The "lifespan" of an airliner is not truly measured in time, but pressurization cycles. Each time the aircraft is pressurized during flight it's fuselage is subjected to stress. The "lifespan" of the aircraft is reached the metal fatigue’s and cracks. The "service life of 20 years" is as generalization that figures 51,000 flight hours and 75,000 pressurization cycles for most aircraft. If an aircraft is used on long haul routes it experiences relatively few pressurization cycles in it's "life" it will last far beyond 20 years. Only a few years ago, the "average' age of a jet in the operating fleet of Northwest Airlines was 20 years according to a 1999 article. A 2005 news clip noted NWA continues to operate DC-9-31 aircraft that have an average age of 34 years old! Referance- http://www.freep.com/news/airtravel/qfleet18.htm http://www.mercurynews.com/mld/grandforks/business/10958837.htm?template=contentModules/printstory.jsp Photo- http://www.airliners.net/open.file/814913/M Interstingly enough, one of those references point out: "The U.S. Air Force unveiled plans to keep the nation's fleet of B52 bombers in the air for as long as 40 more years. The long-range bombers, built in 1960 and '61, already are older than most of the crews that fly them. With proper maintenance, most of the B52s in the fleet will be close to 80 years old before they're finally retired from service around 2040, the Air Force said." Imagine that! On the other hand, aircraft that are repeatedly flown on short hops will reach their useful life much sooner. For instance, the famous Aloha Airlines incident on April 28, 1988 where part of the fuselage broke away due to metal fatigue is an example of the latter. The aircraft had accumulated 35,496 flight hours and 89,680 flight cycles and was 19 years old. Very close to that 20 year mark, but her actual cycles flown was the amount expected of a much older aircraft. Due to the low altitudes their inter-island flights were flown, we can be sure the maximum cabin pressure differential of 7.5 p.s.i. was not reached on every flight, so the number of full pressurization cycles on the accident airplane can be interpolated to be less than the total 89,680 cycles (a takeoff & landing) accumulated on the airplane. However, it was still enough to cause significant fatigue in the airframe and is still a much higher number than 75,000 cycles predicted for a 20 year service life. Aloha Flight 243- http://as.wm.edu/Faculty/Hinders/aloha.html Aloha Airlines N73711 a/c history- http://www.airfleets.net/ficheapp/?app=b737&msn=20209 Here is a message board discussion about the 'oldest' airliners in service- http://www.airliners.net/discussions/general_aviation/read.main/1250174

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