Why is my WAAS equipped aircraft telling me to descend sooner than the FAF (KDVT 25L LPV)

Question

I ran into a weird issue today with our G1000 WAAS-equipped aircraft. The aircraft wanted me to begin decent .9nm before the FAF on the KDVT 25L LPV approach. The LPV is considered a non-precision approach (Edit: it is not an NP approach, it is a APV which would have you follow GS when intercepted at 4700), so you should fly at 4700 feet to FEGEV and then go capture the glide slope, so this has the potential to screw up a student or anyone for that matter by following the glide slope almost a mile early and getting a potential Pilot Deviation.

I asked ATC what they showed our altitude at and was correct all three times.

I could not find the proper FAA source quickly, but Honeywell had this to say:

An LPV glidepath is calculated entirely using SBAS altitude. That is, the GPS constructs the glidepath using GPS altitude and height above earth ellipsoid to produce a path that unlike baro-VNAV, is not susceptible to pressure and temperature variations. This means that like an ILS, an LPV glidepath is essentially a fixed path in space being tracked using GPS altitude to a DA that the pilot reads from a baro-altimeter. This is a component to how an LPV can achieve minimums comparable to an ILS.

So if they are both fixed paths in space, why is one of them having me descend so early?

I didn't know if anyone else had run into this issue before. Is it due to a faulty GPS receiver? (both GPS 1 and 2 were showing the same)

Any insights are appreciated.

(Edit: Concern is how early it is having us capture the GS where someone on the LNAV for the same approach would be at 4700 feet for almost a mile after we begin descending on the LPV)

On ground GPS signal and altimeter altitude match:

KDVT 25L LPV approach on glide .9nm ahead of FAF:

KCGZ ILS is right on (.1nm off) at correct altitude for FAF:

Approach plate:

Answer 1

The G1000 is correct. Just like you would on an ILS, once you have intercepted the glidepath you should follow it down. The altitude restriction at FEGEV (the final approach fix) and the stepdown altitude at GIGEC apply to the LNAV version of the approach, but not the LPV.

The AIM touches on this in 5-4-5(b)(5):

Altitude restrictions depicted at stepdown fixes within the final approach segment are applicable only when flying a Non-Precision Approach to a straight-in or circling line of minima identified as an MDA (H). These altitude restrictions may be annotated with a note “LOC only” or “LNAV only.” Stepdown fix altitude restrictions within the final approach segment do not apply to pilots using Precision Approach (ILS) or Approach with Vertical Guidance (LPV, LNAV/VNAV) lines of minima identified as a DA(H), since obstacle clearance on these approaches is based on the aircraft following the applicable vertical guidance. Pilots are responsible for adherence to stepdown fix altitude restrictions when outside the final approach segment (i.e., initial or intermediate segment), regardless of which type of procedure the pilot is flying. (See FIG 5-4-1.)

This mentions that you can ignore altitude restrictions on the final approach segment. So the question then becomes- is FEGEV part of the final approach segment, or not?

The answer is that the final approach segment of an LPV begins as soon as you have intercepted the glidepath, just like an ILS. From TERPS 2-6-1:

The [final approach] segment begins at the PFAF and ends at the MAP and/or DA.

The PFAF is the "precise final approach fix," defined as:

a calculated WGS84 geographic position located on the final approach course where the designed vertical path (NPA procedures) or glidepath (APV and PA procedures) intercepts the intermediate segment altitude (glidepath intercept altitude). The PFAF marks the beginning of the FAS. The calculation of the distance from LTP to PFAF includes the earth curvature.

The LPV is an APV (approach with vertical guidance). So if you are at the glidepath intercept altitude (4700 feet) and you intercept the glidepath, then you have identified the PFAF. This means you have started the final approach segment and can start your descent along the glidepath, ignoring any further altitude restrictions.

You are still required to adhere to the altitude restrictions at CIPLU and DUYAC, as these are not on the final approach segment.

On hot days, your altimeter reads lower than your true altitude. Thus you are farther from the ground that your altimeter would lead you to think, and you will intercept the glidepath farther out than you would on a standard day. The size of this error can be determined using this lovely formula:

$$ H\times\frac{15-(T+L\times E)}{273.15 + (T+L\times E) - 0.5\times L\times (H+E)} $$

where $H$ is your height above the aerodrome, $E$ is the aerodrome elevation, $T$ is the aerodrome temperature in degrees Celsius, and $L$ is the lapse rate. Plugging in 1475 feet for the aerodrome elevation, 3225 feet for the glidepath intercept height, 39 degrees celsius from yesterday's METAR for KDVT, and assuming a standard lapse rate of $1.98$ degrees per 1000 feet, this gives about 280 feet, so your true altitude MSL when your altimeter reads 4700 feet is actually 4980 feet.

With a glidepath angle of $3^\circ$, this difference in altitude gives you an expected glidepath intercept $280~\text{feet}/\tan(3^\circ)\approx .88~\rm nm$ outside FEGEV. So your equipment appears to be working perfectly!