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The Boeing 787 Dreamliner's fuselage is almost completely made of composite carbon fibre material, which is not susceptible to metal fatigue.

The main reason why the cabin pressure in a pressurized aircraft is kept as low as possible is to reduce the expansion and shrinking of the fuselage due to changes in pressure differences, reducing metal fatigue in the long run.

Does the 787 use higher cabin pressures than other commercial aircraft? Boeing touted this as one of the revolutionary new features of the aircraft back in 2006, but does it actually use higher cabin pressure now that the plane is in use by airlines?

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3 Answers 3

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According to this document by Boeing:

Altitude: How High Is Just Right?

Today's airplanes are pressurized to a typical cabin altitude of 6,500 to 7,000 feet (1,981 – 2,133 m), with a maximum certification altitude of 8,000 feet (2,438 m). Because the advanced composite materials that make up the 787’s fuselage do not fatigue, the 787 can be pressurized more, which allows for lower cabin altitude levels.

Studies at Oklahoma State University explored the effect of altitude on passengers to determine optimum levels. After testing at various altitudes, it became clear that lowering the cabin altitude to 6,000 feet (1,830 m) provided meaningful improvements. Lowering the cabin altitude further, however, provided almost no additional benefit. Based on that knowledge, Boeing designed the 787 to be pressurized to a maximum cabin altitude of 6,000 feet.

So if we assume a constant altitude of 43,000 (the 787's service ceiling1), we would get a maximum differential of:

  • 6,000 ft Cabin Altitude = 9.06 psi
  • 8,000 ft Cabin Altitude = 8.11 psi

This would indeed be a higher cabin pressure than what Boeing considers "typical" at maximum altitudes.


1 It is probably actually an even bigger difference since the ceiling of a lot of airplanes isn't this high so the max differential would be even lower.

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Would that also decrease discomfort during descent for people sensitive to ear/sinus problems? –  Philippe Leybaert Feb 13 at 23:39
    
@PhilippeLeybaert Absolutely! If the "altitude" in the airplane is lower, that means that it has to descend less in order to get back to the destination airport altitude. It's the amount of "difference" that affects people, so the lower the cabin altitude the better! –  Lnafziger Feb 13 at 23:42
    
Wasn't one of the other key comfort benefits to having a composite fuselage that cabin air could be at a higher humidity versus in a traditional aluminum fuselaged plane? –  Nick T May 25 at 18:54
    
@NickT Yes! With a traditional aluminum airframe, the humidity had to be kept low so that it didn't cause corrosion. With a composite airframe, the humidity can be higher! –  Lnafziger May 25 at 20:12

It may surprise you to know that the 6000 feet cabin pressure wasn't even a new feature when the Boeing 787 released! The Aérospatiale-BAC Concorde, the world's first and last profitable supersonic airliner maintained its cabin pressure at 6000 feet ASML.

From Wikipedia:

Airliner cabins were usually maintained at a pressure equivalent to 6,000–8,000 feet (1,800–2,400 m) elevation. Concorde’s pressurisation was set to an altitude at the lower end of this range, 6,000 feet (1,800 m).[98] Concorde’s maximum cruising altitude was 60,000 feet (18,000 m); subsonic airliners typically cruise below 40,000 feet (12,000 m).

The Concorde's airframe had to deal with several factors that most subsonic aircraft aren't even designed for, including thermal expansion of the fuselage, stress due to contraction when returning to subsonic flight, and the pressure difference at 60000 feet AMSL.

Based on several 787 trip reports, the pax do report that they felt comparatively much better in the 787 after an ultra long haul flight, compared to other contemporary airliners. This is due to the higher cabin pressure and higher artificial humidity maintained in the cabin. The revolutionary feature of the 787 is the bleedless engines (i.e., air for cabin pressurisation is not compressed by the engines, unlike in almost every airliner).

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The aircraft does use a lower equivalent altitude in the cabin, while usually operating at an altitude higher than most other aircraft. Since it's designed for it, and I don't think it makes a difference it fuel consumption hence there's no reason to opt out. It's highly appreciated by crew and passengers alike since it leaves them feeling more rested and less jetlagged. It's around 6000 feet rather than the usual 7500-8000 feet.

What also contributes substantially to passenger comfort is the fact that the air is considerably more humid, since corrosion is less of a concern, also leaving passengers and crew dehydrated.

I've never flown on one, but it seems to be holding:

"As for the cabin pressure and moisture, it's all true: I arrived both times feeling well-hydrated and without the parched skin that would result from flying in any other airplane." Source

and a little more on the workings of this system:

"Cabin – Pressurization differential pressure maximum is 9.4 psid, so the cabin altitude is only 6000 feet when at the max cruising altitude of 43,000 feet. There is a cockpit humidifier switch, and cabin air humidification is fully automatic." Source

and here's an article from 2011:

Most conventional passenger jets set the cabin pressure at an equivalent of around 7,500 to 8,000 feet above sea level, which Boeing claims is the primary cause of a range of in-flight ills. “There are many passengers problems associated with altitude – headaches, muscle aches, fatigue and even nausea” Craver says. The difference between external air pressure when an aircraft cruises at 40,000 feet and an internal pressure that’s one-fifth of that stresses the plane’s fuselage – and the greater the difference, the more the stress. That’s been the limiting factor in increasing cabin pressure, Craver explains: the metal body of current aircraft wouldn’t safely be able to handle the fatigue induced by maintaining this pressure at high altitudes. That changes due to the use of carbon-fibre composite materials on the 787’s fuselage. Carbon-fibre doesn’t suffer from metal fatigue and in turn allows for lower 'cabin altitude' levels. The 787’s cabin pressure is set to 6,000 feet, a figure arrived upon by Boeing modifying a pressure chamber to look like an airplane cabin which could hold 12 people at a time. “We cycled over 500 people through the chamber, and they stayed there for up to 20 hours of simulated flying time” Craver recalls, and they found that 6,000 feet was the ‘sweet spot’. “Between sea level and 6,000 feet there was almost no difference in the reported symptoms” Craver says, “so we can alleviate or mitigate a lot of symptoms you get at a cabin altitude of 8,000 feet” Boeing claims that one in four travellers experience some form of ‘respiratory distress’ after flying 12 hours in a conventional aircraft with a cabin pressure of 8,000 feet, but this plummets to 5-6 per cent at 6,000 feet. Source

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