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I recently flew the RNP approach for runway 16 into Corfu. While reviewing the approach charts, I noticed something unusual. The LNAV/VNAV minima is set higher than the LNAV minima for this particular RNP approach. Traditionally, I've seen LNAV/VNAV minima being lower due to the added vertical guidance, compared to LNAV which only provides lateral guidance.

I'm curious to know the rationale behind this. Are there specific local terrain or procedural factors at Corfu airport that might explain this anomaly? Or is there a technical explanation related to the performance requirements or equipment capabilities for LNAV/VNAV versus LNAV approaches?

Furthermore, are there other airports or scenarios where the LNAV/VNAV minima are set higher than LNAV minima? What are the general circumstances under which this might occur?

RNP 16 Chart - Greece AIP (https://aisgr.hasp.gov.gr/aipgr_incl_amdt_1123_wef_30nov2023/cd/ais/index.html)

RNP 16 Chart - Greece AIP

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  • $\begingroup$ Related: Why are some LNAV/VNAV mins lower than the LPV mins for the same approach? $\endgroup$
    – Bianfable
    Oct 31, 2023 at 14:58
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    $\begingroup$ It's not totally uncommon. I'm searching for a PANS-OPS explanation . There are several sources explaining it based on TERPS. It's based on the way the obstacle clearance is determined with a glide path (lnav/vnav) vs no glide path (lnav). $\endgroup$
    – user22445
    Oct 31, 2023 at 15:00

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I cannot locate a clear explanation in the PANS-OPS Doc 8168. However, to me it's reasonable that the same general criteria that is used in U.S. TERPS will apply to PANS-OPS. So, the information below responds to your question, but is based on TERPS.

According to the Boldmethod website the reason that on some instrument approaches the LNAV MDA is lower than the LNAV/VNAV DA is because of the height and location of the controlling obstacle upon which the minimums are based.

Since, as shown by the example instrument approach (Harrisburg, PA - GPS RWY 13) in the Boldmethod link above, the LNAV/VNAV glidepath, in order to procedurally clear the controlling obstacle, results in a higher DA than the lower MDA procedurally required by the LNAV approach, the (LNAV) MDA is lower than the (LNAV/VNAV) DA. (a link to the full RNAV (GPS) RWY 13 approach is available on the Boldmethod linked above)

enter image description here

The two illustrations and explantions below are from the Boldmethod website linked above:

How Approach Minimums Are Calculated [...]

First, let's look at approaches with no vertical guidance. For LNAV (lateral navigation) approaches, the FAA looks at the obstacle heights along the approach path, and draws a straight line called the Obstacle Clearance Surface (OCS). Then, they add 250 feet to that line for the LNAV Required Obstacle Clearance (ROC). That altitude becomes the LNAV MDA. So at a minimum, the LNAV MDA on an approach is at least 250 feet above the highest obstacle in your path. But, there are several factors that can cause the MDA to be higher. If the MDA needs to be raised, they do it in 20 foot increments.

enter image description here

Next up, let's look at approaches with vertical guidance. For LNAV/VNAV approaches, the FAA draws the same Obstacle Clearance Surface horizontal line. At the controlling obstacle, they draw a horizontal line away from the runway until it reaches the Obstacle Clearance Surface (OCS) plane, which in our diagram is the the gray angled line. From there, they draw a vertical line that reaches the glide path. That point (the orange arrow) becomes the Decision Altitude (DA).

enter image description here

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