On Cirrus' 2015 aircraft page, they advertise Baro-VNAV capability in the new model. Having never done an LNAV/VNAV GPS approach (only LNAV or LPV), I'm wondering: when would I actually use it, if I already have WAAS and can just use the LPV approach? Typically, I'm used to seeing an LPV minima alongside any LNAV/VNAV minima. Are there exceptions? Is this feature valuable in practice?
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2 Answers
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LPV and non-baro LNAV/VNAV requires that a Space-Based Augmentation System (SBAS) be available to the GPS receiver. In the United States, that system is called Wide Area Augmentation System (WAAS). Similar services operate in other parts of the world (e.g. EGNOS in Europe, MSAS over Japan, GAGAN over India) but coverage is not worldwide, and not all GNSS (Global Navigation Satellite System) approaches are approved for LPV.
Having a baro-VNAV system in the aircraft allows you to shoot LNAV/VNAV approaches if SBAS is not available. While not as useful in the United States, as we have robust and reliable coverage with WAAS, it is nice to have in parts of the world where SBAS is unreliable or non-existent.
FAA fact sheet on RNAV (GPS) approaches: http://goo.gl/cIhIJa
Worldwide SBAS information: http://goo.gl/eooOau
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Baro-VNAV computes vertical navigation based on barometric pressure, rather than on GPS-based WAAS altitude.
The altimeter feeds digital altitude information into the RNAV/VNAV unit, and the RNAV/VNAV unit provides vertical navigation information to the pilot by computing a synthetic glideslope, to guide the airplane from the FAF down to DH above the runway.
One major factor for Baro-VNAV is temperature, which cannot be set in the Kollsman Window of the altimeter, so these approaches have a minimum OAT at which they can be utlized.
Reference: RNAV (GPS) Approaches
