Originally Posted by
2StgTurbine
Here is a fun one.
Explain how LNAV can be lower than LNAV/VNAV but higher than LPV minimums for the same approach.
LNAV and VNAV have fixed course widths and therefore fixed protected areas. LP and LPV courses get narrower the closer you get to the runway (Like an ILS or LOC) and therefore the protected areas also get smaller as you get closer to the runway. Just like an ILS, the glide path of an LPV approach must be constant, but the descent path along a LOC or LNAV approach can have step-downs at various descent angles.
With that in mind, there could be an obstacle off center of the extended runway centerline that is tall enough to enter the protected area of the LNAV/VNAV glide path. Since the LPV protected area gets smaller as you get closer to the runway, the obstacles could be in a position far enough from the protected area of an LPV approach not to matter, but still within the protected area of an LNAV/VNAV approach. As a result, the LNAV/VNAV approach would have to end before and above that obstacle. The LNAV approach could simply have a step-down after that obstacle that would require a steeper descent angle than the LNAV/VNAV glide path.