DGCA CPL / ATPL Study Notes • Radio Navigation • Ch 16
✈ Chapter 16: RNAV
Area Navigation Systems & FMS
📋 Contents
1. Introduction & Benefits 2. RNAV Types & Levels 3. Simple 2D RNAV System 4. Phantom Station (VORTAC RNAV) 5. Level 4 RNAV — FMS 6. CDU & FMC Pages 7. IRS/FMC Operation 8. Kalman Filtering 9. RNAV Summary 10. Practice Questions (15 Q)1. Introduction & Benefits
Area Navigation (RNAV) is a method of navigation that permits aircraft operations on any desired track within the coverage of ground-based navigation signals or within the limits of self-contained systems. Aircraft are no longer required to fly directly over ground beacons.
Fig 16.1: RNAV chapter overview — navigation computer, CDU, HSI/CDI indicator system
✓ Benefits of RNAV
- Reduced distance, flight time, and fuel → lower costs
- Increased route capacity — more direct routes, parallel and bypass routes available
- Reduced vertical and horizontal separation criteria
- More flexible use of airspace — avoids congestion at beacon overhead positions
2. RNAV Types & Levels
📡 B-RNAV vs P-RNAV
| Type | Accuracy | Mandate |
|---|---|---|
| Basic RNAV (B-RNAV) | 5 NM on 95% of occasions | Mandatory for 30+ pax aircraft in Eurocontrol airspace |
| Precision RNAV (P-RNAV) | 1 NM on 95% of occasions | Required for P-RNAV terminal routes |
📡 RNAV Dimensions
| Level | Capability |
|---|---|
| 2D RNAV | Horizontal plane only (lateral) |
| 3D RNAV | Horizontal + vertical (altitude guidance) |
| 4D RNAV | Horizontal + vertical + timing (RTA — Required Time of Arrival) |
3. Simple 2D RNAV System
Simple 2D RNAV — Navigation Computer, CDU, HSI/CDI indicator
📡 Simple 2D RNAV Components
- Navigation Computer Unit: processes VOR/DME inputs to compute position
- Control and Display Unit (CDU): pilot enters waypoints; system computes tracking
- Indicator: CDI (Course Deviation Indicator) or HSI (Horizontal Situation Indicator) — both show deviation from computed track as if flying TO a waypoint
⚡ Simple 2D RNAV Limitations
- Position accuracy limited by VOR/DME coverage and accuracy
- Cannot operate beyond LOS of VOR/DME stations
- Only horizontal guidance (2D) — no VNAV capability
- Limited to pre-set waypoints; no automatic flight phase management
4. Phantom Station (VORTAC RNAV)
Fig 16.1: Phantom station — computed waypoint positioned at offset from real VOR/DME
📡 Phantom Station Operation
The RNAV computer calculates a phantom VOR/DME station (also called a waypoint) at a defined range and bearing from a real VOR/DME. The aircraft's CDI/HSI then indicates course and distance to this phantom station as if it were a real NAVAID.
- Pilot programs phantom station position as range & bearing from VOR/DME (rho/theta)
- Computer uses real VOR bearing (theta) and DME distance (rho) to calculate aircraft position
- Computes QDM and distance to phantom station (waypoint)
- Displays deviation from computed track on CDI/HSI
⚡ Exam: Range Read-out on 2D RNAV
Q: Aircraft is 15 NM from VOR, phantom station 30 NM from VOR. Range readout = 20 NM (from phantom, not VOR).
5. Level 4 RNAV — Flight Management System (FMS)
Level 4 FMS — combines FMC, CDU, IRS/GPS inputs for full 4D navigation
📡 FMS Components (Boeing 737-400 / Similar Aircraft)
| Component | Function |
|---|---|
| FMC (Flight Management Computer) | Brain of FMS; processes navigation and performance data; calculates 4D trajectory |
| CDU (Control & Display Unit) | Pilot interface; alphanumeric keyboard + function keys + Line Select Keys (LSKs); scratchpad input |
| IRS (Inertial Reference System) | Self-contained position + attitude + groundspeed; degrades with time (drift) |
| External references | DME/DME, VOR/DME, GPS — FMC uses these to update IRS position |
| Performance database | Aircraft performance data for climb, cruise, descent optimization |
| Navigation database | Waypoints, airways, SIDs, STARs, airports — updated every 28 days |
✓ FMS Capabilities (4D RNAV)
- LNAV (lateral navigation): autopilot follows computed lateral track
- VNAV (vertical navigation): computed climb/cruise/descent profiles
- RTA (Required Time of Arrival): timing function for 4D flight profiles
- Continuous optimization: adjusts speed/altitude for best fuel economy or ATC-assigned RTA
- Automatic DME/DME, VOR/DME or GPS position updates to IRS
6. CDU Pages
Fig 16.6: CDU layout — alphanumeric keyboard, function keys, line select keys (LSKs), scratchpad
CDU pages: IDENT → POS INIT → RTE (pre-flight sequence)
📡 Pre-flight Initialization Sequence
| Page | Purpose | Key Inputs |
|---|---|---|
| IDENT | Verify aircraft/engine/database identity | Check nav data currency; update if required |
| POS INIT | Initialize IRS position & heading | Gate position or lat/long for IRS alignment; magnetic heading from standby compass |
| RTE | Input the route | Departure/destination airports, SID, airways, waypoints, runway |
| CLB/CRZ/DES | Review computed flight profiles | Check speeds, altitudes, restrictions, time/fuel predictions |
⚡ CDU Waypoint Formats
- 5 alphanumerics: named waypoints, VOR idents, airports (e.g. LOREL, TRN, EGLL)
- SID/STAR: up to 7 alphanumerics (e.g. TURN05)
- Lat/Long: N5000.0E00527.0 format
- Place/Bearing/Distance (PBD): TRN250.0/76 (bearing from TRN, 76 NM)
- Bearing/Bearing (BB): GOW167.0/TRN090.5 (intersection of bearings)
CLB/CRZ/DES pages — VNAV profile management; shows ETA, fuel, restrictions, deviations
CDU cruise page — optimum/max altitude, step climb, wind entry, fuel prediction at destination
7. IRS/FMC Operation
IRS/FMC principle — triple IRS positions averaged, Kalman filtered, then updated by DME/DME or GPS
📡 Twin IRS, Twin FMC (Boeing 737)
- Left FMC: uses left IRS as primary + right IRS cross-check; updated by external references
- Right FMC: uses right IRS as primary + left IRS cross-check
- If one FMC fails: other FMC takes over all functions
- If both FMCs fail: IRS information fed directly to EFIS; no automatic performance management
📡 Triple IRS, Twin FMC (Larger Aircraft)
- All three IRS positions compared → gross error detection → averaged
- Averaged position compared with external reference (DME/DME, VOR/DME or GNSS)
- Above 84° latitude: de-couple IRS — secant of latitude approaches infinity → longitude calculation degrades rapidly
⚡ FMC Position Accuracy Over Time
IRS drift accumulates during flight → FMC most accurate at departure (IRS aligned), least accurate toward end of long flight if no external DME/GPS updates.
8. Kalman Filtering
📡 Kalman Filtering
A mathematical algorithm that optimally combines multiple sensor inputs (IRS position/velocity + external DME/GPS fixes) with their known error characteristics to produce the best estimate of current position and velocity. It:
- Weights each input inversely proportional to its error variance
- Smooths noisy GPS/DME updates against more stable (but drifting) IRS data
- Produces a position estimate with lower error than any individual input
- Continuously updates the position estimate as new sensor data arrives
9. RNAV Summary
| Parameter | Detail |
|---|---|
| Definition | Navigation on any desired track within NAVAID coverage or self-contained system limits |
| B-RNAV | ±5 NM, 95% of occasions; mandatory 30+ pax in Eurocontrol |
| P-RNAV | ±1 NM, 95% of occasions; terminal routes |
| 2D RNAV | Lateral only; uses VOR/DME rho/theta; phantom station |
| 3D RNAV | + vertical guidance (VNAV) |
| 4D RNAV | + timing (RTA) |
| FMS | FMC + CDU + IRS + Nav/Perf Database; full 4D capability |
| Nav database update | Every 28 days (AIRAC cycle) |
| CDU sequence | IDENT → POS INIT → RTE → CLB/CRZ/DES |
| IRS de-coupling | Above 84° latitude (longitude error growth) |
| Kalman filtering | Optimally combines IRS + external references |
10. Practice Questions
Q1. The accuracy required of a precision area navigation system is:
(a) 0.25 NM
(b) 2 NM
(c) 1 NM
(d) 0.5 NM
Answer: (c)
P-RNAV requires position accuracy of 1 NM on 95% of occasions. B-RNAV requires 5 NM.
P-RNAV requires position accuracy of 1 NM on 95% of occasions. B-RNAV requires 5 NM.
Q2. A basic 2D RNAV system will determine tracking information from:
(a) twin DME
(b) VOR/DME
(c) twin VOR
(d) any of the above
Answer: (b)
A basic 2D RNAV system uses VOR/DME (rho/theta — range and bearing) to define position and compute track to waypoints.
A basic 2D RNAV system uses VOR/DME (rho/theta — range and bearing) to define position and compute track to waypoints.
Q3. Aircraft on track WP1→WP2 (45 NM). Aircraft 20 NM from phantom station, phantom 270°/30 NM from VOR/DME. Aircraft 15 NM from VOR. Range read-out:
(a) 15 NM
(b) 20 NM
(c) 25 NM
(d) 30 NM
Answer: (b)
The DME display shows distance to the phantom station (waypoint), not the real VOR/DME. The aircraft is 20 NM from the phantom station → 20 NM.
The DME display shows distance to the phantom station (waypoint), not the real VOR/DME. The aircraft is 20 NM from the phantom station → 20 NM.
Q4. The sequence of displays accessed on FMC initialization is:
(a) POS INIT, IDENT, RTE
(b) IDENT, RTE, POS INIT
(c) IDENT, POS INIT, RTE
(d) POS INIT, RTE, IDENT
Answer: (c)
Correct pre-flight sequence: IDENT → POS INIT → RTE. IDENT first to verify database, then position initialization, then route entry.
Correct pre-flight sequence: IDENT → POS INIT → RTE. IDENT first to verify database, then position initialization, then route entry.
Q5. The IRS position can be updated:
(a) on the ground only
(b) at designated positions en route and on the ground
(c) on the ground and overhead VOR/DME
(d) at selected waypoints and on the ground
Answer: (a)
The IRS position can only be initialized on the ground. In flight, the FMC uses Kalman filtering with external references (DME/DME, GPS) to correct IRS drift, but the IRS itself is not manually updated in flight.
The IRS position can only be initialized on the ground. In flight, the FMC uses Kalman filtering with external references (DME/DME, GPS) to correct IRS drift, but the IRS itself is not manually updated in flight.
Q6. Best action to insert accurate IRS position using POS INIT page (Appendix A):
(a) 3R then 4R
(b) 2R then 4R
(c) 4R then 3R
(d) 3L then 4R
Answer: (a)
Select 3R to transfer gate position to scratchpad, then 4R to insert into IRS position field — this uses the most accurate database position.
Select 3R to transfer gate position to scratchpad, then 4R to insert into IRS position field — this uses the most accurate database position.
Q7. The position used by the FMC in the B737-400 is:
(a) an average of the two IRS positions
(b) an average of the two IRS positions smoothed by Kalman filtering
(c) taken from the selected IRS, smoothed by Kalman filtering and updated to external reference
(d) generated from external reference, updated by IRS as part of Kalman filtering
Answer: (c)
The FMC uses the position from the selected IRS, processed by Kalman filtering and updated by external references (DME/DME, VOR/DME, GPS).
The FMC uses the position from the selected IRS, processed by Kalman filtering and updated by external references (DME/DME, VOR/DME, GPS).
Q8. The FMC position will be at its most inaccurate:
(a) on take-off
(b) at TOC
(c) at TOD
(d) on final approach
Answer: (c)
IRS drift accumulates over time. The position is most inaccurate at Top of Descent (TOD) — the end of a long cruise, furthest from the last IRS alignment. FMC tries to correct with DME/GPS updates.
IRS drift accumulates over time. The position is most inaccurate at Top of Descent (TOD) — the end of a long cruise, furthest from the last IRS alignment. FMC tries to correct with DME/GPS updates.
Q9. Positions input to FMC using maximum 5 alphanumerics:
(a) SIDs & STARs, reporting points and airways
(b) Navigation facilities, reporting points and airways
(c) SIDs & STARs and latitude/longitude
(d) Latitude/longitude, reporting points and airways
Answer: (b)
Max 5 alphanumerics: navigation facilities (NAVAIDS), reporting points, and airways designators. SIDs/STARs use up to 7. Lat/long uses special format N5000.0E00527.0.
Max 5 alphanumerics: navigation facilities (NAVAIDS), reporting points, and airways designators. SIDs/STARs use up to 7. Lat/long uses special format N5000.0E00527.0.
Q10. The FMC navigation database can be accessed by pilots:
(a) to update the database
(b) to read information only
(c) to change between 28-day updates
(d) to change data for sector requirements
Answer: (b)
Pilots can read (interrogate) the database only. The database cannot be modified by pilots; it is updated by maintenance/data service providers every 28 days.
Pilots can read (interrogate) the database only. The database cannot be modified by pilots; it is updated by maintenance/data service providers every 28 days.
Q11. Above 84° latitude, a twin FMS/triple IRS system goes to de-coupled operations because:
(a) to prevent error messages as IRS longitudes show large differences
(b) to ease pilot workload
(c) to improve system accuracy
(d) because magnetic variation changes rapidly
Answer: (a)
Above 84° latitude, the secant of latitude increases rapidly. Longitude calculations from the two IRS systems show increasingly different values → de-coupled mode prevents false gross error messages.
Above 84° latitude, the secant of latitude increases rapidly. Longitude calculations from the two IRS systems show increasingly different values → de-coupled mode prevents false gross error messages.
Q12. Maximum range at which VOR bearing information is used by the B737-400 FMC:
(a) 160 NM
(b) 180 NM
(c) 200 NM
(d) 250 NM
Answer: (b)
The B737-400 FMC uses VOR bearing up to 180 NM from the station. Beyond this range, bearing accuracy is insufficient for Kalman filtering updates.
The B737-400 FMC uses VOR bearing up to 180 NM from the station. Beyond this range, bearing accuracy is insufficient for Kalman filtering updates.
Q13. Which of the following is NOT a waypoint definition format for the CDU:
(a) 5-letter alphanumeric
(b) Lat/Long
(c) range and bearing from NAVAID
(d) bearing from two NAVAIDs
Answer: (d)
The bearing/bearing (BB) format (e.g. GOW167.0/TRN090.5) IS a valid waypoint format. The format NOT available is bearing from two NAVAIDs as a simple format — the CDU uses only the BB format with specific syntax.
The bearing/bearing (BB) format (e.g. GOW167.0/TRN090.5) IS a valid waypoint format. The format NOT available is bearing from two NAVAIDs as a simple format — the CDU uses only the BB format with specific syntax.
Q14. Correct format to input position 50°N 005°27'E to the CDU:
(a) 5000.0N00527.0E
(b) N50E00527
(c) N5000.0E00527.0
(d) N5000E00527
Answer: (c)
CDU latitude/longitude format: N5000.0E00527.0 — hemisphere prefix, degrees and decimal minutes, with decimal point mandatory.
CDU latitude/longitude format: N5000.0E00527.0 — hemisphere prefix, degrees and decimal minutes, with decimal point mandatory.
Q15. The period of validity of the FMC navigation database is:
(a) 28 days
(b) 1 month
(c) determined by national authority (28 to 91 days)
(d) 91 days
Answer: (a)
The FMC navigation database is valid for 28 days (one AIRAC cycle). It must be updated every 28 days to remain current.
The FMC navigation database is valid for 28 days (one AIRAC cycle). It must be updated every 28 days to remain current.
DGCA CPL/ATPL Radio Navigation Study Notes
Chapter 16 — RNAV (Area Navigation Systems)
Capt Pankaj Pahil | www.ghostaviator.com
For personal study use only.
Chapter 16 — RNAV (Area Navigation Systems)
Capt Pankaj Pahil | www.ghostaviator.com
For personal study use only.