Not excusing anything about MCAS, but the "cracks in an aluminum airframe" thing is a VERY normal and established cycle/process. Well known and a normal thing. This news piece is just journalistic piling on to hot news.
There is "no news" here.
There is "no news" here.
Cracks can be a normal and expected thing. These parts were designed to last the lifetime of the airframe (around 90,000 cycles) and are failing in under 30,000 cycles.
This raises eyebrows for two specific reasons: 1.) Boeing was already installing subpar parts (edit: not pickle forks) from Ducommon in the 737 NG 2.) The wings fall off if the pickle forks fail completely.
This news piece is just journalistic piling on to hot news.
MCAS isn't the only fire Boeing is fighting right now (they're also battling poor quality control with their KC-46 and 787 lines among other things). Some guy on PPRuNe is claiming that the pickle fork problem was just improper manufacturing[1], which when put in context would make it appear as if Boeing has some major structural problems with its organization.
1: https://www.pprune.org/showpost.php?p=10595986&postcount=258
Cracks have been found in NGs both with and without winglets, so that is almost certainly not the issue at hand.
The long-standing joke is that MD took over Boeing with Boeing's money
Can these be replaced? If they have the expected lifetime of the airframe they might not be serviceable. Out of curiosity: can these cracks be repaired other than replacing the part?
Yes. The operation takes a couple of days per plane.
The actual crack appearance rate diverging from the modelled appearance rate at design time is a potential worry and indicates an issue somewhere along the line. How bad that issue actually is requires more investigation. It's a complex situation because metal fatigue is a statistical process, so you don't know if the cracks you've found are the best case or worst case.
Cracking and wear and tear on the structure might be normal, but it sounds like the issue here is that the 737NG airframes are showing signs of wear and tear much faster than expected.
The AD sets forth two distinct priorities: high cycle and low cycle airframes. Those with higher cycles required near immediate inspection because they are presumably more at risk for these cracks, in airlines with larger fleets inspections of younger frames may not be complete yet.
Southwest (owner of the largest 737 fleet in the world) grounded their (two, IIRC) NGs that required inspection sooner and failed that inspection. I don't believe Southwest has finished inspecting the other 600 or so planes in their fleet. Qantas (an airline with an enviable safety record) found cracks in much younger planes than expected (even after the expectations had been lowered from these parts lasting a lifetime to maybe lasting 1/3 that). Qantas took appropriate action, and were another airline to experience these cracks with such young airframes I'd hope they'd ground their NGs too.
Cracked pickle forks are absolutely noteworthy. Qantas flies its 737s in relatively mild weather, has a reputation for technically skilled pilots and excellent maintenance staff. If there were any fleet where you'd expect to find no cracks it would be Qantas. Instead Qantas has found a third NG with cracked pickle forks (out of a total of 33 NGs that required inspection).
This appears to indicate (yet another) QC and manufacturing deficiency (Boeing already is dealing with other such issues with the 787 and KC-46).
The claim by OP that this is normal because aluminum cracking is normal is really wide of the facts. This part was engineered to withstand the forces it experiences for a certain timespan, and the fact that some now appear to be failing in less than one-third of the lifespan the materials engineers expected it to calls into question whether Boeing or a subcontractor is failing to engineer these components to spec.
The FAA is going to be investigating Boeing for everything minor and major if they want to gain a fraction of the credibility they lost.
Boeing's originally stated lifetime was 90,000.
After the initial discovery of the unexpected cracking, the FAA ordered an inspection of all pickle forks with more than 30,000 cycles within 7 days -- this is probably the source of your confusion. The "youngest" cracked pickle forks discovered so far was on a plane with 27,000 cycles, which is to say 63,000 cycles early (not 3,000 early) with respect to what Boeing rated it for.
Inspection costs money. It might only take an hour to perform the inspection but that plane will be out of service for days.
For more context, the pickle fork holds up the wings, so if those fail you are SOL
also HN discussion https://news.ycombinator.com/item?id=21304277
Engineers seem to have lost control of the company.