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The FARs require a check of the VOR receiver before IFR flight:

§91.171 VOR equipment check for IFR operations.

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

...has been checked using a procedure described immediately following.

Does this apply to an ILS approach? Specifically, consider a flight conducted entirely using GPS, except that it terminates in an ILS approach. In most (or all?) aircraft, the ILS and VOR receivers are one and the same. However, the mechanism of how an ILS and VOR work are very different.

The FARs speak of "the VOR system of radio naviation". Is that specifically for actual VORs, or does it encompass the ILS by virtue of being the same receiver and CDI?

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  • $\begingroup$ Really good question - my instinct would have been to say "yes of course". Thanks to this question and @digitgopher's answer I've learned something today. $\endgroup$ – IanF1 Oct 11 '15 at 18:02
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This regulation does not apply to an ILS approach.

You are right is saying that "the VOR system of radio navigation" refers to actual VORs.

If the flight is conducted using GPS to an ILS approach then you aren't using VOR for navigation. It does not matter that ILS uses the same instrument display.

Intuitively, though ILS and VOR use the same display and frequency band, they are functionally different. The VOR beacon sends out a omnidirectional signal, from which a bearing (0-359 degrees) is calculated. The ILS sends out a focused, directional signal, which the receiver centers on.

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The VOR receiver check is verifying that the needle can interpret the correct radial within a specified tolerance.

It appears you know how the two separate systems work so a bad VOR check won't affect the accuracy of the localizer system. Hence, there is no regulation prohibiting it.

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