Rapid decompression
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Rapid decompression
Ok folks, a rather strange question perhaps, but one which would help me massively....
The situation is that you're flying a 777 at about 27K, a hole is created in the side of the fuselage (say for arguments sake about 75 x 75cm). On the MFD, CAB ALT ramps up straight away until it equals outside pressure. The pilots then make an emergency descent at approx. 4500 ft/min. My question is: would/should the output shown on the display match the actual rate of descent of the A/C?
Many thanks in advance!
Mc
The situation is that you're flying a 777 at about 27K, a hole is created in the side of the fuselage (say for arguments sake about 75 x 75cm). On the MFD, CAB ALT ramps up straight away until it equals outside pressure. The pilots then make an emergency descent at approx. 4500 ft/min. My question is: would/should the output shown on the display match the actual rate of descent of the A/C?
Many thanks in advance!
Mc
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depends on the hole
If you're getting any ram air then the cabin will effectively lead the actual alt or if there is a (i could be wrong here) linear flow over the hole air will be sucked out so cabin should lag the actual alt.
To be honest, I don't think the update rate in the sensors/ processors will keep up. Certainly in the sim I use (not 777) the aircraft reaches FL100 long before the cabin indications. Having said that my ROD is upto twice what you suggest.
My aircraft px system is incapable of showing cabin rates in excess of 2000fpm, just a nice amber dash---sorry cant help more.
If you're getting any ram air then the cabin will effectively lead the actual alt or if there is a (i could be wrong here) linear flow over the hole air will be sucked out so cabin should lag the actual alt.
To be honest, I don't think the update rate in the sensors/ processors will keep up. Certainly in the sim I use (not 777) the aircraft reaches FL100 long before the cabin indications. Having said that my ROD is upto twice what you suggest.
My aircraft px system is incapable of showing cabin rates in excess of 2000fpm, just a nice amber dash---sorry cant help more.
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Your scenario, if I am reading it correctly, would be considered a explosive decompression as compared to a rapid decompression.
Therefore if the cabin PSI is 0 the cabin rate/descent will show the actual aircraft's decent rate.
Again, I may have misread your question.
Therefore if the cabin PSI is 0 the cabin rate/descent will show the actual aircraft's decent rate.
Again, I may have misread your question.
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Rapid Decomp.
Originally Posted by issi noho
"To be honest, I don't think the update rate in the sensors/ processors will keep up. .... Having said that my ROD is upto twice what you suggest."
& would definitely suggest that you push your ROD closer to the airframes limit
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Interesting points guys. On the sim, the situation is that during an emergency descent to FL100, ROD on the display starts at about half the actual ROD of the A/C (4,500ft/min), slowly increasing towards the true value...but even at FL100 it does not match exactly. My gut feeling is that it would be impossible for them to match, but do you think an actual ROD which is double of what's indicated is realistic? 
777 rapid decompression
Based upon several hundred walks around the outside of the B777, I would guess that the combined area of the forward and aft outflow valves would be almost exactly equal to the size of the hole in your suggested scenario. In this case, should such a hull breach occur, the system would cycle both outflow valves to the fully closed position in an attempt to maintain cabin altitude. If the outflow valve area exceeded the size of the hole, nothing would happen.If the hole exceeded the outflow valve cross sectional area, the cabin altitude would vary from the extreme of explosive decompression ( very big hole) to a cabin altitude climb at a rate determined by the size of the hole. To answer the question, if its an explosive decompression the indicated cabin condition on the MFD will rapidly show ambient aircraft altitude, plus actual aircraft rate of descent.If the hole leak rate exceeds the ability of the cabin pressurisation ( air out exceeds air in) , but at a rate below the 'explosive' level, the indicated cabin altitude will climb at a rate determined by the degree to which the fuselage breach exceeds the area of the outflow valves.
As an example, if a 777 is cruising at 37,000 ft the cabin altitude is around 6000-7000ft. Any hull breach, up to the area of the outflow valves ,can be managed by the system and cabin altitude could be maintained. A bigger hole would cause an excessive cabin leak rate, beyond system capability. The cabin will begin to climb and an immediate aircraft descent is required. From a cruise altitude of 37000ft the B777 can descend at about 6000 ft/min. This gives about 5 minutes to 10,000ft and 4 minutes to the cabin oxygen dropout at 13,500ft. Therefore, the cabin would have to climb at a rate of more than approx 1500 ft/min for the passengers to get the fright of automatic dropout oxygen deployment.
Large wide body aircraft have huge airflows through the pressurised structure and can cope with surprisingly large holes in the airframe before the decompression becomes 'explosive'.
As an example, if a 777 is cruising at 37,000 ft the cabin altitude is around 6000-7000ft. Any hull breach, up to the area of the outflow valves ,can be managed by the system and cabin altitude could be maintained. A bigger hole would cause an excessive cabin leak rate, beyond system capability. The cabin will begin to climb and an immediate aircraft descent is required. From a cruise altitude of 37000ft the B777 can descend at about 6000 ft/min. This gives about 5 minutes to 10,000ft and 4 minutes to the cabin oxygen dropout at 13,500ft. Therefore, the cabin would have to climb at a rate of more than approx 1500 ft/min for the passengers to get the fright of automatic dropout oxygen deployment.
Large wide body aircraft have huge airflows through the pressurised structure and can cope with surprisingly large holes in the airframe before the decompression becomes 'explosive'.
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Assuming the system can't cope and the cabin has risen to ambient, then, as long as you can get air into the hole and the outflow of air is less than that going in you must get a px rise.
I'm not saying the px rise would be much, otherwise we wouldn't be in this predicament, just that it should be something higher than ambient at a given point in time if there is ram air into the cabin and assuming no additional changes to the number or size of outflow ports. It may also be that the px rise is localised within an area of the cabin. Have you ever watched the tide flow into a marina, odd swirls and currents all over the place.
The only way to really measure it would be to hole the aircraft and maintain a level and then check the cabin altitude versus actual. Even then there may be position or instrument error, plus you might even need a standard atmosphere day in real terms since your px system is generally controlled/read with ref to 1013. (trying to get my head round that bit, it definitely controls it to 1013 but I don't know if we are open to atmosphere)
If we are talking about a sim (I hope we are) it all really depends on the model the programmer has created.
Just out of interest my fuel tanks are partially px'd by ram air.
If you have ever travelled on the London underground in summer with all the windows open, when you pass an opposite direction train in the tunnel, the px head of the on-coming train forces air in through the windows of your carriage sufficient to pop your ears.
With regard to the 'twice the ROD you suggest' - all I was trying to say was the aircraft I fly has an actual ROD in an emergency descent from 370 of between 7000 and 8500 fpm. So you've surprised me that 777 is so graceful.
Need to sleep now.
I'm not saying the px rise would be much, otherwise we wouldn't be in this predicament, just that it should be something higher than ambient at a given point in time if there is ram air into the cabin and assuming no additional changes to the number or size of outflow ports. It may also be that the px rise is localised within an area of the cabin. Have you ever watched the tide flow into a marina, odd swirls and currents all over the place.
The only way to really measure it would be to hole the aircraft and maintain a level and then check the cabin altitude versus actual. Even then there may be position or instrument error, plus you might even need a standard atmosphere day in real terms since your px system is generally controlled/read with ref to 1013. (trying to get my head round that bit, it definitely controls it to 1013 but I don't know if we are open to atmosphere)
If we are talking about a sim (I hope we are) it all really depends on the model the programmer has created.
Just out of interest my fuel tanks are partially px'd by ram air.
If you have ever travelled on the London underground in summer with all the windows open, when you pass an opposite direction train in the tunnel, the px head of the on-coming train forces air in through the windows of your carriage sufficient to pop your ears.
With regard to the 'twice the ROD you suggest' - all I was trying to say was the aircraft I fly has an actual ROD in an emergency descent from 370 of between 7000 and 8500 fpm. So you've surprised me that 777 is so graceful.
Need to sleep now.
cwatters: please explain how a hole in an aircraft can 'face forwards'. The airflow over an aircraft skin is always parallel to the surface. Only the loss of the nose radome could give a ram air effect, but that is outside of the pressurised structure.
The BAC 1-11 suffered an explosive decompression which almost removed the Captain from the aircraft, so which way was the air going?. The airflow over the open/missing windshield is more likely to remove air from the cabin than to force air into it. The Antonov freight door is not 'forward facing' , no freight door is, it is flush with the fuselage, and therefore, the airflow. There is insufficient ram-air effect to help keep a cabin pressurised at altitude. The differential pressure is over 8.5 psi and this will overcome any airflow effect. If there is a major hole in the airframe for whatever the reason, believe me, you will be too busy putting on your oxygen mask and getting down to low altitude to worry about how quickly the cabin is climbing.
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Fluid mechanics is certainly one tricky subject, but taking all these suggestions into consideration.... when the ambient altitude and cabin altitude are equal, and an emergency descent is initiated to FL100, should the displayed ROD on the FD match that of the actual ROD, or will they remain approx x2 out of sync until FL100 is reached? That is my predicament...
Cheers again...hope I'm not taking this round in a big circle.
Cheers again...hope I'm not taking this round in a big circle.
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You're missing my point. Once the px has reached equilibrium. ie the air went out of the 1-11 but once that was over some must have been forced back in. My argument is that equilibrium is very slightly in favour of air in, ie the cabin may be 10,20,50 or a 100 ft lower than the actual alt. Some insignificant but technically quantifiable amount. I never mentioned maintaining a normal cabin diff, that would be stupid.
The Antonov I referred to definitely had a forward facing freight door, it was the entire nose of the aircraft and not unsurprisingly the airframe was lost.
I totally agree that this is entirely a theoretical thread and that in any actual event, the situation, workload and discomfort will leave you fresh out of ideas except the hopefully well drilled and rehearsed ED.
I still beleive that the indications you look at on the px system are likely to be in error. You would have to talk to who ever designed the system but it can only compare internal to external px. So there must be some sort of snapshot calculation which will be resolved and displayed but since you've descended it will be technically incorrect. No doubt the rate it takes snapshots will be fast but it will not be instant.
Here are a couple of questions back at you;
If you think air continually goes out of he aircraft will it get cold?
Will the px and temp change cause moisture in the cabin to condense, would you get a mist and would it last long enough to see it go out the hole.
The Antonov I referred to definitely had a forward facing freight door, it was the entire nose of the aircraft and not unsurprisingly the airframe was lost.
I totally agree that this is entirely a theoretical thread and that in any actual event, the situation, workload and discomfort will leave you fresh out of ideas except the hopefully well drilled and rehearsed ED.
I still beleive that the indications you look at on the px system are likely to be in error. You would have to talk to who ever designed the system but it can only compare internal to external px. So there must be some sort of snapshot calculation which will be resolved and displayed but since you've descended it will be technically incorrect. No doubt the rate it takes snapshots will be fast but it will not be instant.
Here are a couple of questions back at you;
If you think air continually goes out of he aircraft will it get cold?
Will the px and temp change cause moisture in the cabin to condense, would you get a mist and would it last long enough to see it go out the hole.
Ok, I'm back on terra firma now.
To answer the last post:
Lets assume that the aircraft depressurises, but does not descend. The cabin will depressurise to ambient. I might agree that if the cabin remains intact and there is some opening which allows some dynamic airflow into the structure, there might be some 'ram air' effect which would keep the cabin marginally above ambient pressure. I am not an expert on this subject, but my feeling is that the pressure rise would be undetectable on any display in the cockpit. You would not delay the descent in order to check that.
In the emergency descent case, following decompression at altitude due to a big hole, the cabin altitude would be expected to follow the ambient pressure during descent, less any lag due to the rate at which the increasing external air pressure could find its way into the aircraft stucture. All of your suggested minimal +/- differential pressure values would probably be unobservable during an actual emergency descent from altitude.
I remember from my distant 'A' level studies ( when it was a struggle to get even a 'C' grade) that there was thing called Boyles' Law. Something about P1V1T1=P2V2T2?? What that means , if I remember correctly, is that if a given volume of air (in a cabin) expands rapidly to a new and much larger volume, its pressure will drop and its temperature will fall rapidly.( Please correct me if I am wrong, I did chemistry in 1964). So, to answer the last question, as the cabin depressurises, the temperature will quickly fall below the cabin dewpoint and a 'mist' will appear throughout the cabin. As the air is sucked out of the fuselage, you would certainly be able to see the vapour as it travelled out of the cabin.
Referring again to Boyles' Law, the pressure reduction and the cabin air volume increase means that the temperature will fall rapidly in the cabin. Following a decompression, if the aircraft is not immediately descended below about 15000ft max, everyone on board without an oxygen supply would be unconcious within minutes and exposed to ambient temperatures well below - 35C . Yes, the cabin would get VERY cold!
To answer the last post:
Lets assume that the aircraft depressurises, but does not descend. The cabin will depressurise to ambient. I might agree that if the cabin remains intact and there is some opening which allows some dynamic airflow into the structure, there might be some 'ram air' effect which would keep the cabin marginally above ambient pressure. I am not an expert on this subject, but my feeling is that the pressure rise would be undetectable on any display in the cockpit. You would not delay the descent in order to check that.
In the emergency descent case, following decompression at altitude due to a big hole, the cabin altitude would be expected to follow the ambient pressure during descent, less any lag due to the rate at which the increasing external air pressure could find its way into the aircraft stucture. All of your suggested minimal +/- differential pressure values would probably be unobservable during an actual emergency descent from altitude.
I remember from my distant 'A' level studies ( when it was a struggle to get even a 'C' grade) that there was thing called Boyles' Law. Something about P1V1T1=P2V2T2?? What that means , if I remember correctly, is that if a given volume of air (in a cabin) expands rapidly to a new and much larger volume, its pressure will drop and its temperature will fall rapidly.( Please correct me if I am wrong, I did chemistry in 1964). So, to answer the last question, as the cabin depressurises, the temperature will quickly fall below the cabin dewpoint and a 'mist' will appear throughout the cabin. As the air is sucked out of the fuselage, you would certainly be able to see the vapour as it travelled out of the cabin.
Referring again to Boyles' Law, the pressure reduction and the cabin air volume increase means that the temperature will fall rapidly in the cabin. Following a decompression, if the aircraft is not immediately descended below about 15000ft max, everyone on board without an oxygen supply would be unconcious within minutes and exposed to ambient temperatures well below - 35C . Yes, the cabin would get VERY cold!
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Originally Posted by 777fly
I remember from my distant 'A' level studies ( when it was a struggle to get even a 'C' grade) that there was thing called Boyles' Law. Something about P1V1T1=P2V2T2?? What that means , if I remember correctly, is that if a given volume of air (in a cabin) expands rapidly to a new and much larger volume, its pressure will drop and its temperature will fall rapidly.( Please correct me if I am wrong, I did chemistry in 1964).
Originally Posted by 777fly
So, to answer the last question, as the cabin depressurises, the temperature will quickly fall below the cabin dewpoint and a 'mist' will appear throughout the cabin.
Originally Posted by 777fly
Following a decompression, if the aircraft is not immediately descended below about 15000ft max, everyone on board without an oxygen supply would be unconcious within minutes and exposed to ambient temperatures well below - 35C . Yes, the cabin would get VERY cold!
But as soon as the pressures equalize, the adiabatic cooling stops, and the body heat as well as heat given off by various systems will start warming the air!
My observations were made assuming that the cabin depressurised from its normal value of around 7-8000ft, to ambient ( anything above 27,000ft, let's say ) so the dewpoint would be reached and a mist would appear.
With reference to the Helois crash near Athens: That aircraft had flown unpressurised at altitude for well over an hour. Wasn't it reported that many of the recovered bodies were very cold, or frozen?
With reference to the Helois crash near Athens: That aircraft had flown unpressurised at altitude for well over an hour. Wasn't it reported that many of the recovered bodies were very cold, or frozen?
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Answer to original post....
I'm glad I've opened up a wee debate here, although I must admit that a lot of it is over my head.
I've found the answer to my original question. Essentially, the cab alt shown on EICAS is calculated in SL ft/min whereas that shown on the vertical speed indicator is in standard ft/min. So when delta p equals zero, and the aircraft makes an emergency decent, the value shown on the EICAS cab alt will be far smaller than that of the actual a/c rate of descent
This is also true on ECAM displays.
I've found the answer to my original question. Essentially, the cab alt shown on EICAS is calculated in SL ft/min whereas that shown on the vertical speed indicator is in standard ft/min. So when delta p equals zero, and the aircraft makes an emergency decent, the value shown on the EICAS cab alt will be far smaller than that of the actual a/c rate of descent
This is also true on ECAM displays.
