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Pressure Bulkhead cutaway view

Image source: FAA

What part of the fuselage aft of the bulkhead would leak pressure? Or is it just there to reduce stresses?

How does a DC-9/MD-80/90 incorporate aft bulkheads when there's a staircase in the way?

enter image description here

(Image source)

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What part aft of the bulkhead would leak pressure?

That's a partial misunderstanding of what a bulkhead is there for.

You could build the aft cone section to keep the pressure, but it would be a much heavier solution.

The shape of the final aft section is not well suited to resist pressurization stresses: the best shape is a sphere; the cylinder (with spherical terminations) comes a close second. The conical shape would require serious stiffeners to survive pressurization cycles for the whole life of the aircraft; the bulkhead solves this problem by using a shape that is naturally more resistant to stresses - and thus can be built with less material - leading to less weight, and hence fuel savings (in addition to the increased safety).

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    $\begingroup$ It's worth making a rough estimate of just how big the force on such a bulkhead is. At sea level, atmospheric pressure of 14.7 psi is roughly the same as 1 ton per square foot. The difference between the internal and external pressure at cruising altitude is about half that value. The fuselage diameter of a B777 is about 20 feet, so the area of the bulkhead is about 300 square feet. So the total force on the bulkhead is about 150 tons. Compare that with the max takeoff weight of the plane, which is about 250 tons - it's a seriously large number. $\endgroup$ – alephzero Sep 7 '16 at 0:39
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    $\begingroup$ Maybe it's also worth to mention that most planes have mechanical systems penetrating the aft cone, like trimmable stabilizers. Those would have to be airtight too if the cone would be pressurized. $\endgroup$ – dronus Jun 17 at 20:04
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You can think of an airliner (or any other pressurized airplane, or a submarine) as a pressurized container with control surfaces and a nosecone stuck to it.

Rather like a submarine, an airliner has a floor with seats, a nose to make it aerodynamic, wings for lift, and a tail section for control (yes, I know I am way oversimplifying it, and that's the whole point)

So the bulkhead IS the "tank" and the tail is just added to it. As for why it is shaped that way, it was answered in the other posts. A sphere is stronger and so it is used for deep subs for instance.

This image might help you picture what I mean; it's easy to see the "tank": enter image description here

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    $\begingroup$ +1 for the cool cutaway picture. Since it's a perfect circle I'm guessing it's a 777/787? $\endgroup$ – dkwarr87 Sep 7 '16 at 13:50
  • $\begingroup$ @dkwarr87 I'm thinking it's an A300, based on the 8-across seating and the shape of the engine pylon. Not to mention having an LD3 with a "Lufthansa" logo. $\endgroup$ – pr1268 Nov 10 '18 at 22:23
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Pressure bulkheads are the primary structure members which combined with a fuselage or cabin provide a sealed pressure vessel and carry the fwd and aft pressure loads when the cabin is pressurized - think of them kind of like the end caps on a cylindrical air storage tanks on an air compressor.

As for the aft stairs on a DC-9 or a 727, the stairs are aft of the aft pressure bulkheads and accessed through a pressure door in the aft pressure bulkhead, as in the example from a 727 below.

enter image description here

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A dome is one of the most resilient and versatile shapes in engineering. It is the ideal shape to resist internal cabin pressure. When you blow a balloon it fills out into a sphere too. So the bulkhead can be built with highest efficiency weighing the least.

It is also an integral structural member of the frame to help stability of fuselage against local buckling and articulate transition to tail section which is a totally different geometry.

As for the steps, they have latches and when closed snap tight, similar to the main doors. And become a continuous integrated shell with the fuselage.

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I think what isn't clearly mentioned in other answers (despite those answers being correct) is that the structure rearwards of the aft bulkhead isn't pressurised.

Whilst at cruise level the cabin will be maintaining a pressure equivalent to roughly the altitude of 8,000ft (cred @ vasin1987); the structure rearward of the aft bulkhead i.e. the tail cone, will be at the cruise altitude's external atmosphere's pressure.

Therefor there is no pressure to leak out of that tail cone section of the fuselage as the outside and inside pressure for that section of fuselage are the same.

As others have mentioned, the justification for this would be the costs and challenges in making a difficult shape capable of resisting the pressure imposed on it.

NB, I wouldn't advise opening the bulkhead's pressure door at cruise altitude because on one side will be the cabin pressure and on the other the external atmosphere's pressure!

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    $\begingroup$ @vasin1987 Yes, thanks :-) $\endgroup$ – Edward Comeau Sep 12 '16 at 12:34

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