Is an airliner fully airtight or does it just leak less than the PACKS can deliver?
Posted by CrappyTan69@reddit | aviation | View on Reddit | 233 comments
To me, it feels like they don't need to worry about 100% seals everywhere and some tolerance is ok as the PACKS can deliver more than enough.
Is my thinking correct?
usmcmech@reddit
Yes airliners leak all over the place.
The PACKs (pneumatic air conditioning kit) push air into the cabin constantly. At low altitude the outflow valve is opened to allow that air to escape. At a higher altitude the outflow valve is closed and the pressure finds an equalilibum that results in a "cabin altitude" of approximately 8000 feet
FalconX88@reddit
I cannot believe they would rather use leaking seals rather than an outflow valve to get rid of the excess pressure that builds up.
Vessbot@reddit
They don't "use" leaking seals, the leaking seals are just there. But the outflow valve is used to regulate the pressure in a way that cannot be regulated via the rest of the leaks.
FalconX88@reddit
Exactly. That statement
Is BS. But I played along and showed that it makes no sense because closing the valve would mean you deliberately are using leaks for pressure equilibration.
Vessbot@reddit
The poster is correct. The higher the differential pressure (higher attitude) the more closed the outflow valve is. This is how the regulation happens. Maybe he could have been clearer in saying "more closed" so people like you wouldn't interpret it as fully closed.
usmcmech@reddit
They do use the outflow valves.
However they also don’t bother trying to seal every pinhole leak in the fuselage. That would cost way too much in weight and money.
FalconX88@reddit
This was the statement
If the valves are closed but we are still pushing air into it, the excess air needs to go out through the leaks and it would be deliberate.
But the statement is just BS, these vales aren't closed at high altitude.
usmcmech@reddit
They do use the outflow valve to get rid of excess pressure.
but they also don’t bother trying to get the fuselage completely airtight either. That would weigh way too much.
DT5105@reddit
Out of curiosity I switched on my phones barometer in an aircraft at 30,000ft.
It said my altitude was 6000ft based on air pressure
samsaruhhh@reddit
Does this mean it's the same amount of oxygen we would be inhaling on the ground at 8000 feet?
airfree1@reddit
That's exactly what that means.
Sir_Sir_ExcuseMe_Sir@reddit
Why don't we feel short of breath like people do when they arrive to La Paz, etc?
ElmoloKloIokakolo@reddit
Interesting, yes why?
Sorry-Pitch7765@reddit
The plane would have to be stronger (more weight) to maintain sea level pressure at high altitude, so they settle at about 8000ft in the cabin.
It's why your ears can hurt as you descend because it's bringing you back to sea level pressure
GenericAccount13579@reddit
You definitely have less oxygen on an airliner. If you have a watch with a pulse-ox function you’ll see it.
taylortbb@reddit
La Paz is at 12,000 ft, so lower air pressure than an airliner. The difference between 8,000ft and 12,000ft is actually pretty significant.
Also, what /u/spazturtle said about sitting still. Being on an airplane is a pretty low exercise environment.
spazturtle@reddit
Because you are sitting still and not walking around much. You still feel the effects, which is why you feel tired after a flight and can get jetlagged.
CouchPotatoFamine@reddit
So that's why my cigarette burns so fast on flights...
Difficult_Limit2718@reddit
Sir this is shittyaviation, real aviation is over... Uh.... Hmm
FlyingSceptile@reddit
Flown jets for seven years. Never knew pack was actually an acronym
dhc2beaver@reddit
For sure a backronym, or just made up. I've also heard "Pressurization and Conditioning" for PAC for the love of the three letter acronym in aviation. Someone will have to find Mr. Pack himself and ask him what he meant calling it that
usmcmech@reddit
It may be.
But I haven’t heard another good explanation where the term came from.
dubvee16@reddit
What does bleed stand for?
Someone tried to tell me that virga was a acronym once.
I have no idea if you're right. But a lack of an alternative isn't proof one way or the other.
usmcmech@reddit
Air is “bled off” the compressor
polarisdelta@reddit
Hi, what engine are you thinking of that uses bleed air for fuel control?
That's so insane it can't even be Soviet, it would have to be French.
Vessbot@reddit
It's the normal meaning of the word, a pack of several components (compressor, turbine, intercooler, ducting, etc.) working together. In British aircraft they're called "groups."
gefahr@reddit
I've been reading this sub for 10 years (so more experienced than you!) and I didn't know either.
krazul88@reddit
I've been reading English for the better part of 5 decades and I never knew that equalilibum was a word.
usmcmech@reddit
I r pylot i no spel so gud
krazul88@reddit
That's fine. It's not like you ever need to read or write anything important... as a pilot.
gefahr@reddit
Rookie.
this_my_sportsreddit@reddit
If Reddit has taught me anything, it’s that as soon you hit subscribe to a subreddit, you are automatically an expert in that field.
DarkTeaTimes@reddit
It's the unique 10,000 hour compression algorithm.
PhysicalIntern4911@reddit
Embraer calls them Pressurization and Air Conditioning Kits. It’s interesting how many different companies fill the same acronyms with different words
Cd121212@reddit
I feel like it’s got to be a backronym, because I’m in maintenance, never heard this before, and we always refer to them as Air Conditioning Packs (Normally just AC Packs), which doesn’t make sense with the acronym.
NoShirt158@reddit
Perhaps this was once a retrofitting option to install on refurbishing aircrafts? Its common to turn terms like that into slang. Thus creating the kit. Combine that with aviations need for acronyms for procedure and safety requirements. Someone updating a safety check procedure needed an acronym to list it in the checking sequence. And thus creating PACK?
Not a pylote or aviation engineer, still an engineer and process manager.
thesuperunknown@reddit
Because it isn’t.
FestivusFan@reddit
Right like compared to the other types of air conditioning?
wearsAtrenchcoat@reddit
This is incorrect. The outflow valve is never fully closed during normal operations. If it were the cabin pressure would keep increasing until the the safety over pressure valve would open or the pressurized vessel rapture.
Cabin pressure is maintained by modulating the outflow valve opening. When cabin pressure needs to increase (i.e. during a climb) it reduces the amount of air that leaves the pressurized vessel. When cabin pressure needs to decrease, it does the opposite. During level flight the outflow valve is not moving (very little maybe) to keep a constant cabin pressure. Pressurization (cabin pressure higher than ambient pressure) is maintained by having the packs putting out a higher flow than the relatively smaller outlet that is the outflow valve allows
usmcmech@reddit
Depending on the aircraft the outflow valve can be fully closed but that doesn’t mean it is sealed. Air is still leaking out.
wearsAtrenchcoat@reddit
Yes, you are correct. Fully closed does not mean physically shut, thanks for the clarification
beretta01@reddit
Most induction test flights require a leak test where the packs are selected off at altitude and the cabin rate of climb has to be within a certain parameter….if it exceeds it, the pressure vessel is leaking too much in one (or many areas.)
lickmyscrotes@reddit
Why would you conduct the test at altitude when you can achieve the same result on the ground?
Foggl3@reddit
I've always done it on the ground
Ich_han_nen_deckel@reddit
Remember Helios Airways Flight 522
usmcmech@reddit
Yeah I’ve done a few of those post maintenance tests both as a pilot and as a mechanic. Chasing down the small leaks can be a real pain.
sir_thatguy@reddit
It’s sad when the wrong answer has more upvotes than the right answer.
usmcmech@reddit
that’s Reddit for ya.
saxbophone@reddit
Thank you for providing a definition for PACKs, I had no idea wtf OP was on about other than something to do with air for life support
im_the_natman@reddit
One slight correction is that PACK stands for Pneumatic Air Cycle Kit instead of conditioning, so named because it uses the air cycling process to rapidly cool bleed air from the engines.
usmcmech@reddit
Thank you, that sounds more likely
DarkTeaTimes@reddit
Hey Bill, I've climbed 4 8000 foot mountains - you?
That's nothing man, 8000' ?! I breathed that air over 200 hundred times. Pffftt.
Rk_505@reddit
And that kids, is why being on a plane hungover sucks booty cheeks
Unlucky-Jello-5660@reddit
The clear solution is to just not stop drinking, can't be hungover if you keep your buzz going.
Crq_panda@reddit
Once upon a time, when I look at just the right angle from the front FA jumpseat of the glorius MD-88, I could see sunlight between them large flapping seal and the fuselage of the jet. In an unrelated note, I am told my hearing loss is not service related.
Toastman89@reddit
Back when smoking was allowed onboard, maintenance personnel used to be able to find big leaks by the far streak trailing off it. Little streaks were normal, but big ones indicated a problem
concreteandgrass@reddit
When I was 7 years old we flew from Los Angeles to Finland . My mom booked us in the smoking section both there and back. Crazy times.
Battlemanager@reddit
You're in a pressurized tube with recycled air. You were all smoking to some degree.
TheAntiRAFO@reddit
Eh, it’s not recycled primarily, it’s supplied by bleed air.
Battlemanager@reddit
Even worse.
https://www.wsj.com/business/airlines/air-travel-toxic-fumes-64839d6e
pleasexplain@reddit
“If it bleeds, it leads”
Remember that air travel is still the safest way to travel by a wide margin!
ProcyonHabilis@reddit
That statistic is related to injury and death, it's not really related to non injurious exposure to toxic gasses at all.
Flying is definitely not the safest when it comes to radiation exposure, for example.
anally_ExpressUrself@reddit
This is why flight attendants all get cancer in their 40's.
Battlemanager@reddit
True
ZeToni@reddit
Although air pressure is supplied by bleed air, most of the air in the cabin is recycled, by a bunch of filters around the cabin.
That's mainly due to fuel burn, if all the air was just going in and going out through the discharge at the same rate then you would burn so much more fuel.
All bleed air is stolen from the engine (unless it's a 787). More air stolen more fuel burn.
dontevercallmeabully@reddit
Why is this different on the 787?
unreqistered@reddit
it uses a standalone compressor
DrDeke@reddit
And even then, the energy to run the compressor comes from the engines.
telperos@reddit
The thing is, by not using bleed air to pressurize the cabin you can actually achieve higher pressurization without taking air from the engine, which can now use said air to carry out a more complete combustion, increasing fuel efficiency and reducing emissions.
FalconX88@reddit
Not really. recirculated vs fresh air is about 50:50
Airbus states
Source: https://aircraft.airbus.com/en/newsroom/news/2021-01-cabin-air-quality-key-to-a-comfortable-flight
Suspicious-Ask5557@reddit
It is recycled
Koomskap@reddit
recycled blood air
Hugo_5t1gl1tz@reddit
Even as a smoker, I find the idea of smoking on an airplane horrifying.
Mikoriad@reddit
I remember a flight from Italy to the New York as a 11 year old kid in 1989 (dad was USAF). It was on a pan-am 747 and the entire flight I kept sitting up and looking down the long tube of economy. There were just waves and layers of cigarette smoke floating as far as I could see. I had a splitting headache, and ended up throwing up all over my dad. Also, the in flight movie was the Michael Keaton batman.
I remember looking out the window and seeing the statue of Liberty way down there going by on approach. The water kept getting closer, suddenly grass, runway and BAM!. We hit so hard that the air masks came out of the ceiling and there was burning rubber smell throughout the entire plane.
...very memorable flight. But yes, the smoke on airplanes was incredible. It was like that everywhere though, smokers just blowing out all over every establishment.
woyteck@reddit
You've seen the smoke. Bow imagine inf COVID was visible as the smoke...
nasadowsk@reddit
I've heard stories of inadvertent mask deployment while on the ground, or after a hard landing, where people have actually put them on...
Tomero@reddit
Whats wrong with that? Regular Joe passenger is told to put them on when deployed.
Flesh_And_Metal@reddit
And on a smoking flight, you'd be glad for the extra oxygen.
Dragonfruit_4660@reddit
I flew Alitalia from Boston to Rome in 1988 (as a kid) and we were in the back with a bunch of off duty flight attendants who chain smoked the whole way. Very memorable!!
NoBees8008@reddit
It was amazing
CronozDK@reddit
...not as horrifying as a smoking airplane, I bet.
Marsovtz@reddit
I was in Bosnia a couple months ago. I'm a smoker, but walking down the hotel hall for a breakfast while smelling cigarettes everywhere...too much...
GlockAF@reddit
Ashtrays were provided in every armrest, they expected everyone to light up
GGCRX@reddit
I remember when they finally banned smoking on airplanes. Comedians were making jokes that they should bring it back because the smoke covered up the smell of all the farts.
Kappawaii@reddit
Pressurization is much better now, you fart way less than before in new airplanes
SourceBrilliant4546@reddit
My wife didn't get the memo and were not flying.😫
Pynchon_A_Loaff@reddit
And they did. At least it seemed that way in the 1960s. As a kid with severe asthma, airline flying was a nightmare.
Same-Village-9605@reddit
Wants aeroflot smoking on the left and non smoking on the right?
SloaneEsq@reddit
I flew Royal Jordanian for my honeymoon in 2000. It was smoking by section. Our row was non smoking, the row behind was in the smoking section. 🙄
junkDriver@reddit
No, it was by sections, at least on Tu-154. First section was non-smoking, second, towards the rear, was a smoking section.
No_Worldliness643@reddit
My mom was always salty that the back of the plane was generally where you were most likely to survive a crash, but that was where the smoking section was. And “those idiots are going to die anyway.”
foolproofphilosophy@reddit
I remember a BOS to SLC flight and a guy in the row behind us was smoking cigars.
IWantAnE55AMG@reddit
My parents would buy the cheapest non-smoking seats available. That usually meant we were sitting a row or two in front of the curtain that split the smoking and non-smoking sections.
ripped_andsweet@reddit
isn’t that how they figured out how China Air flight 611 went down? there was a huge panel completely covered in tar which uncovered the maintenance issues
matted-@reddit
Admiral Cloudberg does excellent write ups of plane crashes. He has one on China Air Flight 611
kayloulee@reddit
She - Admiral Cloudberg is a woman. And her write ups are a really good time sink!
Zombiehacker595@reddit
That was my thought when I read that too. The plane suffered a tail strike some 20 years earlier, and instead of removing and repairing the damaged section, they just bolted a plate over it and called it a day. Very preventable crash.
quesoandcats@reddit
Iirc it was a crack in the fuselage near the tail with a tar streak?
Ruin369@reddit
This is part of the reason the Japan Airlines Flight 123 disaster was solved. Investigators found nicotine from cigarette smoke trapped in the rear fuselage. This helped them trace where the fatigue cracks formed.
DrSendy@reddit
'Back when smoking was allowed onboard'
They would just open the door at the airport
And you'd walk right out the tarmac
And up those stairs and light a big cigarette
And stick it in the ashtray
And the stewardess would come along
And if she was hot, you can try to pick her up!
--iggy pop
atmatthewat@reddit
I once flew on a DC-9 where it was 3-2 seating… the 3 side was smoking, the 2 non-smoking.
AV8ORA330@reddit
And the outflow valves were disgusting.
DT5105@reddit
Today I Learned
latedescent@reddit
I had a toilet like this once
qdp@reddit
On a Boeing 737, cabin air is completely refreshed every two to three minutes.
fly_awayyy@reddit
The outflow valve is barely open during the pressurization or cruise in fact may not be open for it. Natural cabin leakage requires it to be shut.
joesnopes@reddit
Natural cabin leakage may be quite high for riveted alloy fuselages but the 787 is composite and leakage should be quite a lot lower.
Vessbot@reddit
There can be leakage through both the outflow valve, and other leaks.
ambuscador@reddit
That's not totally accurate. The outflow valve uses set cabin pressure vs actual pressure and is a very simple mechanism, and in practice it's never fully closed in flight at altitude.
BenaiahofKabzeel@reddit
This is good to know! One of my mom’s main reasons for avoiding flying is “other people are breathing my air.”
SeatPrize7127@reddit
Or maybe she's breathing other people's air??
Battlemanager@reddit
She's definitely breathing a few farts.
gefahr@reddit
Sorry.
mustang__1@reddit
I'm not. Crop dusting first class is the only joy left in being self loathing cattle
Longwaytofall@reddit
I was hanging out with some Ethiad flight attendants once and they told me they always walk back to coach to fart before resuming first class service.
devl_ish@reddit
I don't believe that. Etihad first class customers wouldn't want to share that perk with coach.
AltDS01@reddit
Fun Fact, all smells are particulate matter.
astrokat79@reddit
Thanks, I will never bring my phone into the bathroom again.
OffbeatCamel@reddit
Only in the sense that all matter is particulate matter? Plenty of smelly gases, liquids, etc.
Battlemanager@reddit
Gross
bonfuto@reddit
The internet tells me every breath you take includes an air molecule that was part of one of Napoleon's farts with a probability near one.
zwifter11@reddit
If water is a cycle of evaporation and rain, then you would have drank everyone’s piss at some point. Even Napoleons piss would have made it into the water cycle.
Jazzlike-Sky-6012@reddit
She never goes to a crowded place? No bar, no public transport, no concerts etc?
kwajagimp@reddit
There's actually a federal spec for how much fresh air each passenger the aircraft manufacturer has to design into the peoplespace. This is typically met using X number of air changes per hour, which means you have have a hole in the airframe to allow for those changes. It also is typically met these days using HEPA filtration on any volume recirculated, so while that answer might have been right decades ago before that spec and HEPA getting decent, it's almost completely inaccurate now.
This is (part) of what limits an aircraft's maximum cruising altitude - eventually the pressure in the cabin climbs above the maximum of what's safe (8000 ft) with the holes required to provide that number of air changes. It's also (and mainly) required to give folks enough oxygen in the air too, but the changes are part of that limiting factor.
Chappietime@reddit
The “recirculated air” myth is extremely prevalent.
Adjutant_Reflex_@reddit
Also good to remember air is largely pulled top to bottom, it’s not front to back.
It’s really hard to beat how clean the air is in an airliner, especially those without engine bypasses.
BluesFanDeluxe@reddit
"it’s not front to back"
The air is pulled to the BACK where it exists the outflow valve.
Laescha@reddit
There was a study that came out a couple of years into covid which showed that the risk of catching covid on a commercial flight was shockingly low. If there was someone on the plane who had covid, then the little sat directly next to/in front of/behind them were at a high risk of catching it, but everyone else's risk was very low, because of the way airflow works on most commercial airliners.
The risk of catching covid in an airport, on the other hand...
Huge_Animal5996@reddit
Or sitting on the plane at the gate before the engines are running
oh_helloghost@reddit
Most airliners these days have more frequent air turnover than hospitals.
Tenzipper@reddit
Let your mom know all the air has already been breathed by other animals or people, and she can stop.
Breathing, or bitching about "used air," her choice.
massunderestmated@reddit
And really, every drop of water you ever drank has been pee at some point.
kytheon@reddit
Covid was a difficult time
DarkTeaTimes@reddit
Your mother probably has a molecules of Aristotle in her composition. We're all made from, amongst other things, someone else.
chicknsnotavegetabl@reddit
Refreshed not replaced
MudWallHoller@reddit
You just don't want to breathe the dirty back of the plane people air! /s but also 🤔
zedkyuu@reddit
Does she avoid being indoors with people as well? I'm guessing not...
OldEquation@reddit
Or even outdoors on the same planet as other people.
qdp@reddit
Oh yeah. Well crank that air vent open. Its a mix of outdoor air and HEPA filtered air.
IdeaEmbarrassed7552@reddit
The moisture inside the aircraft is however other people's breaths.
nasadowsk@reddit
Is that the one with the angry looking teeth on it, to keep hands out?
SnazzyStooge@reddit
On a wide body there are two outflow valves, each the size of a door (like the front door of your house). Although a wide body is huge, and has a huge volume of air inside, these outflow doors are also massive compared to, say, a tiny crack in a fuselage seal.
Code_Operator@reddit
The 787 has forward and rear outflow valves. The FAA requires 0.55 lbm/min of fresh outside air per passenger, so at least that much will be vented overboard.
FarButterscotch4280@reddit
Outflow valve is bounded by the red rectangle that is below the aft door. Above it and to the right, all red paint, is the inflow valve. Operated by a simple spring still, I think.
Alive-Sir-4606@reddit
I like how you didn’t answer the question at all and just provided some random fact.
whoaitsjello@reddit
There’s a constant controlled leak via outflow valves and that is what regulates cabin pressure. There are definitely other leaks on aircraft such as door seals, emergency exit seals etc… luckily you got those massive air pumps bleeding pressurized air into the cabin.
Chemtrailcreator@reddit
Most planes, especially as they age, leak all over the place but it doesn’t matter. The packs have way more than necessary to compensate and the outflow valve will just adapt as required
SheepherderFront5724@reddit
I heard that the 747 could theoretically maintain pressure with 6 windows missing. Seems like a high estimate, but I guess if we consider the sum of 500-or so cabin vents its maybe possible....
saintofchaos207@reddit
As a 747 mechanic, 6 windows sounds slightly exaggerated, but I could believe 4 maybe 5 windows as you can have one of the 3 packs INOP with no operational limits.
SheepherderFront5724@reddit
Interesting, thanks!
discombobulated38x@reddit
That would be a differential pressure of 10PSI, you would very much not want to be sat next to that window...
toybuilder@reddit
Yikes. DeltaP in the sky.
PhysicsDude55@reddit
The concord's windows were sized such that 1 window could blow out at 60,000ft and the pressure could be maintained at a breathable levels since it would take multiple minutes to descend to ~10,000ft.
Rocketsponge@reddit
Most of the Time of Useful Consciousness charts don't even go up to 60,000 feet. The best info I could find was less than 5 seconds following a rapid decompression.
SnazzyStooge@reddit
Nose down with full takeoff power would probably take less than that.
Bonus would be the 13-g pull at 10k to avoid the ocean!
PhysicsDude55@reddit
According to Google, Concord's emergency descent procedure was 7,000fpm due to it's complicated trim system, and it would need to decelerate considerably from its cruise speed when reaching lower altitudes.
bherman13@reddit
There would be way more than 13 G's when the plane disintegrates and your body hits the water.
SnazzyStooge@reddit
Bigger number = better
zwifter11@reddit
I’ve heard some ships (the maritime kind, not airships) can have a breach in the hull and be leaking water and still pump the water out fast enough to stay afloat.
The_Ashamed_Boys@reddit
It has 3 packs, so I wouldn't doubt it.
Cammy66@reddit
Now ask about submarines!
zwifter11@reddit
I once knew a submariner. He said his job was look after the CO2 levels. He said a high CO2 level was the biggest killer after an onboard fire (which would quickly consume all the oxygen). Interesting that submarines use CO2 scrubbers and light a chemical candle that produces oxygen.
clippervictor@reddit
Fun (although unrelated) fact: high speed trains pressurize/depressurize several times during any given trip. I get an alert in the cab when it’s been pressurized for too long and CO2 levels start to rise. Since we don’t have any PACK other than the normal HVAC system when that happens, there are some trapdoors that open to let fresh air in.
TravelerMSY@reddit
I believe the oxygen generators for the in-flight emergency masks work similarly it’s a chemical process and not every mask connected to a giant tank somewhere
Temporary-Fix9578@reddit
Do they leak a little water inside for the crew to drink? /s
hoveringuy@reddit
I was on USS Puffer as a mid.
12 of the 13 people in the conning station were chain-smoking. (I was #13).
I was controlling the depth or something...
I selected pilot and never looked back ....
AutothrustBlue@reddit
There are more airplanes in the sea than submarines in the sky at any given moment in time.
Python_07@reddit
Just follow the bubbles……..
anactualspacecadet@reddit
The last s doesn’t stand for anything
clippervictor@reddit
What’s a workable ΔP?
PilotBurner44@reddit
The older they get, the more they leak. And when they leak around a door or window seal, they usually make a God awful high pitch shrieking noise. So when you see someone shoot a hole in the side of the plane and it magically explodes into a massive hole with all the air (and chairs) rushing out, it's completely fictitious. A small bullet hole is nothing the packs can't keep up with, and a small round hole in the aluminum skin isn't going to create a catastrophic hull breaching failure. But no, airplanes aren't really "balloon" air tight
Czexican613@reddit
Okay but what was the name of the movie because that sounds hilarious.
CrappyTan69@reddit (OP)
I've really tried to find it over the years because I want to rewatch it for the hilarity of it. I've failed.
The most memorable moment was them standing around the bar in first class, pouring over some technical drawings trying to work out an escape and a menacing drip appears on the paper....
daygloviking@reddit
It’s one of the Airport disaster movies. They’re all a blast so go find them all!
fastwhipz@reddit
As my sailer father says, all ships make water. It’s true for airplanes too. The cargo doors in the planes I fly you can see the light coming through and hear the wind whistling. You’re right that the pressurization system is able to overcome the normal outflow of air.
The idea in movies that you shoot a gun in a plane and it just explodes is crazy in reality a couple bullet holes wouldn’t be much more than the normal amount of loss and the airplane wouldn’t notice you’d just hear some whistling if you were sat next to it.
pixel293@reddit
So I was on a flight from California to Massachusetts. The captain came on and did the normal spiel, "We at our cruising altitude of XXXX and flight time will be YYY..." They start serving drinks and then suddenly we start descending quickly, like nose down fast.
I'm looking around wondering if anyone else is wondering why they just said we're at our cruising altitude and then started descending. Anyway we level out, the captain comes on and says there's an issue maintaining the air pressure and we'll be traveling at 10,000 feet for the rest of flight.
When I was getting off I overheard the Captain talking with maintenance, "I don't know, there's a hole in it somewhere."
Feeling-Ad-2867@reddit
No it’s not fully air tight. There’s a valve that releases air as more air comes in.
moofie74@reddit
More air changes/hour than any building you’ve ever been in.
LoneGhostOne@reddit
Nothing is completely airtight. Spacecraft, the ISS and space suits all leak air, they just keep up with the leaks.
But as others have mentioned, the air change per hour of a plane is quite high, IIRC an order of magnitude over a commercial building
More_Card_8147@reddit
Yep yep.
There's even a component called an outflow valve which modulates between fully open and fully closed to control the rate of leakage and maintain a set cabin pressure and those outflow valves can get pretty big. A 747 has 2 that are about 8 inches by 12 inches or so.
sir_thatguy@reddit
Unless it’s an Airbus and they’re preparing to go swimming, the outflow valve doesn’t close during normal operation.
More_Card_8147@reddit
Unless you're in a C-130E at 28,000 feet, then it's all the way closed and you're still watching cabin altitude climb.
SkippytheBanana@reddit
Yep! The 145 Galley Door is notorious for leaking badly enough to have a deafening whistle at cruise. The FAs would often stuff the seals with towels or napkins to try and lessen the leak. I’ve also sat next to doors that have a noticeable draft.
saintofchaos207@reddit
Yes, almost every airliner is not perfectly airtight. We actually control cabin pressure by regulating the rate air exits the cabin with something called an outflow valve. Its a small door usually located under the tail just before the rear pressure bulkhead.
Any_Table_3591@reddit
In maintenance we would pressurize the aircraft on the ground and watch the leak rate in feet per minute. Depending on the aircraft it was normal to see 2-3,000 fpm leak rate.
Fourteen_Sticks@reddit
Part of a production test flight (the ones done before the aircraft is delivered to a customer) is to measure the leak rate of the cabin.
Climb to a specified altitude, shut off the packs/bleed air valves, start a timer and see how long it takes to go from the max PSIΔ down to a specified PSIΔ.
DarkGinnel@reddit
You don't have to fly the aircraft.
Cabin leak checks can be done on the ground.
Code_Operator@reddit
LOL don’t forget the KC-135 they popped during a ground test.
oopsie!
retard-is-not-a-slur@reddit
Article says “An incident like this is never funny.”
Do not agree with that. It looks like one of those Pillsbury biscuit tins that you explode open.
Apprehensive_Cost937@reddit
They can, but they are also done in flight sometimes.
Variation_Conscious@reddit
From 89-91 I was a Licensed Industrial Radiographer and did NDT(non destructive testing) and shot a few airplanes looking for cracks and other defects. This was part of maintenance depending on flight hours on the airframe. The pressurized and depressurize can do damage to seals along the joints and around windows.
You definitely want to know if there's defects that can cause sudden depressurization which can unalive everyone on board. That's what happened to the professional golfer Payne Stewart's plane. After the rapid depressurization the windows of the small passenger jet he was on along with several others on board.
The military scrambled 2 jets to check on the private jet and the military members who got eyes on the plane knew that's what happened due to the windows being frosted up from the inside. The passenger jet finally ran out of fuel and crashed afterwards.
Hope this helps you understand why aircraft go through the NDT that's part of normal maintenance depending on hours of flight.
Vee-One-Rotate@reddit
A good cabin leak rate is 1,500 FPM. A typical airliner or holding around 7,000 at normal cruise altitude. Doesn’t take long to get to a cabin altitude of 10,000-15,000’ where the mask drop.
I used to do pre-purchase test flights and the cabin leak rate was always one of the things they asked us to check. Anything above 3,000 and it was a hard pass on the purchase.
britishmetric144@reddit
Aircraft are not airtight. If they were, carbon dioxide would build up to unhealthy levels quite quickly. (In fact, this became a problem during the Apollo 13 spaceflight).
Instead, the aircraft's pressurisation system constantly pulls in outside air, as well as "bleed" air from the engines, to keep the cabin pressure at a safe and comfortable level.
There is an opening on the outside of the aircraft called the "outflow valve", which regulates how much air enters versus leaves the aircraft, and maintain a constant cabin pressure.
rygelicus@reddit
The section labeled Valve is the 'leak'. The system, or the crew, opens that valve as needed to increase or decrease pressure in the plane. They can also stop the airflow into the cabin by turning off the packs if keeping that valve open doesn't get pressure to reduce quickly enough.
There will be other leaks in the structure but that's the official one controlling cabin altitude.
Pynchon_A_Loaff@reddit
There’s at least one large outflow valve that regulates cabin pressure. It allows a controlled amount of air to escape overboard. This is the “leak” that balances the ECS pack inflow.
zwifter11@reddit
As an engineer we‘d pressurise certain systems and then monitor a pressure gauge over, 15 minutes / 30m minutes etc etc. I remember there was an acceptable leak rate, a drop of so many psi after so long.
Like you said, although the system is not air tight, the leak isn’t massive and it’s within what the bleed air / environmental system could provide.
Ive heard of pilots snagging window seals / canopy seals for “being excessively noisy”.
Mendo-D@reddit
Probably leaks some, but there's an outflow valve at the back of the plane.
CL350S@reddit
Several years ago a buddy of mine was flying a long leg across the country in a Lear in the middle of the night . As people sometimes will, he got to talking with his SIC about “wonder what would happen if…”
Well, in this case the discussion centered around this very subject, and they were wondering how fast the cabin would leak down if they shut off both engine bleeds. Mind you, they’re at 45,000 feet.
They talk themselves into trying it. My buddy said the switches had no more than hit their limit of travel when all the moisture in the air condensed, and they couldn’t see shit. Of course he panicked and turned them back on immediately. Unfortunately the bleed system in that plane would recognize a high cabin altitude, and go to max bleed automatically, so now they’re getting hot unconditioned air while the pressurization system tries to recover the cabin. So they went from freezing cold to 60 degree Celsius air in a matter of moments. They ended up having to come up with a ruse with ATC to descend to let the cabin get back up to pressure.
TL:DR some of them leak a lot.
jorblale@reddit
Nah, it's just a glorified eacig at this point.
LostPilot517@reddit
You are correct in your thinking. The packs are constantly delivering airflow for conditioned air and pressurization.
The fuselage utilizes this air pressure and differential for numerous purposes, but it is constantly dumping essentially an equal amount of air being pumped in, overboard. The main source for dumping excess air and controlling the pressure in the cabin is the " Main outflow valve." Some aircraft have multiple outflow valves and some of these close in flight utilizing fewer valves when airborne to stabilize the pressurization.
Most modern aircraft toilet systems are vacuum powered, this system takes advantage of the pressurization differential of the cabin and outside pressurization in flight by dumping air overboard via the vacuum toilet system. Solids and liquids stay contained in the waste system.
The lavatories and galley sink drains, and the floor drains near the doors are constantly venting air overboard, which is where this excess liquid goes overboard. The lavatories also have air vents to constantly pull air out of the lavatories like a fan in a bathroom to pull odors and exchange the air with fresh air.
There are other uses for bleed air, such as pressurizing the potable water system, or providing pressurization to the hydraulic reservoir to keep the hydraulic fluids from boiling and frothing. Anything being pressurized must have a vent.
The fuselage is otherwise tight, but minor leaks are generally inconsequential, however leaks near seals and places not designed to be a vent can create loud buzzing, squeals, and other nuisance noises, or be a source for water, deice fluid intrusion when the aircraft isn't pressurized and can be a future source and location for corrosion, so being sealed and tight is important.
Raguleader@reddit
Fond memories of riding in the back of a C-17 returning from Kandahar and figuring out that you could feel a draft blowing out through the edge of the cargo door.
jjamesr539@reddit
It’s not airtight, but it can be. Pressurization is maintained by the engines constantly dumping air into the pressure vessel through the packs, typically with 2 or more outflow valves, one or more controlled by a CPAM computer, and one or more that opens mechanically when the limit pressure differential is exceeded. If the packs were shut off in flight, the pressure vessel is airtight as both valves will close. There will be some incidental leakage around door seals and the valves themselves, but thats a result of wear over time and is one of the things checked during regular maintenance intervals. There’s an envelope of allowable air loss, but it’s nominally airtight.
Masked_Squirrel@reddit
I’ve sat next to an emergency exit and could hear the whistling of air escaping the whole flight. Figured it had made the previous flight ok, but it was loud enough to be a bit annoying.
wild-and-crazy-guy@reddit
There is a pressure relief valve set to a specific value to prevent the cabin from being over pressurized . The cabin will leak a little all over the place but mostly around the door seals.
sir_thatguy@reddit
Yes but it’s a safety valve in the event of a failure of the outflow valve system.
Stinkin_lincoln42@reddit
Yes your thinking is correct.
Alive-Sir-4606@reddit
They do leak and the maintenance manual provides the acceptable leakage rate when you do a cabin leak test on the ground. You get it to a certain psi, close the outflow valve completely and then turn off the packs and time how long it takes to bleed down.
dumbassretail@reddit
If it was fully airtight, the pressure would just keep increasing until something burst.
Icy_Huckleberry_8049@reddit
there's always leakage around the window exits and around the doors.
griplooma@reddit
Nah, planes leak more than my old Nokia battery.
DoomWad@reddit
Adjustable outflow valve. It's automatic 99.9999999% of the time
MTX-Prez@reddit
I went on the G Force One 727. Zero Gravity flights that use a modified B-727. During the neg G maneuvers I could see outside sunlight and hear a lot of wind noise through the fuselage gaps. I was on the left side right in front of the wing. During the higher G maneuvers it would go away :)
YamComprehensive7186@reddit
The older ones leak when you pull the thrust to idle.
gottatrusttheengr@reddit
Every mechanical assembly leaks, even space stations. The leak rate is a defined requirement that can be inspected during assembly by seeing how much air volume needs to be added
Pitaraq@reddit
Hmm, I work for a company building deep sea robots. It would be disastrous if ours leaked, so I’d say not all mech assemblies leak
gottatrusttheengr@reddit
A watertight assembly isn't always airtight. Also we're talking leak rates on the order of .1 gram over a day. I highly doubt your seals on rotating assemblies and cable passthrough don't leak a microscopic amount like that.
Pitaraq@reddit
That’s a good point, water molecules are somewhat larger that most gasses. Interestingly enough we perform initial seal tests by drawing a vacuum equivalent to around 10k altitude, so not unlike a typical cruise alt. We expect to see no detectable pressure change over the few hours of testing. We use magnetic couplings for rotating assemblies, and glass sealed penetrators for electrical pass throughs. So we are actually pretty airtight as well.
gottatrusttheengr@reddit
Depending on your test volume, it can be a long time before your leak rate is even detectable. Most of our sealed volumes need to be under vacuum for 10+ hours before the theoretical leaked volume exceeds the margin of error on the pressure gauge.
YU_AKI@reddit
In the cockpit of an old Delta DC-9 in 2010 I could clearly see the gap under the windshield surround in the cockpit.
The captain said it barely ever made a noise and didn't leak much. Makes sense when the pressurised air was pushing outward.
Python_07@reddit
In a basic sense, yes. As long as the engines are running to supply the bleed air…..
Go_Loud762@reddit
Except for the B787. It doesn't use bleed air.
Python_07@reddit
Yes. I always forget that….
Every-Progress-1117@reddit
Except if the engines stop running you don't really worry about air for the pax, there are other things to worry about.
Habsin7@reddit
Your think is correct. When I was in flight test we tested leak rate at altitude with packs off to make sure cabin climb rate wasn’t was not excessive (don’t remember the exact spec). After that check I seem to recall we opened the outflow valves to check masks dropped on schedule.
Eastern-Ad-3387@reddit
We once had a brand new MD80 that had a loud whistle at specific differential cabin pressures. It was at the rear of the cabin and, due to pack noise, wea imposed to locate, so we decided to try to soap the aft fuselage to see if we could find the leak causing the whistle. The hangar looked like an old Lawrence Welk rerun. (YouTube search it out). They leak like a sieve. Packs definitely overcome leakage.
Strega007@reddit
Yes, pressurization is controlled by the outflow valves.
pjlaniboys@reddit
I was once flying europe to SFO and somewhere north of Chicago we suffered a double outflow valve failure. Uncontrollable decompression followed be a rather calm emergency descent to 10000' where we continued to SFO. Rather cool. Except for the pax and crew for the return flight as it took a substantial delay waiting for the replacement valves to get found, flown in and installed.
punkslaot@reddit
Leaks less than the packs push. Theres an outflow valve that is always letting air go out
External-Creme-6226@reddit
Supposed to leak, otherwise the cabin air would get refreshed! Just needs to leak slower than the packs can replenish. It’s the reason most planes have an altitude restriction for single pack operation. Most jets I’ve flown can go up to the mid to upper 20k’s on a single pack if the second is on MEL
Apprehensive_Cost937@reddit
It is definitely not 100% sealed, not even when it comes from the factory. As aircraft age, the seals deteriorate, but generally isn't an issue.
There is an acceptable level of leakage with packs off that is tested either during a test flight or during a ground pressurisation test. Newer aircraft might leak at a rate of around 1500ft/min (cabin vertical speed), while older ones will be more than 2000ft/min.
the_real_hugepanic@reddit
I just remember a hole in the bracket for the lever of the waste-water system on the A380.
We, primary structure design, try to seal every riv-nut, and the "systems-guys" drill holes to drain water.... We had our fun....
radarlock@reddit
They have an outflow valve to regulate pressure so no, they are not airtight normally.
69Hugh_Janis69@reddit
Yea some minor leaks are normal