Originally Posted by
Ian W
This is an extraordinarily poorly written note by the FAA.
If the aircraft is on the glide slope (assuming that it is accurate at that range) then the aircraft will always have the same vertical distance from terrain as the glide slope is a 3 degree path from a fixed point on that terrain.
What they should be saying is that the glide slope obstacle clearance surface is only calculated to 20 miles outside that distance there is no guarantee that the glide slope does not penetrate terrain.. The minimum safe altitudes for the fixes on the STAR are calculated to ensure that an aircraft at that altitude will be clear of terrain regardless of the pressure setting or temperatures and therefore should be obeyed.
These glideslope/step-down fix/barometric altimetry violations are almost always about airspace construction and traffic separation, not about terrain.
The actual height of the electronic glide slope is not the issue. The issue is that traffic below the glide slope will be at higher ACTUAL heights on hot days, because they are flying based off of barometric altimetry. The step-down fixes are also based on barometric altimetry, so aircraft on the approach must comply with the step-down altitudes for separation to be assured, regardless of where the electronic glide slope might actually be.
Traffic below Class B airspace are not monitoring the ILS glide slope. They are only minding their barometric altimeters.