I am new to instrument flying and am trying to wrap my head around the different types of GNSS approach minimums. I am having an issue understanding why LNAV minimums exist. My understanding is, if you're GPS is not WAAS-certified, then you are flying your approach down to LNAV/VNAV minimums. In this case, the lateral guidance is provided by SBAS (your GPS), and the vertical guidance is provided by your barometric altimeter. So...my confusion here is...since most aircraft have barometric altimeters, when would you ever use the LNAV minimums? Since every aircraft contains some sort of altimeter (your vertical guidance), then why does an LNAV approach even exist? Is it only for the case of an aircraft having a GPS and a non-barometric altimeter (i.e. a radio altimeter)?
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$\begingroup$ Welcome to Av.SE! A small point, a baro altimeter is required for lots of things beyond approach minimums, and a radio altimeter is expensive & only required for some fairly specific things like Cat II / Cat III minima, plus some more advanced GPWS systems (more advanced than terrain warning that you can get with GPS + an iPad, at least). So anybody that has a radio altimeter will certainly have a baro altimeter (or two) as well. $\endgroup$– Ralph J ♦Commented Jun 3, 2023 at 19:20
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2$\begingroup$ Adding to this… The misunderstanding in the question is due to thinking that LNAV/VNAV requires only a barometric altimeter and not a Baro-VNAV system. $\endgroup$– TimboCommented Jun 3, 2023 at 21:12
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$\begingroup$ Exactly what Timbo said. I am often told by people that they can fly to the Baro-VNAV minimums because their autopilot “obviously” understands barometric altimetry because it has altitude hold, vertical speed modes, and altitude preselect. This is not correct. The slope precision required for a glidepath can only be met by a navigator that has a baro-VNAV approved Air Data Computer. Common in transport aircraft, but not common in GA aircraft. The ubiquity of WAAS GNSS approaches has greatly diminished the justification for the cost of certifying an aircraft for baro-VNAV. $\endgroup$– Max RCommented Jun 12, 2023 at 5:25
1 Answer
In LNAV/VNAV approaches, the vertical guidance can be provided by WAAS, or it can be provided by a baro-VNAV system.
On an LNAV approach, there are step-down waypoints just like on any other non-precision approach. The pilot uses the altimeter to manually adhere to minimum altitudes.
On an LNAV/VNAV approach, the avionics compute a glideslope and provide vertical guidance all the way to the decision altitude, so the pilot can "chase the needle," just like a precision approach. The vertical guidance can be provided either by WAAS or by the barometric altimeter ("baro-VNAV"). Not every airplane with a barometric altimeter is capable of using it in this way.
So LNAV approaches are the only ones that can be done by aircraft without baro-VNAV or WAAS available. There also can be restrictions on the other approaches- for example, baro-VNAV approaches are often not allowed if it is too cold, due to the altimeter errors caused by low temperatures.
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1$\begingroup$ One reason Baro-VNAV might be unavailable: "Baro-VNAV and VDP not authorized with _______ altimeter setting" - note on an approach chart for an airport with a part-time tower.. Going from local altimeter setting + LNAV/VNAV mins to remote altimeter + LNAV mins changes the mins from a DH that's 282' above the runway to an MDA that's 458' above (if baro VNAV is your only option), so that's a significant difference. $\endgroup$– Ralph J ♦Commented Jun 4, 2023 at 12:02