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How does landing with slight positive pressure ensures structural integrity?

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How does landing with slight positive pressure ensures structural integrity?

Old 6th Sep 2017, 03:24
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How does landing with slight positive pressure ensures structural integrity?

Hi,

As the topic stated, more so for the B777, why does the aircraft lands with a slight positive pressure?

Thanks
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Old 6th Sep 2017, 03:52
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Ever try to crush a full Coke can vs. an empty one?

Now I can't say if that's the reason any plane lands that way as opposed to some other, less obvious reason; nor can I say whether it makes any non-negligible difference in a semi-monocoque airliner, but it's true. It's well known that the first generation Atlas rockets had such thin tank walls that they could not stand unsupported while empty, for they'd collapse. They relied on the pressure from the propellants (or tank pressurizarion inert gas) for their structural integrity. They called them "balloon tanks."
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Old 6th Sep 2017, 15:27
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I'd say, structural integrity is not the reason.

The way it was explained to me, positive px on takeoff and landing is to avoid uncomfortable pressure bumps, that could happen on rotation and flare, due to changes in the airflow around the fuselage.
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Old 6th Sep 2017, 15:42
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(Note to myself .. RTFQ....trying again)

Can't see why structural integrity enters into it, for some emergencies, possibly when you want things stiffest.. you are deliberately going to land completely depressurised. FWIW the 777 FCOM states: "For takeoff, the system supplies a small positive pressurization prior to rotation to cause a smooth cabin altitude transition to the cabin altitude climb schedule" and one would expect similar logic to apply on landing, to stop any of those pressure bumps/transients.
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Old 6th Sep 2017, 16:42
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All small business jets back to the Citation II in my experience put a small positive pressure at some point prior to take-off, so it's related to pressure bumps, not structure. The B727 had the outflow valves in the shaft and it was explained to me, on rotate the valves could be opened by airflow if the differential were not there causing bumps in the cabin.

OTOH, cabin pressure was a requirement for flight in the C-5 for structural reasons. Unpressurized flight required a Lockheed/USAF waiver.
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Old 6th Sep 2017, 17:03
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Therer could be a structural argument (I don't know, but I can see how it might work). In the upressurised form the fuselage resolves bending loads into tension/compression in the frames, stringers, skins etc. Tension loads are no problem because they are handled by the tensile strength of the material, but compression loads in thin structural elements are primarily handled by the *stiffness* of the structure.

The failure mode of a compression failure in an aircraft component is usually buckling rather than a material failure (which would produce the pyramid forms you see in stone and concrete building failures). A sudden bending load - like the way the fuselage reacts to less-than-soft contact with the runway - will tend to produce compression and buckling failures in the lower fuselage. The harder the landing the greater these compression loads. Pressuring the fuselage biases the tension and compression such that the the smaller loads become "reductions in tension" rather than "compression". The internal pressure would also provide lateral support to skins and stringers that effective forms extra bracing that opposes buckling.

Fuselage bending also tends to "oval" the round fuselage tube, reducing its volume. If the fuselage is pressurised this tendency will be opposed by the pressure of the internal air because (again) it provides additional internal "bracing".

There are several of the liquid-fuelled ballistic missiles which use the pressure in their propellant tanks as a structural component to the point that they literally cannot be moved, or laid flat, without pumping up the pressure in the tanks with nitrogen or similar.

So I can see how it might work, but do not know whether that's the specific reason in this case.
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Old 6th Sep 2017, 22:41
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s the topic stated, more so for the B777, why does the aircraft lands with a slight positive pressure?
Please elaborate. Positive pressure, altitude pressure?

There are plenty of airports in the World where the typical altiitude pressure is beyond the typical cabin pressurization.

Landing at Palm Springs is different than landing at Cuzco (10,800) or Lhasa (11,800)

Last edited by underfire; 7th Sep 2017 at 00:00.
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Old 7th Sep 2017, 00:27
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I've never heard the structural integrity argument before.

The only reason I can think of is that a bit of positive pressure stops anyone opening the cabin doors. This is in the days before flight locks.
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Old 7th Sep 2017, 00:49
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I've read about light aircraft with tubular steel frames that are sealed and pressurized. As long as the frame stays pressurized, you know there are no cracks (or at least no full-thickness ones). That doesn't, of course, explain why a 777 would land with a slight positive pressure. But maybe it could be the source of a confused association between pressure and structural integrity?
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Old 7th Sep 2017, 10:08
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Nothing to do with structure; all about pax comfort.
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Old 7th Sep 2017, 10:16
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If it was structural you would have to think about the unpressurised failure mode anyway. So no, it's not structural. It's pax comfort.
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Old 7th Sep 2017, 10:33
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Absolutely agree with Hoppy, releasing pressure gently doesn't hurt peoples' ears. The other way round can.

That's why it was always ridiculous to dish out boiled sweets for the take-off and climb as used to happen eons ago . . .
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Old 7th Sep 2017, 12:11
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Think of the location of the outflow valves - so if fully open/unpressurised as the aircraft approaches flare and enters ground effect....
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Old 7th Sep 2017, 14:36
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Maybe it's best to have some internal supporting pressure inside the tube...




.
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Old 8th Sep 2017, 04:56
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Positive Cabin Pressure Differential on ground.

I can't comment on modern two-pilot RPT aircraft, however in the days when we old coffee drinking, sandwich eating Flight Engineers were carried I know we did not pressurise the airframe during ground operations. Depending on which aircraft being flown, e.g. C130 Hercules to B747 and others in between in my case, the pressurisation was not introduced until airborne. This may have been via manual selection or automatically once airborne. There was no structural reason ever mentioned, primarily the operation of doors was the consideration. Plug type doors would be difficult to open with positive differential pressure applied. Flight engineers, at least those I know and worked with took pride in making the cabin environment as comfortable as possible by handling the temperature and pressurisation systems with thought and care. In some cases where performance requirements demanded it we would do "No Bleed Offtake" take-offs to get maximum engine performance, which I would suggest rules out any structural considerations in relation to the pressurisation system during ground operations.
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Old 8th Sep 2017, 05:06
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Outflow Valves.

Originally Posted by PW1830 View Post
Think of the location of the outflow valves - so if fully open/unpressurised as the aircraft approaches flare and enters ground effect....

PW1830. By "Outflow Valve location" I assume you meant "Outflow Valve position in relation to being between Open and Closed". If you operatd DC3's or C47's as your Pen name suggests you had no such worries. Cheers Old Fella.

Last edited by Old Fella; 8th Sep 2017 at 05:17.
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Old 8th Sep 2017, 06:22
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Originally Posted by Chu Chu View Post
I've read about light aircraft with tubular steel frames that are sealed and pressurized. As long as the frame stays pressurized, you know there are no cracks (or at least no full-thickness ones).
Been around tube framed aircraft a long time, even own one, and I have not heard of pressurizing structural tubing. So, if it's true what you say that the pressurization is a check on cracks, how exactly does that work? Is there also a pressure gauge on each tubing segment? If not, how does one tell if the tube remains pressurized or not?
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Old 8th Sep 2017, 07:32
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The Zlin Trenner (226/526 etc) aircraft (and the later 142/242 series) have this system on the tubular mainspars IIRC. From the Pilot flying review:

There is also a canny main spar pressure gauge fitted in the front cockpit and this illuminates should the sealed, gas tight, metal spar crack from being over stressed.
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Old 8th Sep 2017, 14:24
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Originally Posted by Old Fella View Post
PW1830. By "Outflow Valve location" I assume you meant "Outflow Valve position in relation to being between Open and Closed". If you operatd DC3's or C47's as your Pen name suggests you had no such worries. Cheers Old Fella.
Its is not valve position (open or closed) it is the valve location on the aircraft. Boeings typically have them at the back where the tail starts to taper. On rotation and flare that area typically suffers a pressure bump due to the distance change to the ground. To ensure that the pax are not inconvenienced by this sudden change in pressure a small positive pressure is programmed into the pressurization software.


This is from the 737 but it also applies to the 777:

"The cabin begins to pressurize on the ground at higher power settings. The controller modulates the outflow valve toward close, slightly pressurizing the cabin. This ground pressurization of the cabin makes the transition to pressurized flight more gradual for the passengers and crew, and also gives the system better response to ground effect pressure changes during takeoff."

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Old 8th Sep 2017, 21:33
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Jet II,

Thanks for this...I was always wondering how the cabin pressure would be regulated when going to high alt airports. Typical cabin pressure is meant to be from 6000 (B787) to 8000 (others) feet.
You get to Cuzco (10,800) or Lizha, (11,700) and levels on approach to 200' above field level... (with cabin warnings at 8000') All good for airports less than the respective pressures...but. My brain hurtz.. Thank goodness for the automation, because if that ever gives up and one has to adjust this manually....damn.

They had oxygen bottles everywhere (and coca leaves) but nothing on the flightdeck.

Back to OP. It is not a structural issue, in fact, the structure would be far better off without the stress associated with overpressurization

Last edited by underfire; 8th Sep 2017 at 21:44.
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