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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)
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.
