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Assuming my aircraft's navigation equipment is a "Qualified/Suitable RNAV System," as defined by AC 90-108, is it legal for me to fly the LDA Y Rwy 19 IAP to DCA with ATC Radar out of service?

Here is a copy of FAA AC 90-108

Paragraph 7. a. (AC 90-108) states: (emphasis is mine)

Uses of Suitable RNAV Systems.

a. Usage of Suitable RNAV Systems. Subject to the operating requirements in this AC, operators may use a suitable RNAV system in the following ways.

(1) Determine aircraft position relative to or distance from a VOR (see first note in subparagraph 7b), TACAN, NDB, compass locator (see second note in subparagraph 7b), DME fix; or a named fix defined by a VOR radial, TACAN course, NDB bearing, or compass locator bearing intersecting a VOR or Localizer (LOC) course.

(2) Navigate to or from a VOR, TACAN, NDB , or compass locator.

(3) Hold over a VOR, TACAN, NDB, compass locator, or DME fix.

(4) Fly an arc based upon DME.

Here is the approach plate in question:

enter image description here

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  • $\begingroup$ I have a feeling that the "radar required" has more to do with the DC SFRA than it does the RNAV approach... $\endgroup$
    – Ron Beyer
    Nov 5, 2018 at 14:47
  • $\begingroup$ @RonBeyer - that's a thought, but the RNAV (RNP) RWY 19 approach to KDCA does not require radar. $\endgroup$
    – user22445
    Nov 5, 2018 at 14:55
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    $\begingroup$ I agree with @RonBeyer. It's also interesting to note that all STARs into DCA require radar and they all end with 'expect vectors to final'. I expect if there's no radar, you'll never get a clearance. $\endgroup$
    – Gerry
    Nov 5, 2018 at 14:55
  • $\begingroup$ @Gerry - you could be right, but "Radar Required" is (according to TERPS and AIM 5-4-5) a requirement when, among other things, there is no published transition to the approach course, which seems to be the case at DCA for the LDA rwy 19 (note the RNAV RNP rwy 19 does not require radar). Anyway, AC 90-108 allows appropriate RNAV equipment to substitute for inoperative or not available navaids, etc. My question in this case is does AC 90-108 allow for the "radar" to be "not available" and therefore permitting the approach to be flown with the onboard RNAV equipment. $\endgroup$
    – user22445
    Nov 5, 2018 at 15:31
  • $\begingroup$ AC 90-108 allows for RNAV to substitute for fixed navaids, it doesn't provide for a blanket substitute for radar. AOPA has an article about charting standards which says the FAA has a new standard that will phase in to clarify the equipment requirements. The new standard will put statements such as: RNAV/GPS or RADAR required for transition. This should remove any ambiguity, but obviously the DCA charts haven't been updated. It is interesting that all approaches other than RNAV say RADAR REQUIRED, including the visual approaches. I suspect the FRZ rules will overrule the TERPS here. $\endgroup$
    – Gerry
    Nov 5, 2018 at 16:10

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Since "radar" means "ATC radar", I'm not sure how a pilot could substitute anything for it. The issue isn't that the pilot needs radar to navigate the approach; it's that's ATC needs radar in order to allow the approach in the first place.

Per this answer, ATC can only clear aircraft for an RNAV approach without an IAF if radar monitoring is available. I don't think we've found out why that's the case, but it's what the ATC orders say (4-8-1(h)). Since the approach above has no IAF, that means ATC can only clear you for it if they have radar monitoring.

4-8-1(g) also instructs controllers not to let pilots choose their approach if there's no radar available:

g. Where instrument approaches require radar monitoring and radar services are not available, do not use the phraseology “cleared approach,” which allows the pilot his/her choice of instrument approaches.

In other words, ATC has to make sure that they clear pilots for approaches only if their own requirements are met.

But as it happens, ATC does have a possible substitution for radar. The ATC orders 5-5-4 say:

The use of ADS-B only information may be used to support all radar requirements associated with any published instrument procedure that is annotated “Radar Required”.

I assume that means that - at least theoretically - if ATC lost radar but could still receive ADS-B, they could clear you for the approach. But that would be ATC substituting equipment, not the pilot :-)

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    $\begingroup$ Re your last point: at least in the terminal environment, we run using FUSION, which is a combination/best-average of the primary ASR site(s), long-range ARSR site(s), and ADS-B. We can enable an indicator to show if a target has operational ADS-B or not, but as far as the actual "radar scope," we don't know nor care if the information is coming from ADS-B or traditional primary/secondary radar. So "Radar required" just means that ATC has at some point called your aircraft "radar contact" and has not subsequently called "radar contact lost." $\endgroup$
    – randomhead
    Oct 26, 2021 at 1:02

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