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Cabin Pressurisation Question  
User currently offlineFlymad From South Africa, joined Jun 2006, 207 posts, RR: 0
Posted (6 years 4 weeks 16 hours ago) and read 3060 times:

Hi guys, first time I'm posting on Tech/Ops. I'm not in aviation, just an avid enthusiast.
On the main forum is a discussion about a F9 A320 that had problems with pressurisation in-flight and one of the comments was that pax in the rear of the a/c felt the effects more than those in the front. The question I posed there was why would this be? As I said there, I would like to be educated. Can anyone help?
Thanks

25 replies: All unread, showing first 25:
 
User currently offlineZeke From Hong Kong, joined Dec 2006, 8643 posts, RR: 75
Reply 1, posted (6 years 4 weeks 8 hours ago) and read 3019 times:



Quoting Flymad (Thread starter):
On the main forum is a discussion about a F9 A320 that had problems with pressurisation in-flight and one of the comments was that pax in the rear of the a/c felt the effects more than those in the front.

No physical reason for this, the pressure exerted on the inside of a pressure vessel is equal.



We are addicted to our thoughts. We cannot change anything if we cannot change our thinking – Santosh Kalwar
User currently offlineJetlagged From United Kingdom, joined Jan 2005, 2532 posts, RR: 24
Reply 2, posted (6 years 4 weeks 6 hours ago) and read 2994 times:

Possibly a problem with air quality rather than cabin pressure. It's possible some parts of the cabin might get more recirculated air rather than fresh air.


The glass isn't half empty, or half full, it's twice as big as it needs to be.
User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 3, posted (6 years 4 weeks 5 hours ago) and read 2984 times:

I assume that the A320, the specifics of which I am not familiar with, like most other aircraft has the valves regulating the cabin pressure at the rear of the cabin.

If so, the comment is not implausible. The air in the cabin has a mass which is not insignificant. For the air pressure in the front of the cabin to drop due to the regulating valve acting up, the air in the front of the cabin has to expand. The gas law, pV=nRT. If the air in the frontmost meter of the cabin is to expand by 20%, it has to push all the air in the rest of the cabin 20 cm aft. This means accelerating the mass of the air in the rest of the cabin. This acceleration will not by any means be instantaneous. This will slow the expansion down and with it the rate of pressure drop.

Thus, pressure spikes will be evened out somewhat. If someone cares to crunch the numbers, go ahead. I have no idea if this evening out is enough to make a significant difference on the degree of discomfort felt. The effect will be more noticeable the more abrupt the pressure changes are next to the valve.

If there's discomfort due to audible effects of the valves acting up, that discomfort will decrease rapidly with increasing distance from the source of the noise.

Cheers,
/Fred



I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlineLongbowPilot From United States of America, joined Jan 2005, 577 posts, RR: 4
Reply 4, posted (6 years 4 weeks 5 hours ago) and read 2983 times:
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I am by no means a expert on here but do know, from prior ramp experiance, that the pressure relief valves on MD-88s are in the rear of the cabin. So it would make sense there could be a pressure differential in the aircraft between the front and rear if it were not sealing properly.

I have also sat in the aircraft during a rapid de-pressurization of the aircraft. The effect was actually pretty neat, with a "fog" coming from the back to the front in a couple of seconds. I could be wrong, but that might explain the reasons.

-Attack


User currently offlineJetlagged From United Kingdom, joined Jan 2005, 2532 posts, RR: 24
Reply 5, posted (6 years 4 weeks 4 hours ago) and read 2975 times:



Quoting FredT (Reply 3):
If so, the comment is not implausible. The air in the cabin has a mass which is not insignificant. For the air pressure in the front of the cabin to drop due to the regulating valve acting up, the air in the front of the cabin has to expand. The gas law, pV=nRT. If the air in the frontmost meter of the cabin is to expand by 20%, it has to push all the air in the rest of the cabin 20 cm aft. This means accelerating the mass of the air in the rest of the cabin. This acceleration will not by any means be instantaneous. This will slow the expansion down and with it the rate of pressure drop.

Surely not. The air in the frontmost meter of the cabin (assuming this was indeed where all the air inflow is) is not constrained in any way and the increased pressure will be distributed along the cabin at the speed of sound, more or less. Also air inlets to the cabin are distributed throughout it's length, not concentrated at the front.

Because the air is flowing through the cabin towards the rear there will be a slight pressure gradient along it. But if cabin pressure is increased by 1 psi that increase will be the same along the cabin. So pax at the rear will, generally speaking, experience the same cabin pressure and pressure fluctuations as those at the front.

As I hinted in my first reply air quality at the rear may well be very much worse, and that coupled with an 8000ft cabin altitude might make things worse back there.



The glass isn't half empty, or half full, it's twice as big as it needs to be.
User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 6, posted (6 years 4 weeks ago) and read 2943 times:



Quoting Jetlagged (Reply 5):
Surely not.

Jetlagged,
surely. You cannot argue with physics. The effect is there, exactly as I described. If any pressure change would propagate through the cabin instantly (or, for that matter, at the speed of sound), you either have to explain why the gas law does not apply (i e how the pressure can drop without the air expanding) or how the inertia of all the air in the cabin can be ignored (enabling instant expansion).

The question is not if the effect is there, as described, but if it is significant enough to be the cause of the subjectively experienced differences in the OP.

Quoting Jetlagged (Reply 5):
Also air inlets to the cabin are distributed throughout it's length, not concentrated at the front.

I am aware of this but it is insignificant in the situation discussed, which clearly pertains to a valve acting up.

Quoting Jetlagged (Reply 5):
Because the air is flowing through the cabin towards the rear there will be a slight pressure gradient along it

Movement of a fluid is not linked to a pressure gradient. Acceleration in a fluid is. If the pressure is lower at the back of the cabin, the pressure difference will exert a force on the air in the cabin, which will accelerate towards the aft of the aircraft until the mass flow has equalized the pressure and equilibrium is restored. If there's a constant inflow (a source) at the front of the cabin and a constant outflow (a sink) of an equal mass of air at the rear of the cabin, equilibrium and a constant pressure throughout the cabin will only be reached if there is a flow from the front to the rear equalling the in- and outflow.

Rgds,
/Fred



I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlineEx52tech From United States of America, joined Dec 2006, 559 posts, RR: 1
Reply 7, posted (6 years 3 weeks 6 days ago) and read 2837 times:



Quoting Flymad (Thread starter):
F9 A320 that had problems with pressurisation in-flight and one of the comments was that pax in the rear of the a/c felt the effects more than those in the front.

I have been wondering if the pax. were feeling uncomfortable due to cabin temperature problems, and was it brought up as a pressurization problem, when it was actually a trim air problem.

When I worked line maint. on A320s we had some pressurization problems that revolved around dried up and brittle O rings, that are used to seal the slip joints on the pac. plumbing in the pac. bay, leaking so badly that the cabin altitude could not be maintained.

Quoting LongbowPilot (Reply 4):
I am by no means a expert on here but do know, from prior ramp experiance, that the pressure relief valves on MD-88s are in the rear of the cabin.

I don't think I have ever seen one of those open and then stay open.



"Saddest thing I ever witnessed....an airplane being scrapped"
User currently offlineTdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 8, posted (6 years 3 weeks 5 days 17 hours ago) and read 2786 times:



Quoting LongbowPilot (Reply 4):
So it would make sense there could be a pressure differential in the aircraft between the front and rear if it were not sealing properly.



Quoting FredT (Reply 6):
The effect is there, exactly as I described. If any pressure change would propagate through the cabin instantly (or, for that matter, at the speed of sound), you either have to explain why the gas law does not apply (i e how the pressure can drop without the air expanding) or how the inertia of all the air in the cabin can be ignored (enabling instant expansion).

The effect is real, but it's *tiny*. Airflow velocities inside the cabin are very low, much less than Mach 0.3, which is where compressibility starts kicking in. One of the properties of low speed flows is that you need extremely small pressure gradients to cause very large velocity gradients. Conversely, small velocity gradients (like you have in an aircraft cabin) are caused by very very very small pressure differences. I'd be pretty shocked if the fore-aft pressure differential was detectable by humans.

Keep in mind that weather systems live in the same type of speed environment...60 kph winds are caused by minuscule pressure differentials.

Tom.


User currently offlineAnalog From United States of America, joined Jul 2006, 1900 posts, RR: 1
Reply 9, posted (6 years 3 weeks 5 days 5 hours ago) and read 2737 times:



Quoting Tdscanuck (Reply 8):
I'd be pretty shocked if the fore-aft pressure differential was detectable by humans.

For steady-state I agree.

What about transient & periodic variations? Is there a potential mechanism for an oscillation in cabin pressure? If the control system (ECS) has some sort of limit-cycle, either because of the use of hysteresis, or because of some issue with the outflow valve, or something else, the pressure oscillations might be more significant in the rear (or front) of the aircraft.

Note: I'm presenting this as a question (Can this happen?), not to say that this does happen.


User currently offlineTdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 10, posted (6 years 3 weeks 4 days 22 hours ago) and read 2688 times:



Quoting Analog (Reply 9):
What about transient & periodic variations? Is there a potential mechanism for an oscillation in cabin pressure? If the control system (ECS) has some sort of limit-cycle, either because of the use of hysteresis, or because of some issue with the outflow valve, or something else, the pressure oscillations might be more significant in the rear (or front) of the aircraft.

Note: I'm presenting this as a question (Can this happen?), not to say that this does happen.

I've never heard of it myself, but I don't see any reason it couldn't happen. The pressure vessel is a nice shape to set up a standing wave if you were unlucky enough to hit the right frequency, although the resonant frequency of an air column that long would be way below the threshold of hearing...maybe enough to feel though. Our bodies can react to sound frequencies lower than ones we hear.

Tom.


User currently offlineAvioniker From United States of America, joined Dec 2001, 1109 posts, RR: 11
Reply 11, posted (6 years 3 weeks 4 days 2 hours ago) and read 2614 times:

Quoting Tdscanuck (Reply 10):
I've never heard of it myself, but I don't see any reason it couldn't happen. The pressure vessel is a nice shape to set up a standing wave if you were unlucky enough to hit the right frequency, although the resonant frequency of an air column that long would be way below the threshold of hearing...maybe enough to feel though. Our bodies can react to sound frequencies lower than ones we hear.

You are all dancing around the issue. There is a variable that must be considered i.e. the laws of Depletion and Replenishment. There is most indeed a wave effect in a cabin experiencing any fluctuation in the outflow valve and it will be most measurable or noticable at the points of termination or change of the wave. In an 80 that's at the rear.
In the case of an outflow valve suddenly opening the pressure in the rear of the aircraft must decrease before the flow can start from the front. Same rules apply.
Splash some water in a bowl or look at one of those lava wave machines and you'll see what happens.
 

[Edited 2008-03-26 12:44:32]


One may educate the ignorance from the unknowing but stupid is forever. Boswell; ca: 1533
User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 12, posted (6 years 3 weeks 4 days 2 hours ago) and read 2600 times:

Interesting discussion with some knowledgeable participants!  Smile

Quoting Tdscanuck (Reply 8):
The effect is real, but it's *tiny*.

Certainly. Detectable by the sensitive human ear, which will detect very tiny pressure variations as sound? Possibly.

Quoting Tdscanuck (Reply 8):
Airflow velocities inside the cabin are very low, much less than Mach 0.3, which is where compressibility starts kicking in.

That's true, and you are right in pointing out that it is also completely irrelevant as the velocities involved will be nowhere near those required for compressibility to be anything of a factor.

Quoting Tdscanuck (Reply 8):
Conversely, small velocity gradients (like you have in an aircraft cabin) are caused by very very very small pressure differences.

But when we have a dump valve acting up, the pressure gradients will be very significant.

I must also admit that I fail to understand which velocity gradients you are talking about. I'm concerned with the pressure gradients and resulting accelerations.

Quoting Tdscanuck (Reply 8):
I'd be pretty shocked if the fore-aft pressure differential was detectable by humans.

Oh, it most certainly would be. At the instant of the valve opening, the pressure at the valve will be that of the ambient atmosphere but the pressure in the rest of the cabin will be whatever the pressurization altitude dictated. That's a noticeable difference if ever I saw one.

However, that is again not the subject of the discussion. The facts are

1) The pressure will drop significantly in the entire cabin rather abruptly

2) This will be easily noticeable by anyone on board

The question is if an increasing distance from the valve will make the pressure drop less abrupt to such a degree that the discomfort experienced will lessen.

Quoting Tdscanuck (Reply 8):
Keep in mind that weather systems live in the same type of speed environment...60 kph winds are caused by minuscule pressure differentials.

That's a totally different setting, and I wouldn't say that the speed environment is the same either. Remember that the Coriolis forces are the result of moving air around on a rotating sphere where the rotational speed varies from 0 to almost 1700 km/h.




I see a need for visualizing what is going on better.

The cabin of an A320 is 27.5 m long with a diameter of 3.7 meters. Let us assume a cylindrical cross section of the same diameter for the sake of this discussion. Assume an atmosphere inside the fuselage equal to the ICAO standard atmosphere at 8000'. That's 752.6 hPA and 0.963 kg/m^2.

Cut the air inside the cabin one meter from the forward bulkhead and replace the air behind with a moveable piston. Let this piston have the same weight as the air behind the 1 m cutoff point. Volume times density gives us a mass of 285 kg. Anyone surprised?

Assume that the valve will instantly drop the pressure at the rear of the cabin to the equivalent of 12000' altitude in the standard atmosphere. That's 644.4 hPa and 0.849 kg/m^3.

Visualize this 285 kg piston with 1 m of 752.6 hPa in front and 644.4 hPa behind. Now you have a situation reminding of what we have in the cabin when the dump valve opens, albeit rather simplified.

For the pressure at the front of the cabin to instantly drop to 644.4 hPa, as it did at the valve, the volume ahead of the piston would have to instantly increase equivalent to moving the piston from 1 m from the bulkhead to 1.13 m.

Is it likely to have this 285 kg piston instantly accelerate to infinite velocity, jump 13 cm aft and instantly decelerate to a standstill, or will it in fact accelerate slower and act as a damper on the pressure reduction?




The initial pressure drop (i e the fact that the pressure is dropping, not the magnitude of the drop) will propagate throughout the entire cabin, starting at the valve, at the speed of sound. The rate of pressure drop will however decline with increasing distance from the valve.


Cheers,
/Fred



I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlineMD11Engineer From Germany, joined Oct 2003, 13797 posts, RR: 63
Reply 13, posted (6 years 3 weeks 3 days 23 hours ago) and read 2584 times:

Even then, while the outflow valve is indeed located at the rear end of the pressure hull, the airflow through the passenger cabin (due to cabin floor and liners) runs from the aircon outlets on top of the cabin downwards to grilles at foot level on the dado panels. From there the air is routed around the cargo hold liners and aft to the outflow valve.
If there was a measurable pressure gradient in the cabin, (beside the one causing the circulation of air from the aircon ducts to the grilles), it would be in vertical direction.

Jan

Quoting FredT (Reply 12):
Oh, it most certainly would be. At the instant of the valve opening, the pressure at the valve will be that of the ambient atmosphere but the pressure in the rest of the cabin will be whatever the pressurization altitude dictated. That's a noticeable difference if ever I saw one.

However, that is again not the subject of the discussion. The facts are

1) The pressure will drop significantly in the entire cabin rather abruptly

2) This will be easily noticeable by anyone on board

The question is if an increasing distance from the valve will make the pressure drop less abrupt to such a degree that the discomfort experienced will lessen.

I have been carrying out pressure runs on A320s after D-checks to verify the operation of the positive overpressure valves and could not feel any difference in pressure. The only indication that the valves had opened was the ECAM message.

Jan


User currently offlineTdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 14, posted (6 years 3 weeks 3 days 22 hours ago) and read 2568 times:



Quoting Avioniker (Reply 11):

You are all dancing around the issue.

snip...

Quoting Avioniker (Reply 11):
In the case of an outflow valve suddenly opening the pressure in the rear of the aircraft must decrease before the flow can start from the front. Same rules apply.

I'm not dancing around the issue...the point is that the pressure drop due to opening the valve is so small as to be meaningless.

Quoting FredT (Reply 12):
But when we have a dump valve acting up, the pressure gradients will be very significant.

I must also admit that I fail to understand which velocity gradients you are talking about. I'm concerned with the pressure gradients and resulting accelerations.

Any time you have a pressure gradient in a fluid with part of the fluid stationary, you have a velocity gradient, or else you're not at steady state. Given the typical dimensions of a fuselage you hit steady state very quickly, so if you're got a pressure gradient in the fuselage you're going to have a velocity gradient too.

Quoting FredT (Reply 12):
Oh, it most certainly would be. At the instant of the valve opening, the pressure at the valve will be that of the ambient atmosphere but the pressure in the rest of the cabin will be whatever the pressurization altitude dictated. That's a noticeable difference if ever I saw one.

At the valve, there is a huge pressure differential. But that drops off *extremely* fast as you move away from the valve. Anywhere in the cabin I would expect functionally constant pressure.

Quoting FredT (Reply 12):
1) The pressure will drop significantly in the entire cabin rather abruptly

2) This will be easily noticeable by anyone on board

No, it won't. If you crank the outflow valve full open it's going to very quickly go sonic and choke (some aircraft actually use sonic chokes in place of the outflow valve). It takes a very tiny pressure difference to get that to happen in the immediate vicinity of the outflow valve, so the instantaneous drop in overall cabin pressure will be small.

Quoting FredT (Reply 12):
The question is if an increasing distance from the valve will make the pressure drop less abrupt to such a degree that the discomfort experienced will lessen.

I doubt it, but I'm willing to be proven wrong.

Quoting FredT (Reply 12):

The cabin of an A320 is 27.5 m long with a diameter of 3.7 meters. Let us assume a cylindrical cross section of the same diameter for the sake of this discussion. Assume an atmosphere inside the fuselage equal to the ICAO standard atmosphere at 8000'. That's 752.6 hPA and 0.963 kg/m^2.

Cut the air inside the cabin one meter from the forward bulkhead and replace the air behind with a moveable piston. Let this piston have the same weight as the air behind the 1 m cutoff point. Volume times density gives us a mass of 285 kg. Anyone surprised?

Assume that the valve will instantly drop the pressure at the rear of the cabin to the equivalent of 12000' altitude in the standard atmosphere. That's 644.4 hPa and 0.849 kg/m^3.

This assumption is invalid, which is where the rest of the example goes wrong. The valve will instantly drop the pressure at the valve outlet to ambient. The pressure loss across the valve is huge, so the pressure drop even small distances inside the cabin is small. The example you're positing is as if the tail fell off the aircraft and opened the whole back of the fuselage to ambient at once. Obviously, that would cause a huge pressure differential and noticable effects.

The area of the outflow valve compared to the area of the cabin is very very small. As a result, the velocity in the cabin to support even very fast flows in the valve is very very small. This means the pressure gradient in the cabin is very very very very small. It's only in the immediate vicinity of the valve (we're talking inches here) that you're going to pick up significant velocities and pressure gradients.

Tom.


User currently offlineJetlagged From United Kingdom, joined Jan 2005, 2532 posts, RR: 24
Reply 15, posted (6 years 3 weeks 3 days 7 hours ago) and read 2529 times:



Quoting FredT (Reply 6):
You cannot argue with physics. The effect is there, exactly as I described. If any pressure change would propagate through the cabin instantly (or, for that matter, at the speed of sound), you either have to explain why the gas law does not apply (i e how the pressure can drop without the air expanding) or how the inertia of all the air in the cabin can be ignored (enabling instant expansion).

Fred,

In an aircraft cabin, the volume of air is constant. Pressure changes are related to changes in air mass. Less air in the cabin means less pressure and vice versa. The outflow valve controls pressure by allowing more air out or keeping more air in. At steady state the airflow into the cabin equals the airflow out of it.

Ask yourself this, how did the front section of the cabin in your example get pressurised? If it was by a piston acting on a fixed mass then pressure and volume would be directly related, but in an aircraft cabin pressure increases because of excess air inflow into the cabin. So the mass of the air has increased too. Pressure waves propogate at the speed of sound. The mass of air in the cabin is basically stationary, so accelerating it in the manner you describe is not significant and the pressure throughout the cabin will change at almost the same time.

Regarding pressure differences along the cabin, any air flow though a tube will resuilt in some pressure loss along it. That is the pressure difference I was referring to.

The fact that air inflow is distributed along the cabin [u]is/u] significant, because it means some air inflow is very close to the outflow valves. In other words the source of the pressure increase (air inflow) is not concentrated at one end of the cabin.



The glass isn't half empty, or half full, it's twice as big as it needs to be.
User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 16, posted (6 years 3 weeks 13 hours ago) and read 2431 times:

If anyone reading this but the three of us still haven't made up his or her mind, please speak up. If not, I think it is time to agree to disagree and let this thread die.

While theoretical discussion is fun, I think the practical point made by MD11Engineer is more interesting than all the theory. The plenum chamber and flow restriction of the cargo bay/dado grilles as described, if the arrangement is similar in the A320, should dampen out any sudden pressure changes enough for theoretical effects such as those discussed here to be effectively masked.

Quoting Tdscanuck (Reply 14):
This assumption is invalid, which is where the rest of the example goes wrong.

Yes, the assumption made is similar to the rear of the fuselage dropping off, and that is not the real life situation. However, that's the beauty of an illustrative example as opposed to, say, a true-to-life calculation (which I'm still offering anyone the joy of making). You can make representative assumptions, say assume an instant pressure drop rather than the more realistic "very sharp pressure drop", to make it clear to the casual reader that e g, as in this case, the inertia of the air in a closed volume is a factor in the rate of pressure drop throughout the volume.

Quoting Tdscanuck (Reply 14):
Any time you have a pressure gradient in a fluid with part of the fluid stationary, you have a velocity gradient, or else you're not at steady state.

And steady-state conditions are exactly what we are not discussing here.

Jetlagged,
we're not talking about how the cabin gets pressurized. Completely different process over a much longer time span.

1) We are not talking steady state.

2) We are talking very short time spans, where cabin air inflow will not be a factor as the volume over this time span will be completely insignificant.

Quoting Jetlagged (Reply 15):
Pressure changes are related to changes in air mass.

Which means moving mass around, hence inertia. My point exactly.

Quoting Jetlagged (Reply 15):
The mass of air in the cabin is basically stationary,

You just stated (correctly) that a pressure drop will mean a reduction of the mass of air in the cabin. In a depressurization, just how do you propose that a mass of air equal to the mass reduction will get to the outflow valve while being stationary?



I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlineJetlagged From United Kingdom, joined Jan 2005, 2532 posts, RR: 24
Reply 17, posted (6 years 3 weeks 8 hours ago) and read 2405 times:



Quoting FredT (Reply 16):
Jetlagged,
we're not talking about how the cabin gets pressurized. Completely different process over a much longer time span.

1) We are not talking steady state.

2) We are talking very short time spans, where cabin air inflow will not be a factor as the volume over this time span will be completely insignificant.

I disagree, the mechanism for pressure change is the same, whether over the long term or short term.

1. We were talking basically steady cabin pressure, with small fluctuations due to the outflow valve acting up. You said a 20% change in pressure at the front would require a large movement of air aft first. Very true, but a 20% pressure change is huge, and would be obvious to the flight crew on ECAM as a cabin altitude change and a cabin vertical speed indication. If you scale your pressure fluctuation back to something realistic then the amount of air movement necessary is correspondingly very much smaller.

2. Very short time spans? You talked about a 20% pressure reduction in the front section. Was that In a short time span too? There'd be a few ears popping as a result of that. In the short space of time you speak of the outflow valve cannot do much to local pressure at the rear of the cabin. If it were a large change over a long period of time you would be tlaking about a failure of the entire system, not some minor fluctuation.

Quoting FredT (Reply 16):
Which means moving mass around, hence inertia. My point exactly.

You aren't moving the entire mass of cabin air around very much, otherwise you'd feel the air moving while sitting in the cabin. Inertia is barely relevant to this scenario.

Quoting FredT (Reply 16):
You just stated (correctly) that a pressure drop will mean a reduction of the mass of air in the cabin. In a depressurization, just how do you propose that a mass of air equal to the mass reduction will get to the outflow valve while being stationary?

Now you introduce rapid depressurisation, which I had not talked about, to discredit what I said. I have always had the greatest respect for you and your posts, but moving the goalposts around like this is silly.



The glass isn't half empty, or half full, it's twice as big as it needs to be.
User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 18, posted (6 years 3 weeks 7 hours ago) and read 2393 times:



Quoting Jetlagged (Reply 17):
1. We were talking basically steady cabin pressure, with small fluctuations due to the outflow valve acting up.

No, we were not. I was talking about very abrupt pressure changes (spikes) due to a dump valve opening or similar major malfunctions at altitude. I see now that the OP does not mention anything about how abrupt the pressure changes were.

As we have apparently been talking about completely different things, small wonder we've been unable to reach any kind of consensus!

Cheers!
/Fred



I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlineJetlagged From United Kingdom, joined Jan 2005, 2532 posts, RR: 24
Reply 19, posted (6 years 2 weeks 6 days 22 hours ago) and read 2341 times:



Quoting FredT (Reply 18):
As we have apparently been talking about completely different things, small wonder we've been unable to reach any kind of consensus!

Easily done. Now it all makes sense.



The glass isn't half empty, or half full, it's twice as big as it needs to be.
User currently offlineHAWK21M From India, joined Jan 2001, 31573 posts, RR: 57
Reply 20, posted (6 years 2 weeks 6 days 14 hours ago) and read 2308 times:



Quoting FredT (Reply 18):
No, we were not. I was talking about very abrupt pressure changes (spikes) due to a dump valve opening or similar major malfunctions at altitude

window blow out at Altitude.
regds
MEL



Think of the brighter side!
User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 21, posted (6 years 2 weeks 6 days 5 hours ago) and read 2286 times:



Quoting Jetlagged (Reply 19):
Easily done. Now it all makes sense.

Now let us argue about who was the first to misinterpret and thus responsible for picking up the tab for the first round of conciliatory malt-based beverages...  Wink



I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlineJetlagged From United Kingdom, joined Jan 2005, 2532 posts, RR: 24
Reply 22, posted (6 years 2 weeks 6 days 2 hours ago) and read 2224 times:



Quoting FredT (Reply 21):
Now let us argue about who was the first to misinterpret and thus responsible for picking up the tab for the first round of conciliatory malt-based beverages...

Would that be Ovaltine or the hard stuff (Horlicks)?



The glass isn't half empty, or half full, it's twice as big as it needs to be.
User currently offlineTWAL1011727 From United States of America, joined Mar 2006, 619 posts, RR: 0
Reply 23, posted (6 years 2 weeks 4 days 22 hours ago) and read 2151 times:



Quoting LongbowPilot (Reply 4):
I am by no means a expert on here but do know, from prior ramp experiance, that the pressure relief valves on MD-88s are in the rear of the cabin.



Quoting Ex52tech (Reply 7):
I don't think I have ever seen one of those open and then stay open.

The positive and negative pressure relief valves on the MD88s are located fwd of the left wing.
The outflow valve and ground pressure dump valve (the big hole with the rotating door) is located under the left engine
just in front of the rear lav service panel.

KD


User currently offlineAnalog From United States of America, joined Jul 2006, 1900 posts, RR: 1
Reply 24, posted (6 years 2 weeks 4 days 21 hours ago) and read 2145 times:



Quoting Jetlagged (Reply 22):
round of conciliatory malt-based beverages...

Would that be Ovaltine or the hard stuff (Horlicks)?

MMmmmmm beer.


User currently offlineJetlagged From United Kingdom, joined Jan 2005, 2532 posts, RR: 24
Reply 25, posted (6 years 2 weeks 4 days 7 hours ago) and read 2106 times:



Quoting Analog (Reply 24):
MMmmmmm beer.

I think Fred might have had something more "Scottish" and "spirited" in mind, and I'd go along with that idea.  Smile



The glass isn't half empty, or half full, it's twice as big as it needs to be.
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