Is it possible that, especially during descends with high v/s in an airliner, the differential pressure between the cabin and the environment exceeds the allowed range under normal circumstances?The presure controller works with limited v/s itself, so I could imagine these v/s may be too low to cope with steep descends in some situations...Andreas

Don't quote me on it but I think most modern airliners can cope with maximum structural descent as far as pressure goes, which will for safety reasons be around 2500 - 3000 fpm. I can't imagine any airliner descending faster than this anyhow.

G'day Andreas,All pressurised aircraft have negative pressure relief valves that will open and enable cabin PRESSURE to increase at the same rate as ambient pressure should the cabin/ambient differential try to go negative. Aircraft are structurally designed for a positive differential (cabin pressure higher than ambient).Under normal operation the pressure controller can cope but in emergencies the negative pressure relief valves will function.An emergency decent in a B707 - will result in a 6000ft/min decent Cheers,Roger

Yes it is. If the relief valve remains closed to pressure vessel will maintain any pressure you put into it. Most systems have alarms and safety systems that prevent this, but if they do not kick in...Rob

>Is it possible that, especially during descends with high v/s>in an airliner, the differential pressure between the cabin>and the environment exceeds the allowed range under normal>circumstances?Not sure what the problem is. If the aircraft descends rapidly the outside pressure goes up - helping with the pressure differential rather than making it worse.Michael J.http://www.precisionmanuals.com/images/for...argo_hauler.gifhttp://sales.hifisim.com/pub-download/asv6-banner-beta.jpg

G'day Michael,>what's the problem..Imagine an aircraft at 12,000ft flying pressurised with a cabin altitude of 8000 ft under isobaric control. At this stage the cabin/ambient pressure differential is postive - cabin>ambient. If the pilot was to dive the aicraft then as you say the cabin/ambient differential would be degreasing all the way down to zero. That's fine nothing wrong so far. (cabin=ambient, the point where flight altitiude and cabin altitude are the same 8000 ft). From a structural viewpoint the aicraft is now essentially flying unpressurised. Should however the aircraft continue to decend then ambient will have a higher pressure (lower altitude) than the cabin and a negative pressure differential start to build up causing a reversal of the structural loads on the fuselage and that's a BIG no no. Should ambient pressure ever be higher than cabin pressure then the fuselage would be crushed. Aircraft are designed to withstand a tensile stress in the skin and hoop stress in the frames. Under compression there is very little strength. Aircraft are not submarines. :-) Negative cabin/ambient differential pressures are prevented by negative differential relief valves that open and allow ambient to flood into the cabin, thus preventing a build up of negative differential. On a 747 the valves commence opening at 0.25 psi negative differential pressure and are fully open by 1.0 psi should the negative differential ever get that high.. Under normal operation these valves don't open as the pressurisation controller controls the outflow valves to take care of the cabin pressure but in case of rapid decent where pressure changes are beyond the response control of the pressurisation system then they will "open" to preserve structural integrity of the fuselage.Cheers,Rogeredit: spelling

G'day Andreas,>Is it possible that, especially during descends with high v/s>in an airliner, the differential pressure between the cabin>and the environment exceeds the allowed range under normal>circumstances? Aircraft have protection systems that limit the maximum pressure differentials, for both positive (cabin>ambient) and negative (cabinThe presure controller works with limited v/s itself, so I>could imagine these v/s may be too low to cope with steep>descends in some situations... As the aircraft decends there's no problem UNTIL ....The aircraft flight altitude and cabin altitude are the same. The differential pressure is zero. BUT...If the aircraft now continues to decend then ambient would become higher than cabin pressure and a negative pressure differential start to build up.Under NORMAL operation the pressurisation controller and outflow valve can cope with this and maintain a zero differential pressure. HOWEVER - under certain circumstances; low air supply to cabin and or high aircraft rate of decent, the pressurisation system may not be able to cope and a negative differential actually may occur. Aircraft fuelage's cannot withstand very much negative pressure differentials at all. They are not designed to. To preserve structural integrity negative pressure relief valves will open at very low settings and allow ambient to flood into the cabin. The B747 has 4 off these valves (flapper doors actually) 2 on each cargo door.Cheers,Roger

>Imagine an aircraft at 12,000ft flying pressurised with a>cabin altitude of 8000 ft under isobaric control.Sorry, the numbers used in this example are nonsensical. Instead of 12000 ft lets put 35000 instead and this changes everything. Also there are valves that prevent outside pressure to exceed internal pressure. Michael J.http://www.precisionmanuals.com/images/for...argo_hauler.gifhttp://sales.hifisim.com/pub-download/asv6-banner-beta.jpg

G'day Michael,>Sorry, the numbers used in this example are nonsensical.>Instead of 12000 ft lets put 35000 instead and this changes>everything.Ok I'll humour you. you can start at 35,000 ft if you must. It changes absolutely nothing but the starting cabin/ambient differential. On the decending aircraft we were in agreement so what's changed??The rest of the post stands.Just saving you time and fuel on the decent from 35,000 to 12,000.One day when you have the time I'd be interested to hear how you anticipate landing the aircraft at 35,000 without first flying at 12,000 ft.Find an airport above 12,000 did you?? :-lol> Also there are valves that prevent outside>pressure to exceed internal pressure. Sorry. Not even close! :-) but not a bad try :-lolyou did read my post didn't you before this "shot accross the bows".Cheers,Roger

>Ok I'll humour you. you can start at 35,000 ft if you must. It>changes absolutely nothing but the starting cabin/ambient>differential. It changes everything because by the time you reach 8000 ft while doing rapid descent your cabin pressure will have no problem catching up with what's going on the outside. Also to your other point - the pilot can always depressurize the cabin hence equalizing with outside pressure so you really had to either be dumb or working really hard to ever reach a lower pressure inside than outside.Michael J.http://www.precisionmanuals.com/images/for...argo_hauler.gifhttp://sales.hifisim.com/pub-download/asv6-banner-beta.jpg

G'day Michael,Mate we are not on the same page! :-)The posters question asked if it was possible to get a higher ambient pressure than cabin pressure. Could this perhaps occur on a fast decent??>It changes everything because by the time you reach 8000 ft>while doing rapid descent your cabin pressure will have no>problem catching up with what's going on the outsideDon't equate rapid decent to an emergency decent after a pressurisation failure. The pressurisation system in my example and the posters question is 100% serviceable. I stated the conditions at the beginning of the post.This situation can occur under ONE condition. AND that condition is that if the pilot ever tries to take the aircraft flight altitude lower than the pressurised cabin altitude.If the pilot is flying the aircraft at 35,000ft (just for you :-) ) and the cabin is at 8000 ft UNDER ISOBARIC CONTROL then as the aircraft decends THE CABIN WILL REMAIN AT 8000FT under the control of the pressurisation system At a flight altitude of 8000ft there will be NO cabin/ambient differential. If the pilot continues his decent then a NEGATIVE cabin/ambient differential will start building up. NEGATIVE PRESSURE RELIEF VALVES will crack open at a very low delta P (approx 0.5 psi) to prevent the fuselage being CRUSHED. THE LOADS ARE ENORMOUS. Negative pressure relief valves are ESSENTIAL protection for all pressurised aircraft.That's why they exist :-) OK time to turn the page :-)Whilst the scenario I described is NOT standard operating procedure that doesn't make the scenario nonsensical. The whole purpose was to set up scenario that the average layperson could follow and comprehend. At TOD the pilot normally should set his landing altitude (for argument sake 0) and cabin rate of change (300ft/min) into the pressurisation controller. Therefore cabin will go from 8000 to zero in approx 26 minutes. Once set the pressurisation controller is now in "cabin rate of change mode" not isobaric.As the aircraft has to go from 35,000 to 0 in the same time which means the pilot should decend the aircraft at approx. 1350 ft/min.If the pilot decends too slowly; or maybe put on hold at 25,000 then it is possible that the cabin/ambient differential will increase all the way up to max diff and the max diff valves open to dump cabin to ambient.The opposite to that is if the pilot decends the aicraft too fast then the flight altitude may catch up with the decending cabin altitude. As soon as the flight altitude goes below the cabin altitude the negative relief vales open to allow ambient to flow into the cabin to prevent an any higher build up of negative differential.Its all about differential pressures (absolute) and is not as straight forward as it may appear at first glance. To get a negative pressure differential the rate of decent must be reasonably fast. Sorry for the long post ; tried to be a little more explicit. Cheers,Roger