Aircraft Cabin Pressure
 

I recently toured the Boeing facility. They mentioned that the 787 has a lower cabin altitude than other aircraft.

Apparently, one of the big needs for pressurization (aside from passenger comfort) is that it reduces stress on the fuselage.

As the cabin pressure gets lowered at altitude, passenger comfort increases. It looks like newer planes are getting cabin pressure around 5000-6000 feet at cruising altitude, with the Bombardier getting pressure to 4500 feet.

It appears that a key component to lowering cabin pressure at cruise is the structural strength of the fuselage. Boeing indicated that carbon fiber is stronger than aluminum, so that allows a lower cabin pressure due to a stronger aircraft structure.

My question is: what is the limiter on fuselage strength and lowering cabin pressure? Is it weight (more strength means more weight)? Is it cost?

I would think that cruising in an aircraft at a sea level cabin pressure would offer plenty of benefits to an airline and passengers.

Do you see any breakthroughs in lowering cabin pressure on the horizon? Is cruising at sea level cabin pressure optimal?

All of the articles that I find online talk about fuselage structure and cabin pressure. I'm wondering if there are other ways to lower the cabin pressure.

It seems that lower cabin pressure will have excellent travel benefits. I'm not an engineer-type.

My son just started college at Embry Riddle this year, studying aerospace engineering. I told him if he can figure out a way to get aircraft cabin pressure to sea level, he'd have an incredible career.

If you have any ideas, please let me know.

Thank you!

Mark

Cabin pressure is a differential pressure when you're talking about maintaining the same cabin altitude and flying at different altitudes. In other words, if you want to maintain a sea level cabin and climb to a cruise altitude, the amount of differential pressure (pressure inside the cabin vs. what's outside) increases, the higher you go.

When the structural limit for pressurization (or anticipated pressurization cycles) has been reached, that either limits the altitude of the aircraft, or any climb higher than that requires an increase in cabin pressure altitude.

The actual cabin pressure altitude isn't so big a deal so much as the rate of change. Generally we try to keep the climb and descent of the cabin pressure altitude to a comfortable 500 feet per minute, or less. Unless someone has a head cold, that's a good value for nearly anyone. If someone does have a head cold, they shouldn't be flying.

Pressurizing a cabin doesn't decrease stress on the fuselage structure. It adds stress. It also adds strength and rigidity if properly applied.

Think about the stress on even a small section of cabin. A door that's three feet wide and six feet tall has an area of 18 square feet, or 2,592 square inches. If a differential pressure is maintained of 6 psid, that's over fifteen thousand pounds of pressure just on the door. A 10X12" window has over seven hundred pounds pushing out on the plexiglass at the same differential pressure. Now think about the area of the entire pressure vessel; the airplane cabin is a big aluminum balloon that gets stretched with tremendous pressure with every pressure cycle.

I'm not sure that there's any great advantage to getting cabin pressure to sea level. Most people don't live at sea level. Take off in Denver or Salt Lake City and the cabin pressuring going down to sea level wouldn't be a big plus. Take off in Phoenix and the field elevation is already fifteen hundred feet. Why go down to sea level unless landing there? If landing there, the cabin pressure will be reduced to sea level prior to landing (we generally ensure that field elevation pressure is reached by 1,000' above ground during a descent to landing, and that the airplane lands unpressurized).

Aircraft structures are built just strong enough, plus a margin. Weight is a big issue in any airframe.

Carbon fiber offers certain advantages, but it also offers unknowns, and a conservative finite life due to some of those unknowns.

The entire fuselage isn't pressurized; the pressurized portion is called the "pressure vessel." Short of finding ways to strengthen the pressure vessel, what else is there to be done other than limit altitude? Differential pressure is the limitation; either strengthen the fuselage with different materials and construction at the same weight (new technologies), increase weight at the cost of payload, fuel, and performance, or limit the altitude of the aircraft. Those represent your options.

My question would be why it's so important to achieve a sea level cabin. What's the point?

Looking at the population centers of the world, I'm pretty sure that most people DO live within 1000' of sea level.

I think the bigger issue is that the cost-to-benefit doesn't make sense after a while, otherwise the fuselage needs to be incredibly strong, which adds weight and decreases payload, so there's a point at which the returns are marginal, which is what I think you were trying to say.

The world's average elevation is 2,700.'
Average elevations by country...
USA: 2,500'
China: 6,000'
India: 525'
Russia: 2,000'
South Africa: 3,400'
Indonesia: 1,200'
Brazil: 1,000'
Pakistan: 3,000'
Nigeria: 1,200'
Mexico: 3,600'
Japan: 1,400'
Phillipines: 1,400'
Ethiopia: 4,300'

That's over 58% of the world population by country living at an average of 2,400 as established by word population by country, and average elevation of those countries.

I didn't try to say anything. I said it.

Another issue with increased pressurization is increased humidity. Humidity is really bad for aluminum aircraft, and definitely not good for all the electronics/wiring in composite aircraft. (airplanes get parked in deserts for a reason).

Just because the average elevation of a country is a certain figure, that doesn't mean all the people, or even the majority of the people live at that figure or higher... The most populated areas of many of those countries are sea level cities in coastal areas.

There is no advantage, or need, to run cabin pressure to sea level; the majority of the world's population does not live at sea level. Some world population centers are there, and eventually flights landing there will have cabin pressures matching field elevation, as is always the case; to suggest that all flights should have cabin pressure altitudes that descend to sea level, especially when the flight may never actually go to sea level, makes no sense.

If you'd prefer to figure out world population by cities and location, cite each one, and do the math, more power to you.

I think you get the point.

How does increasing cabin pressure increase humidity?

Why do composite aircraft have more wiring than conventional airframes?

How does it follow that just because the average elevation of a country is x, that y of the population lives at the average?

Read above.

Mark, this statement above is not correct. Pressurization of an aircraft is the equivalent of inflating a balloon. The pneumatic system of an airliner pumps a high volume of air into the aircraft interior. This in-flow of air is regulated by an outflow valve that continuously allows a certain amount of the air in the fuselage to vent overboard as "new" air is constantly introduced to the cabin environment. So, at any time during high altitude cruise, every molecule of air in the "inflated" fuselage is straining against the interior of the aircraft - pushing outward. The lower (closer to sea level pressure) you make the cabin altitude, the more air molecules are present within the fuselage and the higher the interior pressure on it. "Inflating" the fuselage to a higher pressure and then "deflating" it as the aircraft lands is what causes the stress and fatigue on the airframe.

It's a combination of both. In older (non 787) aircraft, in order for an aluminum fuselage to be structurally capable of withstanding the additional stress of being pressurized to lower cabin altitude, it would need to have all aspects of its structure strengthened. This would obviously mean more material during construction (cost) and more weight during the operation of the aircraft. More weight within the aircraft structure means less available for fuel, passengers and cargo (less profit). The other factor involved is the source of the air being used to pressurize the aircraft. Engine bleed air is typically used to operate the pneumatic systems involved with pressurizing the aircraft. So, using more of this bleed air from the engines to increase the pressurization within the fuselage means the engines will operate less efficiently and use more fuel.

The 787 carbon fiber fuselage allows Boeing to pressurize the aircraft to a lower cabin altitude than a similar aluminum design. Since that aircraft also uses an electrical pneumatic system rather than the traditional bleed air system on most aircraft, they can accomplish this increased pressurization with no additional demands on the engines. Nothing come for free, so I would imagine the electrical demands are considerably higher. This may demand more from engine driven electrical components but obviously that was deemed to be an acceptable tradeoff by the designers. From what I understand, in actual practice, this electric-centered aircraft design has had some growing pains in terms of system reliability.

Cabin pressurization is a fairly simple concept and the physics involved with actually accomplishing it aren't going to change. You have to pump enough air into the fuselage to sustain life inside a tube that spends hours above 30,000 feet. It seems like Boeing was able to make some minor "breakthroughs" in fuselage structure and system architecture that allow them to get cabin altitude lower without major sacrifices in efficiency. They were able to drop it from typical ranges of 6000-7000 feet down to 5000-6000.

Getting it to sea level is probably not a worthwhile trade off for the extra cost involved. It's definitely not optimal when it comes to efficiency and aircraft operation.

That doesn't make sense (above); what if everyone in those countries live on the coast. I looked on a couple searches an I saw anywhere from 40-80% of folks live by the ocean. Looking at pictures of earth from space at night, a good portion of the densely populated areas are on a coast line. Historically a good portion if not a majority of humans have lived by coast lines. I am not sure if it is still most based on a couple quick searches, but it might be. I realize all coastlines are not at sea level with cliffs and such, but most likely under 500' MSL.

Mark H. I think you are confusing lower cabin altitude with lower pressure; or at least it looks like you are using them interchangeably. Lower cabin altitude would be higher pressure relative to higher cabin altitude.

I do not see an advantage of having cabin pressure remain at sea level. It actually might be worse if you are having fluctuations; as far as effects on the body. Big swings in pressure can do bad things.