16
$\begingroup$

GPS is more accurate than a VOR or NDB signal. Can we use a GPS to fly a VOR or NDB approach from the initial approach fix through the missed approach and hold?

I know there are two types of approaches that deal with GPS and VOR/NDBs:

I'm specifically asking about the first type or approach without the wording "or GPS in the title.

$\endgroup$
1
  • $\begingroup$ GPS can be more precise, this is not necessarily more accurate or more reliable. There is more involved with determining suitability for a particular end goal. There are locations where the GPS signal is frequently lost (eg a mountain valley) and thus unreliable, there are also differences in ground distance vs slant range DME for holding pattern obstacle clearance and conversion errors with different survey datums. $\endgroup$
    – Max Power
    Nov 16, 2020 at 4:59

4 Answers 4

14
$\begingroup$

In the US they can (Effective: May 26, 2016). This change allows for the use of a suitable RNAV system as a means to navigate on the final approach segment of an instrument approach procedure (IAP) based on a VOR, TACAN, or NDB signal. The underlying NAVAID must be operational and monitored for the final segment course alignment.

Still can’t fly an ILS or localizer approaches.

Update: 2017-04-19 Max Trescott has a good explanation of how to use GPS in conjunction with ground-based navaids on ILS and VOR approaches.

AIM Section 2. Performance−Based Navigation (PBN) and Area Navigation (RNAV)

1−2−3. Use of Suitable Area Navigation (RNAV) Systems on Conventional Procedures and Routes

  1. Use of a suitable RNAV system as an Alternate Means of Navigation when a VOR, DME, VORTAC, VOR/DME, TACAN, NDB, or compass locator facility including locator outer marker and locator middle marker is operational and the respective aircraft is equipped with operational navigation equipment that is compatible with conventional navaids. For example, if equipped with a suitable RNAV system, a pilot may fly a procedure or route based on operational VOR using that RNAV system without monitoring the VOR.

NOTE− 4. The navigation database should be current for the duration of the flight.

c. Uses of Suitable RNAV Systems. Subject to the operating requirements, operators may use a suitable RNAV system in the following ways. 1. Determine aircraft position relative to, or distance from a VOR (“VOR” includes VOR, VOR/DME, and VORTAC facilities and “compass locator” includes locator outer marker and locator middle marker.), TACAN, NDB, compass locator, DME fix; or a named fix defined by a VOR radial, TACAN course, NDB bearing, or compass locator bearing intersecting a VOR or localizer course.

  1. Navigate to or from a VOR, TACAN, NDB, or compass locator.

  2. Hold over a VOR, TACAN, NDB, compass locator, or DME fix.

  3. Fly an arc based upon DME.

None of the substitutions mentioned above include the localizer portion of an ILS. So you may not substitute GPS/WAAS for and ILS or localizer approach, but you may still use it for situational awareness.

You need to dig into the AIM to find out what they mean by “suitable RNAV system” but basically both certified GPS or WAAS count.

$\endgroup$
5
  • 3
    $\begingroup$ I believe a key factor in the "suitable RNAV system" is that the desired approach must be in the nav database for the system. You can't manually enter approach fixes to create an approach. $\endgroup$
    – Gerry
    Feb 28, 2017 at 15:26
  • $\begingroup$ @Gerry That’s true, the approach has to be in your database, but what they say about "Suitable RNAV Systems’ is "the following systems qualify as a suitable RNAV system: 1. An RNAV system with TSO−C129/ −C145/−C146 equipment, installed in accordance with AC 20−138,… and 2. An RNAV system with DME/DME/IRU inputs that is compliant with the equipment provisions of AC 90−100A" For most people, that means certified GPS or WAAS systems. $\endgroup$
    – JScarry
    Feb 28, 2017 at 19:45
  • $\begingroup$ Agreed, but I raised the point as it's entirely possible to have an approved "Suitable RNAV System" installed in your aircraft and not have the desired approach in your database. I think it's an important factor when trying to determine "Can I fly a particular approach?" Having an approved system won't help if the approach you want isn't in the database. $\endgroup$
    – Gerry
    Mar 1, 2017 at 12:35
  • $\begingroup$ Reading at section in the AIM, the first note stipulates to read AC 90-108. That AC says you cannot substitute a GPS/RNAV system on a Final approach segment. What does that mean? $\endgroup$
    – wbeard52
    Mar 1, 2017 at 19:32
  • $\begingroup$ AC No: 90-108 Change: 1 is Dated: 4/21/15. The AIM change is Effective: May 26, 2016. I’m guessing that the AC has not been updated yet OR there is another AC that the AIM is based on. $\endgroup$
    – JScarry
    Mar 1, 2017 at 19:55
9
$\begingroup$

No, you still have to monitor the underlying NAVAID. You just have to read further in that same section of the AIM. Reference the most recent edition of the AIM, which has Change 3 dated April 27, 2017. In section 1-2-3-c-5 it says:

Use of a suitable RNAV system as a means to navigate on the final approach segment of an instrument approach procedure based on a VOR, TACAN or NDB signal, is allowable. The underlying NAVAID must be operational and the NAVAID monitored for final segment course alignment.

That last sentence in the quote above is the important part.

$\endgroup$
1
  • $\begingroup$ To clarify further, the named fixes and named procedures must be included in the GPS receiver's database of published nav data. Using lat and long waypoints or manually connecting waypoints into a procedure is not acceptable for IFR. $\endgroup$
    – Max Power
    Nov 16, 2020 at 5:04
0
$\begingroup$

In reference to the OP’s original question about VOR and NDB procedures:

Yes for a Missed Approach Segment or Hold.

&

NO for a Final Approach Segment WITH LATERAL GUIDANCE.

The answer as it pertains to the final approach segment is NO if the Navaid is providing lateral guidance. And, you do not have to monitor the underlying Navaid EXCEPT for the final approach segment.

In other words, you can fly to, from, and around a Navaid without tuning in to the Navaid. You can even use GPS to fly to the runway while monitoring the Navaid. As soon as you are using Lateral guidance to align the plane with the runway for landing purposes, you have to use your Nav radio.

I imagine that the only time a VOR would provide lateral guidance by scaling would be if the VOR were located on the field. This is probably making a distinction between VOR/DME and NDB approaches not providing alignment guidance, and all other approaches. Probably since a VOR not located on the field will point you to the runway. But, it will not funnel or lead/guide you down to it (scaling).

And, your database must be current and contain the desired procedure to legally use the GPS.

See the below excerpt from the latest FAR/AIM as of 26 Jan 20. Specifically AIM 1-2-3 Notes section 2 & 4.

The FAR/AIM says:
Notes

  1. The allowances described in this section apply even when a facility is identified as required on a procedure (for example, “Note ADF required”).

  2. These operations do not include lateral navigation on localizer-based courses (including localizer back-course guidance) without reference to raw localizer data.

  3. Unless otherwise specified, a suitable RNAV system cannot be used for navigation on procedures that are identified as not authorized (“NA”) without exception by a NOTAM. For example, an operator may not use a RNAV system to navigate on a procedure affected by an expired or unsatisfactory flight inspection, or a procedure that is based upon a recently decommissioned NAVAID.

  4. Pilots may not substitute for the NAVAID (for example, a VOR or NDB) providing lateral guidance for the final approach segment. This restriction does not refer to instrument approach procedures with “or GPS” in the title when using GPS or WAAS. These allowances do not apply to procedures that are identified as not authorized (NA) without exception by a NOTAM, as other conditions may still exist and result in a procedure not being available. For example, these allowances do not apply to a procedure associated with an expired or unsatisfactory flight inspection, or is based upon a recently decommissioned NAVAID.

  5. Use of a suitable RNAV system as a means to navigate on the final approach segment of an instrument approach procedure based on a VOR, TACAN or NDB signal, is allowable. The underlying NAVAID must be operational and the NAVAID monitored for final segment course alignment.

  6. For the purpose of paragraph c, “VOR” includes VOR, VOR/DME, and VORTAC facilities and “compass locator” includes locator outer marker and locator middle marker.

d. Alternate Airport Considerations. For the purposes of flight planning, any required alternate airport must have an available instrument approach procedure that does not require the use of GPS. This restriction includes conducting a conventional approach at the alternate airport using a substitute means of navigation that is based upon the use of GPS. For example, these restrictions would apply when planning to use GPS equipment as a substitute means of navigation for an out-of-service VOR that supports an ILS missed approach procedure at an alternate airport. In this case, some other approach not reliant upon the use of GPS must be available. This restriction does not apply to RNAV systems using TSO-C145/-C146 WAAS equipment. For further WAAS guidance, see paragraph 1-1-18.

$\endgroup$
3
  • $\begingroup$ In #5 you quoted above it explicitly allows one to use a GPS to navigate on the final approach segment but the pilot must monitor the underlying navaid and follow that if the GPS and the navaid disagrees. $\endgroup$
    – wbeard52
    Jan 26, 2020 at 14:54
  • $\begingroup$ That’s the sticky wicket. Number 4 & 5 contradict each other. I think the key phrase is “lateral guidance”. Most of the examples of VOR and NDB IAPs don’t guide you down to the centerline of the runway using lateral scaling. You could be off as much as a mile and still have your CDI centered depending on how far away from the field the VOR is. On more than half the IAPs sampled, the final approach is either not even aligned with the runway and/or Circling only. I think that is the difference between the two sub-sections. $\endgroup$
    – Dean F.
    Jan 26, 2020 at 19:28
  • $\begingroup$ Something else that comes to mind is that the VOR IAPs sampled directed flight from the FAF as a Mag Course with no explicit mention of the Radial. Which is a point of irrelevant minutia since in all actuality, the Mag Course is the same or the reversal of the Radial. This might be interpreted as not explicitly getting the lateral guidance from the VOR. A more important point is that all of them have a waypoint as the MAP. So, technically the GPS is giving guidance to a waypoint and not lateral guidance for the approach runway. Thereby, falling under the rule of subsection 5. $\endgroup$
    – Dean F.
    Jan 26, 2020 at 23:36
0
$\begingroup$

Good info here, but I'd add a little interpretation and implied requirements as follows, as I don't see conflict between AIM 1-2-3(c) Notes 4 & 5 - though that is less than completely obvious.

  1. AIM 1-2-3 (c) note 4: "lateral guidance" has nothing to do with runway alignment. It is simply the left/right guidance provided by the NAVAID to follow a final approach course as depicted on the chart. Runway alignment does not impact this note; the NAVAID must be referenced as provided in the note during the final approach segment, period. So we are not allowed to simply fly a VOR or NDB approach using GPS needles and ignore the "raw" VHF data. That's always been true.
  2. AIM 1-2-3 (c) note 5: we can navigate with the GPS provided "the underlying NAVAID [is] operational and the NAVAID monitored for final segment course alignment." So here's an example of a practical way of doing this: some AP systems can be flown in GPSS mode (navigating directly off the GPS) while the primary CDI still displays the VHF course information (i.e. the Avidyne PFD w/ separate GPS heads can do this; however, many systems, including G1000-based systems, cannot). That gives a rock steady flight path (don't we love GPSS?) while showing the "raw" VOR data right on the HSI's CDI. Why do I think it should be a CDI type display (for VOR approaches) showing the "raw" data and not just the RMI style bearing pointer on your PFD pointing to the VOR? Because pilots aren't expected to do math in their heads much and the requirements for "on course" (and "established") during the final approach segment are based largely on CDI deflection (3/4 max deflection for VHF approaches) and stability on the course and converting a bearing pointer to an off-course number of degrees or a rate of departure from a course (that is, a rate of lateral tracking error) and then knowing when too much is too much and time to go missed is much more difficult. So without two CDIs available (and many do have that, but not all) then I think one has to go with the VOR indication on the one CDI. Our local DPE's expect to see the VHF device on a CDI during an approach (FAF inbound), including VOR approaches - not just a bearing pointer. So depending on your equipment you may or may not be able to actually fly the GPS and still monitor the VOR appropriately. My $0.02; good thread, all.
$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.