They're not connected - ish, within reason. It's all very dependent on conditions as to whether you can achieve those rates or not. If by full you mean full PAX, full Cargo, full Fuel, then no, you're not very likely to see rates of climb. With say full pax, full cargo but only enough fuel for a one hour hop in an A330, then you'd certainly be more than capable of getting rates like that if you tried...

 

Regards,

Ró.

 

So, exactly as I thought then. Yes, by 'full' I meant 'full pax / cargo'

 

Umm ok, do you have some 10 spare min for a short lecture about A/C pressurization? I always wondered is the pressurization done all the way to 8000 ft (that being cabin altitude) before/right after takeoff (depending on T/O with or without PACKS ON) or is it done gradually as you climb/descend? I mean I know it is done automatically but how it works in practice? And if it stays the same through all the flight, how come ears pop and heads ache? If it's not same, what is its connection to the different phases of the flight?

 

thanks in advance

Ignat

It is not unsualy to have high rates and ascent and descent. It all depands on aircraft weight and set thrust. So an initial descent rate for a B744 from FL380 of 6800fpm is generally quite common.

 

As for the pressurisation, if the cabin altitude is 10000, then the cabin will maintain that altitude until the aircraft reaches 10000 and the cabin will reduce altitude at the same rate as the aircraft, This is the reason why passengers have ear problems when the aircraft is at lower altitudes.

Umm ok, do you have some 10 spare min for a short lecture about A/C pressurization? I always wondered is the pressurization done all the way to 8000 ft (that being cabin altitude) before/right after takeoff (depending on T/O with or without PACKS ON) or is it done gradually as you climb/descend? I mean I know it is done automatically but how it works in practice? And if it stays the same through all the flight, how come ears pop and head aches? If it's not same, what is its connection to the different phases of the flight?

A lecture on pressurization eh?

 

Right so first things first, typical cabin altitude in the cruise is round and about 6,000-7000', with a maximum of 8,000' for most jet aircraft. This Cabin Altitude (CA from here on out) is achieved through the use of both the packs and the outflow valves.

 

So, for simplicity's sake, lets just talk about my A320 and how it behaves. In the 320 there are 4 stages of pressurization, ground mode, pre-pressurization, pressurization and depressurization.

 

Ground mode is basically all the outflow valves open, keeping the cabin at the ambient pressure that is outside the aircraft, this mode is active from about 55 seconds after landing to takeoff.

 

Pre-pressurization occurs on the TO roll where by the aircraft is pressurized at a rate of 500fpm until Delta pressure is equal to 0.1.

 

Pressurization during the climb happens at a pre-programmed rate that is calculated based on a number of parameters that the system takes into account including the aircraft's VS. During the cruise the outflow valves are actuated to control the CA and ensure that it is comfortable for all passengers.

 

During the descent, the cabin altitude is adjusted such that landing altitude pressure is achieved just before landing. A maximum of 750fpm is allowed for depressurization.

 

This is all done automatically for the most part.

 

Hope this helps your understanding of aircraft pressurization.

Regards,

Ró.

whoa, awesome answer. Thanks! Because what's better than pressurization stuff on a lazy sunny Sunday afternoon for an airhead?

 

Now, to see if I'm parsing this correctly:

 

Ground mode - valves fully open, thus outside and inside pressures are equal.

 

Pre-pressurization mode on take-off roll - valves open as much as to allow for higher outflow volume than the packs bring in, thus decreasing the CA at a rate of 500 fpm.

 

Pressurization during the climb - the decrease of the CA continues gradually until it gets to 8000' (max. value), where valves open so much as to allow for the same outflow volume as the packs bring in, thus maintaining those 8000'.

 

Descent - the valves allow for lower outflow volume than the packs bring in, thus increasing the CA at a rate of 750 fpm.

 

Rollout - ~55 sec after landing valves fully open, thus increasing the CA to be equal to outside.

 

it's little baffling, but there can be no knowledge without poking the people who bear it to spill the beans

regards, thanks again

Ground mode - valves open, thus outside and inside pressures are equal.

Correct.

 

Regards,

Ró.

Nice intro to pressurisation Ro.

 

Question though. On airbus aircraft, in the doors there is an indicator to say the cabin is still pressurised and so don't open the doors. Is it normal after landing for there to be a pressure differential between the cabin and the outside?

 

Or is it a case where the valves have stuck and the cabin altitude is stuck above the ground?

Nice intro to pressurisation Ro.

 

Question though. On airbus aircraft, in the doors there is an indicator to say the cabin is still pressurised and so don't open the doors. Is it normal after landing for there to be a pressure differential between the cabin and the outside?

No, not normal, but in the event it did happen, that warning is there to stop the door slamming into someone's face...

 

Regards,

Ró.

Ok, where's the thumb up feature gone now? I wanted to put +1 on Ró's post. :wacko:

 

Either way, I have a question regarding the TO roll pressurization.

Pre-pressurization occurs on the TO roll where by the aircraft is pressurized at a rate of 500fpm until Delta pressure is equal to 0.1.

What's the trigger for the system to begin with that phase? Thrust lever position, flap setting, wheel spin-up above a certain margin, all of those, something else? Just out of interest. Results will not be forwarded to the Aer Lingus HQ (well, this time). :Peace:

 

Edited. How about a 'packs off' TO? I don't know if those a common for you folks. Maybe you have at least the APU on the packs in that case. Surely an airline dependant item.

Thanks Ró

Ok, where's the thumb up feature gone now? I wanted to put +1 on Ró's post. :wacko:

 

Either way, I have a question regarding the TO roll pressurization.

 

What's the trigger for the system to begin with that phase? Thrust lever position, flap setting, wheel spin-up above a certain margin, all of those, something else? Just out of interest. Results will not be forwarded to the Aer Lingus HQ (well, this time). :Peace:

The takeoff phase is triggered in Airbus aircraft by placing the Thrust levers into either the Flex or TOGA detent/gate.

 

Regards,

Ró.

sorry, had to type in parts. All parts are now there.

The takeoff phase is triggered in Airbus aircraft by placing the Thrust levers into either the Flex or TOGA detent/gate.

Thanks. So when you abort the TO and come out of the detents, the system reverts back, right? So the outflow valve opens again, sooner or later.

 

Look, they've written a song about

. :Whistle:

whoa, awesome answer. Thanks! Because whats better than pressurization stuff on a lazy sunny Sunday afternoon for an airhead

 

Now, to see if I'm parsing this correctly:

 

 

Pressurization during the climb - the decrease of the CA continues gradually until it gets to 8000' (max. value). Is that synchronised with the actual A/C altitude, i.e. dies the CA reaches 8000' pressure by the time the A/C is at 8000?

 

Not normally! The "CA" is the target cabin altitude for the flight depending on cruise altitude. Usually depending on a/c type and take off conditions it is achieved at around 300fpm. So from LHR at 80ft ASL that would be 7,900/300= 26 minutes. Descending is often higher at around 500fpm. What is important is the cabin differential pressure. It is this, that effectively sets the cruise cabin pressure. Normal a/c fuselage design allows for a differential of 9.5 psi between the outside pressure and the cabin pressure. Concorde had a CA of 6,000ft due to its design. The VC10 for example normally operated at 6,000ft going up to 8,000ft above FL40 and that was controlled manually. On take-off and landing the spill valves were left open. So if the FE forgot to close them during drift out then the cabin altitude alarm would go off.

Not normally! The "CA" is the target cabin altitude for the flight depending on cruise altitude. Usually depending on a/c type and take off conditions it is achieved at around 300fpm. So from LHR at 80ft ASL that would be 7,900/300= 26 minutes. Descending is often higher at around 500fpm. What is important is the cabin differential pressure. It is this, that effectively sets the cruise cabin pressure. Normal a/c fuselage design allows for a differential of 9.5 psi between the outside pressure and the cabin pressure. Concorde had a CA of 6,000ft due to its design. The VC10 for example normally operated at 6,000ft going up to 8,000ft above FL40 and that was controlled manually. On take-off and landing the spill valves were left open. So if the FE forgot to close them during drift out then the cabin altitude alarm would go off.

 

Yup, I realised that as I was typing, so now the questions are edited. There they are again:

 

whoa, awesome answer. Thanks! Because what's better than pressurization stuff on a lazy sunny Sunday afternoon for an airhead?

 

Now, to see if I'm parsing this correctly:

 

Ground mode - valves fully open, thus outside and inside pressures are equal.

 

Pre-pressurization mode on take-off roll - valves open as much as to allow for higher outflow volume than the packs bring in, thus decreasing the CA at a rate of 500 fpm.

 

Pressurization during the climb - the decrease of the CA continues gradually until it gets to 8000' (max. value), where valves open so much as to allow for the same outflow volume as the packs bring in, thus maintaining those 8000'.

 

Descent - the valves allow for lower outflow volume than the packs bring in, thus increasing the CA at a rate of 750 fpm.

 

Rollout - ~55 sec after landing valves fully open, thus increasing the CA to be equal to outside.

 

it's little baffling, but there can be no knowledge without poking the people who bear it to spill the beans

regards, thanks again

Edited. How about a 'packs off' TO? I don't know if those a common for you folks. Maybe you have at least the APU on the packs in that case. Surely an airline dependant item.

Packs off TO are rare, usually if you needed the engine bleeds off on TO, the Packs will be on the APU, if no bleed air is available then that phase of pressurization is skipped over.

 

 

Thanks. So when you abort the TO and come out of the detents, the system reverts back, right? So the outflow valve opens again, sooner or later.

Correct.

 

Regards,

Ró.