A question for our aircraft pilot(s).

When a big aircraft is pressurised for flight, how much air leaks out?

Or to put it another way, if an aircraft was pressurised and then sealed, how long would it take the internal pressure to decrease to ambient?

mollwollfumble said:

A question for our aircraft pilot(s).

When a big aircraft is pressurised for flight, how much air leaks out?

Or to put it another way, if an aircraft was pressurised and then sealed, how long would it take the internal pressure to decrease to ambient?

With the 747 it would take about five minutes I guess. Each of the engines serves as an air source, the air goes from them into three pneumatic air-conditioning units (called packs) where it’s filtered and cooled to provide cabin air.
The inflow of air is relatively constant – quite often when the aircraft is cruising one pack will be shut down to save fuel, as the less bleed air from the engines that is used the fuel burn is also decreased.
There’s two outflow valves, as they’re called, in the rear of the pressure hull, and they are a simple flap that opens & closes to control the amount of air escaping, hence the overall pressure inside. This also makes the airflow typically go from the front to the back.
Anyway, if the engine air bleeds are all closed, the packs shut-off, and the outflow valves closed, then there’s not a great deal of leakage in a good plane, I saw it done once on one of the freighter flights I was on and that’s how I guessed about five minutes. Gotta go now, back later.

Spiny Norman said:

mollwollfumble said:

A question for our aircraft pilot(s).

When a big aircraft is pressurised for flight, how much air leaks out?

Or to put it another way, if an aircraft was pressurised and then sealed, how long would it take the internal pressure to decrease to ambient?

With the 747 it would take about five minutes I guess. Each of the engines serves as an air source, the air goes from them into three pneumatic air-conditioning units (called packs) where it’s filtered and cooled to provide cabin air.
The inflow of air is relatively constant – quite often when the aircraft is cruising one pack will be shut down to save fuel, as the less bleed air from the engines that is used the fuel burn is also decreased.
There’s two outflow valves, as they’re called, in the rear of the pressure hull, and they are a simple flap that opens & closes to control the amount of air escaping, hence the overall pressure inside. This also makes the airflow typically go from the front to the back.
Anyway, if the engine air bleeds are all closed, the packs shut-off, and the outflow valves closed, then there’s not a great deal of leakage in a good plane, I saw it done once on one of the freighter flights I was on and that’s how I guessed about five minutes. Gotta go now, back later.

Perfect, Spiny Norman, exactly what I wanted to know.

It affects the work on aircraft corrosion that I’m starting to do again.

That answer really surprised me. I had no idea how long it would take, but I thought it would be a long time. A 747 is after all pretty big.

I had no idea of a all the mechanisms described by SN.

Amazing the stuff we just take for granted.

p.s. From a pedantic viewpoint, I still think the time period to exact equalisation of pressure should be “almost infinite”.

mollwollfumble said:

Spiny Norman said:

mollwollfumble said:

A question for our aircraft pilot(s).

When a big aircraft is pressurised for flight, how much air leaks out?

Or to put it another way, if an aircraft was pressurised and then sealed, how long would it take the internal pressure to decrease to ambient?

With the 747 it would take about five minutes I guess. Each of the engines serves as an air source, the air goes from them into three pneumatic air-conditioning units (called packs) where it’s filtered and cooled to provide cabin air.
The inflow of air is relatively constant – quite often when the aircraft is cruising one pack will be shut down to save fuel, as the less bleed air from the engines that is used the fuel burn is also decreased.
There’s two outflow valves, as they’re called, in the rear of the pressure hull, and they are a simple flap that opens & closes to control the amount of air escaping, hence the overall pressure inside. This also makes the airflow typically go from the front to the back.
Anyway, if the engine air bleeds are all closed, the packs shut-off, and the outflow valves closed, then there’s not a great deal of leakage in a good plane, I saw it done once on one of the freighter flights I was on and that’s how I guessed about five minutes. Gotta go now, back later.

Perfect, Spiny Norman, exactly what I wanted to know.

It affects the work on aircraft corrosion that I’m starting to do again.

To expand a bit. I’m working with someone who thinks that there’s a lot of air leakage between pressurised and unpressurised compartments on a C130 Hercules, so much so that it far exceeds the air leakage between the unpressurised compartments and the outside. I think the opposite.

Whichever is true has a big effect on the temperature and humidity of the unpressurised compartments, and therefore on the corrosion of components within the unpressurised compartments.

Now I can say – set up your wall porosity so that air would leak out of the pressurised compartment in a time of the rough order of 5 minutes.

After the C130, we’ll be looking at helicopter corrosion.

> Almost infinite

Yes. Where infinite is defined as when the fuel runs out.

Ah, a C130. I’d expect them to leak air quite badly. The rear door would be a nightmare to keep airtight due to the shape & size of it.

I went downhill somewhat after flying the 747, I was based much closer to home (to look after my ageing father better) and my last year of flying was in a Fairchild Metroliner. Or, as it’s often called, the San Antonio sewer pipe. The vid just below shows how we could play music with the air leaks on the front door.

https://youtu.be/k4n—6-T_xg

The damn thing were so leaky that they could barely get up to the rated normal PSID of 5.5, I think ~4 psi was more typical with both bleeds runing, and about 1.5 with just one bleed.

Spiny Norman said:

Ah, a C130. I’d expect them to leak air quite badly. The rear door would be a nightmare to keep airtight due to the shape & size of it.

Unfortunately I had the ‘pleasure’ of flying on a C130 a few times. I was quite surprised to find that they we pressurised at all.

FWIW a couple of schematics I dug up from my systems books.

Spiny Norman said:

FWIW a couple of schematics I dug up from my systems books.

So I’m looking at the AC Pack here?

The rear door in a C130 would be a pressurisation disaster. But I suspect that’s not relevent for air leakage from the pressurised body into unpressurised compartments above and below the main open area, which is what I need my ballpark estimate for.

I wish I had a C130 to look at, to see whether access to the main unpressurised instrument bays is from outside or inside the aircraft, or both.

mollwollfumble said:

Spiny Norman said:

FWIW a couple of schematics I dug up from my systems books.

So I’m looking at the AC Pack here?

The rear door in a C130 would be a pressurisation disaster. But I suspect that’s not relevent for air leakage from the pressurised body into unpressurised compartments above and below the main open area, which is what I need my ballpark estimate for.

I wish I had a C130 to look at, to see whether access to the main unpressurised instrument bays is from outside or inside the aircraft, or both.

I’d bet the cabin remains pressurised the rest of the plane doesn’t. Do some googlin’

“Both the flight station and the cargo compartment can be pressurized to maintain a cabin pressure-altitude of 5000 feet at an aircraft altitude of 28,000 feet.”

https://fas.org/man/dod-101/sys/ac/c-130.htm

poikilotherm said:

mollwollfumble said:

Spiny Norman said:

FWIW a couple of schematics I dug up from my systems books.

So I’m looking at the AC Pack here?

The rear door in a C130 would be a pressurisation disaster. But I suspect that’s not relevent for air leakage from the pressurised body into unpressurised compartments above and below the main open area, which is what I need my ballpark estimate for.

I wish I had a C130 to look at, to see whether access to the main unpressurised instrument bays is from outside or inside the aircraft, or both.

I’d bet the cabin remains pressurised the rest of the plane doesn’t. Do some googlin’

“Both the flight station and the cargo compartment can be pressurized to maintain a cabin pressure-altitude of 5000 feet at an aircraft altitude of 28,000 feet.”

https://fas.org/man/dod-101/sys/ac/c-130.htm

and

“AIR CONDITIONING AND PRESSURIZATION: Two independent air conditioning systems for the flight deck and cargo compartment are operated from engine bleed air in flight and by the GTC/APU on the ground.

> Both the flight station and the cargo compartment can be pressurized to maintain a cabin pressure-altitude of 5000 feet at an aircraft altitude of 28,000 feet.

Many thanks, I didn’t know that.

> GTC/APU

Brings back memories.

Oops, oops, oops.
I should think before opening my mouth.

The two largest unpressurised compartments on a C130 are the tail and the landing gear bay. NOT instrument bays.

For the 747, am I right in saying that some cargo compartments are always heated and pressurised.
But that some cargo compartments need not be pressurized. What about heated?

What you said earlier, about:
“two outflow valves, as they’re called, in the rear of the pressure hull, and they are a simple flap that opens & closes to control the amount of air escaping, hence the overall pressure inside. This also makes the airflow typically go from the front to the back”
is extremely important and I missed it first time. It means that the air inside the tail comes from the cabin but the air inside the landing bay comes from outside. So the rate of corrosion in the two compartments may be completely different.

mollwollfumble said:

Oops, oops, oops.
I should think before opening my mouth.

The two largest unpressurised compartments on a C130 are the tail and the landing gear bay. NOT instrument bays.

For the 747, am I right in saying that some cargo compartments are always heated and pressurised.
But that some cargo compartments need not be pressurized. What about heated?

What you said earlier, about:
“two outflow valves, as they’re called, in the rear of the pressure hull, and they are a simple flap that opens & closes to control the amount of air escaping, hence the overall pressure inside. This also makes the airflow typically go from the front to the back”
is extremely important and I missed it first time. It means that the air inside the tail comes from the cabin but the air inside the landing bay comes from outside. So the rate of corrosion in the two compartments may be completely different.

On the 747 the entire fuselage is pressurised from the rear wall of the radar nose cone (Smallish flat panel about a metre or so in diameter) right to the rear pressure bulkhead, about two metres or so rearwards of the last rear cabin doors ->

There’s two large cargo bays, one in front of the wing and the other to the rear. They are accessed by means of large doors that are hinged at the top and opened with electric motors that also move the locking latches. The rear bay has a smaller extension that’s tucked up into the fuselage under the rear cabin doors, it’s mainly for last-minute stuff that’s gently placed (thrown in quickly) without being in a container. All of the cargo bays are pressurised and heated but I suspect it still gets a little chilly down there.
The main bays are large enough to fit a reasonable sized car. All the bags & gear that goes into them are put into containers.

These are the outflow valves, they are around a metre long … ? Not sure, but they are a fair size.

All of the landing gear bays are unpressurised and have virtually no air flowing through them.
This is the nose gear bay, the insulated box from the middle to the right of the photo.

I’ve got a heap more photos that might be of interest, have a look here -> Aviation

Spiny Norman said:

mollwollfumble said:

Oops, oops, oops.
I should think before opening my mouth.

The two largest unpressurised compartments on a C130 are the tail and the landing gear bay. NOT instrument bays.

For the 747, am I right in saying that some cargo compartments are always heated and pressurised.
But that some cargo compartments need not be pressurized. What about heated?

What you said earlier, about:
“two outflow valves, as they’re called, in the rear of the pressure hull, and they are a simple flap that opens & closes to control the amount of air escaping, hence the overall pressure inside. This also makes the airflow typically go from the front to the back”
is extremely important and I missed it first time. It means that the air inside the tail comes from the cabin but the air inside the landing bay comes from outside. So the rate of corrosion in the two compartments may be completely different.

On the 747 the entire fuselage is pressurised from the rear wall of the radar nose cone (Smallish flat panel about a metre or so in diameter) right to the rear pressure bulkhead, about two metres or so rearwards of the last rear cabin doors ->

There’s two large cargo bays, one in front of the wing and the other to the rear. They are accessed by means of large doors that are hinged at the top and opened with electric motors that also move the locking latches. The rear bay has a smaller extension that’s tucked up into the fuselage under the rear cabin doors, it’s mainly for last-minute stuff that’s gently placed (thrown in quickly) without being in a container. All of the cargo bays are pressurised and heated but I suspect it still gets a little chilly down there.
The main bays are large enough to fit a reasonable sized car. All the bags & gear that goes into them are put into containers.

These are the outflow valves, they are around a metre long … ? Not sure, but they are a fair size.

All of the landing gear bays are unpressurised and have virtually no air flowing through them.
This is the nose gear bay, the insulated box from the middle to the right of the photo.

I’ve got a heap more photos that might be of interest, have a look here -> Aviation

Am getting around to sending copies of these to the person making a half-metre by half-metre mockup of the aircraft wall.