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

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:

• Looks like Chris added the approach plate. Thank you, Chris, and happy to be here. Seems like a great place to learn. Sep 8, 2023 at 19:25
• Was this approach flown recently and in the afternoon? Given that KDVT temps are currently 20-35°F above "typical", that means when flying at 4700 ft baro. alt. you're 100+ feet above "typical" when measured geometrically. Wouldn't that push back how soon you intercept the glideslope, which doesn't change with temp, by at least a quarter nautical mile? Sep 8, 2023 at 19:34
• @CodyP, Yes it seems that from what Chris calculated the equipment is actually working perfectly and that since it was approx ISA plus 20-25 yesterday at that altitude the GPS altitude was almost 300 feet different than our indicated (Static) in the aircraft causing us to intercept the GS .9nm early. It's one thing to learn that going through training and then seeing it in practice. Was shocked by how "far" off it was from the standard LNAV FAF point. Sep 8, 2023 at 20:30
• That's what being 27 degrees above ISA will do! For a quicker estimate of how much effect the tempertaute has- ISA temperature is about 285K at TDZE and the actual temperature was around 312K, about 9% higher. So it makes the glideslope intercept about 9% farther. FEGEV is 9.7 NM away from the threshold, so that 9% is about .9nm. Roughly every 3 degrees from ISA changes things by 1%. Sep 8, 2023 at 21:45
• RE: your concern added at the end. This is an advantage of the LPV. Flying the LNAV on a hot day requires the pilot to fly a steeper descent than they otherwise would have to in order to avoid busting the altitude restrictions at FEGEV and GIGEC. The LNAV pilot will fly a higher (and thus steeper) approach as the temperature rises, while the LPV pilot can fly basically the same approach regardless of temperature (aside from the DA, which will be higher on a hotter day). Sep 8, 2023 at 21:54

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!

• I appreciate the insight on the TERPS. I missed that in there and got lost in the fact that the LPV is APV (not a Non-PA as I mentioned) and you would follow the GS. It can not be a PA since it is created onboard the aircraft, not from an external source. My concern is that it should be closer than .9nm though correct? Unfortunately, the other aircraft we have that has WAAS is in MX so can't go up and see what that one does with the same day/conditions currently. Sep 8, 2023 at 19:15
• @JonathanWilkinson The glidepath angle is 3 degrees. .9nm times tan 3° gives 287 feet error at 3225 feet AGL. Yesterday's metar gives KDVT's temperature as 39 degrees C, which gives a temperature error of about 280 feet. So that's right on the nose! You can expect large errors like this from airports in the summer in Arizona. Sep 8, 2023 at 19:39
• @Chris Thank you so much for your help. Out of curiosity, is there any reference you know that would point out the temperature error of the LPV system (only the intercept point) that I could reference/add in the answer above from the FAA, or is the formula sufficient? I know of the snowflake on charts for cold temperature airports but didn't even think of AZ +25 ISA at times causing similar issues. Again, thank you for your help. Sep 8, 2023 at 20:15
• @JonathanWilkinson No problem! People don't worry about high temperatures causing issues because they mean you are farther from the ground than expected, so they just give you "bonus" obstacle clearance. The formula is the same for both but the FAA doesn't bother allowing you to lower MDA/DAs on hot days. Sep 8, 2023 at 20:26
• @Chris - cool! Something about "5 out of 3 pilots getting things backwards" comes to mind! Since you've edited your post removing the swap, I've deleted the other comments addressing the (now removed) confusion. Thanks!
– Ralph J
Sep 9, 2023 at 0:11