HSI accuracy using GPS for NDB during approach

[photofly]

If I load an NDB approach into my GPS then the HSI scale changes when the approach is loaded from 5nm full scale deflection to 1.0nm fsd in Terminal mode and then again 0.3nm in Approach mode.

Can I use the NDB as a waypoint in the flight plan of the GPS (or use "direct to") to identify the beacon as part of the approach without loading and activating the approach? If so, am I required manually to change the scaling on the HSI? and to what, and when?

I suppose the question is, is it permitted to use the GPS to identify/locate the NDB, without formally flying the GPS-overlay approach? If my GPS is certified for enroute only (no approaches) then can it still be used to identify the NDB for an approach?

[Hedley]

Read AIM COM 3.16, specifically 3.16.9 and 3.16.10:

http://www.tc.gc.ca/eng/civilaviation/p ... htm#3-16-9

Subject to any overriding conditions or limitations in the aircraft flight manual (AFM) or AFM Supplement, GNSS may be used to identify all fixes defined by DME, VOR, VOR/DME and NDB, including fixes that are part of any instrument approach procedure, to navigate to and from these fixes along specific tracks, including arcs, and to report distances along airways or tracks for separation purposes. This can be done as long as there is no integrity alert, and provided that all fixes that are part of a terminal instrument procedure (arrival, departure, or approach) are named, charted and retrieved from a current navigation database. GNSS may be used to identify fixes defined by ground-based aids, even if they are temporarily out of service.

[ahramin]

There is a conflict in your question. If you are using the GPS to identify a fix on the approach, as Hedley has shown that is perfectly legal. However if the GPS is not approved for approaches why are you asking about the HSI and the CDI scale on the GPS? The HSI would be on the VOR or the ILS for the approach no?

[photofly]

3.16.9 says

Note that for NDB or VOR approaches that are not part of the GPS overlay program described in Section 3.16.5.2.2, pilots shall use ADF or VOR as the primary source for final approach track guidance.

So if you want to use the GPS for approach track guidance from the FAF (in the case of an NDB or VOR approach) you must be using the overlay, and therefore must have the whole approach loaded, along with consequent changes of HSI accuracy.

But - if you're using the GPS to substitute for the NDB only to locate the IAF and MAP hold for an ILS or LOC approach, then you don't (in my case can't) load the approach, and en-route accuracy is fine?

[photofly]

There is a conflict in your question. If you are using the GPS to identify a fix on the approach, as Hedley has shown that is perfectly legal. However if the GPS is not approved for approaches why are you asking about the HSI and the CDI scale on the GPS? The HSI would be on the VOR or the ILS for the approach no?

Yes you're right, the question was badly worded. My GPS is approved for approaches and I shouldn't have asked about a non-approach approved GPS.

The situation I have in mind is using the GPS to locate the NDB-based IAF and missed approach hold for an ILS approach. Practically speaking I'm wondering if I should be changing the sensitivity. That's 5nm fsd as it stands; If I loaded the NDB approach based on the same navaid, I'd have 1.0nm fsd to the IAF and from the MAP to the hold.

[pilotidentity]

Because you aren't formally doing an overlay approach you are really using the VOR or ADF or LOC for tracking. Because of this there is no requirement to set the GPS track bar to a certain width. I'd set it to 1 mile though instead of the default 5 miles if that is the case. It would be nice to go to .3 but some GPS's don't go there unless in approach mode or to a user waypoint. I'm not an expert on this stuff but this is my take on it.

[CID]

GPS approaches must be derived from the database as an entire procedure. So, you can't just build a procedure by making a flight plan with all the waypoints. I think that's understood.

If you are using the GPS to navigate to an initial fix for an ILS, there isn't much in the way of guidance but even 5 miles full scale would be more than enough resolution for such a thing. Systems that have the "nav-to-nav" capability that you are alluding to usually use 1 mile FS for all terminal area navigation but that's based on GPS navigation performance standards using a track deviation indicator, not a bearing pointer which arguably doesn't give you much in the way of accuracy and resolution.

[Cap'n P8]

First of all, if you have the ability to set the gps manually to 1 mile, I would do it. Second, as long as you have a bearing pointer up with the ndb tuned and identified set in the adf no one should be able to say boo to you, because that is your required primary reference. If the GPS info and the NDB info don't agree...go around.

[photofly]

First of all, if you have the ability to set the gps manually to 1 mile, I would do it. Second, as long as you have a bearing pointer up with the ndb tuned and identified set in the adf no one should be able to say boo to you, because that is your required primary reference.

I don't have an ADF - that's why I have to use the GPS to navigate to the NDB, and so I want to make sure I'm doing it the right way.

[old_man]

I don't have an ADF - that's why I have to use the GPS to navigate to the NDB, and so I want to make sure I'm doing it the right way.

If the NDB is part of the approach (no overlay) and you don't have a functioning ADF then you can't shoot the approach, even using the GPS as a substitute.

At least that is my take on it.

[Hedley]

Read AIM COM 3.16.9 and 3.16.10.

For example, if the DME associated with an ILS/DME approach is unserviceable, traditional aircraft would be denied the approach; however, under these rules, the pilot of a GNSS-equipped aircraft may request and fly the approach

all fixes that are part of a terminal instrument procedure (arrival, departure, or approach) are named, charted and retrieved from a current navigation database

[photofly]

If the NDB is part of the approach (no overlay) and you don't have a functioning ADF then you can't shoot the approach, even using the GPS as a substitute.

I think the text Hedley points out means you can use GPS to substitute a DME or ADF even if the approach doesn't have a GPS overlay.

Also, ILS, LOC and LOC(BC) approaches never have overlays because GPS isn't approved as a means of replacing a localizer for navigation FAF to MAP. (Some GPS have these approaches in the database and flag them for monitoring only; my GPS doesn't have them at all.) I'm about 94% certain that even in that case, you can still use the GPS to locate the IAF.

My interpretation of the second paragraph that Hedley quotes is that as long as the NDB IAF is retrieved from the waypoint database by name (and you don't have to input it's lat/long) that's ok. It's not a requirement in this situation that the NDB is loaded as part of a whole approach. And in fact it can't be, because LOC and ILS approaches aren't in my database (because there are no overlays for them).

[Go Juice]

If the approach is not a NDB (GNSS OVERLAY) approved approach, and you do not have a fonctionning ADF in the airplanem you CANNOT shoot the approach using your gps.

[CID]

GoJuice, I agree. But I believe the question here is whether or not you can use GPS to navigate to an NDB position as the IAF during an ILS. In that case, it's permitted.

Of course you need to transition to the ILS receiver(s) once you intercept the inbound course.

[cooty monster]

You miss 100% of the shots you don't take.

[MackenzieMountaineer]

hey Cooty No Guts no Glory!

[Big Pistons Forever]

The CAP GEN has the relevant reference.

Page 34 "Area navigation procedures" note 3 says "Conventional procedures annotated (GNSS) denote that the instrument approach can be flown using approved GNSS equipment".

Therefore if you want to use your approach approved GPS to fly an segment of a conventional non precision approach, the approach Identifier (ie NDB RWY 16) must have (GNSS) after it. If you were to fly an NDB approach on a non (GNSS) annotated approach procedure, when your aircraft did not have an NDB then you would be breaking the CARs.

ILS approaches do not normally have the GNSS designation so if any part of the approach requires an NDB (ie to identify the FAF or as a part of the missed approach procedure) and your aircraft is not fitted with an NDB, then again I believe you are in violation of the CARs. However what is safe and practicable and what is legal are often unrelated so while using the GPS to locate the beacon is reasonable it is not legal.

[CID]

CAP GEN isn't a regulatory document. It's just a companion to the CAPS. The CAPS are a little flawed when it comes to GNSS approaches too. If you relied on them for regulation, you couldn't perform a GNSS approach without a flight director and a map display.

Hedley has the correct relevant reference in this matter.

[Big Pistons Forever]

CAP GEN isn't a regulatory document. It's just a companion to the CAPS. The CAPS are a little flawed when it comes to GNSS approaches too. If you relied on them for regulation, you couldn't perform a GNSS approach without a flight director and a map display.

Hedley has the correct relevant reference in this matter.

There is a current national NOTAM which references AIC 16/08 and AIM para 9.2.2

The relevant part of the AIC for this discussion is as follows

3.2 GNSS Overlay Approaches
Overlay approaches are traditional very high frequency omnidirectional range (VOR), or non-directional
beacon (NDB) based approaches (not localizer [LOC] based) that have been approved to be flown using the
guidance of an IFR approach-certified GPS or WAAS. These are identified in the CAP with the letters GNSS
in parentheses and in small capitals after the runway designation [e.g. NDB RWY 04 (GNSS)].
3.3 Approach Database
Approaches flown using GPS or WAAS must be retrieved from a current navigation database. The pilot-incommand is responsible for ensuring that the navigation data matches the current CAP information as
amended by NOTAM. If the loss of GPS or WAAS navigation performance accuracy or integrity results in the
inability to support the planned flight operation, the pilot-in-command shall advise the air traffic service (ATS)
as soon as practical

Note para 3.3 says approaches, not individual way points or nav aids must be retrieved from the data base. If the, in this case NDB approach, does not have the (GNSS) not then it is not by definition in the database and you must have a working NDB to legally fly the procedure. Now you would be crazy not to use GPS track data as the primary nav aid when actually flying the appraoch but you still need a working ADF in the airplane to be legal.

As for your reference to flight director and moving map requirements the NOTAM is pretty clear. It gives the provisions of AIM 9.2.2 (which requires a FD and moving map to fly any RNAV STAR) the force of law.

[55+]

"The CAPS are a little flawed when it comes to GNSS approaches too......"

[Hedley]

Reading this thread, one can't help but wonder at how some approaches in Canada are being conducted.

[CID]

Big Pistons forever,

That information is related to non-precision approaches. There are no GNSS "overlay" approaches for ILS procedures. When it comes to GNSS approaches, the only mandatory leg is the FAS (final approach segment) which is the leg between the FAF and MAP.

So...if you fly an overlay approach, and you extract it from the database like a good law abiding pilot, your entry point can be all the way to the FAF. Yes, you can forgo the arc, or the procedure turn and even take vectors to final. So how you get to the FAF is (in most cases) entirely up to the pilot.

For an ILS, it's much the same except of course, you can't use GPS for navigating the FAS. How often do you see people fly the full procedure on an ILS? More often than not, it's vectors to final. How you intercept the inbound course is not written in stone and like the AIM says, you can ask for clearance to navigate to the IAF using GPS.

Do you have any idea how many airliners use GPS based FMS/FMC guidance all the way to the FAS on an ILS procedure? And do it quite legally? It happens several times a day. Ask an airline pilot how they fly an ILS. They'll tell you they cue it up on the FMS and it will fly the published procedure to the FAS where the ILS is intercepted and control is transfered from the FMS/FMC to the ILS system.

On smaller airplanes, this "nav-to-nav" feature is complicated by AFCS limitations so you often have to switch from LNAV to ILS manually but on bigger iron, you can select LNAV and press "APP" (Approach) and it will intercept the ILS from LNAV much like you would intercept ILS from HDG mode in smaller airplanes.

[CID]

55+, FYI, the "flaw" I'm alluding to is that the AIM and CAPS contain equipage requirements that don't have any regulatory basis. None of the certification standards, policy or guidance material require the installation of a flight director or map display for installation or operational approval of GNSS systems.

The AIM is supposed to be a reflection of the certification and operational standards and the CAPS are supposed to be harmonized with those standards but they're not. Nav Canada has been trying to refine the guidance and subtly amend the wording occasionally but the fact remains that they aren't talking to TC with respect to equipment requirements.

So you can easily get a GNSS system installed and approved in an airplane in Canada that doesn't have a flight director or moving map and even if you did, the AIM and CAPS don't specify the minimum standards for such systems to meet the requirement.

You may not be aware of this but there are plenty of non-TSO displays out there that can be installed in aircraft as long as they aren't required to meet a specific rule that contains specific operational standard without harmonization between the regulations and the CAPS and the AIM, you could use an Apple iPod to meet the requirement. See the flaw?

[Big Pistons Forever]

Big Pistons forever,

That information is related to non-precision approaches. There are no GNSS "overlay" approaches for ILS procedures. When it comes to GNSS approaches, the only mandatory leg is the FAS (final approach segment) which is the leg between the FAF and MAP.

So...if you fly an overlay approach, and you extract it from the database like a good law abiding pilot, your entry point can be all the way to the FAF. Yes, you can forgo the arc, or the procedure turn and even take vectors to final. So how you get to the FAF is (in most cases) entirely up to the pilot.

For an ILS, it's much the same except of course, you can't use GPS for navigating the FAS. How often do you see people fly the full procedure on an ILS? More often than not, it's vectors to final. How you intercept the inbound course is not written in stone and like the AIM says, you can ask for clearance to navigate to the IAF using GPS.

Do you have any idea how many airliners use GPS based FMS/FMC guidance all the way to the FAS on an ILS procedure? And do it quite legally? It happens several times a day. Ask an airline pilot how they fly an ILS. They'll tell you they cue it up on the FMS and it will fly the published procedure to the FAS where the ILS is intercepted and control is transfered from the FMS/FMC to the ILS system.

On smaller airplanes, this "nav-to-nav" feature is complicated by AFCS limitations so you often have to switch from LNAV to ILS manually but on bigger iron, you can select LNAV and press "APP" (Approach) and it will intercept the ILS from LNAV much like you would intercept ILS from HDG mode in smaller airplanes.

The person who started this thread is flying a GA aircraft with an approach approved GPS and no NDB. He also stated that ILS's are not in the Data base of his unit, or in other words waypoints which represent parts of the ILS like the IF, FAF, or termination of the missed approach procedure; can't be accessed by selecting the ILS procedure, like the more sophisticated GPS or airline FMS units.

Therefore I believe that if he legally cannot fly an ILS where NDB's were part of the approach (ie to identify the FAF or where more where one or more NDB's were part of the missed approach procedure).

The AIM is advisory only except where its provisions are referenced to a regulation or put into force by a NOTAM, which is the case with respect to required equipment to fly an RNAV STAR.

[photofly]

More research...
TC AIM 3.16.9 says

Note that for NDB or VOR approaches that are not part of
the GPS overlay program described in Section 3.16.5.2.2,
pilots shall use ADF or VOR as the primary source for
final approach track guidance. For these approaches, and for
approaches based on a localizer (LOC) for lateral guidance,
pilots shall not use GNSS as the primary source for missed
approach guidance when the missed approach procedure
requires flying a published track to or from an NDB or VOR.

So I guess that's that.

Edit: I see that's the same AIM section I was looking at (and quoted) above - but the wording's changed. This one is the PDF version on the TC website valid to April 7 2011. Looks like someone at TC has removed the ambiguity.

I wonder what the problem with using GPS for that purpose is?