Cessna T210M Wing Spar Shear between top rivets and lower I-beam Flange Narrowing Point (and Lower Forging Lightening Spotface Indentation.) The white foam may have collected water, aiding corrosion. Photo credit: ATSB. (click image to enlarge)
I’ve read thousands of airplane accident reports, but this is one of the most concerning.
The Cessna 210 (and 177) family has a complex wing design made of aluminum components with around 100 features, mostly rivet holes, that can become stress risers.
Since 2012, several spar cracks have been found, and in 2019, there was a fatal accident in Australia after losing a wing, which is not survivable. (Note that this plane was used for aerial survey grids at 200′ with full fuel plus wing tip fuel tanks and 2 pilots, for as many as 5,000 hours. Two survey planes were operated. It would be fascinating to do a teardown on the other one.)
The fatal 2019 accident in Australia appears to be a combination of corrosion on a rivet hole and metal (aluminum) fatigue, possibly combined with the round shape of the lower spar forging spotface (indentation.) Aluminum can shear instantaneously once a crack forms, though in this case the ATSB suspects previous cracking.
Cessna 210 Wing Lower Spar Forging. Photo credit: tennesseeaircraft.net.
Example of a fail-safe spar with a riveted spar web. This might have arrested the crack, but at the cost of being twice as heavy since both the top and bottom half-spars would need to be capable of carrying the load. And all bets are off with corrosion. Diagram credit: aircraftsystemstech.com.
(In the 1995 Kobe earthquake, some of the freeway supports that failed had a steel jacket around the bottom third of the concrete supports, causing a stress riser in a ring around the support. Japan subsequently updated their building code to do full steel jackets. This spar structure failure appears to be very similar if a forging spotface was involved. Under a sideways load, the weakest part would be at the curved spar cap spotface indentation, acting as a strees riser. Under a vertical load, it would still be the same location, since both the spar caps and forgings narrow in thickness at that point.)
This is a tough one to fix, let alone inspect. The spar and other attachments are no longer made, and even if they were, that’s $86,000 into an old airplane. For these to continue to fly, somebody needs to operate these outside the USA and come up with a “massive spar” modification that’s affordable, if not TSO’ed.
Cessna 210 Spar Links
Wings – Aircraft Structures
federalregister.gov: Airworthiness Directives; Textron Aviation Inc. (Type Certificate Previously Held by Cessna Aircraft Company)
tennesseeaircraft.net: 210 Wing Lower Spar Cap Bulletin Sel-57-01 Revision 1 (2012)
avweb.com: Mask Shortage Results In Cessna 210 Spar AD Extension
generalaviationnews.com: Cessna 210 spar caps subject of new Airworthiness Directive
flyingmag.com: FAA Calls for Cessna 210 Wing Spar Inspections
aopa.org: Cessna 210 owners weigh compliance options for corrosion AD
asrs.arc.nasa.gov: Smoking Rivets
nasa.gov: THE STRENGTH AND STIFFNESS OF SHEAR WEBS WITH AND WITHOUT LIGHTENING HOLES (1945)
faa.gov: FAA Request for Information on Cessna 201 and 177 Airplanes
pprune.org: Cessna 210 Accident Mt ISA
1995 Kobe Earthquake Freeway Collapse