Remaining within approach RNP could put you low on approach?
 

Hi everyone,

If you were flying a standard RNAV approach with a requirement RNP of 0.3NM. Let’s say the ANP is less than RNP but for argument sake the ANP is 0.29NM. I can continue the approach as the ANP<RNP of 0.3NM.

This could mean though that the aircraft’s true position is 0.29NM behind where it says it is. If this were true then if you started descent at the FAF you would be descending early given the aircraft’s TRUE position 0.29NM behind where it says it is.

0.29NM is about 100 feet altitude. Surely this could put you dangerously low down the approach and at MDA??

Not really. The maximum ANP is obviously taken into account in the design of the procedure.

Those approaches also tend to have higher minima. In the 300-400ft range I think, so you'd still be fine.

This is taken into account with procedure design. Hence the RNP

Hi applecrumble

Every fix, whether based on NDB/DME, DME/VOR etc. or RNP has a 'fix tolerance area'. In the case of an RNP 0.3 fix, this is given by a 'cross-track tolerance' (XTT) equal to the RNP value of 0.3nm and an 'along track tolerance' (ATT) equal to 0.8 x the RNP value i.e. 0.24nm. This produces a rectangle 0.3nm either side of the fix and 0.24nm before and after the fix.

The procedure designer will assume the aircraft starts a descent at the earliest point of the fix tolerance area and so will start the protection of the next leg of the procedure from 0.24nm prior to the actual fix position.

Cheers
TeeS

Hi again applecrumble

In the attached screenshot, I have overlaid the fix tolerance areas and protection areas for an LNAV approach on runway 27 at EGBJ in the UK.

The fix tolerance area at the Intermediate Approach Fix (NIRMO) is RNP 1 so is 1nm on each side of the fix and 0.8nm before and after. The complete rectangle of the fix tolerance area is included in the obstacle assessment of the protection areas of the intermediate segment (shown in black).

The fix tolerance area at the Final Approach Fix has reduced to RNP 0.3 so 0.3nm each side of the fix and 0.24nm before and after. The complete area shown in orange is assessed for the required minimum obstacle clearance (246ft in the final approach segment) so even if you descend at the earliest fix tolerance then you are still protected.

Hope that helps.
TeeS
https://cimg5.ibsrv.net/gimg/pprune....e75984ccfb.png
.

Thank you so much for such a great answer. Just to make sure I’ve understood correctly, the area within the black “funnel” is the area used to asses obstacles and terrain. Then the fix tolerance 1X or 0.8X rectangle is used to determine worst case for tracking and descent with regards to terrain and obstacles.

Yep, that's about it apple.

The way I think about it is that the rectangle shows the area that, when your nav system tells you that you are over the waypoint, the geeks have calculated that there is a 95% probability of being inside of that box. Protection areas ('1/2 A/W' value), based on a formula and listed in PANS-OPS (example below), are then applied either side of the nominal track with the first half of the protection on each side given full protection for the segment and then decreasing from full protection to zero on the outer edge. Ignoring 'mountainous terrain', full protection for initial approach segment is 300m (984ft), intermediate approach segment is 150m (492ft) and final approach segment is 75m (246ft).

https://cimg0.ibsrv.net/gimg/pprune....63f2e032cd.png
Cheers
TeeS

0.3nm at 140kts is right around 9 seconds. So, if you're 100 below where you should be in 9 seconds, I should think that's pretty close enough.

That’s really good gen, thank you for that.
cheers,
Apple