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RNP Approach - additional question


Katja Mueller
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Hello!

I know, the CRJ is not able to fly an RNP approach. But what this mean in detail? If I look to approaches in europe, the most GPS approaches are declaird as "RNP" approaches but if you look to the vertical profile like in the attachement, there are 3 options:

 

"LPV" (very new, only in few new aircrafts)

"LNAV/VNAV" (the standard like in an A320)

"LNAV" (all other airctafts without special approach guidance but LNAV)

 

all this with different minimas.

So, I think the "LNAV"  only option means, you have laterial guidance via FMC and vertical is on your descursion (V/S for example)

 

Under the line, the CRJ should be able to fly the "LNAV only" version. Even wether the approach chart is defined as "RNP approach chart"

 

Please let me know if anything wrong here.

 

 

rnp.JPG

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Your general thought process is correct, i.e., that the CRJ cannot fly LNAV/VNAV approaches because it has no VNAV but it is supposed do LNAV only.

 

However, the issue with the CRJ and RNP approaches is that the CRJ cannot even handle the LNAV part. In particular, there a specific leg types that cannot be flow by the CRJ, e.g., curved parts. This is true for both the real plane as well as the Aerosoft one. As far as I know, this is not only a consequence of the FMC's ability to decode those segments but also of the required navigation performance.

 

With that said, note that you can easily fly all approaches that don't contain special leg types, i.e., that only consist of straight lines between waypoints. This is usually the case for simple approaches to airports that often also have an ILS. But for approaches through mountains or along other natural features, this usually doesn't apply. Furthermore, I cannot say whether it would be okay from a legal point of view to fly RNP approach with the CRJ even though they just consist of straight element.

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2 hours ago, SinusJayCee said:

curved parts

 

Excellent post!  My only statement is that curved parts are actually Radius to Fix which is I believe (strictly speaking) part of the RNAV Approach and not really part and partial RNP.  There are aircraft which have RNP but do not have Radius to Fix capability.

 

As much as a nuiance, the Dash 8 Q400 (made by the same company as the CRJ) has both RNP and RNAV Approach capability which includes Radiis to Fix.

 

Many thanks for the post!

 

 

 

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Some more info… The CRJ can fly most RNP approaches down to at least LNAV minima, provided it does not contain radius to fix (RF) legs as posted above. If weather conditions are ISA or better (basically just temperature of 15C or higher at sea level) you can also go down to LNAV/VNAV minima. 

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At my company we are flying RNP departures and approaches regularly with the CRJ900 since 2017. One example is EDDF, MARUN2W (RW25C) and RNP Z 25C. The departure consists of two RF legs, while the RNP Z has one RF turn to final. However, it's correct that the FMS is not approved for Aircrew Authorization Required (AR) approaches. I attached an excerpt of the FMS 4200 Operational Approvals.

 

I talked about this with JRBarrett a few days ago and he wanted forward it to the devs.

 

 

255635565_RNPCapability.thumb.png.dcd1255ecd03b261ea3f8be3b3132d52.png

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There are different types of curved segments in RNP approaches: One are the already mentioned radius-to-fix segments. As the name suggests, you fly in a constant radius to a given fix (usually a VOR). Radius-to-fix segments can also appear in RNAV arrivals. Here is a good example from LGKR:

936917920_Screenshotfrom2021-08-2609-30-55.thumb.png.3c1fbbfd7046aef1d9a728f33b4371f4.png

 

The other type of curved segment are arbitrarily shaped ones. I don't know a special name for them, but maybe they have one. As far as I know, those can only appear in RNP approaches. Here is one example from KDCA RNP approach to RW19, where the segments follow a river:

1829974512_Screenshotfrom2021-08-2609-36-00.thumb.png.bf0c6f7f48966706e6245ed75c200653.png

 

And another one from LPMA RNP Z approach to RW05, where the segments describe a circle but not wrt. a fix:

1736949526_Screenshotfrom2021-08-2609-37-05.thumb.png.9fafee4b5d40d0ca581e478871e62189.png

 

 

The CRJ is supposed to be able to fly the RNAV arrival into LGKR, because here we have a radius-to-fix segment with a constant radius to a single fix.

 

In theory, you also have a good chance that it flies the KDCA approach quite fine, because even though the segments are in fact curved, they roughly assemble straight lines between waypoints. However, it is likely that the FMC does not understand the approach description in the nav database and thus is not able to load this approach (or mess it up). In addition, note that the CRJ is not certified to fly those approaches.

 

Finally, it will certainly fail on the last approach to LPMA, because here you have segments with a varying radius that are not tied to any specific fix.

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vor 7 Minuten, KuntaKinte sagte:

At my company we are flying RNP departures and approaches regularly with the CRJ900 since 2017. One example is EDDF, MARUN2W (RW25C) and RNP Z 25C. The departure consists of two RF legs, while the RNP Z has one RF turn to final. However, it's correct that the FMS is not approved for Aircrew Authorization Required (AR) approaches. I attached an excerpt of the FMS 4200 Operational Approvals.

 

I talked about this with JRBarrett a few days ago and he wanted forward it to the devs.

 

 

255635565_RNPCapability.thumb.png.dcd1255ecd03b261ea3f8be3b3132d52.png

 

Thanks for the additional insights! For clarification, does this apply to "simple" RF legs only or also more complicated ones?

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I tried to fly a simple RNP stright-in approach in LOWL RWY26.

I tried to activate APP butten and there is a green GP indicatior visible in PFD. But the aircraft is not able to fly the VNAV segment until MAF. I think V/S is the only solution for descend on the glide path

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1 hour ago, Katja Mueller said:

I tried to fly a simple RNP stright-in approach in LOWL RWY26.

I tried to activate APP butten and there is a green GP indicatior visible in PFD. But the aircraft is not able to fly the VNAV segment until MAF. I think V/S is the only solution for descend on the glide path

 

APP mode is used only for ILS approaches.

 

You should fly this (Non-precision) approach with FMS as NAV source and with NAV mode and V/S. You should arrange descent according to the advisory VNAV (snowflake) and the vertical profile on the approach plate.

 

2 hours ago, SinusJayCee said:

 

Thanks for the additional insights! For clarification, does this apply to "simple" RF legs only or also more complicated ones?

 

AFAIK, approaches with complex RF legs like the RNP 19 into KDCA are developed e.g. for airports where limiting obstacles exist that have to be avoided laterally. For this reason a higher navigational accuracy is required, which can be achieved with RPN AR procedures. I assume you will find a corresponding remark on the approach plate stating "Special Aircraft & Crew Authorization Required".

 

RNP AR approaches require a Total System Error as low as +/- 0.1 NM on any segment of the approach procedure. As you can see from the Operational Approval above, the CRJ does not meet this requirement (lowest value for RNP approaches: 0.3).

 

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Vor 1 Stunde, KuntaKinte sagte:

AFAIK, approaches with complex RF legs like the RNP 19 into KDCA are developed e.g. for airports where limiting obstacles exist that have to be avoided laterally. For this reason a higher navigational accuracy is required, which can be achieved with RPN AR procedures. I assume you will find a corresponding remark on the approach plate stating "Special Aircraft & Crew Authorization Required".

 

RNP AR approaches require a Total System Error as low as +/- 0.1 NM on any segment of the approach procedure. As you can see from the Operational Approval above, the CRJ does not meet this requirement (lowest value for RNP approaches: 0.3).

Thanks for clarification! That is what I would have expected.

 

Regarding the approach into KDCA, the chart is saying:

Zitieren

1. AUTHORIZATION REQUIRED. 2. RF and GPS required. 3. For uncompensated Baro-VNAV systems, procedure not authorized below -9°C or above 54°C.

 

The LPMA chart is saying:

Zitieren

RNP AR Apch. RF required.

Baro-VNAV not authorized below +7.5°C.

 

The LGKR chart is stating nothing wrt. to that. The same applies to the RNP approach to EDDF you mentioned earlier, while the MARUN3W departure out of EDDF says:

Zitieren

1. RNP 1/A-RNP required.   2. RF required.   3. GPS required.

 

Finally, having a look at the LOWL charts, I only find a statement regarding temperature limitations, but nothing about the navigation performance.

 

So in conclusion, the CRJ is supposed to be able to fly RNP 1 procedures even when they have RF segments, but it is not able to do RNP AR approaches.

 

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  • CRJ Forum Moderator

There is also an STC which can give a CRJ the ability to fly an LPV approach with a GPS-derived vertical glide path.


I do not think this is common in the airlines, as the modification is rather expensive, requiring new GPS receivers and hardware and software modifications to the FMS. It is only applicable to the WAAS system used in the USA - not the SBAS system used in Europe. 
 

The three corporate CRJ-200s I work on have the LPV mod.

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