I wonder how many people think of airliners they fly in, very much like the can in the picture above! In fact it started out much like that in the early 1950’s with the De Havilland Comet aircraft. The Comet became quite (in)famous for tragic in-flight structural failures, almost complete and rapid decompression at 35000ft above sea level.
There were a total of 3 such disasters with the Comet (and a loss of many lives), all of them within a short time after entry with the airlines at the time. This indicated to the designers that there was a flaw in the basic structural design of the aircraft-A ‘stressed skin’ fuselage construction design that imposed or transferred, almost the entire structural load on the aircraft skin. That is, the aircraft skin became the primary load carrying structure. Though that might sound absurd, it wasn’t really a case of structural overloading on a relatively slight aircraft skin that caused those catastrophes. In fact, we may be quite tempted to ask “What were the designers thinking?! How can the aircraft skin be asked to withstand structural loads of the magnitude relevant to an airliner??”
To be sure, the aircraft skin was built many times stronger, heavier, and thicker than today’s airliners. The flaw lay in the fact that repeated pressurization and depressurization (called pressurization cycles*) of the fuselage, induced fatigue in the structure (in this case, the outer skin) of the aircraft. This in turn created a mini fatigue fracture or a fault line on the aircraft skin resulting in localized stress concentration. The rest of the account lies in the coke can theory! A further diagnosis of this theory in future posts.
There were a total of 3 such disasters with the Comet (and a loss of many lives), all of them within a short time after entry with the airlines at the time. This indicated to the designers that there was a flaw in the basic structural design of the aircraft-A ‘stressed skin’ fuselage construction design that imposed or transferred, almost the entire structural load on the aircraft skin. That is, the aircraft skin became the primary load carrying structure. Though that might sound absurd, it wasn’t really a case of structural overloading on a relatively slight aircraft skin that caused those catastrophes. In fact, we may be quite tempted to ask “What were the designers thinking?! How can the aircraft skin be asked to withstand structural loads of the magnitude relevant to an airliner??”
To be sure, the aircraft skin was built many times stronger, heavier, and thicker than today’s airliners. The flaw lay in the fact that repeated pressurization and depressurization (called pressurization cycles*) of the fuselage, induced fatigue in the structure (in this case, the outer skin) of the aircraft. This in turn created a mini fatigue fracture or a fault line on the aircraft skin resulting in localized stress concentration. The rest of the account lies in the coke can theory! A further diagnosis of this theory in future posts.
*Pressurization cycles- One pressurization cycle consists of one complete pressurization and one complete de-pressurization of the fuselage. In plain terms that means one complete sequence of the events involving,
1. A Take-off
2. Climb to cruise altitude
3. Descent from cruise altitude, and
4. Landing
Pressurization and Air conditioning
1. A Take-off
2. Climb to cruise altitude
3. Descent from cruise altitude, and
4. Landing
Pressurization and Air conditioning
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