Outflow valves stuck during cruise
 

Hi,

What are your thoughts if the valves stuck at cruising altitude with cabin altitude maintaining at 5500ft. What will happen if the aircraft starts to descend? Will the outside pressure eventually equalize cabin pressure? There's a few variables to consider.

With Regards

My guess would be that cabin pressure would start to rise during descent to the point where the relief safety valves will maintain max Differential Pressure, provided you don't turn off the Packs.
Quite uncomfortable, from what I have heard.

There are relief valves should this happen.

there’s an alert and a procedure to follow. No big deal

In some aircraft the lower bleed pressure that might be delivered from an engine LP bleed source at engine idle (power off descent) is supplemented by the opening of an HP bleed source. This can provide a slightly counter-intuitive increase in engine bleed sourced cabin air inflow at idle. Every aircraft has it quirks.

Know your particular system but don't overthink things. If there is a procedure follow it.

As the aircraft descends cabin pressure will rise. The procedure will ask you to open some ventilation bleed to leak pressure. If it still goes higher it will ask you switch off packs.

This question seems related to another one.
If the cabin pressure increases above 1atm, just switch off the packs.
But on the opposite, if the packs completely fail, and the outflow valve is kept closed, at cruise altitude, how fast will the depressurisation be ?

it will depend how good the seals are. Probably anything between 200 fpm to 900 fpm cabin altitude.

As far as pressure differential is concerned, pressurised aircraft are designed so that the pressure is exerted from inside to out & not the other way around i.e. higher pressure inside the cabin.

So if you end up in a position where the pressure becomes greater outside the aircraft than inside, the manufacture (of the type I fly anyway) has negative pressure relief valves installed, that automatically let air into the cabin to equalise the pressure in the event this occurs. There is also a QRH procedure to address this in the event the -/ve presuure relief valves do not work as advertised...

I would say the leakage rate is more like 1500 to 2000 ft/min as there is quite a bit of leakage - especially in older airframes. I have a recollection (on my type once again) of a maximum permissible leakage of 2000 ft/min during ground pressurisation testing. I'll try & chase that up & confirm...

411 A NG has correctly answered. I have experienced it myself in A300b4 as both packs failed. Since the aircraft was old it was about 900ft/mt. I didn't need to do EMER DES. But in new aircraft conservatively could be around 400ft..

This is sort of question that should be covered in groundschool for your type, but typical systems incorporate both an inward and outward relief valve
and your qrh should detail what to do.

I think it’s a great question. To the posters that suggest you “follow the qrh procedure”, I would point out that none of the 3 Boeings I have flown have any checklist for this. Maybe some aircraft types have a specific checklist. But without one, it comes down to systems knowledge. It happened, not to me, but on a company 767 and they found in their situation that turning one pack off was enough to reduce airflow to allow the cabin to descend at the appropriate rate. I would imagine they may have had to adjust the aircraft vertical speed to keep the appropriate cabin differential.

There won't be a procedure for OFV stuck but I would be surprised if there is no procedure for overpressure. A320 ECAM will tell you what to do. In Boeing you need to remember NNC titles.