Pressure loss in airplane

[BigJetDriver]

Actually, guys,

Sudden decompression of an aircraft is kind of like a bounce dive in reverse. You will be at lower pressure for only a short period, and then you will be returned to near "normal" ambient pressure.

As you may remember from my notes on "Aircraft Pressurization 101", the modern jet aircraft will have been running with an internal pressure equivalent to a range from 6000 to 7,900 ft msl. After such an incident, we will return to 10,000 ft msl or below that if possible.

For the ordinary passenger, this does not seem to cause all that much trouble in terms of classical DCS, at least in the limited amount of information we have on such occurrrences. They are, after all, fairly rare.

For a diver with a significant inert gas load in his or her system, this is indeed going to be very traumatic. Someone who, for instance, follows the "Flying With Your Hair Wet" rule is going to be bent and twisted like a pretzel!:wacko:

 

[pipedope]

I do see that the pilots short prayer is used;
"S**T"

Interesting to note that this same prayer is used nearly everywere and in many different languages.

 

[Diver0001]

BigJetDriver69 once bubbled...
Actually, guys,

Sudden decompression of an aircraft is kind of like a bounce dive in reverse. You will be at lower pressure for only a short period, and then you will be returned to near "normal" ambient pressure.

As you may remember from my notes on "Aircraft Pressurization 101", the modern jet aircraft will have been running with an internal pressure equivalent to a range from 6000 to 7,900 ft msl. After such an incident, we will return to 10,000 ft msl or below that if possible.

For the ordinary passenger, this does not seem to cause all that much trouble in terms of classical DCS, at least in the limited amount of information we have on such occurrrences. They are, after all, fairly rare.

For a diver with a significant inert gas load in his or her system, this is indeed going to be very traumatic. Someone who, for instance, follows the "Flying With Your Hair Wet" rule is going to be bent and twisted like a pretzel!:wacko:

If I remember my decotheory-101 you can make a 1.6ata ascent after any amount of time. Since the atmosphere is 1ata (funny how that worked out) and since a jet-liner isn't in orbit then even the total decompression of the aircraft isn't going to cause the bends in someone who has a normal level of tissue saturation..... Or am I missing something?

R..

 

[Charlie99]

Diver0001 once bubbled...
If I remember my decotheory-101 you can make a 1.6ata ascent after any amount of time. Since the atmosphere is 1ata (funny how that worked out) and since a jet-liner isn't in orbit then even the total decompression of the aircraft isn't going to cause the bends in someone who has a normal level of tissue saturation..... Or am I missing something?

R..

Yes. What you are missing is that decotheory 101 does NOT say that you can make a 1.6ata ascent after any amount of time. What it says is that you can make a 1.6 to 1 PRESSURE RATIO of ascent. If you are headed to a 1ata surface, then you can come up from 1.6ata (or if you wish to be a bit more agressive, from 2ata per Haldane). With a sea level starting point, a 2 to 1 ratio takes you to 18,000 feet. DCS is a probabalistic event. For a 2:1 pressure change (33' fsw to sea level or sea level to 18,000' altitiude) you have significantly increased your risk of DCS, but more likely than not you will not have signficant problems.

DCS was a problem noted even with WWII bombers.
I don't have the references handy, but IIRC DCS was ocassionally noted from prolonged exposure in the mid 20,000ft range unless they prebreathed O2 for a while to clean out their system. As BigJetDriver has noted, though, in a cabin pressure loss event, the exposure will be very short, thereby dramatically reducing the probability of DCS.

 

[Diver0001]

Charlie99 once bubbled...
Yes. What you are missing is that decotheory 101 does NOT say that you can make a 1.6ata ascent after any amount of time. What it says is that you can make a 1.6 to 1 PRESSURE RATIO of ascent. If you are headed to a 1ata surface, then you can come up from 1.6ata (or if you wish to be a bit more agressive, from 2ata per Haldane). With a sea level starting point, a 2 to 1 ratio takes you to 18,000 feet. DCS is a probabalistic event. For a 2:1 pressure change (33' fsw to sea level or sea level to 18,000' altitiude) you have significantly increased your risk of DCS, but more likely than not you will not have signficant problems.

DCS was a problem noted even with WWII bombers.
I don't have the references handy, but IIRC DCS was ocassionally noted from prolonged exposure in the mid 20,000ft range unless they prebreathed O2 for a while to clean out their system. As BigJetDriver has noted, though, in a cabin pressure loss event, the exposure will be very short, thereby dramatically reducing the probability of DCS.

Right (slaps forehead). Now I have it. Thanks.

R..

 

[String]

According to standard tables your average jet airliner is pressurised to 8,000ft or approx 360 hPa (0.36 bar). In other words just over 1/3 atmospheric pressure.

30,000ft is much lower, ive seen quoted around 12hPa only.

The point im failing to make is that the depressurisation from 8,000ft to 30,000ft is less than the change in going from 20ft to the surface pressure wise. As this doesnt happen instantly and is usually very short lived the effects are nowhere near as bad as they might initially seem.

 

[BigJetDriver]

(1) The upper limit for pressurzation for modern jet aircraft is 8000 feet msl. The range is generally 6000 to 8000 feet.

(2) The current generation of jets is most comfortable in the altitude range of 32,000 feet msl to 37,000 feet msl, and up to 41,000 for the big units (the B777 ceiling is 43,100).

(3) The altitude the crew will descend to depends upon the terrain, the traffic, and the distance to go to a safe landing site, but will generally be 10,000 feet msl, or lower if possible.

Just some food for thought!=-)

 

[Charlie99]

String once bubbled...
According to standard tables your average jet airliner is pressurised to 8,000ft or approx 360 hPa (0.36 bar). In other words just over 1/3 atmospheric pressure.

30,000ft is much lower, ive seen quoted around 12hPa only.

The point im failing to make is that the depressurisation from 8,000ft to 30,000ft is less than the change in going from 20ft to the surface pressure wise.

Both your numbers and your understanding of the concepts are wrong. Are you sure the table you looked at isn't in meters??

Pressure at 8,000' is more like 3/4 bar, not 1/3. Pressure at 30,000' is far more than the 12hPa you mention. My guess is that you are looking at a table that has heights in meters ---- at 8,000 meters the pressure is a bit more than 0.36bar at 300K.

As to concepts, please read my earlier post in reply to Diver0001. To the first approximation,what counts is pressure RATIO, not the absolute change.

If your logic on pressure changes was correct, then you would get bent going from saturation at 132 fsw depth(5 ata) up to 99 fsw depth (4 ata).

Charlie

p.s. If you don't have an altitude table handy, a pretty accurate equation for pressure at room temp is P = 1ata * EXP(-0.038*altitude in feet/1000)

 

[Charlie99]

This article was written by the FAA office of aviation medicine.

Crude summary: Altitude DCS rare below 18,000 feet except after scuba diving. Low ocurrence rate between 18,000 and 25,000 ft --- an Air Force study reported that only 13% of cases occurred below 25,000' (Of course, the flip side of this statement is that 13% of the cases DID occur below 25,000').

It has a nice table of DCS signs and symptoms.

 

[String]

http://www.usatoday.com/weather/wstdatmo.htm

was the source for the altitude pressure tables.

I found a few on the internet that agreed with those values although was a little confused as 12hPa even on 100% O2 shouldnt sustain life and people have gone much higher.