ILS approach

[DrewBlows]

The legal usability of the glideslope extends as far as the outer limit or fix defining the approach. Typically this may be the outermarker, but may also be the published GSIA. Beyond that, technically the glide slope is not approved for guidance.

However, it's acceptable to follow the glideslop so long as it doesn't conflict with any intermediate fixes or step down retrictions on the localizer.

I think you may be mistaken about the legal usability of the glideslope. Firstoff the FAR's give no "legal" guidelines to glideslopes (or any other types of navigation for that matter), so we have to rely on the trusty AIM for service volume infomation which says, "The glide slope is normally usable to the distance of 10 NM. However, at some locations, the glide slope has been certified for an extended service volume which exceeds 10 NM." So legally speaking I think your second paragraph is more correct.

Thoughts...

 

[DrewBlows]

2) in some few cases, the glideslope may take you below a minimum stepdown altitude. Again, uncommon, but it happens. Be alert for this, just because you're on the GS doesn't mean that you can ignore minimum altitudes. outside of the outer marker, use the GS for a descend *aid* and use your altimiter and published minimum altitudes as *primary* descent information.

Does anyone know of any specific approaches that have a minimum stepdown altitude below the glideslope. Do the plates have some sort of cautionary (sp?) statement in the notes or planview section, or do they leave it up to us to figure it out when we see the flashing beacon of a cell tower through the clouds.

 

[A Squared]

Does anyone know of any specific approaches that have a minimum stepdown altitude below the glideslope. Do the plates have some sort of cautionary (sp?) statement in the notes or planview section, or do they leave it up to us to figure it out when we see the flashing beacon of a cell tower through the clouds.

I assume you meant a minimum stepdown altitude *above* the glideslope.

I don't know of any in real life. In fact, it would be mathematically impossible on a 3 degreee GS if it were designed in accordance with the TERPS. The TERPS specifies a maximum design descent gradient on the intermediate segment of 300 ft/nm. A 3 degree glideslope descends at 318 ft/nm. If an intermediate minimum altitude took you above the GS, the approach next stepdown wouldn't be enough to get you back underneathe hte glideslope. It would be possible on any GS less than 2,8 degrees. They can be designed as flat as 2.5 degrees.


There are, however, ILS approaches with MANDATORY altitudes before the GS intercept. I believe that there is one in Houston. The altitude is depicted on the NACO plates with a solid line above and below.

 

[100LL... Again!]

A squared is correct. would be basically impossible.

 

[Tinstaafl]

There is one possibility under PANS-OPS (not sure about TERPS): If the approach plate charts two different type of approaches ie an ILS and a LOC or LOC/DME. They're two different approaches, with separte obstacle clearance criteria.

It can happen - theoretically, I can't think of an example - that an obstacle that doesn't impinge on an ILS obstacle free gradient can still impinge the obstacle free area required to be considered for the non-precision approach.

 

[SkywayFO]

I didn't read all the posts so sorry if someone said this already. But watch out for manditory step downs. There are someplaces that have them do to airspace constraints for airports near by.

 

[avbug]

think you may be mistaken about the legal usability of the glideslope. Firstoff the FAR's give no "legal" guidelines to glideslopes (or any other types of navigation for that matter), so we have to rely on the trusty AIM for service volume infomation which says, "The glide slope is normally usable to the distance of 10 NM. However, at some locations, the glide slope has been certified for an extended service volume which exceeds 10 NM." So legally speaking I think your second paragraph is more correct.

"Service volume" is irrelevant for the use of the glideslope, as it pertains ONLY to the approach. You may be able to receive it at farther distances, but you are NOT guranteed obstacle protection or signal integrity beyond that which is charted. You are not gauranteed anything beyond that for which the approach has been flightchecked, and that's going to be the published portion of the approach only. Typically to the published GSIA.

The "FAR's" give no legal guidelines, do they? Are you not familiar with Part 97?

Have you not viewed TERPs?