Legal Localizers and Such

[stevekirks]

Not intending to complicate, just contribute to the conversation:

If you have two NAV radios and two VOR dials, then why use the GPS except as a tertiary backup? Isn't the whole point of an non-GPS approach to provide a way for aircraft without a GPS to safely navigate and land? Todd mentions accuracy, but the point of the localizer would be lateral guidance (yay I learned that Peter!) and the altimeter vertical guidance. So as a dumb non-pilot here, if I get the outer marker tone and the VOR1 needle (tuned to I-SBP) is centered and the VOR2 needle is showing MQOR355 and I'm at 2300 feet, I know that I'm 6.1 miles from the runway.

Not having real IFR training, I'm sure I'm missing something. I need to buy a book and Keith's RW courses....

[Peter Grey]

If you have two NAV radios and two VOR dials, then why use the GPS except as a tertiary backup? Isn't the whole point of an non-GPS approach to provide a way for aircraft without a GPS to safely navigate and land? Todd mentions accuracy, but the point of the localizer would be lateral guidance (yay I learned that Peter!) and the altimeter vertical guidance. So as a dumb non-pilot here, if I get the outer marker tone and the VOR1 needle (tuned to I-SBP) is centered and the VOR2 needle is showing MQOR355 and I'm at 2300 feet, I know that I'm 6.1 miles from the runway.

This is a 100% ok method to fly the approach. The benefit of the GPS is that you won't have to twist each progressive radial of MQO to id each fix (as the GPS just tells you when you are at them).

[RyanK]

Not intending to complicate, just contribute to the conversation:

If you have two NAV radios and two VOR dials, then why use the GPS except as a tertiary backup? Isn't the whole point of an non-GPS approach to provide a way for aircraft without a GPS to safely navigate and land? Todd mentions accuracy, but the point of the localizer would be lateral guidance (yay I learned that Peter!) and the altimeter vertical guidance. So as a dumb non-pilot here, if I get the outer marker tone and the VOR1 needle (tuned to I-SBP) is centered and the VOR2 needle is showing MQOR355 and I'm at 2300 feet, I know that I'm 6.1 miles from the runway.

Not having real IFR training, I'm sure I'm missing something. I need to buy a book and Keith's RW courses....

The benefits of GPS include automatic sequencing, turn anticipation, more info in one place, etc.. Depending on the procedure it can make your life a lot easier.

[Ray Salmon]

I am going to throw a wrench in the works here and say you can use GPS for lateral guidance (or other qualified RNAV system) on the Final Approach Segment of a VOR approach, provided its use is as an alternate means of navigation vs a substitute means of navigation.

If you look in the definitions section of the AC it differentiates between alternate and substitute means of navigation. In the section that prohibits RNAV use on a final approach segment, it prohibits it is a substitute, however it leaves open the door for use as an alternate means of navigation (I will let you look up the difference in meaning to save me some typing).

Case in point: On the high-tech $38m jet I fly, the autopilot is not capable of tracking a VOR course at all (yep, you read that right). As a result, we fly all of our VOR approaches in "FMS" (GPS/RNAV) mode, and also display the "raw" data from the ground based VOR...however the primary means of navigation and the one the autopilot is using is the GPS/RNAV system, not the ground based navigational aid. Having said that, in order to do this, the VOR must be operational (not NOTAMed) and our VOR receivers must be working per the AC. As such we are using the GPS as an alternate means of navigation. This is all FAA approved and blessed of course.

[Ryan B]

I'm a huge fan of airplanes... and I tend to believe I know a lot about a lot of different aircraft. But I've never heard of a bizjet that couldn't fly a radial on autopilot (using VOR as the nav source).

Are you able to hint away at the $38m bizjet you do fly? I'm just very curious what it is.

I'm always the nerd in the tower at work asking pilots about equipment I see on their aircraft, or asking what new fancy avionics they're flying with lol.

[Peter Grey]

I am going to throw a wrench in the works here and say you can use GPS for lateral guidance (or other qualified RNAV system) on the Final Approach Segment of a VOR approach, provided its use is as an alternate means of navigation vs a substitute means of navigation.

If you look in the definitions section of the AC it differentiates between alternate and substitute means of navigation. In the section that prohibits RNAV use on a final approach segment, it prohibits it is a substitute, however it leaves open the door for use as an alternate means of navigation (I will let you look up the difference in meaning to save me some typing).

Case in point: On the high-tech $38m jet I fly, the autopilot is not capable of tracking a VOR course at all (yep, you read that right). As a result, we fly all of our VOR approaches in "FMS" (GPS/RNAV) mode, and also display the "raw" data from the ground based VOR...however the primary means of navigation and the one the autopilot is using is the GPS/RNAV system, not the ground based navigational aid. Having said that, in order to do this, the VOR must be operational (not NOTAMed) and our VOR receivers must be working per the AC. As such we are using the GPS as an alternate means of navigation. This is all FAA approved and blessed of course.

Yes, I omitted this possibility for simplicity but it is possible and legal.

This starts to get into airplane specifics and the AFM, but in some cases it's legal to not use the primary navaid as the main source as long as it's displayed, in other cases it's technically legal to not have it displayed even. This tends to apply more for larger aircraft (most GA GPS's have AFM restrictions requiring you to use the proper navaid on the final segment of an approach).

It's a fairly technical distinction, but a valid one. We did the same thing in the fancy regional jet I flew (our autopilot could track a VOR radial, but not well enough to use on an approach).

[Ray Salmon]

I'm a huge fan of airplanes... and I tend to believe I know a lot about a lot of different aircraft. But I've never heard of a bizjet that couldn't fly a radial on autopilot (using VOR as the nav source).

Are you able to hint away at the $38m bizjet you do fly? I'm just very curious what it is.

I'm always the nerd in the tower at work asking pilots about equipment I see on their aircraft, or asking what new fancy avionics they're flying with lol.

Sure...it's an Embraer 175.

[Ryan B]

Interesting. So there's no NAV1 freq? It's all in the FMS?

[Ray Salmon]

No, we have VOR receivers with frequencies etc, and we can display VOR info like any aircraft...it's just when you ask the autopilot to track the VOR (by hitting the "NAV" button), it won't do it. We would either have to hand fly or use "HDG" mode to manually keep the aircraft on course.

[Talan2000]

Very interesting, thanks for sharing that Ray. It's kinda funny the mishmash of technologies we have now - partially digitial partially analog etc.

I had some instrument training in the 90s pre GPS and then a loong break. So I've been trying to backfill my knowledge on the GPS procedures with a mind to getting my instrument rating in the real world. I stumbled onto the rather confusing AC 90-108 after googling "how to use a GPS on a Localizer approach" and I was off to the races, confusing myself in the process. A little knowledge can be a dangerous thing. I've just picked up the Instrument flying handbook and Instrument procedures handbook to do some thorough home study.

The planes in the local flying club are an even more interesting bag ... with a mish mash of old instrumentation, radios etc. And of course no autopilot...