Comment by londons_explore
20 hours ago
I assume such forces are calculated and added in when deciding hot thick to make those mounting brackets.
20 hours ago
I assume such forces are calculated and added in when deciding hot thick to make those mounting brackets.
Yes, obviously; MD-11s aren't flinging engines off the wing every single takeoff. A 34 year old airframe may or may not actually match design strength, though.
Yep. Now do 3 decades of metal fatigue.
Did I understand the report correctly that the part was scheduled to be replaced in the future after a certain number of hours, it just hadn't hit the threshold yet ?
If you're referring to this quote (excerpted from the AVHerald article linked elsewhere in the thread), I don't think so:
> At the time of the accident, N259UP had accumulated a total time of about 92,992 hours and 21,043 cycles [..] A special detailed inspection (SDI) of the left pylon aft mount lugs would have been due at 29,200 cycles and of the left wing clevis support would have been due at 28,000 cycles
This isn't talking about replacement, only inspection; and it wasn't going to happen in the near future: 7k cycles at four flights/day means inspection is due in 5 years.
1 reply →
Yes, but the point is that this moment of the takeoff is when a failure that's been waiting to happen is most likely both because of the thrust and the gyroscopic resistance.
Aluminum has limited loading cycles
I'd be very surprised to read that the aft lug that cracked (and the bearing it contained) were made of aluminum. They were almost certainly steel or Inconel.
Wouldn't that be true of all cast metal objects?
Or are some metals impervious?
No; roughly, yes. Based on the crystal structure of the metal, fatigue works differently.
> The fatigue limit or endurance limit is the stress level below which an infinite number of loading cycles can be applied to a material without causing fatigue failure.[1] Some metals such as ferrous alloys and titanium alloys have a distinct limit,[2] whereas others such as aluminium and copper do not and will eventually fail even from small stress amplitudes.
https://en.wikipedia.org/wiki/Fatigue_limit