View Full Version : Ils Terrain Clearance

29th May 2002, 10:51
Could anyone tell me what terrain clearance is afforded on an ILS approach typically for a 3.0 degree slope.I have looked everywhere in the Jepps and cant find a answer.
Thanks in advance.

29th May 2002, 16:37
See United States Standard for Terminal Instrument Procedures (TERPS)


The Boy Lard
29th May 2002, 19:37
Straight out of the Oxford Air Law syllabus:

On leaving the enroute phase Minimum Obstacle Clearance (MOC) 300m

Inital Approach Fix (IAF) set QNH - MOC 300m

Intermediate Fix (IF) (QFE advised) MOC 300 down to 150m

Final Approach Fix (FAF) (optimum 5nms - maximum 10nms) where specified the MOC is 75m (this is for a non precision approach!)

The spiel for a precision approach is:

"The lowest altitude (OCA) or height above the elevation of the relevant runway threshold (OCH), at which a missed approach must be initiated to ensure compliance with the appropriate obstacle clearance criteria"

And it was just starting to make sense!

I'm just a dumb ATPL student struggling to get to grips with Air Law, perhaps someone with real experience at the sharp end could give a better answer!



29th May 2002, 21:11
The standard answer is....300 feet (USA). The glidepath angle is adjusted accordingly to provide the required clearance. "Duckunder" maneuvers are NOT recommended.

Speaking of the Instrument Landing System, can anyone here mention where, when and by whom the system was devised?
Hint: a very well known gentleman, who later on, did a "not too bright" maneuver with an Electra.

Pub User
1st Jun 2002, 00:59
The clearance can't be that simple. How can it be 300' when DH can be 200' even on a Cat I system?

1st Jun 2002, 08:15
'Tis a sliding scale down to minimums...I think. :confused:

1st Jun 2002, 09:47
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. :rolleyes:

Lateral Clearances
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.

Vertical Clearance
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. :)

11th Jun 2002, 16:26

Noting that the above quote is for TERPS and thus not necessarily the same for ICAO PANS OPS designed approaches.

I seem to remember a thread herein where a Captain Phil Simon of UAL offered up about 25 pages of differences between the FAA and the rest of the world.

Be warned....

11th Jun 2002, 17:33
Now wait just a second, are you actually implying that aviation, the most international of endeavors, is being conducted differently in various parts of the world. Come now, different standards and procedures could only lead to confusion. Certainly this couldn't be the case.

11th Jun 2002, 18:33
Perish the thought...