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CDI and RNP Values in the Approach NAV Database

Sep 20, 2018
By: Bob Bruce, Pilot Instructor

Image 1

Image 1

Until recently, Course Deviation Indicator (CDI) and Required Navigation Performance (RNP) values in the Flight Management System (FMS) Navigation Database were only in Authorization Required (AR) Area Navigation (RNAV) procedures.

  • CDI values represent the distance off course, referred to as crosstrack (XTK), when the CDI is displaced two dots.
  • RNP is the required navigation accuracy computed by the FMS.

Airspace planners are now coding many non-AR RNAV procedures with CDI and RNP values in the Navigation Database.

Navigation Database values have a higher priority than default values and are used by the FMS in lieu of default when available. As approaches update, you will find approach transition legs containing imbedded values in the Navigation Database. In the UA Satellite-Based Augmentation System (SBAS)-FMS with Software Control Number (SCN) 100X/110X, both CDI scaling and RNP values are imbedded. Legs without an imbedded CDI and RNP value display as the default value for the phase of flight.

 

Image 2

Image 2

As shown in Image 2, these procedures will have the CDI scaling and RNP values displayed with symbology of (R) for the CDI and (P) for the RNP. If the Navigation Database identifies CDI and RNP for a specific leg, the CDI (two dots) will equal the RNP value, so the symbol (R) refers to RNP. The symbol (P) refers to procedure. If the approach leg code is to 1.00, upon entering that leg the CDI full scale deflection will equal 1.00nm.

 

Image 3

Image 3

For approaches with a Level of Service (LOS), when the approach is active, the scaling reverts to default value of (A) and the Horizontal (HP) and Vertical (VP) Protection Level calculations display, with the Alert levels displayed upon passing the Final Approach Fix (Image 3).

 

Image 4

Image 4

In earlier FMS models with SCN 90X/80X, only RNP values are imbedded.

Starting with the approach transition legs the pilot will see the RNP (P) 1.00 for the terminal phase of flight. The CDI scaling is default (T) for terminal (Image 4).

 

Image 5

Image 5

When approach is active the CDI scaling will be default (A) approach, but the RNP will be (P), and for the leg outside of the FAF it is coded 1.00 (Image 5) . For approaches without imbedded RNP the FMS will use default.

 

Image 6

Image 6

Upon passing the FAF, the imbedded RNP will be set to 0.30 for the approach (Image 6). An RNP value of 0.30 is only required from the Final Approach Fix (FAF) to the Missed Approach Point (MAP).

 

Image 7

Image 7

Approaches with an LOS of LNAV or LP are no different.

 

Image 8

Image 8

On the transition legs the CDI and RNP values are displayed as (R) and (P) – as shown in image 8.

 

Image 9

Image 9

When the approach is active the scaling reverts to default value of (A) and the Horizontal (HP) Protection. Because this LOS uses a baro-VNAV glidepath, only dash lines are presented for the VP (Image 9).

 

Image 10

Image 10

The transition to imbedding CDI and RNP in the NAV database is worldwide.

Notice in image 10 under the LNAV minima, the acronym ‘CDFA’ – this stands for Constant Descent Flight Angle. With the use of the CDFA the pilot may choose a Decision Altitude (DA) instead of a Minimum Descent Altitude (MDA). Many countries are allowing the pilot, whose aircraft has no VNAV display, upon crossing the Final Approach Fix (FAF) to descend at a constant rate using the Ground Speed/Rate of Descent chart and use the DA.

 

Although the chart shows LNAV/VNAV which should produce a LOS of LNAV/VNAV, for many of the RNAV approaches in Europe a GEO (GPS) glideslope is not approved. Therefore, UA has removed the LNAV/VNAV LOS option from the NAV database on those approaches and only LNAV, using a baro-VNAV, will be available for annunciation. However, for those aircraft certified, a baro-glideslope may be used by the aircrew to fly to LNAV/VNAV DA minima (images 11-13).

Image 13

Image 13

Image 12

Image 12

Image 11

Image 11

 

Image 14

Image 14

As the ICAO Performance Based Navigation (PBN) is implemented, the traditional RNAV approaches will be renamed as RNP on the chart (image 14).

 

The current FMS software does not recognize the approach name RNP, so these approaches are renamed as RNV when the NAV database is processed by UA (images 15-17).

Image 17

Image 17

Image 16

Image 16

Image 15

Image 15

 

Image 18

Image 18

RNAV (RNP AR) procedures will and have had the CDI and RNP values imbedded in the NAV database (image 18). Because these procedures do not have an LOS, the CDI and RNP values will show up in the final approach segment and often in the missed approach segment.

 

Image 19

Image 19

These procedures normally have the CDI scaling and RNP values in the NAV database and will display symbology showing an (R) and (P) (image 19).

 

In some parts of the world, RNAV (GPS) is replaced with RNAV (RNP). These approaches do not require authorization, but only that the avionics equipment is able to fly the procedure. If the CDI and RNP are identified in the NAV database for a specific leg, the CDI (two dots) will equal the RNP value. If the approach leg is coded to 0.10, upon entering that leg the CDI full scale deflection will equal 0.10nm and retain linear scaling.

More Information

For additional information, please contact UA Training.

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