In article .com,
"M" wrote:

I don't understand your calculation. At 2.5 miles from the touch-down
zone (assuming that's what it is), the GS should be about 750 feet
above the touch-down zone elevation. The pilot was way below the
glideslope.

(Simple and quick approximate calcuation method: 2.5mi = 15000 feet.
The 3 degree ILS is approximately 1:20 approach ratio. So 15000 / 20 =
750 ft).

20:1 is the floor of the protected airspace along the approach. The G/S
centerline is going to be above that by 100 ft or more.