NCYankee wrote:
A vertically guided approach has a single GP angle, so if there are obstacles close in to the threshold, the DA is adjusted upwards and backwards along the approach course so as to put the obstacle in the visual segment and to allow the missed approach from the higher DA to take into account sink thru and still avoid the obstacle. An NPA can simply pass the obstacle and step down to a lower altitude. If the final is clear of obstacles, then the LPV will generally be lower than an LNAV/VNAV which will be lower than an LNAV, but not always. The obstacle environment matters, including the missed approach path.
Thanks, this makes a lot of sense. The terrain is flat like a pancake on final here but given your explanation I am wondering if the difference is related to the 2 obstacles NW of the runway (hence the MDA of 1700ft for the circle to land)
I think the limiting obstacle is in the missed approach segment. The LOC OCH is lower, but the MAPt is further away (3.4) vs the LPV (2.4). Thus you have an extra mile to climb the 100 ft to get to the same starting level as the LPV missed approach.
(Sorry, I think I just repeated what NCY said)
bookworm wrote:
I think the limiting obstacle is in the missed approach segment. The LOC OCH is lower, but the MAPt is further away (3.4) vs the LPV (2.4). Thus you have an extra mile to climb the 100 ft to get to the same starting level as the LPV missed approach.
But that means that if you fly the LOC as a CDFA, then you will hit the MAPt before you hit the DA!
Indeed, I calculate the nominal altitude at the LOC MAPt to be 1258’, which is almost exactly the same as the LPV DA!!
Airborne_Again wrote:
But that means that if you fly the LOC as a CDFA, then you will hit the MAPt before you hit the DA!Indeed, I calculate the nominal altitude at the LOC MAPt to be 1258’, which is almost exactly the same as the LPV DA!!
I use the GS calculations that are used by the FAA TERPS. My calculator says that at the MAP, if you are using EGNOS for the vertical, it would be 1153 feet or within 3 feet. Using a Baro-VNAV system for the vertical, I calculate 9 feet lower at 1144 feet. The difference is due to the curvature of the earth and the different way the two vertical paths are determined.
NCYankee wrote:
I use the GS calculations that are used by the FAA TERPS. My calculator says that at the MAP, if you are using EGNOS for the vertical, it would be 1153 feet or within 3 feet. Using a Baro-VNAV system for the vertical, I calculate 9 feet lower at 1144 feet. The difference is due to the curvature of the earth and the different way the two vertical paths are determined.
That is an interesting point. If the Baro-VNAV is using a WAAS GPS rather than actual barometric altitude for the GP, will it still be curved? I would think not.
JasonC wrote:
That is an interesting point. If the Baro-VNAV is using a WAAS GPS rather than actual barometric altitude for the GP, will it still be curved? I would think not.
No, WAAS vertical is a straight line in space. Accomodations in how the vertical CDI works were made in order to satisfy the FAA TERPS that WAAS vertical could be used for LNAV/VNAV. Baro VNAV is curved in space, so in a sense accounts for the curvature of the earth. LNAV/VNAV vertical flown by Baro-VNAV for the vertical also has temperature limits, whereas WAAS does not. LPV is a straight line in space, so altitudes of the GP centerline appear to curve upwards from the point of view of an observer on the surface of the earth. The vertical FSD determined by WAAS is limited to a maximum of +/- 150 meters, but is otherwise angular and FSD is +/- 0.25 X GS angle. The minimum FSD can optionally be fixed at +/- 15 meters for LPV and +/- 45 meters for LNAV/VNAV. RTCA DO229D has this note
Note: Manufacturers using the deviations described in figure 2-16b should understand that such deviations are not compatible with existing PANS-OPS for barometric VNAV, and until this issue is resolved the equipment may not be operationally approved to use SBAS vertical guidance on approach procedures published for barometric VNAV.
Ended up flying the LOC ;-)
LFHNflightstudent wrote:
Ended up flying the LOC ;-)
Why?
