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What is the reason for the numerical difference between the MDA altitude in VOR approaches and the DA altitude in ILS approaches?

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The simplest answer is that non-precision approaches (e.g. LOC or VOR) nearly always have higher minima than precision approaches (e.g. ILS).

Precision approaches provide strict vertical guidance: the pilot flies a constant descent angle down a precise glide slope/path, and if they don’t see the runway by the time they reach the DA, they immediately climb and execute the Missed Approach Procedure.

In contrast, for a non-precision approach, the pilot descends at various step-down fixes until the MDA, and then flies level (aka “dive and drive”) until they either see the runway or reach the MAP. Allowing an IFR aircraft to be flying down low for a longer period of time necessarily requires a much larger, flatter OCS.

The entire OCS must be raised to a certain height above any obstacle or terrain within its lateral boundaries, so a large OCS will naturally be higher than a small OCS. For instance, a radio tower or tall building three miles from the runway may be within the large, flat OCS for a VOR approach, forcing a high MDA, but not the small, tilted OCS for an ILS approach, allowing a low DA.

There are rare IAPs with an MDA lower than the DA, but they usually require a step-down fix with a very high descent angle after a close-in obstacle. IAP designers avoid this, however, unless there’s no other way to create a procedure with at least one decent minimums line.

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  • $\begingroup$ Out of curiosity: could you add an example for an approach with a lower MDA than DA? $\endgroup$
    – Bianfable
    Aug 23 '21 at 8:43
  • $\begingroup$ @Bianfable I know I’ve seen one, which is why I mentioned it, but I can’t think of where. Edits welcome. $\endgroup$
    – StephenS
    Aug 23 '21 at 12:22
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    $\begingroup$ Harrisburg, PA RNAV (GPS) RWY 13 approach LNAV MDA is 1180 and the LNAV/VNAV DA is 1572. $\endgroup$
    – 757toga
    Aug 23 '21 at 14:48

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