FDX_Mech: While it is regretable that the spar cracked, one significant point is that you saw the aircraft land. Well-done, redundant design, wasn't it? The L1011 is a good, rugged aircraft. They aren't being retired because of worn out structure, but because of economics. So it appears that the complaint is more about customer support than it is about the design.
The worst customer support that comes to my mind was McDonnell Douglas and the DC-9. Back when the "new" regulations came out for fireproof cargo compartment liners (1990 timeframe), Douglas specified one company with their very expensive liner repair patch in the DC-9 tech data. We had found a similar patch with a much lower price, but Douglas wouldn't certify it. When we asked why, they said it was because their vendor would not appreciate it.
Cracks: All jetliners get them. If they haven't, they will. It is nearly impossible to design something like that without a weak point(s) somewhere. The key is that they either discover them in fatigue tests and devise repairs, or they discover them in service and do the same. The manufacturer will issue service bulletins, and if the FAA is sufficiently concerned then they will make them into Airworthiness Directives. Older structure is designed using the "fail-safe" method where if one part fails, the ones around it take up the load. The L1011 with the spar crack didn't actually crack the entire spar, but just a part of it, such as the lower chord (angle). Newer aircraft are designed for this kind of thing:
- Calculate critical crack length for a particular location. This is the point where the crack quits being predictable and jumps in length.
- Calculate the time it takes for the crack to reach critical length (usually in flight hours).
- Inspect for the possible crack in one half that time.
The C-5 was developed to a specific contracted weight. Lockheed notified the USAF that their design was overweight and recommended that a stronger engine be used to keep the performance. USAF directed Lockheed to make the original weight, or face action for violating their contract. So, Lockheed shaved the airframe to get the weight down. The result was the fragile aircraft. The USAF has paid far more in repairs and lost operations from getting their way about the weight than they would have if they had just gone for the upgraded engine. The C-141, where Lockheed and the USAF worked well together in the design, is a very successful aircraft. They exceeded their design life by 50% before structural problems forced the retirement of many of them.