ASPEN VOR DME or GPS-C questions

[BigFlyr]

Ok hot shots... For anyone that knows the ASPEN, CO DME VOR or GPS-C approach... or has a plate to look at... Maybe you can answer a question I have. What is meant by note 4 under the briefing strip..."VGSI and decent angles not coincident"?
Anyone? Thanks.

 

[boscenter]

Don't quote me on this, but I think by "VGSI" they mean "visual glide-slope indicator," i.e. PAPI, VASI, or whatever that airport is equipped with.

They are probably saying that the VASI is not calibrated to the 10-degrees indicated on the chart.

That's a pretty ugly looking approach though.. !!

 

[avbug]

The note you're looking at is a common one on many proceedure plates, and it only indicates that the visual glide slope indicator angles are not the same as the descent angle at which the approach meets the airport.

At aspen, you will have a reasonably steep approach from Red Table VOR, with a series of stepdown fixes. The MDA is high, and you must get down early enough to be able to pick up the VGSI early...gettting close to the airport will put you far above it.

Many approaches, including instrument landing system approaches, do not have a descent angle that coincides with the VASI, PAPI (etc) angle. On these approaches, you will see the proceedural note "VGSI and descent angles not coincident." A smooth transition from the electronic to the the visual isn't always possible, and this note warns the pilot to expect it.

In the case of Aspen, the PAPI will be at a much shallower angle than occurs with the descent from Red Table. The airport diagram also includes the note that the PAPI is not useable beyond 4 nm from the runway threshold, or 7 degrees right of the runway centerline. If the PAPI is used beyond these distances and restrictions, terrain clearance is not provided.

You'll note that the final stepdown fix to MDA occurs only three miles from the runway threshold, at is 2980' AGL. This is well above where the glideslope will be visual, requiring under visual conditions a descent that is steep, followed by a shallowing and increase of power, to pick up the visual indicator. Additionally, at the 4nm limit for the use of the PAPI, the 3 degree angle of the PAPI places that path well below the stepdown fix at D9.5 DBL on the approach.

 

[vja217]

I just got off the phone with my buddy that is a controller for the Aspen tower/tracon, he said that note was for two reasons:

First, he said basically what boscenter said about the descent profile requiring much more than the 3 degress that the papi will give. If you were to break out right at minimums, DBL 11 DME at 10200, you will still have to descend 2400 feet in 1.4 miles. Because of this, and some other factors (i.e. terrain), all approaches are considered "circling", even if you are able to land on 15 without turning. On windy days, he noted that the faster traffic won't be able to descend as rapidly as is required to land, even on 15, so they will fly somewhat of an overhead pattern, circling over or north of the airport while descending.

The other component that he mentioned relating to this note has to do with the PAPI usable area. If you look in the AFD, or on the Jepp Aspen Facility chart, it says that the papi is only usable within 4 miles of the runway. This generally isn't a problem if you follow the step downs as published to the DBL 9.5 DME (which is within 4 miles), but can be troublesome if you start the approach, then get the airport in sight and cancel your IFR (taking the visual), and follow misleading papi indications from several miles out.

He made another interesting comment about this approach that I don't think I would have picked up on otherwise, and that is the offset localizer that you use in the missed app procedures and in some of the DPs. Even though labeled "back course" on all the charts, it is "normal sensing". In other words, even though you are flying outbound on the localizer, away from the antenna, it does not reverse sense.

Hope all this helps,
Max

 

[vja217]

Sorry about that avbug, you were first... I guess you were typing while I was still on the phone!
Max

 

[BigFlyr]

Thanks for your replies. Being mostly a fair weather flyer from Florida, I have zero experience flying those high atltitude approaches out west, but all that may change soon. Just one comment for vja217 (nice pic by the way)... You stated that the LDA BC on the MAP "does not reverse sense"... I think you meant that your receiver would not reverse sense even if you had your OBS set to 300 degrees. Is that right? I figured you'd have to have it set to the front course or 120 degrees on your HSI OBS in order to get direct sensing, just as you would if you were shooting a LOC BC approach.