Why don't airplanes explode due to pressure difference?

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Discussion Overview

The discussion centers around the question of why airplanes do not explode due to the significant pressure difference between the pressurized cabin and the low external pressure at high altitudes. Participants explore various aspects of aircraft design, structural integrity, and historical incidents related to pressure differentials.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Historical

Main Points Raised

  • One participant questions why airplanes do not explode given the pressure difference, noting that cabins are pressurized at about 0.8 atm while external pressure is around 0.2 atm at cruising altitude.
  • Another participant references the de Havilland Comet, which experienced structural failures due to design flaws, raising questions about the implications of such failures.
  • Some participants discuss the concept of "delta P" (pressure differential) and how aircraft are designed to withstand these forces, mentioning safety features like pressure-driven cabin controllers and outflow valves.
  • A later reply mentions the Aloha Airlines flight 243 incident as an example of pressure-related failure, highlighting the risks of fatigue in aircraft materials.
  • Participants express curiosity about the structural materials used in aircraft and their ability to resist pressure differences, with one noting the importance of elasticity in this context.
  • There are mentions of design considerations for cargo bay doors and the implications of square corners in window designs, suggesting a focus on safety and structural integrity.

Areas of Agreement / Disagreement

Participants generally agree that aircraft are designed to handle pressure differentials, but there are multiple views on the historical failures and the specifics of design safety features. The discussion remains unresolved regarding the implications of past incidents and the effectiveness of current safety measures.

Contextual Notes

Some participants reference specific incidents and design features without reaching a consensus on the broader implications for aircraft safety. There are also mentions of historical failures that highlight the complexity of aircraft design and the challenges of material fatigue.

thommy
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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!
 
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The first passenger jet, the de Havilland Comet did just that several times. It was a beautiful airplane.
 
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hutchphd said:
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... :wink:
 
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Welcome to the PF. :smile:
thommy said:
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? :wink:
 
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berkeman said:
Wait, how does an airplane explode several times? And why did crews keep trying to fly it? Just curious... :wink:
Different crews. :wideeyed:
 
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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
 
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thommy said:
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.
 
hutchphd said:
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... :wink:
 
We'll just say they cracked up...
 
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  • #11
hutchphd said:
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. :smile:
 
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  • #12
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  • #13
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.
 
  • #14
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:

-Stratotanker-Exploded-due-to-a-Stupid-Mistake-1-1.jpg
 
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@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 . . .
 
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