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Garmin / Avidyne / Jeppesen +V Advisory Glidepath / Glideslope, does it ever breach stepdown fixes, and does it exist for non-GPS IAP airports?

Does the Jepp advisory glideslope ever breach stepdown fixes?

This is the “glideslope” drawn as a straight line from FAF to MAP. It appears increasingly on terminal charts, and if the GPS (has to be a “W” box) has it encoded, it can even be flown, even on autopilot.

I have just read somewhere that it is possible that the “glideslope” may breach the previously surveyed stepdown fixes.

It would seem astonishing.

Obviously there is no guarantee of obstacle clearance below the MDA (it isn’t an LPV or iLS) but that isn’t the same thing at all.

Administrator
Shoreham EGKA, United Kingdom

It wouldn’t make sense. You can technically descent vertically after a step down fix to the platform of the next step down fix and then continue horizontally until passing that next fix. Therefore a direct line remaining above all step down fixes must be safe.

The “advisory glide slope” feature of the 430W etc. is a fantastic feature. It makes flying non precision approaches so much easier. Larger aircraft have always used this technique although manually setup in the FMS usually.

Last Edited by achimha at 24 Aug 20:29

The only issue I have seen is a risk in going below the MDA following the advisory glideslope. I think it is always done on the shallowest angle that allows all stepdowns to be obeyed.

EGTK Oxford

The advisory GS will nominally be above the MSL height of the step downs inside of the FAF. I chose to use the word nominally because step down altitudes are based on the barometric altimeter and sensitive to temperature at the airport where the altimeter setting is determined. That means that the MSL altitude at the step down moves up and down with temperature, whereas the WAAS advisory glidepath remains independent of temperature and relatively fixed in space. This means that the advisory GS may cross a step down fix on a hot day below the MSL minimum altitude. There is no danger of hitting the rocks when this occurs, but the altimeter will read a lower value when crossing the step down than the minimum permitted. In the US, there are case where pilots were getting pilot deviations for being below the charted minimums as determined by their Mode C output. This is only likely to be an issue when the step downs are outside of the FAF as the temperature error is more pronounced the higher one is above the airport or the further away they are and the error has to cause at least a 300 foot deviation to be considered a busted altitude. The other scenario where this could be an issue is on a check flight where an examiner is observing the deviation.

In the case above that peter shows, the advisory glidepath will be nominally be at 969 feet at 1.9 NM from RW20, 1500 feet at 3.0NM from RW20, and at 2227 feet at 4.5 NM from RW20. If the temperature at the airport is 35 C, the MSL indication at the three points will be respectively at 906, 1403, and 2082 feet on the altimeter.

KUZA, United States

Where is that Peter? 4.5° seems a fairly sporty slope for an instrument procedure.

London area

Shoreham EGKA.

I don’t like to post the whole of a Jepp chart, in case somebody doesn’t like it….

I am fairly sure that one does appear as an LNAV+V “glideslope” with a “W” GPS, though I don’t know of anybody based there who has a “W” GPS. I know of a bizjet which can fly that procedure but it probably has BARO VNAV (Collins avionics).

Last Edited by Peter at 26 Aug 11:26
Administrator
Shoreham EGKA, United Kingdom

I know of a bizjet which can fly that procedure but it probably has BARO VNAV (Collins avionics).

BARO-VNAV capability is rarely found on smaller bizjets. Mostly Falcon/Challenger and upwards and airliners can do it.

EDDS - Stuttgart

Of course it is. I recognised ADURI but couldn’t remember where from.

I can tell you it does indeed have a glide slope on the 430W. My logbook tells me I did it in May 2012 in IMC. I think it was just after all these things were activated.

London area

It also has the glideslope on the GTN – I flew it a few months back.

With the hills left and right, its not an approach I would be happy flying with the ADF.

EGEO

Peter, what is the name of the approach and the date it was last updated. I can look it up in the database and determine if it is coded for +V. Only the GPS approaches will have this coding, if it is a ground based navaid, the WAAS GPS won’t generate a +V.

KUZA, United States
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