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VGSI and descent angles not coincident

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General Aviation

A few approach plates show a note stating that ‘VGSI and descent angles not coincident’.

I have read about this note on various aviation websites including this one, but I am still looking for more information.

Could somebody kindly answer the following questions:

  • What are the typical reasons why it is not possible at certain airports to make the VGSI and ILS descent angle coincident?
  • How the pilot is supposed to use the information provided by the note during an approach down to the minimums? Is the note just intended not to make the pilot feel surprised for being somewhat high or low when he or she sees the approach lights?

Thanks

1 Answers



  1. Russ Roslewski on Oct 13, 2016

    Your last sentence is exactly right – the whole purpose of that note is so that a pilot is not surprised when they think they are on glideslope according to the ILS or LPV but when they break out and see the lights, it shows them high or low. It’s supposed to prevent overreacting or thinking that your equipment is out of tolerance.

    As for why they are not coincident:

    “Coincidence” is this case means within 0.2 degrees of glidepath OR within 3 feet of Threshold Crossing Height. These are pretty small differences.

    ILSes of course use fixed transmitters and antennas to send out the signal. (LPVs are usually matched to the ILS to the same runway for fairly obvious reasons of redundancy.) VGSI units are also fixed hardware mounted to the ground somewhere. So, the approach is designed based around the ILS which is already installed, or an LPV is designed based on obstacles and optimum glideslope/TCH. Often, this does not match the VGSI for any of a number of reasons – primarily that the VGSI was installed decades ago.

    To match the ILS/LPV in glidepath is relatively easy – ever seen a VGSI up close? Not complicated to adjust, a couple of wrenches will do it. But the trick is the TCH, which you need to adjust as well or the glidepaths would just be paralleling each other but still offset (one would be higher at the threshold than the other).

    So to fix the glideslope AND the TCH, the VGSI may very well need to be physically moved, which would take time and money. Since there’s no real tangible benefit of having them match exactly, it’s like the lowest priority for airport funding. I mean, are airplanes crashing everyday due to the VGSI and ILS glidepaths differing by 0.3 degrees or the TCHes being off a few feet? Of course not. So there’s no real incentive for an airport to move them.

    That’s pretty much all it is – funding.

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