If you must have noticed, some aircrafts fly for many years compared to others.
Aircraft technology has progressed in such a way that if aircrafts are maintained properly then they can technically fly forever, without having a need to replace them.
If experts are to be believed modern aircrafts can go on forever IF maintained properly and taken care off but there is an argument to this statement, I personally think this statement is only true to a certain limit but there are successful example of people using 67+ year old DC-3 aircrafts for cargo use even though this aircraft is a piston engine and is notorious for its thirst for oil.
Aircrafts life span is not measured in years but pressurisation cycle. This is specified by the manufacturer. Each time an aircraft is pressurized during flight, its fuselage and wings are stressed. Both are made of large, plate-like parts connected with fasteners and rivets, and over time, cracks develop around the fastener holes due to metal fatigue.
The life of an aircraft depends on the usage or even the range of flights it is put through. When a pressurised aircraft flies, it’s fuselage acts as lungs, compressing and expanding with the change in atmospheric pressure outside, so in my opinion aircrafts which are used over “short distances and frequent flights” have their mechanical life shortened if compared to aircrafts flown over “long distance not so frequent flights” because the compression and expansion suffered by the aircraft over “short, frequent flights” is much more and severe than encountered by “long distance not so frequent flights”.
When aircraft is on the ground ready to fly, the pressure inside the aircraft is a bit lower than the air pressure outside so the fuselage suffers compression (high pressure outside), as it flies at or above 10,000ft air pressure inside stays at around 6000-8000 ft (depends on the aircraft.), this pressure is higher than the pressure outside (since air pressure decreases with height.) here the fuselage suffers expansion.
So any regional jet will encounter more structural stress through its lifetime than say a Boeing 777 or a 747.
Daily scheduled flight say from LA to San Diego will encounter more stress flying up and down number of times if compared to a non-stop LA-London flight because the number of times a fuselage compresses and expands is drastically reduced when flying long distance at one decided altitude.
Why I am talking about all this is because during maintenance pretty much everything can be overhauled and replaced with new parts except the fuselage and wings itself, if someone decides to do it why not buy a complete new aircraft?
I would love more feedback on this topic since I am no technical ace neither am I an engineer.
Hmm.. I am not sure how much of a 'compression' factor there is -- Before the doors are closed, there is no pressure difference between inside and outside. After the doors are closed, I would be very surprised if the plane ordinarily suddenly decreases cabin pressure (ouch my ears). If the cabin is set to pressurize at 5000ft, what I believe must be happening is that the pressure controlling agent will keep the pressure matching to the outside pressure until they cross 5000ft, and after that, maintain the pressure. Similarly, I suppose that before starting their descent, pilots must be telling the computer -- hey, we are not landing at the same altitude we started from, in the end we'll go to 1700ft instead of 37. Then again, until the plane reaches 5000ft, cabin will have a pressure differential, and later on, probably the pressure will fall off gradually with final descent. Note that again, immediately after landing, there must not be any cabin pressure more than outside -- otherwise, as they say, the doors won't open (since they need to be pulled in first).
ReplyDeleteWell.. All of the above is a ramble, since I am no pilot. But I do agree that the fuselage suffers from a lot of stress -- they usually allow nothing more than 9psi.. Your car tyre would have 30psi, so in comparison to that 9psi might sound less -- but oh. I wouldn't want to be inside my car tyre, and planes are not made of rubber :P
Akshay, can you talk more about metal fatigue in another post? Yes -- fuselage is under stress -- but so are the tiny nuts and bolts that hold the plane together.. The screws that balance tonnes of weight of the engines must be under tremendous stress.. how frequently are these millions of nuts all over the plane inspected and replaced?