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.
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.
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?
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.
