Ryanair 737-800 cabin altitude

kyeev

I got off a Ryanair flight to Faro yesterday.
On a whim I asked one of the cabin crew what was the cruising cabin altitude. He said 14000 feet.
English would have been his second language and I probably should have asked what was the cabin pressurised to?
But if his answer was cabin pressure altitude or cruising altitude, 14000 seems too high or too low respectively.
Does anyone know what altitude Ryanair pressurises the cabin to? Can't find the answer on google. As I walked away from the plane converting feet into metres, one of the pilots walked past me, so I guess I missed my chance to get a definitive answer...

IngazZagni

All 737's would have a cabin altitude at cruise of around 6 to 8 thousand feet. This would be lower for short flights to the UK where the cruising altitude would be lower.

goingnowhere

You would be rather uncomfortable at 14,000 and in a maze of oxygen mask

6-8000 feet is the norm

faoiarvok

kyeev wrote: »

English would have been his second language and I probably should have asked what was the cabin pressurised to?

Could he have been answering the cruising level as 40,000 and probably taking a total guess?

Dardania

Or maybe FR don't pressurise their jets as much, to not stress the airframe! :P

bkehoe

goingnowhere wrote: »

You would be rather uncomfortable at 14,000 and in a maze of oxygen mask

6-8000 feet is the norm

I wouldn't imagine most cabin crew would normally know this but they would know their emergency procedures which is where 14,000 comes from... 14,000 is when the oxygen masks automatically drop....long before this (at about 10,000ft) though the pilots will have an obnoxious warning horn alerting them to excessive cabin altitude and can choose to deploy the masks manually if required before the auto system kicks in.

8000ft is approx the highest you'll have on a 737 at typical higher cruising altitudes and all 737NGs have the same automatic pressurisation system so an airline or pilots can't choose how they pressurise the cabin. If you're on a short hop to the UK from Dublin (e.g. Liverpool/Manchester/Bristol/Scotland) and the aircraft is cruising around 20,000ft then it'll be much lower, in fact if you're cruising at 18,000ft or lower then the system maintains sea level cabin altitude (i.e. zero)!

kyeev

Is it nearer 6000 or 8000 feet though, is what I'd like to know?
At 6000 feet the effective oxygen percentage is 16.6% and at 8000 is 15.4%.
16.6 is considered a medium altitude category and 15.4% is considered high.
Dammit, I should have asked the pilot when I had the chance.

Gaoth Laidir

kyeev wrote: »

Is it nearer 6000 or 8000 feet though, is what I'd like to know?
At 6000 feet the effective oxygen percentage is 16.6% and at 8000 is 15.4%.
16.6 is considered a medium altitude category and 15.4% is considered high.
Dammit, I should have asked the pilot when I had the chance.

The percentage oxygen in the air does not change with altitude. The relative composition remains the same. Only the partial pressures of each gas reduce.

A few years ago I measured pressure drop on an Aer Lingus A320 flight from Dublin. At FL370 the cabin pressure was 770 hPa, so an altitude of 7300 ft.

It surprised me at the time that there was no pressure drop at all until climbing through I think 4 or 5000 ft.

martinsvi

my watch has a baro (chances are your smartphone also has one!), I think lowest pressure on a Ryanair plane I've ever seen was 749hPa, which is roughly 8100ft. A more common figure is around 760, so roughly 7700ft

Gaoth Laidir wrote: »

The percentage oxygen in the air does not change with altitude. .

OP is talking about effective oxygen percentage, the figures are correct, it's not the same as the actual ratio that stays constant..

kyeev

That's what i meant by "effective" oxygen percentage, sorry if that wasn't clear.

They begin pressurising the cabin on the ground and then slowly increase as they climb (from what I read) so perhaps that's why there was no pressure drop initially?

Captain_Crash

kyeev wrote: »

They begin pressurising the cabin on the ground and then slowly increase as they climb (from what I read) so perhaps that's why there was no pressure drop initially?

Maybe someone with more technical knowledge can clarify this, but with a packs off take off the cabin cant be pressurized until airborne and packs have been turned on.

martinsvi

Captain_Crash wrote: »

Maybe someone with more technical knowledge can clarify this, but with a packs off take off the cabin cant be pressurized until airborne and packs have been turned on.

I think, when you climb through 1000-2000 ft the differential pressure is tiny enough to not cause air escape the aircraft at a quick rate so for some time aircraft should be able to maintain ground level pressure even with the packs off.

Captain_Crash

martinsvi wrote: »

I think, when you climb through 1000-2000 ft the differential pressure is tiny enough to not cause air escape the aircraft at a quick rate so for some time aircraft should be able to maintain ground level pressure even with the packs off.

Yes this is my understanding of it. When climbing out the pressure in the cabin will maintain to a certain degree that wont be noticed by the passengers despite the packs being off.

I think were in agreement that in the event of a packs off t/o, the aircraft is unpressurised until the packs are turned on, which happens pretty quick anyway.

bkehoe

Captain_Crash wrote: »

Maybe someone with more technical knowledge can clarify this, but with a packs off take off the cabin cant be pressurized until airborne and packs have been turned on.

Its extremely rare to do a packs off takeoff on a 737. We would take off with the engine bleed air supply off for extra takeoff performance if there's a performance issue (e.g. in ACE) but will use the APU to supply bleed air for the air conditioning and pressurisation in that case so the cabin remains pressurised. Most will only do it a handful of times per year.

Gaoth Laidir

On my flight, taking off from Dublin R10, there was no pressure drop until we were out over the sea, which I estimated to be 3-4000 feet. I was using a Suunto Vector watch but I had forgotten to set QNH on the altimeter mode before takeoff , so not sure of that exact altitude.

Irishweather

IngazZagni wrote: »

All 737's would have a cabin altitude at cruise of around 6 to 8 thousand feet. This would be lower for short flights to the UK where the cruising altitude would be lower.

Don't UK flights cruise at around 38,000 feet?

highlydebased

Irishweather wrote:

Don't UK flights cruise at around 38,000 feet?

A flight to or from Manchester or Birmingham would be not much more than FL200. Much higher from London

Brennus335

On a normal the off, the cabin will be pressurised to about 200 ft BELOW airfield elavation. That's just the way the system works to pressurise the doors closed.

Brennus335

On a normal the off, the cabin will be pressurised to about 200 ft BELOW airfield elavation. That's just the way the system works to pressurise the doors closed.

On a packs off take off, the cabin altitude will climb with the aircraft. However, the outflow valves will be fully closed, so cabin climb rate will be minimal. The actual value will depend on the aircraft. Newish aircraft will be able to maintain pressure, older ones will leak.

I've experienced a decompression at altitude. Luckily it was a pretty new airframe. With both the forward and aft outflow valves fully closed, we only had a cabin climb rate of +700 feet per minute.

smurfjed

 Brennus335 
I'm curious, do you have a reference for the 200 feet below airfield elevation? I can't find it in the FCOM.

Skuxx

Brennus335 wrote: »

On a normal the off, the cabin will be pressurised to about 200 ft BELOW airfield elavation. That's just the way the system works to pressurise the doors closed.

On a packs off take off, the cabin altitude will climb with the aircraft. However, the outflow valves will be fully closed, so cabin climb rate will be minimal. The actual value will depend on the aircraft. Newish aircraft will be able to maintain pressure, older ones will leak.

I've experienced a decompression at altitude. Luckily it was a pretty new airframe. With both the forward and aft outflow valves fully closed, we only had a cabin climb rate of +700 feet per minute.

Was that on a 737cl, no forward outflow valve on the NG!

tu2j2

smurfjed wrote: »

 Brennus335 
I'm curious, do you have a reference for the 200 feet below airfield elevation? I can't find it in the FCOM.

On takeoff the NG pressurises the cabin to 0.1psid below take off field altitude.

Takeoff being N1 on both engines >60% for at least 1.5 seconds or N2 >89% for at least 1.5 seconds.


The system is automatic (bar a failure or being set to manual) so all the crew enter is their planned cruise altitude and the landing field elevation.

Dardania

How can it de-pressurise with respect to airfield? Is there reverse flow, where there is suction? Is it caused by the airflow over the outflow valve when the jet is moving, and insufficient makeup air from the bleed to maintain pressure?

Irish Steve

My understanding of it is that the cabin is pressurised to below the airfield level, so positive pressure in the cabin, which ensures that the door seals are in place, and that the doors are fully latched and secured, if there was anything adrift, the pressure inside should result in a switch activation and appropriate light on the flight deck showing where the issue is, and that also then serves to ensure that the cargo doors have been correctly closed and sealed.

tu2j2

Dardania wrote: »

How can it de-pressurise with respect to airfield? Is there reverse flow, where there is suction? Is it caused by the airflow over the outflow valve when the jet is moving, and insufficient makeup air from the bleed to maintain pressure?

It can be hard to get your head around but to decrease the altitude in the cabin you need to increase the pressure relative to ambient.

If you start to pressurise an aircraft on the ground the cabin altitude will start to go down, meaning you begin to go to minus altitude.

If you could somehow draw air out of the cabin you would increase the cabin altitude.

Hopefully that makes sense, if not I'm sure someone else can actually explain it better.

Dardania

Irish Steve wrote: »

My understanding of it is that the cabin is pressurised to below the airfield level, so positive pressure in the cabin, which ensures that the door seals are in place, and that the doors are fully latched and secured, if there was anything adrift, the pressure inside should result in a switch activation and appropriate light on the flight deck showing where the issue is, and that also then serves to ensure that the cargo doors have been correctly closed and sealed.

Makes sense -
Sorry, I was being blond - of course the pressure is positive with respect to airfield (indicating below airfield elevation) as pressure decreases as you go up.