Why aren't aircraft cabins pressurized to sea level pressure?

Question

From an article on WHO's website:

Although aircraft cabins are pressurized, cabin air pressure at cruising altitude is lower than air pressure at sea level. At typical cruising altitudes in the range 11 000–12 200 m (36 000–40 000 feet), air pressure in the cabin is equivalent to the outside air pressure at 1800–2400 m (6000–8000 feet) above sea level.

Why aren't cabins completely pressurized, but instead to 6000-8000', seeing that many passengers wouldn't have to endure sometimes painful popping in the ears?

Answer 1

Two reasons: Longevity and weight. Which really come down to money.

Airframes have a limited fatigue life, measured in flight cycles. The main driving factor for airliner wear is pressurizing and depressurizing them. Each millibar of difference between cabin pressure and outside pressure effectively consumes some percentage of the airframe's fatigue life.

Reducing the cabin altitude means increasing this pressure difference, and thus consuming more of the airframe's life. This could be compensated for with sturdier construction, which adds weight. It would also consume a little more bleed air, requiring slightly heavier packs, which, as well as weight, means a loss of efficiency.

Luxury business jets often maintain a lower cabin altitude, such as 4,000 ft. This eats into their flight cycles, so they can still be switched to the usual 8,000 ft for flights without the owner/VIP inside.

Carbon fiber has a much longer fatigue life, so CFRP fuselages can afford to lower the cabin altitude to 6,000 ft, without reducing their life expectancy. This pressure altitude can also be maintained in other airliners at flight levels well below their ceiling.

The optimum compromise point is subject to a lot of debate. The highest cabin altitude that could be permitted is 15,000 ft, above which hypoxia-induced loss of consciousness can occur. The regulatory bodies have settled at 8,000 ft, so that's what the manufacturers target with most of their aluminum airliners, and that's where airlines prefer to run them even if they have a choice, to get more life out of their planes.

Answer 2

The higher the pressure differential between inside and outside, the more stress is put on the plane, which reduces lifespan, and the stronger it needs to be, which increases weight. More weight means more fuel burn and shorter range. All of these factors would combine to increase the overall cost of operation--and eventually fares.

Many passengers don't even notice the higher cabin altitude, especially the frequent fliers who account for the vast majority of airline revenue, so there is little financial incentive to change.

Answer 3

I stumbled upon this thread whilst looking for information regarding maximum operating altitudes for commercial air ambulances with seal level cabins. Whilst Therac's answer is great I would suggest that its not the whole picture.

The majority of the work on the physiological consequences of aircraft cabin altitude was done by a Professor John Ernsting (who I had the great pleasure of meeting on several occasions) at Kings College in London. His work in aviation medicine was extensive and he was responsible for working with aircraft engineers to set the maximum acceptable cabin altitude for commercial cabins. Hypoxia is, as Therac points out, the physiological driver for this decision, but the goal was to set the bar well below the threshold for hypoxia induced cognitive impairment.

When we skydive above 10,000 ft AGL (in unpressurised aircraft) we tend to stick to the EASA guidelines of greater than 30 minutes of planned exposure above 10,000 ft requiring individual access to supplemental oxygen, but this is widely flouted in my experience. It is well known that there is considerable variation in the susceptibility to hypoxic environments from person to person.

Back to the commercial cabin. The basis of the 8000ft decision for commercial cabins was that at this altitude the haemoglobin / oxygen dissociation curve is in its flat portion. The curve represents the saturation of haemoglobin in the blood at any given partial pressure of oxygen in that blood. As the cabin altitude rises, the pressure falls and, if you like, less oxygen is driven across the alveolar membrane into the blood. This leads to a lower arterial oxygen pressure (Pa02) and therefore less saturated haemoglobin.

The curve is sigmoid (s shaped) in nature and flat at the highest Pa02. In other words, decreasing the Pa02 does not, initially, have much effect on the percentage saturation of haemoglobin. This is the area that is targeted by the 8000 ft cabin altitude. Raise that altitude significantly and you are on the steep part of the curve and haemoglobin saturation falls precipitously, lower it and you don't get a significant physiological advantage (because the haemoglobin is already between 92 and 98 % saturated).

So yes, its all about hypoxia and the cost benefit of maintaining a particular cabin wall differential pressure (and therefore cabin altitude). Of note is that modern aircraft tend to run their cabins at slightly lower altitude than 8000 ft - in the order of 4000 - 5000 ft. This gives slightly more physiological tolerance for those passengers with pulmonary disease without being over burdensome on the pressurisation packs or cabin wall life.

In line with the guidelines (I just read before posting) I'm an aviation physician with an interest in commercial patient transfer in the UK.

Answer 4

Because the extra air would add extra weight!

In all seriousness the weight of the extra air might well be on par with a layer of paint, or heavier, and we are often told that airliners are sometimes left partially unpainted (e.g. on undersides) to save weight.

Of course the extra structure required by the extra pressurization would also add weight.