I'm not meaning to be critical of anyone who perceives that a plane can seem to be "perfectly good" - sorry if I gave that impression.

You are correct - regulators will ground aircraft that are known to be over schedule for major maintenance inspections, once they find out about it. Remember the unfortunate Ansett story? Whether flight crew will refuse to operate an aircraft or not, I don't know - I do not know how much knowledge they have of the individual service histories and cycles flown of the equipment they operate on any given day.

The fact remains that all these aircraft were designed with specific fatigue cycle limits set down, and 20,000+ cycles begins to approach those original set limits. Major inspections being to be scheduled with increasing frequency, which are costly and time consuming, and the economics of an airline like QF keeping them in the air start to wear a little thin, and hence they retire them.

To be a little more indepth: the essence of metallurgical fatigue is that it starts as a microscopic problem, and grows from there. Depending on where the defect is located in the structure, by the time it becomes readily visible, it could be a serious problem. Materials engineers know this, they know the average lifespan that a certain material will have under a certain cyclical load, and this is where the "rules" come from regarding design lifespan.

Aircraft manufacturers like Boeing continue to do research work on the fatigue performance of these older aircraft. I know they had/have at least one 7-series aircraft that they purchased from an operator who was retiring it at about 20,000 cycles, and they have continued cycling it on a rig to better understand and predict where the problems will occur, in order to give additional directives on maintenance for older aircraft.

Of course, newer generation aircraft use better materials variants with a higher fatigue lifespan. I recall a paper from a few years ago where Boeing's design service objective for the next generation 737 was 75,000 cycles, and they were halfway through conducting safety tests to 3 times that limit.

Fatigue is of course not the only issue for aircraft design, but it's one fo the big ones. And it's one that you don't want to get wrong in an aircraft of all things - it's what makes it the classic study example for young engineers.

Hope that is of interest.

[This message has been edited by MrsDrD (edited 07-10-2003).]