What happened
The FAA’s narrative, in sentence case. The FAA writes it in capitals and removes private details.
An experienced pilot was receiving instruction towards a tailwheel endorsement in this tailwheel aircraft. Runway marks showed a normal landing which appeared to be drifting slightly sideways. This drift to the left appears to have opened up a significant pre-existing metal fatigue crack in the lower portion of the right side, main landing gear spring axle. This caused what is normally a vertical compression load to be a side-load that opened the hinge point around the minimal remaining metal, and caused failure. The complete brake and wheel assembly departed the aircraft. The aircraft fell roughly 8 inches onto the broken axle stub, struck the prop, and pivoted consistent with a ground loop. Both pilots were uninjured and exited the aircraft.
As the FAA filed it
AN EXPERIENCED PILOT WAS RECEIVING INSTRUCTION TOWARDS A TAILWHEEL ENDORSEMENT IN THIS TAILWHEEL AIRCRAFT. RUNWAY MARKS SHOWED A NORMAL LANDING WHICH APPEARED TO BE DRIFTING SLIGHTLY SIDEWAYS. THIS DRIFT TO THE LEFT APPEARS TO HAVE OPENED UP A SIGNIFICANT PRE-EXISTING METAL FATIGUE CRACK IN THE LOWER PORTION OF THE RIGHT SIDE, MAIN LANDING GEAR SPRING AXLE. THIS CAUSED WHAT IS NORMALLY A VERTICAL COMPRESSION LOAD TO BE A SIDE-LOAD THAT OPENED THE HINGE POINT AROUND THE MINIMAL REMAINING METAL, AND CAUSED FAILURE. THE COMPLETE BRAKE AND WHEEL ASSEMBLY DEPARTED THE AIRCRAFT. THE AIRCRAFT FELL ROUGHLY 8 INCHES ONTO THE BROKEN AXLE STUB, STRUCK THE PROP, AND PIVOTED CONSISTENT WITH A GROUND LOOP. BOTH PILOTS WERE UNINJURED AND EXITED THE AIRCRAFT.