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According to 91.171, I am not allowed to fly IFR unless:

"...unless the VOR equipment of that aircraft -

(1) Is maintained, checked, and inspected under an approved procedure; or (2) 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."

When it says "VOR equipment" am I to assume that requires ALL VOR receivers to be functional? Or is this assuming a single VOR receiver, therefore if one of mine is working, would I legally be allowed to fly?

Of course the 91.213 statement also says that "no person may take off an aircraft with inoperative instruments or equipment installed" and then goes on to list a variety of requirements to meet if it is to take off.

Thanks for the help!

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There are a few things going on here:

First off you don't need any VOR receivers to fly IFR as per FAR 91.205 which lists out the minimum equipment for flying IFR.

Lets look a bit closer at 91.171

(a) No person may operate a civil aircraft under IFR using the VOR system of radio navigation unless the VOR equipment of that aircraft -

So they only need to be in alignment if you are using the VOR system for your flight (which in many cases you will be). With that in mind, 91.171 does not specify how many receivers you need, simply that if you have one and want to use it, its aligned.

If you look closely at the clauses in 91.213 you will find that it deals with minimum equipment lists (which your GA plane is unlikely to have). As such we should look at 91.213 (d)(2)(ii) and 91.213 (d)(2)(iii) which points us back to 91.205 and the "Kinds of Operations Equipment List for the kind of flight operation being conducted for which only a single VOR is required for IFR navigation if you intend to use the VOR system (as discussed above).

Keep in mind 91.213 (d)(3)(ii) indicates we must placard the inoperative or out of alignment unit

Deactivated and placarded “Inoperative.”

In some cases two VOR's may be required depending on what you want to do, and the approaches you intend on flying. There may be some things that are not available to you (or may not be practical) with only a single VOR receiver like the ILS/LOC into JFK's 13L which offers lower minimums if you can identify points of the LGA VOR (see UXHUB MINIMUMS).

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  • $\begingroup$ Awesome, thanks Dave. Appreciate the thorough explanation. With 91.213 stating that "deactivation" is required, I'm curious how a VOR is deactivated without impacting your comms. (And yes, forgot to add that I am not req. to have a VOR for basic IFR day/night flight as per 91.205, was more focused on the VOR issue and its impact.) $\endgroup$
    – Tigre222
    Feb 18, 2022 at 20:57
  • $\begingroup$ @Tigre222 keep in mind you need the radio, antenna, and CDI for the whole shebang to work. Ive often seen CDI's throw flags or needles get stuck rendering it INOP even if the radio works. Depending on the radio the NAV section and COM sections may be independent internally and simply mounted in one housing. $\endgroup$
    – Dave
    Feb 18, 2022 at 21:09
  • $\begingroup$ Dave, good call. Definitely robust situations and lots to go offline. Appreciate it! $\endgroup$
    – Tigre222
    Feb 19, 2022 at 23:38

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