What's different about RNP approaches - just curious ...

[chairman 27]

I was wondering what was so different about RNP approaches to warrant this warning in the pinned topic - "The best advice is to use current charts, and if you see a particular approach marked RNP then “proceed with caution”. Either avoid selecting the approach in the sim, or if you do load it, be aware that the aircraft will likely not fly it correctly. It may track the horizontal profile - but it definitely will not automatically descend on the vertical profile the way an RNP-capable aircraft will." I have tried RNP approaches in the CRJ before realising that they were a problem, in fact that's what started me looking at this, and what's described there is exactly what happened.

 

So I looked at the difference between RNAV and RNP and as far as my civilian brain can tell it comes down to RNP is RNAV with higher accuracy requirements.  I can't work out how "you need more accurate gps gear" translates into "sorry eh, not gonna fly that vertical profile", obviously I'm not getting something, can anyone it sum it up ? 

 

Cheers

[Puuhbear 114]

Not knowing anything at all about this my conclusion would be:

 

At point x be at 100ft

but at point x + 50m in any direction at 100ft is a Mountain. 
 

being of by 50m would result in a crash. 
 

Obviously exaggerating here but that’s what my brain translates from „need better GPS accuracy“. 

[Crabby 28]

A Required Navigation Performance app is one that is only available if the equipment in the aircraft is capable of the RNP minimum performance requirement.  For example if you look at the RNP Z for 16R at KSEA you will se two minimums listed.  One has a DA of 770 ft MSL (355 RA).  To fly this minimum you must have an minimum RNP of 0.15.  The right side is still RNP but the RNP is 0.30, not as precise.  So the DA is 854 MSL (439 RA).  The RVR is also different.  The RNP numbers mean that the aircraft can maintain a course of +/- 0.15 nautical miles at least 95% of the time (RNP 0.15) and +/- 0.30 NM 95% of the time for RNP 30.  RNP approaches to NOT always get you as low as a standard GPS approach.  For example the RNAV LPV (Localizer Performance and Vertical guidance) for 16R will can take you down to 630 MSL (200 RA).  LPV approaches have the ability to project a Glide Path (NOT a glide slope).  To fly an LPV you need a WAAS receiver.

 

As a practical note, in real life, the CRJ cannot fly an RNP RNAV that has curved segments like the one on 16R at KSEA.  This is because the autopilot uses a fixed bank to make turns and cannot fly the lateral portion within RNP limits.  It also cannot do GPS LPV approaches because it does not have the WAAS gear.  Unless it has the coupled VNAV equipment, it cannot do an LNAV/VNAV approach either.  It can do an LNAV.  So for 16R at KSEA your typical CRJ can only do the LNAV and if the ceiling is below 920 MSL then the only option is ILS. 

[chairman 27]

Puuhbear I got the impression it's more about having equipment that will tell you when it's not where it should be - the RNP specs say it has to be on point 95% of the time AND it has to tell you when it's not. RNAV doesn't have that requirement, so I presume RNP is used to basically poke aircraft through smaller holes where early corrections if it's not 100% are important.

 

Crabby thanks for the tip about curved segments, that explains a whole lot about why mine was doing what it did. But I still don't see why (in the sim, and if the RNP approach has no curved segments) the CRJ can't follow a vertical profile. The RNP rules seem to be more about better reporting than greater accuracy, I couldn't see anything about you must be able to go up and down faster ?

[Crabby 28]

4 hours ago, chairman said:

Puuhbear I got the impression it's more about having equipment that will tell you when it's not where it should be - the RNP specs say it has to be on point 95% of the time AND it has to tell you when it's not. RNAV doesn't have that requirement, so I presume RNP is used to basically poke aircraft through smaller holes where early corrections if it's not 100% are important.

 

Crabby thanks for the tip about curved segments, that explains a whole lot about why mine was doing what it did. But I still don't see why (in the sim, and if the RNP approach has no curved segments) the CRJ can't follow a vertical profile. The RNP rules seem to be more about better reporting than greater accuracy, I couldn't see anything about you must be able to go up and down faster ?

It can if you have coupled RNAV.  Other wise you would fly, autopilot coupled, from the FAP to the DA(H) using vertical speed mode.  The chart will give you the FPM descent rate for given ground speeds.  You can calculate/estimate this also by dividing your GROUND speed in half and adding a zero.  So if you are approaching at 140 ground speed, that would be 700 fpm.  This assumes a 3.0 degree descent from the FAP.   What they don't want you doing, in real life, if flying a coupled fixed arc because the AP will be unable to quickly adjust for changing winds and you could end up wide or narrow of the intended path and that can be no bueno.  

[chairman 27]

Ah, gotcha.  Could you do it without coupled VNAV following the snowflake and the blue dot on the PFD ? I must try that this evening !

 

Cheers !

[Crabby 28]

8 hours ago, chairman said:

Ah, gotcha.  Could you do it without coupled VNAV following the snowflake and the blue dot on the PFD ? I must try that this evening !

 

Cheers !

Well, I don't know... would that be, as some say, the way it's done in real life or permissible.  I don't know.  I mean, I in the sim it doesn't matter other than what you want to do.  I have always just treated an RNAV approach with the CRJ in the sim like I would a localizer approach, like a step down to visual acquisition of the runway, except with the GPS flying the lateral nav.  This would be a good time for a real life CRJ pilot to chime in.  We have a guy in our VA that flies the A320 now but he flew the CRJ for Envoy.  I will ask him if he wouldn't mind chiming in.  I'll get back to you.