Things you always wanted to know the answer to [Vol. 5]

[mko9]

I have been wearing a Garmin watch for about 10 years now, and I have never seen a pressure altitude above around 5500-5600ft on a commercial flight.

[popeyewhite]

mko9 said:

I have been wearing a Garmin watch for about 10 years now, and I have never seen a pressure altitude above around 5500-5600ft on a commercial flight.

Yeah it's not much. I don't know the sums but 10 feet underwater without equalising is more painful than any flight I've been on.

[Cotty]

stemll said:

Submarines don't carry all the oxygen either. They extract it from the water

Thats called electrolysis. The other method is when they burn a chlorate candle
You might want to skip to 10minutes

[Rich Boy Spanner]

C n C said:

captain_cynic said:

Clockwork Cupcake said:

Granadier said:

Probably just showing my stupidity, only just thought of this question: I always assumed an aircraft cabin in flight was airtight (and obviously rigid in shape/size), so how is the air pressure inside the cabin affected by the air pressure outside? Why doesn't being in a rigid, sealed, airtight container prevent you from feeling the change in external air pressure?

A sealed airtight aircraft would not contain enough air for all the occupants for the duration of the flight.

Yep. It's not a completely sealed system. You're still cycling air from the outside for oxygen (bleed air from the engine compressors in most aircraft, a separate pump system in B787s)

The pressure changes as you climb, even though it's at a different rate. It is possible to maintain pressure at sea level but we don't becuse of the amount of power it would take and risks that would occur in the event of a sudden depressurisation.

Ok - so am I correct that from reading the various replies on this thread, taking that a common airliner is pressurised to 8000 feet, assuming that it takes off from sea level, the changes in air pressure which can cause discomfort/ears popping etc.. should all occur due to the pressure drop as it climbs from take-off to 8000 feet? Once it climbs above 8000 feet, the cabin air pressure is stabilised, so there should be no further air pressure changes as it goes beyond 8000 feet up to whatever its cruising altitude is?

The reverse happens when it descends - no presure change until it gets down to 8000 feet, then the air pressure will increase from 8000 feet down to whatever altitude the runway is at.

By the sound of it, some more recent planes are pressurised to lower altitudes (5000 feet), so there is less of a pressure change on these?

Yes, less difference. It is able to be done because aircraft like the B787 use a carbon fibre reinforced plastic airframe (CFRP) which is stronger than the traditional aluminium airframe and can withstand greater differential air pressure. It's also how a B787 or A350 can cruise at 42,000 FT whereas a B767, B777 or A330 will be down at high 30's. More efficient for fuel consumption to be higher. The 787 also uses some hydration to reduce fatigue. On a general level* air into the cabin comes from air cycle machines/air conditioning packs that mix cold air from outside and heated with bleed air from the engines. It exits via the outflow valve at the rear of the aircraft. The control of air entering the cabin and leaving via that outflow valve is how the cabin pressurisation is managed.

• Again more modern stuff like a B787 use electrical heating as they have no engine bleed and use electricity instead to increase efficiency. Engine bleed costs increased fuel burn.

Edited by Rich Boy Spanner on Wednesday 17th April 21:49

Edited by Rich Boy Spanner on Wednesday 17th April 21:51

[NRG1976]

captain_cynic said:

Clockwork Cupcake said:

Granadier said:

Probably just showing my stupidity, only just thought of this question: I always assumed an aircraft cabin in flight was airtight (and obviously rigid in shape/size), so how is the air pressure inside the cabin affected by the air pressure outside? Why doesn't being in a rigid, sealed, airtight container prevent you from feeling the change in external air pressure?

A sealed airtight aircraft would not contain enough air for all the occupants for the duration of the flight.

Yep. It's not a completely sealed system. You're still cycling air from the outside for oxygen (bleed air from the engine compressors in most aircraft, a separate pump system in B787s)

The pressure changes as you climb, even though it's at a different rate. It is possible to maintain pressure at sea level but we don't becuse of the amount of power it would take and risks that would occur in the event of a sudden depressurisation.

I noted that in certain medical emergencies planes will fly patients at sea level, all very sophisticated!

[beagrizzly]

NRG1976 said:

captain_cynic said:

Clockwork Cupcake said:

Granadier said:

Probably just showing my stupidity, only just thought of this question: I always assumed an aircraft cabin in flight was airtight (and obviously rigid in shape/size), so how is the air pressure inside the cabin affected by the air pressure outside? Why doesn't being in a rigid, sealed, airtight container prevent you from feeling the change in external air pressure?

A sealed airtight aircraft would not contain enough air for all the occupants for the duration of the flight.

Yep. It's not a completely sealed system. You're still cycling air from the outside for oxygen (bleed air from the engine compressors in most aircraft, a separate pump system in B787s)

The pressure changes as you climb, even though it's at a different rate. It is possible to maintain pressure at sea level but we don't becuse of the amount of power it would take and risks that would occur in the event of a sudden depressurisation.

I noted that in certain medical emergencies planes will fly patients at sea level, all very sophisticated!

Maybe just above. :-)

[Cotty]

beagrizzly said:

Maybe just above. :-)

I assume he ment pressure at sea level, not literally sea level, might be a bit damp.

[Strangely Brown]

Cotty said:

beagrizzly said:

Maybe just above. :-)

I assume he ment pressure at sea level, not literally sea level, might be a bit damp.

They would have to careful to avoid all of the parrots at that level.

[President Merkin]

I'd always understood they're pressurised to 8-10k feet as a compromise between what the body can take & not blowing the aircraft up like a beach ball/overstressing the airframe etc. Is that wrong then?

I am not (obviously) a well read aircraft guy.

[Cotty]

Strangely Brown said:

They would have to careful to avoid all of the parrots at that level.

[RizzoTheRat]

Cotty said:

I assume he ment pressure at sea level, not literally sea level, might be a bit damp.

I was diving at a quarry in Snowdonia many years ago when a diver in another group had a problem and was evacuated by a Search and Rescue Sea King. Apparently the flight was rather exciting as as they wanted to keep the pressure as high as possible to avoid further complications...in an unpressurised aircraft. Apparently his buddy who went with him was scared of flying

[48k]

President Merkin said:

I'd always understood they're pressurised to 8-10k feet as a compromise between what the body can take & not blowing the aircraft up like a beach ball/overstressing the airframe etc. Is that wrong then?

You could always scroll up to captain_cynic's post from yesterday if you want to read a post by an actual commercial pilot giving you the actual values to see if you're wrong or not.

Edited by 48k on Thursday 18th April 10:12

[popeyewhite]

Cotty said:

beagrizzly said:

Maybe just above. :-)

I assume he ment pressure at sea level, not literally sea level, might be a bit damp.

There are emergency aircraft that can accurately adjust onboard air pressure to copy that at sea level.

[captain_cynic]

popeyewhite said:

Cotty said:

beagrizzly said:

Maybe just above. :-)

I assume he ment pressure at sea level, not literally sea level, might be a bit damp.

There are emergency aircraft that can accurately adjust onboard air pressure to copy that at sea level.

The Syberjet SJ30 is a business jet designed to maintain a sea level atmospheric pressure at altitude.

https://en.m.wikipedia.org/wiki/SyberJet_SJ30

[AstonZagato]

beagrizzly said:

NRG1976 said:

captain_cynic said:

Clockwork Cupcake said:

Granadier said:

Probably just showing my stupidity, only just thought of this question: I always assumed an aircraft cabin in flight was airtight (and obviously rigid in shape/size), so how is the air pressure inside the cabin affected by the air pressure outside? Why doesn't being in a rigid, sealed, airtight container prevent you from feeling the change in external air pressure?

A sealed airtight aircraft would not contain enough air for all the occupants for the duration of the flight.

Yep. It's not a completely sealed system. You're still cycling air from the outside for oxygen (bleed air from the engine compressors in most aircraft, a separate pump system in B787s)

The pressure changes as you climb, even though it's at a different rate. It is possible to maintain pressure at sea level but we don't becuse of the amount of power it would take and risks that would occur in the event of a sudden depressurisation.

I noted that in certain medical emergencies planes will fly patients at sea level, all very sophisticated!

Maybe just above. :-)

My wife was, back in 1990, one of the doctors on a "Make a Wish" flight to Florida with a plane load of terminally ill children. Apparently the 747 flew much lower than normal (though maybe she got it wrong and the plane was pressurised to a lower altitude).

[Clockwork Cupcake]

I was on a flight once, years ago, where we partially lost cabin pressure and the aircon failed. Not "air masks dropping down" stuff but we had to descend to a much lower altitude and it got uncomfortably hot, with the stewards distributing free iced water.

The Captain told us we might have to divert to another airport as we were burning much more fuel at lower altitude but we actually landed at our destination without further incident.

[Nethybridge]

Watching one of the many MH370 docs, and was surprised that
the on/off control switch for cabin pressurisation was under
the control of the flight crew, obviously there must be
a very good operational reason for this control,
but a handy way of killing all the passengers
maybe not what the designers envisaged.

Sorry but crazy bd pilots intent on suicide is another,
if we didn't have enough already, reasons for
worrying about doing 550mph in an aluminium tube full of paraffin.

[mko9]

popeyewhite said:

mko9 said:

I have been wearing a Garmin watch for about 10 years now, and I have never seen a pressure altitude above around 5500-5600ft on a commercial flight.

Yeah it's not much. I don't know the sums but 10 feet underwater without equalising is more painful than any flight I've been on.

And was referenced earlier in the discussion, I always found descending (from altitude or SCUBA) to be much more painful than ascending.

[MadCaptainJack]

If I put a cardboard box in tne recycling that’s got sellotape and/or staples all over it, does it get recycled okay or does it end up in landfill?

[borcy]

Why do male sportsman wear those sports bra things?