In 1996 the FAA changed the obstacle clearance criteria for the ILS approach. After years of field studies they found the lateral clearance for a localizer was excessive, while the vertical obstruction clearance for the electronic glideslope was much too narrow. The new criteria apply to ILS IAPs that have been brought online or revised since 1996. The obstacle criteria is similar to the criteria used for the multitudinous MLS approaches that we have available in the United States.
Not that straight forward, you may want to sketch this. There are three cross-sectional areas: one has 0 gradient, the next has a 4:1 gradient, and the last has a 7:1 gradient. These expand, as does the ILS, the farther from the transmitter one travels. The problem is they donít expand at the same degree. So the sketching part (okay, remember this is supposed to be fun). At 200 feet from the threshold the 0 gradient clearance plane is 800 feet wide, the next is the 4:1 plane and that is 300 feet on either side on the 0 plane. The third plane (7:1) is also 300 ft wide. (Great - donít get cocky.) At 5 miles from the threshold the 0 gradient plane is 2974 ft wide, the 4:1 plane is 2459 feet wide on either side, and the 7:1 plan is 1628 ft on the outside of the 4:1 plane. Connect the points and you have your profile view of the ILS Ė I bet that could have been explained better.
This is the tough one, remember this one increased. At 200 feet from the threshold 122 ft of clearance, at 5 miles 755 ft.... the buzz word here is connect the dots, so if you do that youíll have your gradient (and you can apply to the FBI). Now thatís within the localizer primary area which is the 0 gradient plane and also the 4:1 slope. I think you have all the data points to build a conceptual model.
Thanks to Wally Roberts for the technical info, I alone am responsible for obfuscating the data to the point of uselessness.