Comment by tremon
19 hours ago
> On the aft lug, on both the inboard and outboard fracture surfaces, a fatigue crack was observed where the aft lug bore met the aft lug forward face. For the forward lug's inboard fracture surface, fatigue cracks were observed along the lug bore. For the forward lug's outboard fracture surface, the fracture consisted entirely of overstress with no indications of fatigue cracking
If I'm parsing this correctly, they're saying that fatigue cracks should have been visible in the aft pylon mount, and that the forward mount was similarly fatigued but showed no damage on the outside?
> If I'm parsing this correctly, they're saying that fatigue cracks should have been visible in the aft pylon mount, and that the forward mount was similarly fatigued but showed no damage on the outside?
If you can get to the report, Figure 7 shows the left pylon, with the forward and aft lug enlarged in the inset. Both lugs cracked on two sides. They're saying both cracks on the aft lug as well as the inboard crack on the forward lug were observed to be fatigue cracks, but the forward lug outboard fracture was observed to be entirely a stress crack.
Outboard and inboard are just away from and towards the center of the plane. On the left pylon, that's left and right, respectively. So, it looks like the left side crack in the forward lug developed from overstress, but the other three cracks were from fatigue. My expectation is that fatigue should be apparent upon the right kind of inspection, if timely, even if the metal has yet to fracture.
It sounds like the aft lug failed first, and then the not quite as compromised forward lug failed in overload.