Why don't airplanes explode due to pressure difference?

[thommy]

My question might sound trivial (I'm just a first year physics student anyways) , however I really feel the need to get an answer.

Since the pressurized cabins inside an airplane are regulated at about 0.8 atm while the pressure outside doesn't surpass 0.2 atm (at 35 000+ ft of altitude), shouldn't the airplane's body simply explode (stronger forces pushing outwards)? Why doesn't such an event ever occur?

Thanks to everyone in advance!

 

[hutchphd]

The first passenger jet, the de Havilland Comet did just that several times. It was a beautiful airplane.

 

[berkeman]

The first passenger jet, the de Havilland Comet did just that several times. It was a beautiful airplane.

Wait, how does an airplane explode several times? And why did crews keep trying to fly it? Just curious...

 

[berkeman]

Welcome to the PF.

My question might sound trivial (I'm just a first year physics student anyways) , however I really feel the need to get an answer.

Since the pressurized cabins inside an airplane are regulated at about 0.8 atm while the pressure outside doesn't surpass 0.2 atm (at 35 000+ ft of altitude), shouldn't the airplane's body simply explode (stronger forces pushing outwards)? Why doesn't such an event ever occur?

Thanks to everyone in advance!

Why don't all submarines implode? Why don't all vacuum chambers implode? Why don't my scuba air tanks explode? What could be going on here?

 

[russ_watters]

Wait, how does an airplane explode several times? And why did crews keep trying to fly it? Just curious...

Different crews.

 

[hutchphd]

Wise guys. Allow me to rectify (and expand for the OP) the statement (see you'll be sorry...)
The story of Comet is one of those "every engineer should know by heart" tales of just a little too much in one design bite. The airplane was a marvel of new technologies including new aluminum alloys and stretched skin structure in addition to the jet power. The fleet was initiated and flew well for about a year (?as I recall)) and then the planes started coming apart at altitude. It turns out some window casings were not rounded enough and fatigue cracks developed. How this was tracked down is a great tale. Check it out.
I think this unfortunately initiated the sad demise of British commercial aircraft industry

 

[sysprog]

My question might sound trivial (I'm just a first year physics student anyways) , however I really feel the need to get an answer.

Since the pressurized cabins inside an airplane are regulated at about 0.8 atm while the pressure outside doesn't surpass 0.2 atm (at 35 000+ ft of altitude), shouldn't the airplane's body simply explode (stronger forces pushing outwards)? Why doesn't such an event ever occur?

Thanks to everyone in advance!

You're right to wonder about the consequences of the pressure difference $-$ it's a constraint in the design of aircraft that requires that they be made sufficiently robust to withstand that difference $-$ similarly to @berkeman's air tank.

 

[berkeman]

The fleet was initiated and flew well for about a year (?as I recall)) and then the planes started coming apart at altitude. It turns out some window casings were not rounded enough and fatigue cracks developed.

So which is it young feller? "Coming apart" and "cracks" or "Exploded"? I not a wise guy...

 

[hutchphd]

We'll just say they cracked up...

 

[Lnewqban]

... shouldn't the airplane's body simply explode (stronger forces pushing outwards)? Why doesn't such an event ever occur?
...

Welcome, thommy!

The metal that forms the structure and skin resists that tendency.
Please, see:
https://www.efunda.com/formulae/solid_mechanics/mat_mechanics/pressure_vessel.cfm

https://www.engineeringtoolbox.com/stress-strain-d_950.html

 

[phinds]

It turns out some window casings were not rounded enough and fatigue cracks developed.

Reminds me of what I tell people about my undergrad course in Fourier Analysis and Boundary Value Problems: All I learned from the course was that if you ever get on a plane with square windows, get off again immediately and trampolines don't make good musical instruments.

 

[thommy]

Welcome, thommy!

The metal that forms the structure and skin resists that tendency.
Please, see:
https://www.efunda.com/formulae/solid_mechanics/mat_mechanics/pressure_vessel.cfm

https://www.engineeringtoolbox.com/stress-strain-d_950.html

Yeah, I was wondering if it had to do anything with elasticity. That's what I was basically asking. I'm currently reviewing my (classical) mechanics textbook!

Thanks to everyone for welcoming me and taking part in the discussion!

 

[phyzguy]

It does happen. Another example was Aloha Airlines flight 243. After repeated pressure cycling from many flights the aluminum fatigued and the roof blew off the plan in mid-air. The plane landed safely, but one flight attendant was literally blown out of the airplane in mid-air and was killed.

 

[Flyboy]

A bit late, but something to add to the discussion here as an A&P...

All pressurized aircraft have a design limit to the pressure differential between the inside and the outside, usually shortened to the phrase "delta P". To ensure the pressurization system never exceeds this limit, there's a pressure-driven portion of the cabin pressurization controller that will actuate at a certain point just under the design limit. Even within a manufacturer's family of designs, like the Cessna Citations for example, that precise value will vary from design to design. For the Citations, that can be anywhere from 8.3 psi on the smaller birds like the Mustang and the CJs, all the way up to 9.3 on the Citation X. The outflow valve(s) that manage cabin pressure can sense the delta P, and will open up rather abruptly if you attempt to overpressurize the cabin. These valves are checked regularly during scheduled inspection to ensure they work properly. Sometimes, they don't, usually because of damage, like a mouse deciding to nibble its way through the diaphragm of the valve to get into the contents of the snack bar!

And it's absolutely possible to overpressurize the plane if you disable those safety valves... as a maintenance crew working on a KC-135R found out the hard way 20ish years ago:

 

[sysprog]

@Flyboy, What do you think about the FAA belatedly mandating that cargo bay doors flange to plug the ports, and open inward instead of outward, despite the cost of interior space, so that a greater pressure inside serves to more firmly keep the door closed, instead of tending to force it open; and what do you think of the comment of @phinds to the effect that portals with square corners are not to be trusted? I agree with the FAA and with @phinds on these . . . and it would have been better if that FAA mandate had been sooner . . .