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On a US IFR flight, it is required to do a VOR check every 30 days and the error tolerance is 4 degree except for airborne check.

Suppose I have 4 degree error on my VOR check. When I fly an ILS and right on the indicated glide slope, what is the maximum degree of deviation from the actual glide slope? (range of my actual glide slope)

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    $\begingroup$ Why would a deviation in a VOR check affect the ILS glideslope? $\endgroup$
    – DeltaLima ♦
    Aug 3, 2016 at 7:53
  • $\begingroup$ VOR/ILS uses the same VHF NAV system, so if there is an error on VOR frequency, it may imply an error on ILS frequency as well. $\endgroup$
    – CrepuscularV
    Aug 3, 2016 at 8:10
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    $\begingroup$ But the VOR system works on a 108.00-117.95 MHz, frequency & amplitude modulated signal. The glideslope works on a 329.15-335.0 MHz amplitude modulated signal. Both have different signal paths to the indicator instrument, and they use different needles on the indicator. $\endgroup$
    – DeltaLima ♦
    Aug 3, 2016 at 8:16
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    $\begingroup$ @skyoasis While VOR and ILS systems do often appear in the same box, the ILS system uses circuitry other than that which degrades in the VOR system and which necessitates the VOR accuracy check. $\endgroup$
    – J W
    Aug 4, 2016 at 2:32

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The deviation detected on your VOR check is likely to be unrelated to the deviation of the glideslope. Whilst they use the same indicator instrument, the radio systems driving the instrument are typically separate.

The VOR system works on a 108.00-117.95 MHz, frequency & amplitude modulated signal. The glideslope works on a 329.15-335.0 MHz amplitude modulated signal.

Both signals have different paths to the indicator instrument, and they use different needles on the indicator.

Therefor it is unlikely that a systematic error causing a deviation in the VOR omni-bearing indicator will cause a deviation in the glide path indicator.

There may be common error sources in both indications, but that depends really on the internal design of the receiver. For example if there is a voltage bias on some part of the system it may affect both systems. But you can't say anything about the maximum degree of deviation from the actual glide slope based on the observed deviation in the VOR receiver.

The localizer receiver works on the same frequencies as the VOR, but the demodulation is different. In an analogue system with needles, they use the same needle. So for localizer there is a higher system commonality with VOR than in the glide path case, which increase the likelihood of a common source of error.

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    $\begingroup$ I am not saying they are completely independent, I am saying it is unlikely that a deviation in VOR will also show as a deviation in glideslope. There may be common error sources in both indications, but that depends really on the internal design of the receiver. For example if there is a voltage bias on some part of the system it may affect both systems. But you can't say anything about the maximum degree of deviation from the actual glide slope based on the observed deviation in the VOR receiver. $\endgroup$
    – DeltaLima ♦
    Aug 3, 2016 at 9:00
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    $\begingroup$ @DeltaLima Does the same hold true for the localizer needle, i.e., if NAV1 fails a VOR check, is it trustworthy for a LOC approach? $\endgroup$
    – Greg Bacon
    Aug 3, 2016 at 13:53
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    $\begingroup$ @GregBacon The ILS LOC works on the VOR frequencies, but the demodulation is different. In an analogue system with needles, they use the same needle. So for LOC there is a higher system commonality with VOR than in the GP case, which increase the likelihood of a common source of error. $\endgroup$
    – DeltaLima ♦
    Aug 3, 2016 at 15:52
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    $\begingroup$ The difference is that the vor receiver has to compare the phase difference between two 30Hz tones so requires the frequencies to be right on the number. The ILS compares the amplitudes of a 90Hz tone and a 150Hz tone. Since it doesn't need to calculate a phase relationship it doesn't require such accuracy. Also, the VOR accuracy is worse at long distances. 4 degrees is a big difference at 100 nm. The ILS is the same way, but you're not using it from 100nm and it gets more accurate as you get closer. $\endgroup$
    – TomMcW
    Aug 3, 2016 at 18:30
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    $\begingroup$ @TomMcW, the two 30Hz tones a VOR receiver compares both come from the received signal and are both derived from the same oscillator, so they trivially match. Of course, the method of comparison is still very different — VOR needs to phase-shift one of the tones and add them to measure interference and the inductive element that does that (which is attached to the OBS knob) is not used for ILS. $\endgroup$
    – Jan Hudec
    Aug 5, 2016 at 17:36

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