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| Fractured wing-spar from a Piper PA-28 Cherokee. Two fatalities, April 4, 2018, Daytona Beach, Fl Source |
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| Wing assembly from same model, not crashed plane. Showing wing-spar position on wing. |
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| Not the wing-spar but a fatigue fracture with regions labeled. |
Initiation points identified with red dots. Order of initiation numbered based on amount of black "smut" or fretting debris at the fractures. The center section (3) is frosty white and the faint beach marks suggest it did not let go all-at-once but fractured progressively but very rapidly.
The plane this wing-spar came from was used to train students. One report suggested that it had approximately 7000 hours on the air-frame. That is a lot of touch-and-goes.
Landing puts unusual stresses on the structure where the wing is attached to the fuselage. The sudden force pulse of the landing gear hitting the ground can be large and excite dynamics that cause stress reversals from tension-to-compression. The instant the tire hits the ground creates a rearward force on the landing-gear as the tire-spins-up and that creates an axial torque along the wing axis.
Corrosion can be a factor on aircraft frames but it was ruled to not be a player in this accident.
Another consideration is that the wing-spar is "nested" into the structural cross-member on the fuselage. If there is a mismatch (or gap) in the fit then stresses are induced when the bolts are torqued and the gaps pulled in. That may be the reason for the "fish-mouth" cut-out in the spar, to introduce some compliance to reduce assembly stresses.
Waiting for you to introduce your analogy to society/economy.....
ReplyDeleteDiffer
Consider the "societal flexing" induced by ("the summer of love" riots)(the persecution of the Jan 6folks)(the covid debacle)(forced vaccination by experimental not-vaccines)(insert other stressors, here).
DeleteConsider the effects upon our social contract (or however you might prefer to denominated that which makes a western society work).
Consider the effects upon community trust of/in "expert" opinion or guidance.
When does that first bolt fail?
What is the next bolt to fail?
What occurs "downstream" of that failure?
I understand there is a support group for wives of widows of piper aircraft pilots.
ReplyDeleteI racked up 5,980 aircrew hours in my USAF career. All in 707 based E3s.
ReplyDeleteI'm never flying again.
That shit just ain't natural.
True of many things.
ReplyDeleteRelationships
Governments,
Societies
finances.
Modern almost anything is a very complex thing that can fail at multiple points, the stress points of which are not always obvious. When it goes, of course, it is generally catastrophic.
ReplyDeleteThis reminds me of the Aloha Airlines failure - either the airplane wasn't designed for those cycles, or it wasn't inspected properly.
ReplyDeleteI don't see a scenario outside flight school where it would see that many hours or that many landing cycles.
Jonathan
This vulnerability is why some of us fly / flew Mooney aircraft. Their wing spars are one solid, full length piece of metal - wings do not come off individually. The fuselage sits on top of the main and a 3/4 length secondary spars. Last I checked never a wing failure in 50+ years. (Yes they used wood in the early days - those models not recommended.) Bolted connections are fine if designed and maintained properly - but when a couple of miles up, I like a solid base to sit on.
ReplyDeleteThis is/was always a concern with Navy aircraft due to salt water corrosion.
ReplyDelete