the locks prime purpose is, prior to cabin pressure forces, ie at "rotation" the locks were to ensure emerg door were not able to open out into airflow and thus not blanket elevators
anyone else heard of this?
bigslim
and they're regularly tested in maint visits.
From what I understand the locks prime purpose is, prior to cabin pressure forces, ie at "rotation" the locks were to ensure emerg door were not able to open out into airflow and thus not blanket elevators
anyone else heard of this?
bigslim
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Just been doing some maths of my own;
If the differential was only 100 feet (is that likely at any point after takeoff?) then (if memory serves me correctly) that would be around 3.3 millibar (30 feet per millibar) (0.0479 PSI) (wolframalpha) times 775 cubic inches (wolframalpha) then that's "only" 37 pounds (or 17kg) -> do-able.
So from that I'd deduce that you'd almost want something in the region of a 300 foot differential before the numbers climbed into the "superman strength required" zone.
Not a biggie - just curious.
If the differential was only 100 feet (is that likely at any point after takeoff?) then (if memory serves me correctly) that would be around 3.3 millibar (30 feet per millibar) (0.0479 PSI) (wolframalpha) times 775 cubic inches (wolframalpha) then that's "only" 37 pounds (or 17kg) -> do-able.
So from that I'd deduce that you'd almost want something in the region of a 300 foot differential before the numbers climbed into the "superman strength required" zone.
Not a biggie - just curious.
