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IFR Chapter-13-V8_Basic Master Frame.qxd 12/19/2019 5:23 PM Page 20

13-20 Rod Machado’s Instrument Pilot’s Handbook
you could use the glideslope for descent to the MDA. For
The Advisory Vertical Glideslope: LNAV+V
instance, suppose you were flying the RNAV (GPS) Rwy 30
approach to Jacqueline Cochran Regional airport (TRM) as
shown in Figure 19. If the WAAS HPL and VPL limits are
met and a glideslope is generated, the approach annuncia-
tor window will indicate LNAV+V as shown in Figure 20,
position A (no LPV minimums are available on this
approach). You simply follow the glideslope down to the
lowest MDA allowed, which
is 260 feet at TRM (Figure
19, position B). Following
the glideslope allows you to
cross FORKI, the stepdown
fix (Figure 19, position C), at
the appropriate altitude and
make an uninterrupted
descent to the LNAV MDA of A
260 feet. If the requirements
for landing aren’t met, you
stop your descent at 260 feet
(i.e., level off or begin a climb)
a n d f l y t o t h e m i s s e d Fig. 20 Courtesy Garmin Fig. 19
approach point. If you do
meet these requirements, you
could remain on the advisory glideslope and ride it all the D
way to touchdown (and I hope you do because it’s obstacle
free, meaning it’s good for your health and it contains no C
fat). Remember, RNAV (GPS) generated glideslopes take
you to a runway intersection point approximately 1,000
feet past the threshold. Nevertheless, even though a
glideslope exists, you must keep in mind that this is still
a non-precision approach. The glideslope is for advisory
purposes only. You don’t have to follow it to and beyond
the MDA, but there’s really no good reason not to if you
have the required visibility for the approach. B
Take note that the glideslope angle is 3.04 degrees
and has a threshold crossing height of 45 feet (Figure The approach to TRM has LNAV minimums but also provides
19, position D). If, when reaching the MDA, you have an advisory vertical glideslope during descent to the MDA.

WAAS Protected Horizontal Limits
When using WAAS for LNAV and LPV approaches, as you approach within two miles of the
FAF, your CDI scale gradually transitions from a fixed horizontal
distance scale of +/– 1 nm to an angular scale of +/– 2 degrees.
In other words, your CDI becomes “localizer like” (thus the rea-
son LPV stands for localizer performance with vertical guidance).
At the FAF, your CDI scaling starts somewhere near +/– .3 nm
then reduces to a CDI scale of +/– 350 feet (for a five dot
deflection, with each dot representing 70 feet displace-
ment from the course centerline) at
the missed approach waypoint and
remains at +/– 350 feet until a
missed approach is initiated. Keep in
mind that the CDI shown electronical-
ly at the bottom of the Garmin 500W
box (Figure 16, position N) only
shows two dots for a full scale deflec-
tion. This means that, on the WAAS’
two-dot built-in CDI Scale, each dot
deflection at the missed approach
waypoint equals 175 feet displace-
ment from the course centerline.