Cabin Pressure

Go to: The Ear, Pressure Transients

Why Aircraft Are Pressurized

You've all heard Flight Attendants say, "The aircraft is pressurized for your comfort.  In case of a rapid change in cabin pressure, ...etc.".  Most of us don’t pay any attention anymore, we’re so “old hat” about flying.  This section is an attempt at an explanation of what they're saying.

Aircraft are pressurized to ensure that the occupants have a livable environment while the aircraft flies at altitudes up to 45,000 feet above sea level for the subsonic jets, up to 51,000 ft for the executive jets, and up to 60,000 feet in the case of Concorde.  At those altitudes the human body would quickly die; thus, the aircraft's fuselage is pressurized to an altitude that people can live and, more importantly, work in without any impairment of their faculties.  The maximum altitude where this is deemed possible on a continuing basis is 8,000 feet.  The Federal Aviation Regulations (FARs) (particularly FAR 25.841), therefore, state that the maximum normal operating altitude for the cabin be limited to 8,000 ft.  If the aircraft flies at altitudes below the maximum allowed (certificated), then the cabin altitude is decreased in some proportion to increase the livability and workability of the cabin environment (i.e. the cabin altitude is not always at 8,000 ft, as is sometimes assumed).

A typical cabin altitude vs. aircraft altitude graph is shown below (and also in the analysis section).

Figure 1

Figure 1 is a typical cabin altitude cruise schedule for a modern jet airliner using a digitally controlled cabin pressure control system.

The schedule, as noted in the main text, is a compromise between obtaining maximum passenger comfort and building an aircraft that is economical to fly and operate.

On the other hand, aircraft cabin altitude is not kept at sea level either for the entire flight for the main reason that the structural weight increases dramatically to handle the increased differential pressure (pressure in the cabin minus the pressure outside – (Pcab - Pamb)), and this additional weight becomes prohibitive from an economic standpoint.  The payload and/or aircraft range would dramatically decrease under these conditions.  There really is no need to keep the cabin at sea level when destinations, such as Denver or Mexico City are at 5,000 to 7,000 ft above sea level.  People live and work in these cities with no detriment to their health.

Pressurization (the ear)

In order to understand the how's and why's of cabin pressurization, it is necessary, first, to understand the workings of the ear.  The ear is the organ that usually suffers the most due to changes in air pressure.

Figure 2

Photo (above) and Explanation: EarHelp

How Our Ears Work

The Outer Ear

The outer ear collects sound from the environment and funnels it down the ear canal to the eardrum where it is converted into mechanical vibrations. The eardrum separates the outer ear from the middle ear.

The Middle Ear

Vibrations from the eardrum are transmitted and amplified by three small connected bones called the malleus, incus, and stapes. These bones carry the vibrations from the relatively large surface of the eardrum to the much smaller opening into the inner ear, called the oval window. The middle ear also contains the eustachian tube, a tiny channel linking the middle ear to the back of the throat. The primary function of the eustachian tube is to equalize air pressure in the middle ear with air pressure outside the middle ear. When your ears "pop", that is the sound of your eustachian tubes opening to let air into the middle ear.


The Inner Ear

The inner ear is made up of two areas, which perform separate functions. First, the semicircular canals are involved with balance and motion. These fluid filled canals detect movement in any direction. This is the mechanism, which tells us if we are standing up or lying down. If the balance organs send mixed or incorrect signals to the brain, then we can feel dizzy. Damage to this area can cause vertigo, a severe debilitating form of dizziness.

Second, the snail-shaped structure called the cochlea is where the sensation of hearing takes place. The fluid filled cochlea contains thousands of special nerve endings called hair cells. The mechanical vibrations in the oval window cause movement of both the fluid and the hairs immersed in the fluid. Movement of the microscopic hairs stimulates the attached cells to send a tiny electric impulse along the fibers of the auditory nerve to the brain.

Beyond The Ear

Our ears, all by themselves, are not capable of hearing and understanding speech. Our ears receive sound, modify it, break it down into its component parts, and transmit that information to the brain. The brain actually translates that information into something meaningful; speech, music, noise, etc.

Looking at the picture above, the eustachian tube is the item that allows the ear to equalize pressure due to changes in air pressure.  When the pressure is reducing (i.e. cabin altitude is increasing during aircraft climb), the air can easily pass down into the nose/throat area to relieve the pressure changes.  Air may also pass out of other body orifices, causing discomfort and/or embarrassment.  When the aircraft is descending from cruise to the destination, the pressure in the cabin increases (i.e. cabin altitude is decreasing).  Increases in pressure can cause the eustachian tube to collapse, thus not allowing equalization of pressure in the ear.  This collapse, and inability to equalize pressure, is the major cause of ear discomfort and/or pain.  People with upper respiratory infections/problems will suffer more.  The use of nasal sprays for these people is very helpful.  Other means of equalizing pressure include: yawning frequently, moving the jaw in many directions, chewing gum, sucking on hard candy (sweets), and, lastly, holding the mouth and nose closed and trying to blow out through the ear (Valsalva manoeuvre).

In the earlier days of air travel the Flight Attendants would pass out candy to everyone at the start of descent; that seems to have gone by the wayside in these times of concentrating only on profit!  The habit should be re-introduced.  Also, passengers should not be allowed to continue sleeping during descent; there is reduced swallowing and, therefore, the chances of eustachian tube collapse increase along with its attendant discomfort.  If the pressure difference increases to too great a value the eardrum can rupture.

Pressure Transients

Realising that the ear can experience problems during both climb and descent of an aircraft, design engineers specify acceptable limits to the rates-of-change of cabin pressure.  Figure 3 shows the specified limits.  Transient pressure changes can occur that momentarily exceed the nominal, continuous allowable rate changes.  This explains why Figure 3 shows rates-of-change of the order of ten to one hundred times the nominal values, with corresponding reductions in the time that they are acceptable. (Note: the chart is log-log, not linear).

Figure 3

Figure 3 specifies the allowable limits on cabin pressure rates-of-change for various durations of the pressure change for passenger comfort.  This figure is figure 4 of ARP-1270 published by the Society of Automotive Engineers Inc., who publish these Aerospace Recommended Practices (ARPs).

These limits are set in order to minimize the noticeable effect in the ears of a normally healthy person during the climb and descent phases of flight.  Also, the limits are in effect for transient pressure changes during all phases of flight.

Following the suggestions noted above should reduce any noticeable effects.

Further detailed reading may be found at the following websites: TBD

Pressurizing an aircraft

So, how is an aircraft pressurized?  Air is brought in from outside, compressed, conditioned for temperature and moisture, distributed throughout the aircraft cabin, and, finally, ejected overboard.  It is really not quite that simple! But the basic principles are described in that short sentence.  I have neither the time nor inclination to go into great detail on all of these items.

The part that I am knowledgeable about and responsible for is the last part - ejecting the air overboard!

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