Why does an airplane's life depend on its pressurization cycles?

Question

There is a related question on Y! Answers about the average life of an aircraft. Why does it depend more on pressurization cycles and no so much on age in years?

Answer 1

Every time the air pressure is raised inside an airliner's cabin to allow passenders to breathe without air masks, the airliner's skin is subjected to pressure. That pressure slightly stetches the metal skin. thee skin moves again when it contracts as the airplane descends and the pressure returns to nomal. This is a pressurisation cycle.Each of these cycles stressest he metal slightly. Eventually the metal begins to suffer from metal fatigue. If this process continues (through many more flights & pressure cycles), cracks will begin to form. if the cycle continues and the problem ius neglected the cracks will spread and some will foin up. The metal will eventually fail and the airplane can break up. About ten years agoo an airplane lost much of the top of the cabin during a flight. It looked like it hadbeen peeled off by a gigantic knife. I believe a flight attendant was killed but the airliner was safely landed. It made quite a picture. Truly a monument to corporate, homical greed.
Dan the Answers-Man.

Answer 2

While cycles are a major factor in determining the -theoretical- life of an airplane, what kills most airplanes is corrosion. It simply becomes uneconomical to continue to repair old airplanes (except for the military, where they don't have to worry about making a profit--which is why the B-52s and KC-135s are still flying).

It's the flexing, the cyclic loading and unloading, that causes metal fatigue. That flexing can come from pressurization, or simply landing and taking off.

While there's probably been one or two somewhere that reached it, I've never heard of a commercial airliner that made it all the way through to its cycle life limit. Most are scrapped while they still have life left on their theoretical limits.

For example, for the Boeing 707, the life-limiting parts were not in the pressurized area, but were the horizontal stabilizer pivot fitting, the elevator aft quandrant assembly, and the quadrant support, all of which had a 200,000 cycle life limit. It's been several years ago, so I may not have all the details right, but I worked for a company that maintained several 707 trainers for the military. Since they were used as trainers, they did a lot of touch-and-go landings, each of which counts as a cycle. On a good day, the crews could put 50 or more cycles on a single airplane. The fleet leader (the 707 with the most cycles ever) was one of these airplanes. This airplane was scrapped with between 55,000 and 60,000 cycles (along with its sister ships), due to extensive fatigue cracks not in the fuselage structure, but in the wing skin splice stringers (the structural elements to which the wing skins attach). Repairing the damage would have involved removing the wing skins, replacing the cracked stringers (which ran from the wing root out to the production break just outboard of the outboard engines), and re-installing the wing skins. We estimated the cost at around $15M per airplane (with a 1 year lead time for the parts), which was too much for the Air Force, so they scrapped them. (A picture of the airplane in question, in its original TWA colors, is listed below.)

Life limits can be found on the airplane's type certificate data sheet (TCDS). A link to the FAA website's TCDS database is listed below.

Answer 3

Pressurization and depressurization stretches the metal in and out a little bit each time. This can cause small cracks to form in the metal. If too many small cracks form, the aircraft could suddenly fracture, and if it happens in-flight, then that would be a disaster. It's kind of how wearing a pair of jeans for a long time can weaken the fibers, which might tear open all at once while you are sitting down.

However, if you leave the plane (or the pants) someplace sheltered and don't use it, then it will keep for a longer period of time. It might still get some expansion and contraction from changing temperatures, or corrosion, but the biggest stress that happens to a plane is going to be the pressurization.

Answer 4

The main reason is metal fatigue. Every time you pressurize an airplane the airframe components "stretch" to accommodate the pressure and forces act over all the main components from the inside out.
Sometimes this pressure can be as high as 8 psi (that is 8 pounds per square inch, which translates to more than 1,000 pounds of force per square foot). When the plane lands, the structure takes its original shape back. This bending of metal produces fatigue. If you grab a piece of metal and bend it back and forth enough times, it will eventually crack and break. The same can happen to an airplane structure, so the airplane is limited to a number of cycles low enough (although the number can be in the thousands) to assure that no component will fail during its normal
lifespan.
Other components, such as engines and avionics, can be replaced, so they don't usually limit the airplanes useful life. Minor corrosion can also be fixed before it becomes a problem over time.

Answer 5

The stretching and relaxing of the airframe stress-hardens and fatigues some alloys. Cracks develop and propagate.

The Aloha Airlines 737 jet in which a section of the roof peeled in 1988 off really brought this home. A few injuries and one flight attendant got sucked out. It could have been much worse. Those jets in Hawaii only fly 20-40 minutes each flight so the pressurization cycles become more important than total hours in airframe life.

Answer 6

Each time the aircraft pressurizes and depressurizes...it puts tremendous strain on the fusalage. If an aircraft cruises at 35,000 feet, the outside airpressure is about 1/3 of sea level. The interior will normally be pressurized to about 6-8 thousand feet.
So the difference between inside and out would typically be around 22,000 - 25,000 feet.
Sea level pressure is 14.7 lb sq inch. Pressure at 6000 ft is typically around 11-12 lbs sq inch. Pressure at 35,000 ft is about 4lbs per sq ft
So the difference in pressure for an aircraft in cruise would be appx 8 lbs per sq inch. ( this apprxiamates to OVER 1200 lbs per sq ft)
This pressure differential is pushing outward on the aircraft skin for the duration of the flight at cruise. The cycle repeats every climb and descent and this stresses the skin and structure until eventually it fails. Therefore there is a limit on the number of cycles an aircraft is designed to undertake.

A few years ago an Aloha Air B737 had the ENTIRE upper roof portion rip off in flight.....cause?..metal fatigue from repeated takeoffs and landings...They did land safely but several were sucked out in the initial failure. Also early Comet jetliners had several tragic failures due to metal fatigue directly attributed to pressurization stress,

Answer 7

Doggzilla's got it. The fatigue life of aluminum alloys is limited and depends on how many stress cycles its been thru, and also the magnitude of the stress. The higher the stress, the smaller the number of cycles.

If you want a light weight craft, then the alum skin is thinner and the stress levels of pressurization are higher. Then you better count the number of stress cycles carefully!

Answer 8

The pressurization and depressurization slowly form cracks in 2024 and 7075 aluminum, which are the most common alloys for aircraft. some other aluminum alloys dont have that problem, but are weaker. since the life of the airplane is still 10's of thousnads of hours, its acceptable.

Answer 9

I never knew it did!! This is a really good question!
Thinking about it moist, pressurised air would be forced into all the nooks and crannies of the aircraft. It might be warm inside but it will be cold outside and this would cause internal condensation. This in turn would lead to corrosion and a shortening of the airframe's lifespan.
Maybe (!)