For every technical product engineers also have to "construct" the average lifespan, if for a car or ship or aircraft. A Mercedes is constructed to live twice as long as an Alfa Romeo for example.

What about aircraft? I know the B747 is constructed to fly a minimum of 60.000 hours/20 years. Quite short for such an expensive product - and it belies reality. At the moment it looks like 2 out of 3 Jumbos will even see their 30th birthday!

The B767 is constructed to fly 30 years plus, 50% longer than the Jumbo. That really astonishes me because I don´t see a reason here.

Can someone tell me why? And does someone have figures of other aircraft, the A340 for example?