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Pay attention that this page is under construction.
Information inside can be read but shall not be taken as input for any
exam or definite information.
¶ Introduction
This article deals with Required Navigation Performance
(RNP) for approach operations (RNP APCH).
During this article, we will focus especially on the LNAV-only
approach type performed with the Airbus A320.
Please be aware that this article will not deal directly with the
management or handling of your aircraft.
For all questions regarding this aspect, you should refer to your
aircraft documentation. No such documentation is published at the moment
by IVAO.
\
¶ Theoretical aspects
¶ Lateral guidance
Like for any RNP approaches, the only sensor available for navigational
guidance is provided by a GNSS.
The procedure must be retrievable from the current AIRAC navigation
database to be flown.
This is a regulatory requirement as some segments are hard-coded in the
database with specific path connections between them (see more: leg
types) which
cannot be specified when manipulating your on-board navigation
management system.
This is the same reasoning with waypoints that may present themselves
with specific constraints: altitudes and/or speed ; which you may alter
from the certified approach.
During the approach, you will monitor the precision of your positioning
system whilst you will try to maintain the intended flightpath by
crosschecking your crosstrack distance to the segment.
Any deviation must be corrected without delay as the certification of
the procedure is valid for an error up to 0.3 NM.
Any total loss of positioning must be considered an emergency situation
whilst flying IMC. A climb to a safe altitude must be initiated
immediately whilst establishing a flight path clear of any obstacle:
this must be briefed before the approach.
¶ Vertical guidance
Officially, there is no medium available to serve as a vertical guidance
during a LNAV-only approach.
Like for any non-precision conventional approaches, the management of
the vertical profile on the final approach segment is based on
dead-reckoning descent.
As you will be flying toward established GNSS-defined waypoints during
the final approach, you will always have the information of distance to
the waypoint. This distance is used by procedure designers to establish
continuous descent profile by crosschecking the altitude versus the
distance.
Depending on the level of sophistication of your aircraft, your
navigation management system may be able to produce an instrumental
descent path deviation indicator such as the one you would find during
an ILS approach with the glideslope indicator.
Such information must be crosscheck with the charts depicting the
vertical profile in a table indicating the altitude versus the distance.
The latter has regulatory precedence.
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¶ Preparatory work
In this article, we will perform the RNAV (GNSS) RWY26 approach at
Gaborone Botswana FBSK/GBE.
/FBSK_RNP26_Extract_Chart.png)
From studying
this chart, you can derive the following information:
- Altitude versus distance to runway threshold waypoint "RW26" table
- The final descent begins at 5100 feet on a 3.00-degree slope at
SK404 - The descent rate versus the ground speed as a recommendation
- The minimums for a CAT-C LNAV approach: MDA 3620 feet /
Visibility 1600 meters
Mind the indication of the missed approach point (MAP) which differs
between the LNAV/VNAV approach and the LNAV approach: the MAP is
overhead RW26.
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¶ Flying the approach
¶ In-flight approach preparation
The following subtitles do not include the full Airbus procedure for
approach preparation. The current IVAO documentation does not offer any
aircraft-specific standard operating procedures manuals.
You may refer to the manual of your aircraft add-on.
¶ FPLN page
Crosscheck the loaded procedures with the corresponding chart:
- Track with a tolerance of 1°
- Distance with a tolerance of 0.1 NM
- Glidepath angle with a tolerance of 0.01°
- All applicable altitude and speed constraints are inserted
¶ PROG page
As part of your approach preparation, check the PROG page:
/Perform_LNAV_APP_-_PROG_page_prep.png)
Check the following item:
- Make sure the waypoint you have set is corresponding to the waypoint
of the altitude-distance crosscheck table - If your MCDU does not update the required precision automatically to
0.3 NM, make sure you set it manually. - As per Airbus procedures, make sure you are flying in GPS Primary
/ Accuracy High
When the charts stipulate a missed approach point set to the threshold
of the runway, Airbus has the following naming convention: XXXXyyy
- XXXX is the ICAO code of the airport
- yyy is the runway identifier
Therefore, for this approach, RW26 becomes FBSK26.
¶ Briefing
The key point to add to your briefing when dealing with any RNP
approaches is the loss of GPS during approach.
You must deal with both case:
- if you are VMC: you may continue visually to land if the ATC
clears you to do so - if you are IMC: you must determine a safe trajectory to reach
the minimum safety altitude as applicable in your area. The
assistance of the ATC in a radar environment can be sought.
¶ Initial and intermediate segments
¶ Selection of guidance modes
The following table summarize the two certified modes to perform an LNAV
approach:
+-----------------+-----------------+-----------------+-----------------+
| Activation | Lateral mode | Vertical mode | Disconnection |
+=================+=================+=================+=================+
| When the |
| the platform | | with the runway | |
| altitude | FINAL APP | threshold, | |
| inbound the | | flight | |
| final approach |
| point, the APPR | | be kept on | |
| push-button can | | until landing | |
| be switched on | | | |
| on the FCU | | | |
+-----------------+-----------------+-----------------+-----------------+
| As per SOPs, | NAV | FPA | Flight |
| NAV mode is | | | directors must |
| engaged. FPA | | | be switched off |
| selection | | | below minimums |
| should be made | | | |
| 0.3 NM before | | | |
| the final | | | |
| approach point | | | |
| to account for | | | |
| inertia | | | |
+-----------------+-----------------+-----------------+-----------------+
Using the FINAL APP mode, you must make sure that the outside air
temperature is above the minimum temperature for which the approach has
been certified.
Such precautions are not applicable when flying the approach in selected
modes (NAV/FPA).
¶ Aircraft configuration
As per Airbus philosophy, if no vertical guidance is available
(i.e.: if you do not use FINAL APP mode or FINAL APP mode is not
available), you shall perform a stabilized approach:
- Speed stabilized at F-speed
- Aircraft configured to fly such speed: Flaps 2/Gear up or
Flaps 3/Gear down
The height of the aircraft at the final descent point will determine the
appropriate configuration.
If you are flying the approach using the FINAL APP mode, you may fly the
approach using the standard decelerated approach technique.
¶ Final segment
¶ Establishing the final descent
The following steps are recommended to establish the aircraft on the
correct vertical profile:
- Switch the FCU selection from HDG/VS to TRK/FPA
- 1.0 NM before the final descent point, pre-select the correct
descent angle on the FCU - 0.3 NM before the final descent point, pull FPA to establish the
aircraft on the vertical profile
Remember that the above steps are not applicable to approaches being
performed in FINAL APP mode as the approach is fully managed.
¶ Crosscheck the final descent
Remember that whatever may be the final approach guidance (selected or
managed), you must crosscheck the altitude of the aircraft versus the
position of the aircraft and compare it to the published vertical
profile.
¶ Final cockpit state
The final cockpit state for a selected-guidance (NAV/FPA) flown LNAV
approach is:
/Perform_LNAV_APP_-_Final_cockpit_status.png)
Remember that we do not deal with LNAV approaches flown with managed
guidance (FINAL APP). More info can be found regarding this guidance by
referring to the corresponding article: Perform a LNAV/VNAV approach
(A320) .
¶
See also
- None
¶
Reference
- None
¶
Author
- VID 192622 - Creation
- VID 200696 - Standardization & Wiki integration