IF the controller intends to say "turn heading 270 intercept the
localizer" but instead actually says "turn heading 170 intercept the
localizer" and the pilot, sensing that 170 is not an approprite heading
and that 270 is probably what was intended, asks "confirm heading 270,
straight west" and the controller says "affermative, heading 270 intercept
the localizer", was the clearance amended?

Yes.

It is in that sense that the OP (I think - it's an old thread I'm not
going to dig up) had used the word "amended", and you claimed that it
did not count as an amended clearance. The two situations are quite
parallel.

Jose
--
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