LPV, LP, LVAV/VNAV CDI sensitivities

4 Answers

1. 
   

   
   

   John D Collins on Nov 27, 2016

   The most common implementation of the FSD (Full Scale Deflection) for an LPV or LP starts out as a fixed value (+/- 1 NM), then transitions to angular (+/- 2 degrees) over a 2 NM distance, finally becomes fixed again at the runway threshold (+/- 350 feet) until the point where the glidepath intersects the runway. The latter point is roughly(not precisely) 1000 feet past the threshold.

   As a practical matter, you fly the procedure the same way. There are several options permitted by RTCA DO 229D. Look in the Pilot Guide to see how the FSD varies for a given GPS system.

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   Charles22 on Nov 27, 2016

   John,
   Many thanks for answering my recent post (and a few other posts before).

   What I am trying to do is to determine the FSD profile along the path of the LPV and LP approaches. Roughly, as there is no point in trying to achieve foot single digit accuracy for something like this.

   From the information that you provided I have now a clear understanding of the geometry of the FSD at the threshold and on the first ~1000 feet of the runway.

   I am still a little confused about what happens between the runway threshold (FSD=350 feet) and the point (2 NM outbound of the FAF) where the FSD becomes 1 NM.

   Let’s assume for discussion purposes that the distance between the threshold and the FAF is 5 NM. Then the distance between the threshold and the point where the FSD becomes 1 NM is 5 NM+2 NM = 7 NM. The angle that you mention (2 degrees) will not bring down the FSD from 1 NM to 350 feet (0.058 NM) over a distance of 7 NM. Over 7 NM such an angle will reduce the FSD only from 1 NM to 0.76 NM. There must be a point where the FSD goes down briskly (perhaps to 0.3 NM?) and then, from that point on, further reduces according to the 2 degree angle down to 350 ft.

   Does the RTCA DO229D describe how the FSD transitions from 1NM to 350 ft? Unfortunately I don’t have that document and I feel that is a little too expensive to purchase for what would likely be a one time use.

   Again, thanks for your help.

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3. 
   

   
   

   John D Collins on Nov 27, 2016

   Charles,

   First, there is not a single answer as the details vary by the type of approach, whether or not VTF is being used, and the specific choices made by the GPS vendor. If you look at the GNS530W Pilot Guide in Appendix C, it has this question and discussion;

   When does the CDI scale change and what does it change to?

   The CDI scale is 1.0 NM (terminal mode) within 31 NM of the departure airport. The CDI will smoothly scale to 2.0 NM (en route mode) once beyond 31 NM from the departure airport. The unit begins a smooth CDI scale transition from the 2.0 NM (en route mode) to the 1.0 NM (terminal mode) scale within 31 NM from the destination airport. When within 45° of the final approach course and the FAF is the TO waypoint, the 500W-series unit switches from terminal mode to approach mode. CDI scaling is tightened from 1.0 NM full scale deflection to either 0.3 NM or 2° full scale deflection whichever is less at the FAF. The scaling change occurs gradually over a 2 NM distance and is completed before crossing the FAF.

   If Vectors to Final is selected, the unit provides CDI scaling appropriate to the approach, which is typically ±2 degrees angular from the origin of the approach. This angular course deviation reaches the max ±1 NM deviation at approximately 27 NM from the MAP. The course width and angular deviation for GPS approaches are similar to what is provided for VHF localizer signals. (see the figure – Full-Scale deflection and defined path for VTF approach).

   If a missed approach is executed, the CDI scale will change to ±0.3 NM or ±1.0 NM, as described in Section 5 – Procedures – Flying the Missed Approach.

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4. 
   

   
   

   Charles22 on Nov 27, 2016

   John,

   Thanks for your help.

   I have now all the information that I was looking for on the FSD profile for LPV/LP approaches!

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