Peter wrote:
It is an interesting point that if you use the altimeter to check the +V glidepath, and it is extra cold, you will think you are below the glidepath when actually you are not. But that would be the case with LPV also, and I would expect LPV to be closer to the terrain than +V.
In the case of cold temperatures, if you are following the GP and comparing the charted altitude at the PFAF with the indicated altitude, the indicated altitude will be higher than the charted altitude. That is, you are actually lower than what the indicated altitude is displaying and if significantly lower temperatures exist, you should preform temperature compensation for determining the DA.
+V only comes into play when the minimums are LNAV. The primary OCS is a fixed level plane that is 250 feet above the highest obstacle in the segment with a fixed lateral dimension of +/- 0.6NM. For an LPV, the OCS involves 3 sloping surfaces (W, X, and Y) based on the GP angle to a point usually about 200 feet prior to the threshold. The OCS slope is based on 102/(GP angle), so for a 3 degree slope, the slope of the OCS is 34 to 1 and the lateral dimensions goes from +/- 2200 feet at the PFAF to +/- 400 feet at the origin of the slope nearest the runway.
For LNAV/VNAV, there are two primary OCS surfaces, one is sloped and when close to the runway there is a second one that is level. They are both +/- 0.6 wide. The sloping OCS has a more complex equation to determine the surface angle that involves the average cold temperature.. A level OCS (89 ft above TDZE) is applied from 0.3 NM past the LTP/FTP to the point of intersection with the sloping OCS. The LNAV/VNAV OCS is designed to be used with Baro-VNAV, so the OCS takes into account the potential effect of temperature.
As a side note, using Baro-VNAV to fly a LNAV/VNAV, one can’t use the cross check on the GP at the PFAF to confirm the altimeter setting is reasonable and therefore detect a miss-set altimeter because the GP as well as the altitude indication are both based on the same altimeter setting.
Peter wrote:
I don’t think +V uses any of this, It is, as you say, a straight line from the FAF (or a point after the FAF) to the start of the runway (or maybe to the TDZ?).
+V is constructed from the FAF to the threshold at the crossing height. A 3 degree GP intercepts a level pavement down the runway at 954 feet from the threshold for a 50 foot TCH and if the TCH is 40 feet, 763 feet from the threshold.
The UK AIPs no longer have LPV data.
Procedures are sourced from AIPs
Pseudo ILS is different functionality
Peter wrote:
When the CAA de-published the LPV approaches (due to the SoL issue discussed in this thread) the hex data block used for LPV would also have been removed from the AIP. Obviously the data will still be “out there” but unless you build your own GPS you can’t make use of it. And building your own GPS is quite tricky because the Jepp databases are encrypted so that only known TSO boxes can use them.
Maybe there is no need to build your own GPS
There are ways to upload LPV procedures into Dynon Skyview. Emergency use only of course
Yeah I did wonder 
Flying using the altimeter is more dangerous in low temps.
Peter wrote:
It is an interesting point that if you use the altimeter to check the +V glidepath, and it is extra cold, you will think you are below the glidepath when actually you are not.
It’s the other way around… In cold weather the altimeter reads too high (that’s why you have to increase the minima in very cold weather) so you will think you are above the glidepath.
When the CAA de-published the LPV approaches (due to the SoL issue discussed in this thread) the hex data block used for LPV would also have been removed from the AIP. Obviously the data will still be “out there” but unless you build your own GPS you can’t make use of it. And building your own GPS is quite tricky because the Jepp databases are encrypted so that only known TSO boxes can use them.
I don’t think +V uses any of this, It is, as you say, a straight line from the FAF (or a point after the FAF) to the start of the runway (or maybe to the TDZ?).
It is an interesting point that if you use the altimeter to check the +V glidepath, and it is extra cold, you will think you are below the glidepath when actually you are not. But that would be the case with LPV also, and I would expect LPV to be closer to the terrain than +V.
Baro-VNAV approaches have existed here in the UK for years. Southend EGMC was one I recall but right now it shows no GPS approaches at all (just 2 × ILS). Old thread here. There was some funny history around that one too, with some boxes not displaying a GPS approach if a Baro-VNAV one existed; that was eventually fixed IIRC.
gallois wrote:
A +V would also need a start point, ok way enough in this case KA20F but what would be your end point. The CDFA line finishes at the Missed approach point which is at a point 40ft above the threshold. Is that point in the +V database or is the threshold the zero point in the database?
What ends up in the database depends if the procedure has an LPV minima specified. In that case, the entry is a FAS datablock and everything in the datablock is in terms of WGS84. In the case of an LNAV/VNAV or LNAV line of minima, they don’t require the FAS datablock. The VDA is specified in the database as is the FAF. That pins one side of the +V. The other side is based on the threshold location and the TCH height. The fact that all terrain, obstacles, and approach procedures have their lateral locations and vertical heights in terms of the WGS84 does not change where or how high they are, just the reference they are measured from. If there are LPV and other minimums such as LNAV/VNAV and/or LNAV as options on the procedure, the LPV FAS datablock provides the navigator with the parameters for the approach.
