RNP approaches

[Cbf]

Hardy,
When I perform a RNP approach (AR or LNAV/VNAV), I am very often high on the glide path when reaching DA.
To illustrate, I performed Bordeaux LFBD RNP 29 overlayed with the ILS. Both approaches glide path is 3° and TCH is 50Ft. Weather is set to STD conditions: 1013HP QNH & 15°C. Airport elevation is 166Ft. It means that vertical path for both approaches are nearly similar.
When I reach DA, this is what I read on instruments:

During VNKT RNP AR 02 & 20 (Glide path 2.80°), I have 4 white PAPI turning on final with vertical deviation path indicator centered. I know that those aproaches are not certified for the 744 but even with a B737 or A320, it would lead to a missed approach procedure.?
Am I missing something and why always high?

[Hardy Heinlin]

The geometry is based on the data in the nav database. I don't know why it's slightly higher than the ILS. Since this isn't a precision approach (DA 500 ft?) it may be a safety margin. Or the threshold for this approach is some feet behind the ILS threshold? I guess the PAPI is for the ILS only or for a visual approach.

[Bluestar]

The PAPI is for a visual approach.  When landing the pilot flying should tell the pilot monitoring which indicator is being used for the landing (PAPI or G/S).  On the above illustration the aircraft is only about a hundred feet high and easily corrected with 500 feet to go (personal opinion).

When landing I normally fly the RNAV to the ILS and then the ILS.  My normal sight picture on landing visually is three whites.  In another life I was told that landing short will get you fired so I will always be long .  I even fly the B737 Max sim this way (old habits are hard to break).

One of my CAF buddies has over 20,000 hours in the B747. He told me that he always used three whites because of the height of the B747 cockpit and how low the gear hangs down in relation to other aircraft. 

[Cbf]

I have tried KSFO RNP Z 28R overlayed ILS (3°/TCH 55' for both) with FAA database. LFBD was performed with Navigraph. The result is the same.
This problem is not specific to RNP APP. Non precision APP (LOC / VORD...) are also concerned.
I will:
- use V/S FD/AP mode instead of VNAV during final APP
- avoid RNP AR APP
- use LNAV minimums instead of LNAV/VNAV

[Hardy Heinlin]

Non precision APP (LOC / VORD...) are also concerned.

Non-precision approaches are usually slightly steeper than ILS glideslopes. Remember that VNAV uses barometric altitudes and these have a greater error range than ground based reference systems. So you need a slightly greater safety distance between the aircraft and the ground.

[Bluestar]

Non-precision approaches are usually slightly steeper than ILS glideslopes. Remember that VNAV uses barometric altitudes and these have a greater error range than ground based reference systems. So you need a slightly greater safety distance between the aircraft and the ground.

I went back and tried the approaches.  The sight picture at minimums was what I expected to see.  The landings were no issue at MGLW (300/85, on centerline, 1850 from threshold).

[Cbf]

Thanks for your answer Bluestar. Did you try VNKT RNP AR 02/20?

[Bluestar]

Thanks for your answer Bluestar. Did you try VNKT RNP AR 02/20?

Just now, it wasn't as bad as I had expected. I used the situation set up by Hardy.

[Cbf]

The procedure in Hardy's situ is RNP Z 02 AR. Airport T° is 7°C so STD +1. The Glide path for this approach is 2.80°. PAPI is set at 3.00°. At 300Ft (Minimum DH published), you should have 2 or 1 white on the PAPI rather than 3.
RNP Y 02 AR is more challenging. Same DH and a RF leg at 1NM from touchdown.

[Bluestar]

I will give it a try.  Currently flying SEQM-KMIA-ELLX today so may be tomorrow.

I normally fly with three whites because short was a CLM in a prior lifetime.  ROFL.

[MFarhadi]

I think I've seen something recently that might have effects on this topic:

During the latter part of geometric descent, in "non-idle path" or "Final x.xº" descent modes in reference to the Instructor Window > Analysis > Navigation, there's a gradual plateau once you get near the next point. The flattening of "DES target V/B" in regards to the "Final x.xº" is evident when the Vertical bearing value starts decreasing as you get near the MAPt or the runway threshold, although DES Target FIX ALT regarding the Runway threshold designator, and touchdown zone elevation plus threshold crossing height.