IFR / GPS Question

[zulutime]

First post. Thx for reading.

As I understand it, you cannot use a GPS overlay for a LOC approach. Only VOR and NDB can be GPS overlaid. If you are therefore flying the LOC RWY 30 at CYHM you must have an ADF on board in case you have to go "missed", since you cannot use the GPS in the case of a missed approach. Since, in this particular case, CYHM also has an NDB RWY 30 (GNSS) one could fly that approach with only the GPS overlay and use the GPS to identify the beacon in the event of a "missed" approach. Assuming I am correct on this, is this something an examiner might try and get a pilot to do, i.e. ask the pilot to fly the LOC when he knows the a/c doesn't have an ADF? The only difference I see between these two approaches is the MDA. The LOC is lower and that could be the trap the examiner is trying to see if you will fall into.

Thx...signed old and confused....

[ahramin]

Ask yourself why you can't use GPS to fly a LOC approach ...

[whoop_whoop]

Hi zulutime,

Welcome to avcanada!!

I believe that one of the reasons for not being able to fly a LOC approach with GPS is because of actual navigation performance (fault tolerance) and obstacle clearance criteria…the GPS will only flag when you get more than 0.3NM from your position. I hope I've done the math correctly here…

Let's assume that your runway is 1 NM long and your LOC approach has minimums at around 300 AGL…which would put you about 1 NM from the threshold (2NM from the LOC antenna). You know most localizers are 5 degrees wide, so a full-scale CDI deflection is 2.5 degrees. The tangent of 2.5 = opposite/adjacent. In this example the adjacent is 2NM so you can rewrite the equation to be: opposite = tan(2.5) * 2NM. The answer is 0.09 NM which means at full-scale deflection on the CDI at 2NM from the antenna you will be approximately 0.09 NM off-position…about 3 times as accurate as your GPS.

Happy New Year!

WW

[200hr Wonder]

You can however take credit for the NDB or DME in some cases. COM 3.15.9 in the AIM. I don't have a YHM apprpach plate handy so I can't say if you can take credit or not in this case.

[photofly]

The AIM is very precisely worded. On a non-overlay approach, you can't use the GPS for missed approach course guidance "when the missed approach procedure requires flying a published track to or from an NDB or VOR."

You can use GPS for the missed approach when the procedure required flying a published course but not direct to an an NDB/VOR.

For the LOC RWY 30 at CYHM, missed approach is climb on track of 297° to 2800 to "ZHA" NDB - this cannot be done with a GPS.

Contrast with the missed approach CYOO LOC/DME RWY 12 which is Climb 4000 on track of 122° LEFT turn direct to "YPQ" NDB. The assigned track is not to the NDB, and the direct portion does not have an assigned track. This CAN be flown with a GPS.

I have that distinction in writing from TC.

As far as the flight test goes: you decide what approaches you want to fly, not the examiner. For an initial flight test you have to show him/her one precision approach and one non-precision approach, to minima. If you want the non-precision approach to be the LOC, that's your choice. If you want to fly the NDB, that's your choice too.

[turbo-prop]

The AIM is very precisely worded. On a non-overlay approach, you can't use the GPS for missed approach course guidance "when the missed approach procedure requires flying a published track to or from an NDB or VOR."

You can use GPS for the missed approach when the procedure required flying a published course but not direct to an an NDB/VOR.

For the LOC RWY 30 at CYHM, missed approach is climb on track of 297° to 2800 to "ZHA" NDB - this cannot be done with a GPS.

Contrast with the missed approach CYOO LOC/DME RWY 12 which is Climb 4000 on track of 122° LEFT turn direct to "YPQ" NDB. The assigned track is not to the NDB, and the direct portion does not have an assigned track. This CAN be flown with a GPS.

I have that distinction in writing from TC.

As far as the flight test goes: you decide what approaches you want to fly, not the examiner. For an initial flight test you have to show him/her one precision approach and one non-precision approach, to minima. If you want the non-precision approach to be the LOC, that's your choice. If you want to fly the NDB, that's your choice too.

That is funny, because in our G1000 equipped aircraft as soon as I push the go-around button it automatically switches to GPS. If I am flying an ILS, LOC, LOC-BC, NDB, or VOR it automatically switches to GPS when I push the go-around button.

[photofly]

As I understand it, you cannot use a GPS overlay for a LOC approach. Only VOR and NDB can be GPS overlaid.

One other comment: "using" an overlay isn't just a case of dialing up the NDB or VOR location in your GPS. You actually need the whole approach in the database, and it has to be loaded and enabled, otherwise the GPS receiver doesn't go into approach mode.

My GPS doesn't even contain any LOC approaches, so being "unable" to use an overlay isn't a pilot decision issue: the approach isn't there to load.

That is funny, because in our G1000 equipped aircraft as soon as I push the go-around button it automatically switches to GPS.

It looks like a funny Canadian rule; I can't see anything in the FAA AIP about it.

[Big Pistons Forever]

The AIM is very precisely worded. On a non-overlay approach, you can't use the GPS for missed approach course guidance "when the missed approach procedure requires flying a published track to or from an NDB or VOR."

You can use GPS for the missed approach when the procedure required flying a published course but not direct to an an NDB/VOR.

For the LOC RWY 30 at CYHM, missed approach is climb on track of 297° to 2800 to "ZHA" NDB - this cannot be done with a GPS.

Contrast with the missed approach CYOO LOC/DME RWY 12 which is Climb 4000 on track of 122° LEFT turn direct to "YPQ" NDB. The assigned track is not to the NDB, and the direct portion does not have an assigned track. This CAN be flown with a GPS.

I have that distinction in writing from TC.

Best explanation I have seen yet.

[CID]

Ahramin, you are out of line and frankly not at all helpful to this poster.

[turbo-prop]

The AIM is very precisely worded. On a non-overlay approach, you can't use the GPS for missed approach course guidance "when the missed approach procedure requires flying a published track to or from an NDB or VOR."

You can use GPS for the missed approach when the procedure required flying a published course but not direct to an an NDB/VOR.

For the LOC RWY 30 at CYHM, missed approach is climb on track of 297° to 2800 to "ZHA" NDB - this cannot be done with a GPS.

Contrast with the missed approach CYOO LOC/DME RWY 12 which is Climb 4000 on track of 122° LEFT turn direct to "YPQ" NDB. The assigned track is not to the NDB, and the direct portion does not have an assigned track. This CAN be flown with a GPS.

I have that distinction in writing from TC.

Best explanation I have seen yet.

G1000 does this in GPS mode and I am flying in Canada.

[photofly]

If it comes to a disagreement between the G1000 and TC AIM about the correct procedure to use in Canada, I know where I'll put my money.

[ReserveTank]

It looks like a funny Canadian rule; I can't see anything in the FAA AIP about it.

As with most rules in Canadian aviation, this one is years behind. In the US, I can load an ILS in the G430W to monitor how I`m doing with the actual ILS. In case of a missed, all the missed approach waypoints and course guidance are there and I can use them as needed. The hold and entry are even published and the autopilot follows along.
Pretty much the only thing you cannot do with the GPS is LOC or ILS final approach course guidance, and that is for a good reason. The localizer performance must come from the actual localizer. On the other hand, flying an LPV is quite handy if available.

[photofly]

There's no difference between Canadian and FAA regulations on that point. In the US also, you may not use GPS for primary guidance at any stage on a non-GPS and non-overlay approach (AIP 17.14. just as in Canada and, in Canada, you may also follow along on a G430 while flying an ILS. In both cases primary guidance both before and after the FAF must be provided by the ILS.

The only difference appears to be the Canadian rule about the missed approach.

[nightbird]

Would you be able to do the approach legally if you request different miss-approach instructions from ATC prior to execute the approach?
This is a very funny rule indeed, is it anywhere in the cars or just mentioned in the AIM

[photofly]

Yes, I think you can ask for alternate missed approach instructions. I've only ever seen it in the AIM; I don't know what its regulatory status is.

[zulutime]

All,

Very much appreciated. Didn't realize this wasn't a clear cut question and answer when I first posted it. Sounds like this would be a good round table discussion topic with some instructors.

Thx again for everyone's feedback. Much appreciated indeed.

[pelmet]

So in the end, one can use the GPS along this track but the ADF/VOR is tuned and indicating properly and monitored. The GPS use is technically your secondary backup. They have to be indicating properly. A good question would be why do we have these restrictions.

[Beefitarian]

Ask yourself why you can't use GPS to fly a LOC approach ...

I did that and I believe there are two reasons keeping in mind I am caraaaazeeee.

1- GPS is still a US military system and can be switched to an encoded mode basically shutting it off. Now what do you do?

2- It's Canada and we don't have the money to set up GPS augmentation at our airports to provide the accuracy and back up to prevent those that choose to become over reliant on a pirated knock off of the foreflight app running on a hacked android device doing their jobs from being lost if GPS was suddenly unavailable. [/runon]

[AuxBatOn]

Beef,

The GPS signal is already encrypted for military application (P(Y) code) while transmitting the unencrypted signal.

[ahramin]

A good try beef, but those answers would also apply to GPS approaches as well, which we can obviously do. A LOC approach provides more precise positioning than non augmented GPS can reliably deliver, so can't be replaced by GPS. Once you are on the missed though, you aren't using the LOC signal anymore, and can go back to GPS (excepting the restrictions others have already pointed out).