My previous post provided of overview on RNAV and RNP navigation. This article will explain what a RNAV approach is, provide incite to the operational requirements for a RNAV approach, and discuss specifically the RNAV (RNP) approach. I will also briefly discuss Approach Procedures and Vertical Guidance (APV) and RNP/ANP values.
The operational criteria for RNAV approaches is complicated and not easy to explain. There are a number of RNAV approaches (often different for differing areas of the globe) and each is defined by the accuracy of the equipment used in the execution of the approach. As such, this article is not all encompassing and I encourage you to read other technical articles available on this website and elsewhere.
LEFT: RNAV 07 L - one of several RNAV approach charts for Los Angeles International Airport (LAX). The most important aspect of an RNAV approach is that it is a Non-Precision Approach (NPA). Note the word GPS is written in the title of the approach plate.
RNAV Approaches - Background Information
The Global Positioning System (GPS) is the brand name owned by the US military. Initially all RNAV approaches were GPS orientated, however, in recent years this has been updated to include Global Navigation Satellite System (GNSS) applications. GNSS applications are not owned (or controlled) by the US military. As such, RNAV approach charts often use the word GPS/GNSS interchangeably.
What is an RNAV Approach
The definition for an RNAV approach is 'an instrument approach procedure that relies on the aircraft's area navigation equipment for navigational purposes'. In other words, a RNAV approach is any non ILS instrument-style approach that does not require the use of terrestrial navigation aids such as VOR, NDB, DME, etc.
Rather than obtain navigational information directly from land-based navigational applications, the aids for the approach is obtained from a published route contained within the aircraft's Flight Management System (FMS) and accessible to the crew by the Control Display Unit (CDU). The approach broardly uses signals that are beamed from navigational satellites orbiting the Earth to determine the position of the aircraft in relation to the information presented from the database.
All Boeing Flight Management Systems (FMS) are RNAV compliant and have the ability to execute a RNAV approach.
A RNAV approach is classified as a Non-Precision Approach (NPA).
Non-Precision Approaches (NPA)
Before writing further, a very brief overview of Non-Precision Approaches is warranted.
There are three ways to execute a Non-Precision Approach.
(i) IAN (integrated Approach Navigation). IAN is a airline customer option and makes a NPA similar to an ILS approach. A separate article has been written that addresses IAN.
(ii) Vertical Speed (V/S). V/S is not normally used when flying a RNAV approach that uses positional information from the aircraft's database. However, V/S can be used for other Non-Precision Approaches.
(iii) VNAV (Vertical Navigation). VNAV is the preferred method to execute a NPA provided the approach is part of the FMS database.
(iv) LNAV (Lateral Navigation). LNAV is mandatory for all approaches that are GPS/GNSS/RNP based.
RNAV Approach Types
The following are RNAV approaches:
(i) RNAV (GPS) approach;
(ii) RVAV (RNP) approach;
(iii) RVAV (RNP) AR approach; and,
(iv) RNAV (GNSS) approach.
The RNAV (GNSS) approach covers an additional three possible types of approach with each identified by a different minima. The approaches are:
(i) RNAV (GNSS) LNAV;
(ii) APV Baro VNAV approach;
(iii) APV SBAS approach.
It's easy to become confused by the various types of RNAV approaches, however, the actual flying of a RNAV approach does not differ greatly between each approach type. The main difference lies in the level of accuracy applied to the approach and the methods used to enable this accuracy that determines what minima can be flown.
Approach Procedures with Vertical Guidance (APV)
APV refers to any approaches which are designed to provide vertical guidance to a Decision Height (DH). An APV approach is charactertised by a constant descent flight path, a stable airspeed, and a stable rate of descent. They rely upon Performance Based Navigation. For an overview of PBN please refer to my earlier post.
The difference between the two APV approaches is that the APV Baro VNAV approach uses barometric altitude information and data from the FMS database to compute vertical guidance. in contrast the APV SBAS approach uses satellite based augmentation systems, such as WAAS in the US and Canada and EGNOS in Europe, to determine lateral and vertical guidance.
I will now discuss RNAV approaches in general and specifically, the RNAV (RNP) approach.
Flying The RNAV (GNSS) Approach
The RNAV (GNSS) approach is designed to be flown with the autopilot engaged. The recommended roll mode is LNAV or HDG SEL. The preferred method for pitch is VNAV. If LNAV and VNAV are engaged, the aircraft will fly the lateral and vertical path as determined by the FMS database; the route is displayed in the LEGS page of the CDU.
The aircraft uses the FMS database to determine its lateral and vertical path. As such, it is very important that the RAW data published in the navigational database is not altered by the flight crew. Furthermore, the data presented in the CDU should be cross-checked to ensure it is identical to that presented on the RNAV approach chart.
As discussed previously, a RNAV (GNSS) approach is classified as a Non-Precision Approach. Therefore, minima is at the Minimum Descent Altitude (MDA). It is good airmanship to add +50 feet to the MDA to reduce the chance of descending through the MDA. If a RNAV (RNP) or APV approach is being flown, the minima changes from a MDA to a Decision Height (DH). Whatever the requirement, the minima will be annotated on the approach chart.
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, with 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.
LEFT: LIDO chart (Lufthansa Systems) depicting the RNAV (RNP) 01 approach into BNE-YBBN (Brisbane Australia). Note that this chart has a Decision Altitude (DA) rather than a Minimum Descent Altitude (MDA). Chart courtesy of NaviGraph (click to enlarge).
RNP/ANP - How It Works
A RNAV (RNP) approach uses RNP/ANP which is the comparison between the Required Navigation Position (RNP) and the Actual Navigation Position (ANP). If the data becomes erroneous such as from the loss of a GPS signal, the ANP value will exceed the RNP value. The use of RNP/ANP enables greater accuracy in determining the position of the aircraft.
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) on the EFIS 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 a 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 a 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 a RNAV approach.
Prior to beginning the approach, the crew must brief the approach and complete needed preparations. These include, but are not limited to, the following items, which may be included in an approach review card or other type of briefing aid:
(i) Equipment that must be operational prior to starting the approach;
(ii) Selection of the approach procedure, normally without modifications from the aircraft's navigation database;
(iii) For airplanes without Navigation Performance Scales (NPS), one pilot should have the map display in the 10 NM or less range. This is to monitor path tracking during the final approach Segment;
(iv) For airplanes with NPS, the map display range may be set as the crew desires;
(v) TERR display selected on at least the Captain or First Officer side of the ND;
(vi) The RNP progress page displayed on the CDU (as needed). 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) If a RNAV (RNP) approach is being executed, ensure that there is no UNABLE REQD NAV PERF - RNP alert displayed before starting the approach.
In addition to the above, airline Standard Operational Procedures (SOPs) may require additional caveats, such as range rings to be set up on the ND to provide enhanced situational awareness (CDU FIX page).
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) has an ‘at or above’ altitude restriction, it may be changed to an ‘at’ altitude restriction using the same altitude. Speed modifications 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.
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.
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