My understanding is that the TSO C146AR WAAS GPS may fly any minimum line approved on the approach chart. The point is there are examples of all these kinds of anomalies. Often this is also caused by an obstacle within a few miles of the runway that penetrates the LNAV/VNAV OCS but with the LNAV, a step down can be used, once the obstacle is past, and the MDA can drop down lower than the LNAV/VNAV. More common is the situation where the LNAV/VNAV results in a higher DA than the LNAV MDA. This may force the LPV DA higher than the LNAV/VNAV DA. But there is a small area in which the OCS for the LPV and the LNAV/VNAV will result in a penetration for the LPV OCS but not affect the OCS for the LNAV/VNAV. In the vast majority of cases, the LPV will have the lowest DA and the LNAV/VNAV the second lowest. There are different geometrical methods used to determine the OCS for LNAV/VNAV and LPV approaches. There is a second obstacle surface that is evaluated between the DA and the threshold at the TCH elevation, but it is narrower than the OCS and picks up where the OCS ends. The way this is accomplished is by projecting the obstacle height back to a point where it intersects the OCS, and then the GS directly above this point becomes the DA. The general concept is that if an obstacle penetrates the surface, the DA must be moved back and up higher so that when you break out at the DA you can see the offending obstacle and avoid it. When there are obstacles close in to the runway that penetrate the OCS (Obstacle Clearance Surface) that provides a surface below the glideslope that provides a ROC (Required Obstacle Clearance) that decreases as you go from the glidepath intercept point down to the DA. If that's the case the discussion about using the lower LNAV/VNAV mins when LPV HAL and VAL are met is moot. I'd love to hear from aTerpSter or anyone else in that line of work, under what conditions could this be correct. There's a diagram of this issue on page 64 in Ekalbar's 'Instrument Flying Update' book.Īnd WRT the title question, everything I've read about the TERPS requirements for LPV and LNAV/VNAV approaches (8260 and Ekalbar's book) suggests that there is no valid reason why the LPV mins should exceed the LNAV/VNAV mins so I strongly suspect that the TTA GPS-21 has them reversed.

The geometry of the OCS (Obstruction Clearance Surface, which lies below the approach path) is slightly different for LPV than for LNAV/VNAV approaches. WRT the title question, upon further review I found that it is indeed possible albeit uncommon for the LPV mins to be slightly higher than the LNAV/VNAV mins on the same approach. I suspect that the verbiage in the 530W PG you quoted was written by someone who naively assumed that LPV mins would never be higher than the LNAV/VNAV mins on the same approach. And FWIW, the AFMS and PG (included by reference in the AFMS) for my GNS480 do not contain any language dictating otherwise. AFaIK the only reason a WAAS navigator would revert from LPV to LNAV/VNAV is when the tighter HAL (horizontal accuracy requirement) for the LPV (40M) was exceeded but not by so much that the HAL for LNAV/VNAV (556M) was also unavailable. The VAL (vertical accuracy requirement) is the same 50M for both types (this is reduced to 35M if the DA is less than 250 AGL) and I can see no technical reason why there would be any issues flying to LNAV mins when the box is happy with LPV operation. Ron, I'm pretty certain that TSO C146 (WAAS) boxes use the exact same mechanisms (software) to generate LPV and LNAV/VNAV vertical guidance.

Lp vs lpv vs lnav