Originally Posted by
sycamore
FW, at the `fix`/step,the a/c flies at an indicated altitude of 5000ft.Another a/c is vectored to 4000 ft in the same area.
The higher a/c is at a true altiude of 4700 ft under the conditions.
The lower aircraft is at a true alt of (4x4x15=240)=3760ft..difference ,940ft..
...
That's correct, if the higher aircraft is flying an indicated altitude of 5000 ft.
But, say that the indicated altitude of 5000ft results in being 2 dots high on the glide slope, or even full-scale. Some flight crews assume that it is okay to follow the glide slope; after all, it is fixed in space!
But this thread is contemplating and describing why complying with barometric altitude restrictions of step-down fixes on the approach is important: because conflicting traffic is separated based on barometric altitudes, not the fixed glide slope.