Yesterday, I also happened to fly the RNP (was: GPS) approach to runway 21 at EDSB. Since the aircraft is not LPV capable, this was flown “to LNAV minimums”.
To be honest, I am still a bit confused about CDFA vs. dive-and-drive and the way the final approach descent is depicted on the charts. Just below the plan view, there is a distance/altitude table. On the 2D approach, are these considered as minimum altitudes, or as precise altitudes I am supposed to “hit”? If yes, with what tolerances? And if yes, and if the approach is flown as a CDFA, what significance do the two checkpoints in the profile view (2.0NM and 5.0 NM) still have?
My thinking was that the latter were restrictions, and that the distance/altitude table was somehow “advisory only”. In fact, I think that approach plates in the US for example don’t have these tables.
Bosco, you are right. The table is recommended altitudes. All altitudes in the Profile view are mandatory minimums in my understanding unless labelled otherwise. The platform altitude (4000) is in bold.
On jepp plates – the tables are advisory. Very useful on CDFA. Actual step down minima are charted in the vertical view with a fix and a grey shades area underneath the minimum altitude.
The one thi I don’t get tis the LPV minimum being so much higher than the LNAV/VNAV
boscomantico wrote:
If yes, with what tolerances?
From an obstacle clearance point of view, the only tolerance is that you must not go below the DA or a step-down altitude. There might be some tolerance requirement on a checkride.
Yep on the stepdown fixes it is Minus zero.
Cobalt wrote:
The one thi I don’t get tis the LPV minimum being so much higher than the LNAV/VNAV
In most cases, the LPV has a lower DA than an LNAV/VNAV in the US. But this is not always true as the DA is determined by a different set of TERPS specification for each approach type. The LPV has a sloped surface below the GS to the threshold that is evaluated for obstacle penetrations. The LNAV/VNAV has a sloped surface down to a point and then a second level surface is evaluated after that point. So there are cases where close in obstacles can affect the LPV but not the LNAV/VNAV. When an obstacle is encountered that penetrates the sloped surface, the height of the obstacle is projected back along the final approach course until it hits the level of the obstacle surface and the DA is established above this point. This is done for two reasons, 1) to protect for missed approach at the DA and 2) to place the obstacle in the visual segment so the pilot may see and avoid it if the approach is continued.
NCYankee wrote:
The LNAV/VNAV has a sloped surface down to a point and then a second level surface is evaluated after that point. So there are cases where close in obstacles can affect the LPV but not the LNAV/VNAV.
Very interesting, thank you.
Since the performance requirements (TTA, HAL, continuity) for flying LPV are always more stringent than those for flying a LNAV/VNAV based on SBAS, is there any practical risk in flying an approach to the LNAV/VNAV minima where these are lower than the LPV minima?
Nevertheless, I’m stll confused about how a non-precision approach can have lower minimums than a precision approach. To me, the tolerances explain the differences between the two. I thought that a precision approach is more precise in leading the aircraft along the glideslope to the threshold, thus permits lower minimums. What you guys are telling is that… it depends
I am also confused about the EDSB chart above: an LNAV/VNAV approach should be considered as a non-precision approach and thus have a MDA. The LNAV approach here has a DA. This is not how I learned it.
@Boscomantico
What the german (DFS) chart looks like?
NCYankee wrote:
So there are cases where close in obstacles can affect the LPV but not the LNAV/VNAV.
A little drawing perhaps?
Flyamax wrote:
Nevertheless, I’m stll confused about how a non-precision approach can have lower minimums than a precision approach. To me, the tolerances explain the differences between the two. I thought that a precision approach is more precise in leading the aircraft along the glideslope to the threshold, thus permits lower minimums. What you guys are telling is that… it depends
An LPV is very accurate, this is not about accuracy. It is about the rules for determining the objects/terrain that affect the minima. LPV and LNAV/VNAV have slightly different rules and this can lead to an anomaly.
I am also confused about the EDSB chart above: an LNAV/VNAV approach should be considered as a non-precision approach and thus have a MDA. The LNAV approach here has a DA. This is not how I learned it.
LNAV/VNAV approaches have DA/DHs. An LNAV approach has an MDA but if presented as a CDFA approach it is flown as a DA/DH. You could fly it dive and drive and use it as an MDA.
(And, although very precise, the LPV is still a non-precision approach)