LP Mins - RNAV (GPS) Approach

[John Collins]

The approach is an old approach from 2009 and is a stand alone GPS approach. The new standard is the RNAV approach and the GPS stand alone approaches will be superseded over the next few years. There are only 290 of these type approaches still in the inventory. L45 is scheduled to have the approach upgraded to RNAV 9/20/12 with LPV minimums. I don't know what the real glidepath and TCH are coded into the database for this approach, but as a rough assumption, with a TCH of 40 feet and a 3 degree glidepath would put the nominal glidepath at 2030 feet at APCUD, which should be a small fly down (just under a half a dot) if you were on the glidepath on an ISA temperature day. Two and a half dots (assuming 5 dots are full scale) is more than I would expect and would represent the glidepath being approximately 200 feet lower than the FAF minimum verses 30 feet above the altitude. What does your CDI show during the startup self test page of your GPS, it should indicate half scale fly up. If this was off, then it would show up during the approach to the same degree.

Regardless, you have to honor the minimum altitude as you were doing rather than using the advisory glidepath, when there is a conflict between the two.

 

[brywd]

John my cdi is correct, but in retrospect I've been leveling off 50 feet high so I don't bust minimums on my checkride on these approaches. So factor that in I guess.

 

[John Collins]

John my cdi is correct, but in retrospect I've been leveling off 50 feet high so I don't bust minimums on my checkride on these approaches. So factor that in I guess.

Do remember what the approximate temperature was at the airport and were there any unusual conditions such as a temperature inversion?

 

[brywd]

I don't remember an inversion or anything unusual about that day and the ground temp was around 60F I'd guess because I remember grabbing a sweat shirt from the baggage compartment after getting out of the plane.

We'll be making this flight again in a couple of weeks and I'll pay closer attention to the glideslope needle next time.

 

[poadeleted20]

In any event, if you don't have WAAS, you use the straight LNAV mins.

 

[jcelso6232]

What's the difference?

Hello all. Long time reader, first time poster.

There is A LOT of good information on this post but I believe there is a major missing piece of information here. Jeppesen in their December 1999 Chart Clinic explained LNAV+V...

http://ww1.jeppesen.com/download/aopa/dec99aopa.pdf

LNAV+V is a Jeppesen only product that any GPS system that uses Jeppesen NavData™ can take advantage of. In the Jeppesen NavData™ database for airborne systems such as GPS and FMS, there is a vertical navigation (VNAV) angle for virtually every non-precision approach procedure in the world. All of the descent angles are based on a series of rules which are written in the ARINC 424 specifications. The rules essentially state that a straight line will be drawn from 50 feet above the runway threshold back up to the altitude at the FAF. A calculation will then be made to determine the angle for the descent line. This is the method specified in both the TERPs criteria and the ARINC specs and is rounded to one hundredth of a degree. The descent angle will be at least 3.00°. If the computed descent angle is less than 3.00°, the angle will be raised to the minimum of 3°.

Look at the profile view of any RNAV approach and note the dotted line from the FAF to the runway threshold. The dotted line will always match the angle in the database. To show that the descent line is computed and in the database, the dotted line is shown in a gray color rather than the dark black lines used for the other profile view information. The computed descent angle is included in brackets to show the database information.

This is a computed glidepath and NOT a transmitted (as in ILS GS) or "carved" (as in LNAV/VNAV, LPV) glidepath. Hence why it is advisory only.

If the profile view does not show the gray dotted line and bracketed info from FAF to MAP or bracketed info above the blackened GP line, then LNAV+V is not a possibility ever.

Hope this helps!