30-day VOR check required?

[Aye Effaar]

So let's say I have a WAAS certified GPS (TSO 145 or 146) and a King KX 165a Nav/Com (which includes glideslope & localizer receivers).

One day, I'm out flying in IMC and decide to request an ILS approach.

While a localizer uses the same frequency range as a VOR, they are very different transmission modes. And 91.171 specifically applies to operations involving "a civil aircraft under IFR using the VOR system of radio navigation"

I can legally use the WAAS GPS to navigate enroute, but I will need to switch navigation source over the King KX 165a to fly the approach. The question is, do I need to have my 30-day VOR check (91.171) legally to shoot the ILS?

 

[chemgeek]

An ILS is not a VOR, and doesn't respond to the radial setting on your VOR CDI. The VOR check attempts to evaluate the accuracy of your CDI setting circuitry. A VOR check would therefore not verify the accuracy of LOC operation. So logic says no VOR check is required, but the FAA legalese is not always clearly written.

 

[luvflyin]

So let's say I have a WAAS certified GPS (TSO 145 or 146) and a King KX 165a Nav/Com (which includes glideslope & localizer receivers).

One day, I'm out flying in IMC and decide to request an ILS approach.

While a localizer uses the same frequency range as a VOR, they are very different transmission modes. And 91.171 specifically applies to operations involving "a civil aircraft under IFR using the VOR system of radio navigation"

I can legally use the WAAS GPS to navigate enroute, but I will need to switch navigation source over the King KX 165a to fly the approach. The question is, do I need to have my 30-day VOR check (91.171) legally to shoot the ILS?

No. But I ain't sure enough to bet a paycheck on it. I'll bet a beer though. While the LOC and VOR share a frequency band, and often the same box in the panel, they are different, and I've never seen a 30 day, or any day, check requirement for the Localizer.

 

[Aye Effaar]

An ILS is not a VOR, and doesn't respond to the radial setting on your VOR CDI. The VOR check attempts to evaluate the accuracy of your CDI setting circuitry. A VOR check would therefore not verify the accuracy of LOC operation. So logic says no VOR check is required, but the FAA legalese is not always clearly written.

True -- and the technique of tracking is completely different (keep the receiver between the two transmission lobes). But I paused for a moment when I considered the receiver still needs to accurately measure the two signals. I still agree the 30-day check is not called for -- but I had a moment of reconsideration.

 

[Aye Effaar]

No. But I ain't sure enough to bet a paycheck on it. I'll bet a beer though. While the LOC and VOR share a frequency band, and often the same box in the panel, they are different, and I've never seen a 30 day, or any day, check requirement for the Localizer.

The other thing that caught me was the overlapping frequency range of localizers and VORs. Clearly there's some shared circuitry in the receiver -- but I'm going with you. The regs mention VORs so I think the check is not required.

 

[luvflyin]

The other thing that caught me was the overlapping frequency range of localizers and VORs. Clearly there's some shared circuitry in the receiver -- but I'm going with you. The regs mention VORs so I think the check is not required.

I dunno. But it seems possible they share the tuner. Like it knows which 'reciever'(internally) to send it it to. Or each receiver only responds to correct frequencies. I'm just shooting from my laymans hip here. There are electro engineer types around here who will probably be along soon.

 

[dmspilot]

A localizer is not a VOR, don't overthink it.

 

[nrm2430]

VOR check is only required if you’re using that to fly a VOR approach or to navigate along an airway

 

[Rgbeard]

putting away legal and pulling out practical.

Whether or not you do a fully-compliant VOR-check, you should have a good understanding if your radio receiver is functioning properly or not.

If it's not doing well, would you want to fly an ILS with it, while you argue (possibly) from the bottom of your smoking hole, that an ILS is not a VOR?

 

[Larry in TN]

The OBS knob is adjusting a phase-shift circuit so that the phases match, and the needle centers, at the desired radial. That phase-shift circuitry is not used in the LOC mode as the LOC is just comparing the relative strengths of the two lobe frequencies.

A VOR check has no relation to the LOC accuracy.

 

[Doug Reid]

There are separate centering adjustments for LOC and VOR. My old KI201C would be right on the VOR radial, but 2 dots off the LOC. AFAIK there is only a 30 day check for VOR, but I highly recommend tracking the LOC and GS to be safe if you plan on using them . If you have 2 receivers, they should agree when flying an ILS .

 

[asicer]

True -- and the technique of tracking is completely different (keep the receiver between the two transmission lobes). But I paused for a moment when I considered the receiver still needs to accurately measure the two signals. I still agree the 30-day check is not called for -- but I had a moment of reconsideration.

The way that the receiver "measures" a VOR signal is also different from the method used for a LOC.

 

[MauleSkinner]

The way that the receiver "measures" a VOR signal is also different from the method used for a LOC.

And to tie this back to the OP, the reg acknowledges that…an ILS isn’t part of “the VOR system of navigation”.

 

[Clip4]

A nav radio can be VOR only, VOR/LOC only, or VOR/LOC/GS. The pilot only need check the VOR function if he intends to use that navigation system.

LOC frequencies are 108.10-111.95
VOR frequencies are 108.00-117.95
VOT is typically 111.00 - above most of the LOC frequencies.

 

[RussR]

I think this regulation is pretty clear. You just have to ask yourself if you are using the VOR system of navigation. If you’re only using ILS, the answer is “no”, therefore the VOR check is not required.

Besides, a VOR check doesn’t check anything on the ILS anyway.

The only reason it’s confusing is because avionics manufacturers (wisely) started putting both systems in the same box and displaying on the same indicator. But I’m sure that in the early days you had to buy a VOR receiver and indicator and a separate ILS receiver and indicator. If you had two separate pieces of equipment doing each thing, you wouldn’t even think to ask the question, so I bet you won’t find this question on any internet message boards before about 1960.

 

[aterpster]

I think this regulation is pretty clear. You just have to ask yourself if you are using the VOR system of navigation. If you’re only using ILS, the answer is “no”, therefore the VOR check is not required.

Besides, a VOR check doesn’t check anything on the ILS anyway.

The only reason it’s confusing is because avionics manufacturers (wisely) started putting both systems in the same box and displaying on the same indicator. But I’m sure that in the early days you had to buy a VOR receiver and indicator and a separate ILS receiver and indicator. If you had two separate pieces of equipment doing each thing, you wouldn’t even think to ask the question, so I bet you won’t find this question on any internet message boards before about 1960.

I've worked with this stuff since 1955 as a USAF tech, then flying starting in 1956. I have never seen separate receivers for the LOC and VOR. GS, yes.

 

[flyingron]

It's an entirely different form of modulation, altogether.

The VOR is phase encoded signals the difference ofwhich gives you the radial you are on. The LOC is a simple strength difference of two signals to yield left-to-right.

 

[Larry in TN]

It's an entirely different form of modulation, altogether.

Okay, "It's an entirely different form of modulation."

 

[John Collins]

Also, just look at the regulation, how could it possibly relate to the Localizer?

91.171(a)(2) says: "Has been operationally checked within the preceding 30 days, and was found to be within the limits of the permissible indicated bearing error set forth in paragraph (b) or (c) of this section". How do you set the bearing for an ILS localizer when there isn't such a setting available?

How would you do this check in 91.171(b)(4) for a localizer?

(4) If no check signal or point is available, while in flight -
(i) Select a VOR radial that lies along the centerline of an established VOR airway;
(ii) Select a prominent ground point along the selected radial preferably more than 20 nautical miles from the VOR ground facility and maneuver the aircraft directly over the point at a reasonably low altitude; and
(iii) Note the VOR bearing indicated by the receiver when over the ground point (the maximum permissible variation between the published radial and the indicated bearing is 6 degrees).

The bearing information is not in anyway connected to a localizer function and has no effect.

 

[WDD]

........, while you argue (possibly) from the bottom of your smoking hole, ......

1) I learned something new about VOR vs ILS
2) New cool phrasing to add to my dictionary

 

[TangoWhiskey]

If you’ve got it on board, why not test it and log it? There’s plenty of time (and VORs in range) to do so in cruise flight. I have that (and “monitor 121.5”) on my cruise checklist for that very reason. Because GPS outages and degradation NEVER happen.

 

[asicer]

If you’ve got it on board, why not test it and log it? There’s plenty of time (and VORs in range) to do so in cruise flight. I have that (and “monitor 121.5”) on my cruise checklist for that very reason.

Easy to do if you have 2 NAV radios. If you only have 1 and are flying D> then it's not quite as easy.

 

[coma24]

Easy to do if you have 2 NAV radios. If you only have 1 and are flying D> then it's not quite as easy.

Adjust your path of flight to include a fix which is on a known radial in relation a VOR. I do this from time to time since I have a single nav radio and an IFR GPS.

 

[asicer]

Adjust your path of flight to include a fix which is on a known radial in relation a VOR. I do this from time to time since I have a single nav radio and an IFR GPS.

Easy to do if you have a Victor airway near your home airport conveniently located near your route of flight. Don't forget that you can't just choose any old radial but that it has to be along an airway. With Victor airways going MON, it's getting harder to do.

 

[coma24]

Easy to do if you have a Victor airway near your home airport conveniently located near your route of flight. Don't forget that you can't just choose any old radial but that it has to be along an airway. With Victor airways going MON, it's getting harder to do.

Fair point about MON, I hadn't considered that.

 

[RussR]

Adjust your path of flight to include a fix which is on a known radial in relation a VOR. I do this from time to time since I have a single nav radio and an IFR GPS.

That certainly would seem to accomplish the intent of the VOR check, but does not comply with the letter of the regulation. That requires you to find a visual landmark on an airway. GPS is not addressed.

 

[coma24]

That's totally fair, Russ. It's such an antiquated rule, having to use a visual landmark instead of correlating with a known fix as reflected on a GPS. I was taking liberties since I don't need to perform the 30 day check as a result relying primarily on WAAS navigation (I just like knowing the VOR is still functional for emergency use) and had honestly forgotten that it made the landmark distinction.

 

[Hang 4]

Without debating the rule itself, has anyone found an issue with a relatively modern nav receiver failing the test? I do test mine, GTN and a GTR225, and have never found an issue.

 

[Larry in TN]

Without debating the rule itself, has anyone found an issue with a relatively modern nav receiver failing the test? I do test mine, GTN and a GTR225, and have never found an issue.

It is not the receiver that you are testing. You are testing the calibration of the OBS.

 

[Hang 4]

It is not the receiver that you are testing. You are testing the calibration of the OBS.

Thanks for the clarification - Same question, different phrasing - In an all glass panel, has anyone seen any test failures?