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Discussion in 'AviatorChat.com' started by Bob Parks, Sep 30, 2019.
One of the 38, no doubt.
Well, honestly, I don't think so. The "Quantas" plane was identified as having this problem only today, Oct. 31st, so could not be one of the 38, furthermore it is not above the erstwhile threshold of 30.000, but under it; and Quantas is now inspecting 32 other airframes.
One big change from the 737 Classics to the NG was increase in service ceiling (37k to 41k). Thus the operating delta p is greater for the NG. The major contributor to fuselage fatigue is pressurization, and it will impact the inner chord of the pickle fork fitting. There is already a supplemental inspection program, but it doesn't kick in (threshold) until 50k flights. Apparently the stresses used in establishing that threshold are too low.
Quantas just found 2 more... total of 3...
Interesting about the delta P...
Interesting thing about crack growth (and fatigue) is that it is not linear. A 10% increase in stress can result in nearly 50% decrease in life.
The higher the ceiling, the higher the delta P IIRC.
Three others at Ryanair, which is not much considering the number of 737NG they have...
From your Ryan link
That is a significant percentage.
I never was a champion in maths, but looks it could very well be 5%...
More than the original 38, but the percentage with problems (~5%) seems to be holding pretty steady.
Should have looked this up earlier, as this is different from what I envisioned. The outbd chord is not as critical for pressurization as the inner chord. For the NG series airplanes Boeing has a defined supplemental inspection program defined for critical areas, although these mandated supplemental inspections do not being until after 50000 flight cycles. I don't believe there are any specific inspections in the area that these cracks are being found.
The outbd chord, and associated failsafe strap, are critical for reacting vertical shear between the wing and fuselage. S-18 is also a major load reaction member for fuselage bending which peaks at the rear spar (Sta 663). The fact the ~5% of the fleet have cracks growing in a critical wing-to-body joint at such low usage should be very concerning. 5% is not random occurrences. Somebody missed something or the models used to predict loads/stresses in this region are not accurate.
Jim- Affirmative on that. Not sure how hard they are, if able at all, to retrofit.
FS strap can be replaced relatively easy. I think there is an SB which does that on the 767 (similar design). The actual pickle fork fitting not so much. Also very difficult, if not impossible, to add reinforcement if any cracks were removed. Full retrofit can always be performed, just a matter of time and $.
The only thing that I would be comfortable with is a full replacement of the forging and that means disassembling the fuselage at that point. I'm no expert on the design but it just doesn't look complete to me. I keep thinking of the 707 that seemed to have everything in a closed circuit. Never a problem on the airplane, even after the Gander Dive. I looked at that airplane when it came back to Seattle to have the horizontal tail replaced. The wing to body fillet hd been compressed and blown off but everything else passed a rigid inspection. No leaks in the wing even after additional degrees of dihedral were bent into it. Sheet metal inspection panels were also blown off in many areas.
I wish that I could find my drawings of the assembly of the fuselage-to-wing of the 707. As I remember it, the inboard wing assy. was joined to the center section that had 5 full depth span wise beams between the front spar and rear spar. Spanwise stringers were joined by upper and lower plus chords with heavy external splice plates on the lower joint . The 45 Section of the fuselage had forged bulkhead rings at the front spar and rear spar locations. It was lowered onto the center section where it was mated to the forgings that joined the wings and center section by four large bottle pins. The lower sections of the front spar and rear spar bulkhead rings joined to a keel beam fastened to the lower skins of the center section. The wing bending loads were reacted by by the center section beams, center section skins and stringers, splice plates, etc. There was no way for the center section to spread or to bend and after numerous AOG incidents it was obvious that it was almost indestructible.