FredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 7, posted (11 years 10 months 1 week 5 hours ago) and read 1647 times:
Fatigue most certainly does not equal stress. Stress is the load put on the structural members. Fatigue, in this context, is metal fatigue, or the reduction in ability to carry loads which occurs if a structural member is subjected to repeated stress above a certain threshold.
Over Va, you will overstress parts of the aircraft, most typically battery or engine mounts. That means inspection and that there is no guarantee you won't damage the aircraft. It won't break instantly though, engineering might be a precise science these days but it's still not THAT precise.
If one wing breaks off, you will have a roll moment and a loss of about half your lift. This will mean an angular acceleration around the roll axis and a downward acceleration. The downward acceleration will momentarily increase the angle of attack, and with it the aerodynamical loads, of the already highly stressed wing on the other side. Perhaps it is not all that surprising to see both wings give up together.
Fatigue, although the understanding of this phenomenon is relatively speaking rather recent, is surprisingly predictive. This means that two members, such as two wing mounts, which have been put through the same number of cycles of the same loads will be similarly weakened by metal fatigue.
I thought I was doing good trying to avoid those airport hotels... and look at me now.
LMP737 From , joined Dec 1969, posts, RR:
Reply 8, posted (11 years 10 months 1 week 1 hour ago) and read 1614 times:
It's just been reported that the aircraft had undergone repairs for cracks in the wing. LMT aircraft tend to have rigid wings. In fact the C-5, C-141 and Electra have had issues with cracks in the wings. Earlier this year the wing on a C-141 fell OFF while being refuelled.
EssentialPowr From United States of America, joined Sep 2000, 1820 posts, RR: 2
Reply 10, posted (11 years 10 months 1 week ago) and read 1597 times:
Fatigue as it relates to a/c design is continued stress cycles accompanied by strain or creep, which is localized and slight elongation (for metals, technically a realignment of the grain structure) of the material, which will ultimately lead to a failure. Stress/Strain curves are basic materials science.
My point is that an apparently symmetrical wing fatigue does not in any way preclude fatigue as a cause, as crack propagation and fatigue are related. The Navy had a similar, symmetrical A6 wing failure due to fatigue years ago. The result was a restricted G limit on the a/c, which extended the airframe life until the a/c were retired or rewinged.
Jetguy From , joined Dec 1969, posts, RR:
Reply 11, posted (11 years 10 months 6 days 23 hours ago) and read 1590 times:
I'm not positive about it being corrosive, but I don't think it is. I say that because I understand that the slurry is a combination of water and some type of fertilizer. Our hangar is adjacent to a tanker base (we have to taxi around their slurry hoses) and the ground crews make no effort to keep if off of the airplanes. (It actually makes a pretty good mess by the end of the season.) The plane that crashed was an early model C-130A operated by Hawkins & Powers. I have seen it here several time during past years. One of our pilots, a former fire pilot, knew some of the crew. The early C-130s are pretty old. Butler aircraft has one that is a year older than one of their fire fighting DC-7s. I believe that those aircraft are operated in the restricted category, so I don't know if they carry "black boxes" or not, but I really doubt if they are required to. It's hard to say what happened, but it sure looks like a combination of fatigue and overstress to me.
MD11Nut From , joined Dec 1969, posts, RR:
Reply 12, posted (11 years 10 months 6 days 23 hours ago) and read 1585 times:
I tend to agree with Jetguy regarding the possible causes. From the video, the crew could not have pulled more than 2g's from that maneuver so I don't think pure overstress was the case. Now, for a firefighting aircraft, the C130 must have had hundreds if not thousand of cycles of the same maneuver, putting stresses on the same points year in and out.
One thing about the causes, we will know definitively soon enough since it will be relatively easy for investigators, when looking at the broken wings to tell the differences between the break lines caused by overstress (45 degrees shear) and fatigue (rings of discolor patterns).
Notar520AC From United States of America, joined Jul 2001, 1606 posts, RR: 4
Reply 13, posted (11 years 10 months 5 days 13 hours ago) and read 1489 times:
Keep in mind, a paperclip can only be bent so many times before it snaps, however in a normal a/c I don't think that would be the case, but with the weight of the fire retardent on board the the g's being pulled leveling off from that maneuver, I would say that was a factor, not the cause.
Also, that same aircraft was involved in a bullet fire fight earlier in it's life, which was also probably a factor.
Now I'm not jumping to conclusions, but I can't think of many more things that could have caused that crash. I viewed the eye-witness video, and I have never heard nor seen anything like that EVER.
L-188 From United States of America, joined Jul 1999, 29706 posts, RR: 59
Reply 14, posted (11 years 10 months 5 days 11 hours ago) and read 1478 times:
I don't have a link to it but apparently this aircraft did have a wing crack repair about four years ago.
The news being the news, I don't know what the repair was for but it is and interesting development.
I watched H&P's PB4Y and their KC-97 doing drops on a fire at North Pole, Alaska about a month ago. I didn't think those guys where really putting that much stress on the aircraft with what they where doing.
OBAMA-WORST PRESIDENT EVER....Even SKOORB would be better.
Paulc From United Kingdom, joined Mar 2001, 1490 posts, RR: 0
Reply 15, posted (11 years 10 months 5 days 9 hours ago) and read 1468 times:
The wings folding up on the C130 is not a new thing - as I understand it when the RAF ordered them they opted not to go for the uprated wings as offered by lockheed despite there having been a similar crash. This was done for cost reasons (about £7000 per aircraft on 66 ordered) However this changed when a test rig of the centre box wing section failed at Marshall in Cambridge much earlier than anticipated. All RAF C130s were grounded for a time while an inspection team check each one before flying them to be modified (£250,000 per plane) Military versions were fitted with G meters to allow crew to monitor this.