Rolf, here's some info on Turkish Airlines quality control, specifically about the accident you mentioned above where the 737 got slow on final. What surprised me was how many known issues there are at Turkish air lines.

http://www.pprune.org/rumours-news-13/

This is the pilot equivilent of Catsailor, ie. it's where pilots from all over the world go to share info and to bitch about...everything.