Originally Posted by
pineapple_juice
http://www.turkishairlines.com/en-int/miles-and-smiles/awards/upgrade
Is IST-JNB defined as a high-demand route? Given that TK never release any limited capacity awards in business class I guess it is...
Its definitely a route where the limited capacity awards in J are hard to find, so upgrading at the airport should be your best bet.
For what it’s worth: when I flew TK there and back earlier this year, the J cabin wasn’t full.