Catastrophic Decompression; Small Place Solo

Mike Rapoport wrote:
"John Gilmer" wrote in message
...

"Mike Rapoport" wrote in message
rthlink.net...

"Ralph Nesbitt" wrote in message
odigy.com...


At altitude A/C cabins are like a pressure vessel. A/C cabins are
pressurized to maintain ~ 12 PSI Gauge, ~ the same as normal

atmospheric

pressure @ 11,000' ASL, in the cabin irrespective of altitude above

11,000'

ASL.
Ralph Nesbitt
Professional FD/CFR/ARFF Type

No, the cabin is not at a constant preasure above 11,000' (and the

number

that you are looking for is 8,000') unless the rate controller is set to

a

rate greater than the climb rate of the aircraft (which is never done).

If

what you say were true then the cabin altitude would not climb with the
airplane above 11,000' (8,000') which it clearly does. If you doubt

this,

go buy an altimeter watch and see for yourself. Or you could buy a
pressurized airplane.

OK, I'll bite.

Just how does the psia in the cabin track the psia "ambient."


What are you asking?

Mike

The comment was made that the pressure inside the cabin is not
necessarily constant above a certain altitude, unless certain
cabin rate settings were made, possibly leading to confusion
over cabin altitude vs. actual aircraft altitude.

It is a simplification to say that cabin altitude
is constant above a given aircraft altitude. In fact, the
cabin altitude will move toward a certain altitude (8,000'
in most cases, it seems) as set by the cabin pressurization
controller (or any of its several other names), IF the
outside pressure altitude is higher than the cabin altitude.
That is to say that if you take off from sea level and climb
to 10,000', the cabin altitude will steadily climb, at a rate
set by the cabin pressure controller, toward 8,000'. If you
were to level off at 5,000' and stay there long enough for
the cabin pressure to catch up, it will maintain 5,000'
until the aircraft begins to climb again, at which it will
again climb at the set rate until it reaches either the set
cabin altitude or the actual ambient pressure altitude. So
the cabin pressure can be below 8,000' (or whatever the
set altitude is) at actual altitudes above the set altitude,
but will be moving toward the set altitude in that case.

Note that cabin altitude cannot be maintained ABOVE the
actual pressure altitude, as air is continually pumped into
the cabin by the bleed air system, and is let out (or leaks
out) to maintain the set altitude or rate of change. If
you descend quickly enough, you may temporarily get a
cabin altitude above ambient, but air will not exit the
aircraft (outside pressure would prevent any air from
leaving through the outflow valve, and in some cases a
safety valve may automatically open to allow air to flow
INTO the aircraft, relieving the negative pressure
differential) and air flowing into the cabin will tend
to increase pressure rapidly until equilibrium is reached.

Having attained the pressure set by the controller,
the system will typically attempt to maintain that
pressure altitude until it is reset to another
altitude, although it may not always be able to.
Descents with low engine power may result in not
airflow into the cabin to match the outflow, due
either to the controller not closing the outflow
valve fast or far enough, or due to leaks in the
airframe letting more air out than the system
can pump in. Also, the cabin controller will
typically not allow a pressure differential above
a certain limit- too much pressure differential
stresses the fuselage. The controller will open
the outflow valve to maintain that maximum
differential, so at that point, if the aircraft
climbs then the cabin altitude will increase as
well.

Prior to or during the descent, the cabin
pressure controller would normally be set to
the field pressure altitude (which is normally
below the cabin altitude during cruise, but
this may not always be the case). The
controller will then begin to decrease the
cabin altitude, again at the set rate. This
prevents rapid pressure changes as the
aircraft descends through the cruise cabin
altitude and is more easier on the passengers
and crew.

Long and convoluted enough for you?

Mike
EC-130H Compass Call
"In Jam, no one can hear you scream"

"Ron Natalie" wrote in message om...
"C J Campbell" wrote in message ...

We do not really know any such thing. If the passenger is bigger than the
window he simply cannot be pulled through it any more than he can sucked up
through a vacuum cleaner hose.

Passengers are pretty flexible. Some might plug up the hole, others will
go through holes that looked like they ought not to fit through.

a midget will

(Aviation) wrote in message du...
Thank you, everyone, for providing lots of helpful answers and info.

Even with slightly inaccurate info (2-seater; 6-seater), I guess
the second Q was easy:
Is the simplified movie solo flight completely
bogus or could it happen that way?

The basic answer is "more or less".

The answers to the first Q, being more tecnically compicated, leave
me asking for a few clarifications. That was about what REALLY happens
when a large pressurized aircraft, e.g., a 747, explosively or
catastrophically decompresses at high (25000+ feet) altitude.

Ron Natalie ) wrote:
On the one hand, passengers need to get denser air to breathe
but large aircraft have oxygen masks that drop down.

The pilots initiate the dive for the reasons you suggest. While
supplemental oxygen helps, it's still better to get down to a reasonable altitude. They presmably notify ATC while they are doing
this that there is an emergency in progress.

So, other than alarms or other signals, there is NOTHING AUTOMATIC
that puts the aircraft into rapid descent. Pilots have to respond
to the signals (explosion, screams from the cabin, meters in their
cockpit, their own ears popping, flashing lights, bells and whistles,
etc.) and initiate the dive MANUALLY. For well trained pilots this
would take, what, only a few seconds at most? Do they put their oxygen
masks on FIRST or start the dive first?

In some movies, the pilots are often unconscious, slumped over the
controls (wedging the 'steering wheel' / joystick full forward) as
the hero struggles to pull them out of the way and get out of the
dive. It sounds like a pilot, if not alert or in good health, could
actually lose consciousness from hypoxia under these conditions, so
maybe those kinds of movie scenes are ALMOST believable?

(This assumes worst case total decompression. It was pointed out
by Mike Rapoport ) that the cabin might
not even go to ambient pressure if the hole isn't too big and the
outflow valves close down and the engines keep pumping air into the
cabin.)

Someone also pointed out my goof about "holding" your breath
upon going from cabin (8000 ft pressure) to ambient (25-35,000
ft pressure). In estimating how much time the average civilian
passenger could go without TAKING a breath of good air (14,000
ft or below), I used the HOLD your breath estimate. Assuming
there is 3-5 minutes of mask-oxygen and one minute of "holding"
the last breath, they've got 3-6 minutes to get down to breathable
(14,000 ft?) air and then below. For the movie Executive Decision,
they were cruising at 39,000 ft. so they'd have to dive 25,000 ft
to 14,000 ft in 5 minutes, 5,000 ft/minute, average. Doable?

nope a person cannot simply hold ones breath. the pressure
differential between the airway cavity and the outside of the body
makes this impossible. in addition, a body of air expands and
contracts under pressure.

if anyone here is a certified diver they will be aware of compression
of air in the human body at depth (and the effects on ascent and
descent), and conversely the expansion of air at altitide.

"John Gilmer" wrote in message ...
"Bob Gardner" wrote in message
news:QODIb.85072$VB2.187788@attbi_s51...
No way. You start off at sea level, of course, and they pump the chamber
pressure down to 25000 feet with masks on...then they do some experiments
with a few of the pilots taking their masks off, just to show how severely
their abilities are affected. Then comes the explosive decompression,
which
takes the chamber from 25K back down to sea level in a second or two.

SIlly question but ...

Why would "they" pressurize the aircraft to sea level? Why not some
compromise "pressure" like that equivalent to, say, 6,000 or 8,000 feet?


EMWTK

"they" dont. cabin pressurization is normally at 8000ft though crew
may elect to use different cabin pressures under certain circumstances
(eg medevac operations where pressurization is a factor)

Michael Williamson wrote in message ...
John Gilmer wrote:



SIlly question but ...

Why would "they" pressurize the aircraft to sea level? Why not some
compromise "pressure" like that equivalent to, say, 6,000 or 8,000 feet?


Typically, they start at whatever the local pressure altitude
happens to be (which of course may or may not be sea level, depending
on where you happen to have the chamber). Also, I've never
experienced in any altitude chamber ride a rapid REcompression
down to sea level- it strikes me as a good way to have sinus
or ear problems.

Mike


nope. when the doors on an aircraft are shut the pressure is ambient.
thats it. when bleed air pressures the cabin its noramlly to about
8000ft.

Michael Williamson wrote in message
...

Note that cabin altitude cannot be maintained ABOVE the
actual pressure altitude, as air is continually pumped into
the cabin by the bleed air system, and is let out (or leaks
out) to maintain the set altitude or rate of change. If
you descend quickly enough, you may temporarily get a
cabin altitude above ambient, but air will not exit the
aircraft (outside pressure would prevent any air from
leaving through the outflow valve, and in some cases a
safety valve may automatically open to allow air to flow
INTO the aircraft, relieving the negative pressure
differential) and air flowing into the cabin will tend
to increase pressure rapidly until equilibrium is reached.

http://www.b737.org.uk/pressurisation.htm has some good information.

Paul Nixon

"running with scissors" wrote in
message om...

"they" dont. cabin pressurization is normally at 8000ft though crew
may elect to use different cabin pressures under certain circumstances

Actually, it depends on the aircraft. Aircraft are certified to certain
"differential pressures" which is the difference between in the inside cabin
pressure and the outside ambient air pressure. An airframe must be designed
to handle the differential pressure required to maintain acceptable cabin
altitude up to the max flight level the aircraft is certified to.

As an example, the King Air I fly has a normal differential pressure of 6.6
psid. This provides for a cabin altitude of 2,700 up to 20,000 cruise
altitude, an 8,700 foot cabin altitude at 31,000 cruise altitude, and a
10,200 foot cabin altitude at 35,000 cruise altitude.

In contrast, the Lear 60 I fly has a normal psid of 9.5 (9.7 max) which
typically results in a cabin altitudes of around aprox 6,500 up in the
40,000 ish cruise altitudes.

On Tue, 30 Dec 2003 22:12:24 GMT, "Bob Gardner"
wrote:

I have no direct knowledge of how big an airliner
outflow valve is, but I am going to guess three inches in diameter

That would not be a good guess! The main outflow valve on a 737 is
much larger than that.

-J


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running with scissors wrote:
Michael Williamson wrote in message ...

John Gilmer wrote:



SIlly question but ...

Why would "they" pressurize the aircraft to sea level? Why not some
compromise "pressure" like that equivalent to, say, 6,000 or 8,000 feet?


Typically, they start at whatever the local pressure altitude
happens to be (which of course may or may not be sea level, depending
on where you happen to have the chamber). Also, I've never
experienced in any altitude chamber ride a rapid REcompression
down to sea level- it strikes me as a good way to have sinus
or ear problems.

Mike



nope. when the doors on an aircraft are shut the pressure is ambient.
thats it. when bleed air pressures the cabin its noramlly to about
8000ft.

While the final (steady state) pressure of the aircraft
(as determined by the cabin pressurization controller) will end
up at its cruise setting, the aircraft is pressurized from
takeoff and the cabin altitude will typically climb slowly
toward its set value (about 8,000' in this case) rather than
follow ambient- the pressurization system is indeed pressurizing
the aircraft its initial field elevation up to the cruise
pressurization setting.

In addition, my above response was poorly worded and I actually
was commenting on the pressurization of the altitude chamber- which
begins at whatever the ambient pressure happens to be. My bad for
not reading the question closely enough.

Mike

"Gary Mishler" wrote in message news:eBoKb.745162$HS4.5776347@attbi_s01...
"running with scissors" wrote in
message om...

"they" dont. cabin pressurization is normally at 8000ft though crew
may elect to use different cabin pressures under certain circumstances

Actually, it depends on the aircraft. Aircraft are certified to certain
"differential pressures" which is the difference between in the inside cabin
pressure and the outside ambient air pressure.

We were talking about transport aircraft. US certificated transport aircraft
must keep the cabin altitude below 8000'.