alpergokgoz:
what is an unstable approach ? what makes a circling approach unstable ??
It depends a lot upon the height above airport (HAA) of the circle-to-land MDA. Where there are only low obstacles TERPs requires the HAA to be not less than 450' for CAT C and 550' for CAT D. The circling maneuvering area (TERPs) is 1.7 miles for CAT C and 2.3 miles for CAT D.
So, you must remain within the circling maneuvering area whether the MDA is 450 or 550, or whether it is, for instance, 2,200 HAA. Those circumstances create very different maneuvering requirements, and both are potentially very dangerous, whether the final approach is offset from the runway by only a relatively few degrees or whether you have to circle to land in the opposite direction.
In the case where the final is lined up, or nearly light up, with the runway but only circling minimums are charted, you have a high descent rate issue facing you. That is just one of many traps in CTL minimums.
That is why "TERPs is Traps."
Well, that and the fact that trapezoids are used to design some of the protected airspace. PANS-OPS provides much more reasonable areas of airspace for the CTL maneuver. TERPs has new, much better circling criteria approved after 20 years of battle, but certain vested interests are delaying implementation.