Flying a nonprecision approach - is checking the altitudes on the way down after the FAF an assumption in the procedure design?

The GP or Advisory GP comes on when the FAF is the active waypoint. On a full procedure, this is when navigating on the last leg ending at the FAF. For VTF, the last leg extends 30 NM outside the FAF and the GP or advisory GP is on when you are essentially aligned with the extended final approach course.

NCYankee wrote:

Outside of the FAF, particularly far outside, a step down may be required for separation from traffic for other airports and one must not use the advisory GP

Thank you! So will a GTN750 display the advisory GS outside the FAF?

NPA for GPS in the US has a MOC of 250 feet inside the FAF. On the intermediate segment, the MOC is 500 feet. An advisory GP will clear a step down inside the FAF on a nominal temperature/pressure day. On warm days, the GS will be below the step down MSL altitude and on cold days will be above. The GPS advisory GP does not move in space, it is the MSL indication that moves. The obstacles don’t move in space. Outside of the FAF, particularly far outside, a step down may be required for separation from traffic for other airports and one must not use the advisory GP or even an ILS GS/LPV GS and they must respect the step down MSL altitude or risk a pilot deviation from ATC.

Although approach designers do not consider obstacles below the MDA on a NPA straight in approach, so it may not be clear on an extension of the advisory glidepath below the MDA, the flight test will determine if obstacles require the flightpath to be adjusted (from one deviation fly up indication) to avoid obstacles to the runway threshold. If this is determined, the VDA is not charted and a warning note is added to the chart.“Visual Segment – Obstacles”.

In the US, NPA straight in approaches flown using CDFA result in higher actual minimums in terms of initiating the missed approach so as to not bust the MDA and higher visibility minimums than the published minimums for the procedure. In the US, there are about 5000+ airports served with instrument approaches and many do not have clear final approach segments and don’t have approach lights. These are usually the non towered airports and the ones not served by the Airlines. Towered airports number around 500.

Peter wrote:

Obviously if you fly a GS of 100kt and a VS of -797 fpm then you will be exactly on the “glideslope” …

That’s only true in a standard atmosphere if the vertical speed is based on barometric pressure.

Peter wrote:

Furthermore, there is no way you will get anywhere near those masts laterally because you have GPS guidance. The distance is about 500m.

Peter showed me the approach chart. It is a RNAV(GNSS) approach with LNAV minima. For that it is assumed that the total navigation accuracy is such that the aircraft will be within 0.3 NM of the final approach centreline at least 95% of the time. This figure includes piloting accuracy, not just guidance accuracy.

The total width of the approach area is 1.5*0.3+0.5=0.95 NM on each side of the centreline. The 1.5 factor is to get the 95% up to 99.7% – three standard deviations – and the 0.5 addition is a “buffer value” to cater for the remaining 0.3%. Half of the distance on each side of the centreline (the primary area), e.g. ≈0.48 NM on each side, has full minimum obstacle clearance of 294 ft. The outer half (the secondary area) has progressively less obstacle clearance reducing to zero at the full distance.

So the masts you mention are considered in the obstacle clearance calculations.

Antonio wrote:

I see…so you must be visual before MDA, which you should anyway

Nope. You shouldn’t descend below MDA until you have the required visual references. You need to be visual by MAP.

Airborne_Again wrote:

You have terrain clearance down too the MDA, of course, but not below that if you follow the nominal glidepath.

I see…so you must be visual before MDA, which you should anyway.Airborne_Again wrote:

Runway 07 had an NDB approach with an advisory ILS! The nominal ILS glideslope did clear a hill in the approach, but not if you were half deflection below, so NDB minima applied.

Wow! That is really interesting! I assume it is no longer active?

I would be more worried about advisory guidance outside of the FAF, where clearance may or may not be good, but I do not know if that guidance is active on GPS approaches or it only activates inside the FAF…do you know?

Peter wrote:

The distance is about 500m

THen the MOC design criteria should be +250ft.
It is an interesting study that you did…I guess you could do it for every approach procedure, but is it not what onboard navigation guidance does anyway?

There is also an additional vertical criteria that descent path should be no more than 3.7 deg or something like that (1000fpm @ 150 kts gs) or else you need especial procedures, so this may also drive some of the step-down altitudes. Perhaps this had an influence in the Shoreham design?

Not that you would be doing that, but if you were to design an IAP, then 250ft MOC should be legal between FAF and MAP if the descent path criteria is met.