My question was not meant to be a dig at Boeing. (I think that perhaps a good case can be made that the last generation of mangers may have fatally damaged the company, but I was not advancing that point in my question above.)
I was just wondering how often planes are checked for these sorts of issues. If they were designed to be crack free for 90,000 flight cycles, when would someone normally inspect them? As reported, these cracks were found only because a plane was being converted. Reports read as if if that single plane was not being converted that the cracks would not have been detected until ...
https://komonews.com/news/local/exclusive-unexpected-cracking-found-on-critical-boeing-737ng-equipment
I thought this might be useful:
Airlines and airworthiness authorities casually refer to the detailed inspections as "checks", commonly one of the following: A check, B check, C check, or D check. A and B checks are lighter checks, while C and D are considered heavier checks. Aircraft operators may perform some work at their own facilities but often checks, and especially the heavier checks, take place at maintenance, repair and overhaul (MRO) company sites.
A check
This is performed approximately every 400-600 flight hours or 200–300 flights, depending on aircraft type. It needs about 50-70 man-hours and usually performs in a hangar for a minimum of 10 hours. The actual occurrence of this check varies by aircraft type, the cycle count, or the number of hours flown since the last check. The occurrence can be delayed by the airline if certain predetermined conditions are met.
B check
This is performed approximately every 6-8 months. It needs about 160-180 man-hours, depending on the aircraft, and is usually completed within 1–3 days at an airport hangar. A similar occurrence schedule applies to the B check as to the A check. However, B checks are increasingly incorporated into successive A checks, i.e.: Checks A-1 through A-10 complete all the B check items.
C check
This is performed approximately every 20–24 months or a specific number of actual flight hours (FH) or as defined by the manufacturer. This maintenance check is much more extensive than a B check, requiring a large majority of the aircraft's components to be inspected. This check puts the aircraft out of service, and the aircraft must not leave the maintenance site until it is completed. It also requires more space than A and B checks. It is, therefore, usually carried out in a hangar at a maintenance base. The time needed to complete such a check is at least 1–2 weeks and the effort involved can require up to 6,000 man-hours.
3C check
Some authorities use a type of check, known as a 3C check or Intermediate Layover (IL), which typically includes light structural maintenance, including checks for corrosion, or on specific high-load parts of the airframe. It may also be used as the opportunity for cabin upgrades (for example, new seats, entertainment systems, carpeting) which would otherwise put the aircraft out of service for a significant time without the need for an inspection. As component reliability has improved, some MROs now spread the workload across several C checks, or incorporate this 3C check into D checks instead.
D check
The D check, sometimes known as a "heavy maintenance visit" (HMV) is by far the most comprehensive and demanding check for an airplane. This check occurs approximately every 6-10 years. It is a check that more or less takes the entire airplane apart for inspection and overhaul. Even the paint may need to be completely removed for further inspection on the fuselage metal skin. Such a check can generally take up to 50,000 man-hours and 2 months to complete, depending on the aircraft and the number of technicians involved. It also requires the most space of all maintenance checks, and as such must be performed at a suitable maintenance base. The requirements and the tremendous effort involved in this maintenance check make it by far the most expensive, with total costs for a single D check in the million-dollar range.
Because of the nature and the cost of such a check, most airlines — especially those with a large fleet — have to plan D checks for their aircraft years in advance. Often, older aircraft being phased out of a particular airline's fleet are either stored or scrapped upon reaching their next D check, due to the high costs involved in comparison to the aircraft's value. On average, a commercial aircraft undergoes two or three D checks before being retired.
And that last item I might add is that cracks can be caused by one or a series of hard landings. Unavoidable overweight landings can do the same. (a landing necessitated by an emergency in aircraft without fuel dump capability) All pilots in reputable air carriers know that they should log such an occurrence and can do so without fear of reprisal. This always leads to an extra inspection.
Hope this helped.
