RNAV (RNP) Approaches
RNP stands for Required Navigation Performance which means that specific navigational requirements must be met prior to and during the execution of the approach.
There are two types of RNAV (RNP) approaches:
(i) RNAV (RNP) approach; and,
(ii) RNAV (RNP) AR approach.
Both approaches are similar to a RNAV (GNSS) approach, however, a RNAV (RNP) approach, through the use of various sensors and equipment, achieves far greater accuracy through the use of Performance Based Navigation (PBN), and can therefore be flown to a DA rather than a MDA.
RNP/ANP - How It Works
An RNAV (RNP) approach compares the position that the aircraft should be in with the actual position of the aircraft. If this value exceeds the prescribed distance (RNP exceeds ANP), the approach must be aborted. The use of RNP/ANP enables greater accuracy in determining the position of the aircraft.
RNP/ANP Alerts
If an anomaly occurs between RNP and ANP one of two RNP alerts will be generated:
(i) VERIFY POSITION - displayed in the scratchpad of the CDU; or,
(ii) UNABLE REQD NAV PERF-RNP - displayed on the Navigation Display (ND) (if EFIS is set to MAP).
It should be noted that different versions of CDU software will generate different alerts. This is because newer software takes into account advances in PBN. To determine which software version is in use, press IDENT from the CDU main page (LSK1L) and check OP PROGRAM. ProSim-AR uses U10-8a.
The variables for RNP/ANP can be viewed in the CDU in the POS REF page (page 3), the LEGS page when a route is active, and also on the Navigation Display (ND).
A second type of RNP approach is the RNAV (RNP) AR approach. This approach enables you to have curved flight paths into airports surrounded by terrain and other obstacles. Hence why special aircraft and aircrew authorization (AR) is required for these approaches. Other than AR and additional flight crew training, the approach is identical to the RNAV (RNP) approach.
Advantages of RNAV and RNAV (RNP) Approaches
The benefit of using an RNAV approach over a traditional step-down approach is that the aircraft can maintain a constant angle (Continuous Descent Final Approach (CDFA)) until reaching minima. This has positive benefits to fuel savings, engine life, passenger comfort, situational awareness, and also lowers flight crew stress (no step-downs to be followed). Additionally, it also minimises Flight Into Terrain (CFIT) events.
A further advantage is that the minimas for an RNAV approach are more flexible than those published for a standard Non-Precision Approach not using RNAV. RNAV approach charts have differing descent minima depending upon the type of RNAV approach.
For example, if flying a RNAV (RNP) approach the MDA is replaced by a DH. This enables a lower altitude to be flown prior to a mandatory go-around if the runway threshold is not in sight. The reason that a RNAV (RNP) approach has a DH rather than a MDA (and its resulting lower altitude constraint) is the far greater accuracy achieved through the use of Performance Based Navigation (PBN).
Approach To Land Using RNAV
The following addresses the basics of what is required to execute an RNAV approach.
Prior to beginning the approach, the crew must brief for the approach and complete ant required preparation. This includes, but is not limited to, the following items:
(i) Equipment must be operational prior to starting the approach;
(ii) Selection of the approach procedure, normally without modifications from the aircraft's navigation database (CDU);
(iii) For airplanes without Navigation Performance Scales (NPS), the map display should be set to the 10 NM or less range. This is to monitor path tracking during the final approach Segment and provide greater navigational awareness;
(iv) For airplanes with NPS, the map display range may be set to whatever distance is desired;
(v) TERR display must be selected on either the Captain or First Officer side of the ND;
(vi) For airplanes without Navigation Performance Scales (NPS), the RNP progress page on the CDU should be displayed. For airplanes equipped with NPS, selection of the CDU page is at the crew's discretion;
(vii) The navigation radios must be set according to the type of approach; and,
(viii) There must be no alerts generated (UNABLE REQD NAV PERF and/or VERIFY POSITION).
In addition to the above, airline Standard Operational Procedures (SOPs) may require additional caveats. For example, the setting of range rings on the ND to provide enhanced situational awareness at specific points (range rings can be set on the FIX page in the CDU).
Important Points:
Select the approach procedure from the arrivals page of the CDU and cross-check this data with that published on the approach chart, especially the altitude constraints and the Glide Path (GP).
If the Initial Approach Fix (IAF) in the CDU has an ‘at or above’ altitude restriction, this may be changed to an ‘at’ altitude restriction that uses the same altitude. Speed modifications (using speed intervention) are allowed as long as the maximum published speed is not exceeded. No other lateral or vertical modifications should be made at or after the IAF.
Beginning the Approach
Select LNAV no later than the IAF. If on radar vectors, select LNAV when established on an intercept heading to the final approach course. VNAV PTH must be engaged and annotated in the Flight Mode Annunciator (FMA) for all segments that contain a Glide Path (GP) angle, as shown on the LEGS page, and must be selected no later than the Final Approach Fix (FAF) or published glide path intercept point.
Speed Intervention (INTV), if desired, can be used prior to the GP. Good airmanship directs that the next lower altitude constraint is dialled into the MCP altitude window as the aircraft passes through the previous constraint. When 300 feet below the Missed Approach Altitude (MAA) re-set the altitude window in the MCP to the MAA.
Final Approach using RNAV
When initiating descent on the final approach path (the GP), select landing flaps, slow to final approach speed, and do the landing checklist. Speed limits published on the approach chart must be complied with to enable adequate bank angle margins.
At minima, or as directed by the airline's SOP, the autopilot followed by the autothrottle is disconnected and a visual 'hands on' approach made to the runway threshold.
Once established on final approach, a RNAV approach is flown like any other approach.
Final Call
The Boeing aircraft is capable of several types of Non-Precision Approaches, however, outside the use of ILS and possibly IAN, the RNAV approach enables an accurate glide path to be followed to minima. While it's true that the differing types of RNAV approaches can be confusing due to their close relationship, the approach is straightforward to fly.
This short article is but a primer to understanding an RNAV approach. Further information can be found in the FCTM, FCOM and airlines SOP.
In my next article we will look some of the possible 'gotchas' that can occur when using VNAV.
References
Flight Crew Training Manual (FCTM), Flight Crew Operations Manual (FCOM) and airline SOP.
Acronyms and Glossary
Annunciator – Often called a korry, it is a light that illuminates when a specific condition is met
ANP - Actual Navigation Position
APV - Approach Procedure with Vertical Guidance
CFIT - Continuous Flight Into Terrain
DME – Distance Measuring Equipment
FAF - Final Approach Fix
FCOM - Flight Crew Operations Manual (Boeing)
FCTM - Flight Crew Training Manual (Boeing)
FMA - Flight Mode Annunciator
FMC – Flight Management Computer
FMS – Flight Management System
Gotcha- An unfavorable feature of a product or item that has not been fully disclosed or is not obvious.
GPS – Global Positioning System
GNSS - Global Navigation Satellite System
IAF - Initial Approach Fix
Korry - See annunciator
LNAV – Lateral Navigation
LPV - Localizer Performance with Vertical Guidance
MAA - Missed Approach Altitude
MCP – Mode Control Panel
ND – Navigation Display
NPA - Non Precision Approach
PBN - Performance Based Navigation
RNAV – Area Navigation
RNP - Required Navigation Performance
SOP - Airline Standard Operational Procedure. A manual that provides additional information to the FCTM and FCOM
SBAS - Satellite based augmentation systems. In the U.S. called WAAS and Europe called EGNOS.
VNAV – Vertical Navigation
VNAV PTH – Vertical Navigation Path
VNAV SPD – Vertical Navigation Speed
VOR – VHF Omni Directional Radio Range
