Indeed, FMS system relies on altimeter to compute arrival altitude and glide slope. Horizontal guidance is identical to LNAV approach: their precision is 5 NM in Enroute mode, 1 NM in Terminal mode and 0.3 NM in Approach mode. These approaches provide lateral and vertical guidance in the same cockpit presentation as an ILS. LNAV/VNAV approaches for FMS receptors (3D) It is pilot's responsibility to check and announce scale transition. Scale transition is made automatically assuming the pilot does not intercept final approach segment at less than 2 NM from FAF, because Intercepting final approach segment at less than 2 NM from FAF, would prevent automatic scale transition. Which is to say that when the needle just arrived at full deflection, cross track deviation is 5 NM, 1 NM or 0.3 NM depending on plane's mode at the time. Their precision is 5 NM in Enroute mode, 1 NM in Terminal mode and 0.3 NM in Approach mode. Non-WAAS GPS systems allow for approach with horizontal guidance only. We shall ignore GBAS procedures which are not yet installed on a regular basis in Europe. Those which benefit from horizontal and vertical guidance and are intended for WAAS equipped GPS (EGNOS as well): LPV. Those which benefit from horizontal and vertical guidance and are intended for baro-integrated FMS equipped aircraft: LNAV/VNAV. A turn too early can have bad results.Those which only benefit from horizontal guidance: LNAV. Also don't forget on any missed approach initiated at the pseudo DH, one must still navigate to the MAP and follow any turn instructions at that point. For turbojet aircraft, this may be required, but for piston aircraft, much steeper descents to the runway are possible to do safely. By initiating the missed approach at the point where the GS intersects the MDA, the visibility requirement is much higher and in low visibility situations, an unnecessary miss may be required when in fact the approach could be safely completed. The MAP on the LNAV is typically at the runway threshold and the visibility requirement assumes the pilot continues along at the MDA until the runway is identified. Some pilots treat a LNAV+V MDA as a DH and add a small buffer of 50 feet so they don't bust minimums and start the missed approach at that point. With a LNAV, there is a MDA, and the aircraft must not descend below the MDA until a normal visual descent can be made. Momentum may carry the aircraft lower than the DH and that is permitted as long as the missed approach has been initiated if the visual criteria is not met. In the US, flying an approach with official vertical guidance is to a DH, and the decision is made at that point. I have never seen +V indicated on an approach chart, although one can normally guess if it will be provided. LNAV+V is a feature and invention of the manufacturer and is not part of the approach evaluation criteria. The Garmin WAAS GPS Units are approved for flying officially vertically guided APV approaches, both LPV and LNAV/VNAV. However, the advisory glidepath annunciated as LNAV+V will only be available on a RNAV approach that only has LNAV minimums. Below the DH, the visual segment must be clear of obstacles and following the glidepath is expected. If the approach has vertical guidance via an APV such as LPV or LNAV/VNAV, then the vertical guidance is official and may be used to the DH. I am sure it works but most German LNAVs where the +V would operate also have a LNAV/VNAV so that will be used if the system is able to. LNAV+V would be an extremely handy tool to do RNAV approaches below the MDA, aehm I mean to reduce pilot workload and make sure to go around on reaching MDA. I will try it again next week and wonder if there's something I have to enable/configure for these approaches to show up. The GNS simulator is too old it seems (there is a way of injecting a current database but that didn't help either). Someone told me yesterday that Straubing EDMS has it now enabled but I was not able to find it last time I went there. Jeppesen used to block the +V component in the European database but when I asked them about it, they told me this limitation was lifted ca. That's exactly what the GNS/GTN boxes do and it is part of the Jeppesen supplied approach database. But there is a mode on the GNS/GTN boxes whereby the "continuous descent" profile (a straight line FAF-MAP) seen on many/most Jepp plates is available as an advisory "glideslope" and the box can output a VNAV deviation signal to a CDI (or HSI etc) and thus to an autopilot.
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