Crack in the main spar upper cap - What causes it and how to avoid.
It happened not only once that the main spar cap shows a crack in the outer end area.
These cracks are caused by hard landings, resulting in an extra stress or loads on the outer rib of the center section l/r that connects main spar with rear spar:
The loads lead to cracks in either the rib, or worse the main spar.
If you find your spar cracked, there is hope now. Recently a DER signed off a repair recommendation for exactly this problem.
A hard landing can ruin your day, but it does not have to. Alon already recommended years ago a fix to that area to mainly avoid the crack in the ribs, but it also serves as a direct load transfer to the thick part of the main spar and not through the stressed skin and the thin outer part of the spar cap where these cracks happen.
It is basically an angle made from .063 aircraft grade steel that is rived to the outer rib and is bolted directly to the main spar.