Area Navigation (RNAV)

Area Navigation (RNAV)

• RNAV allows pilots to fly directly to a destination without needing to use VOR or ground-based facilities.

RNAV Background

• RNAV is traditionally an instrument flight rules (IFR) navigation method.
• Permits flights on any course within a network of navigation beacons for land and inertial navigation systems (INS) for oceanic operations.
• It has evolved with satellite navigation, leading to performance based navigation (PBN).
• RNAV is a navigation technique within PBN, alongside Required Navigation Performance (RNP).
• Navigation involves the route an aircraft takes between points.
• Pilots rely on ground features when flying under visual flight rules (VFR).
• Pilots must maintain positional awareness when not flying under VFR.
• The VHF omnidirectional range (VOR) system helps maintain awareness, involving flying between VOR radials.
• Aircraft overfly VOR stations to maintain navigational awareness.

Evolution of Navigation Systems

• Declining need to follow published flight routes using ground-based beacons let pilots take more direct routes.
• This reduces flight times, fuel consumption, and makes navigating more efficient.
• Systems supporting RNAV include VOR/DME, VORTAC based systems, Inertial Navigation Systems (INS), and Global Positioning Systems (GPS).
• Flight Management System (FMS) Computers provide area navigation capability using an array of navigation systems.
• Under RNAV, aircraft maintain navigational awareness without needing to overfly VOR stations.
• The aircraft flies from waypoint to waypoint using latitude/longitude positions entered into the navigation computer.
• Using modern navigation systems a fix on actual position is obtained to ensure flight path maintenance.
• Waypoints are independent of ground points so flight paths can be direct from point to point.

RNAV Waypoint Navigation

• RNAV waypoint navigation based on VORTAC and VOR/DME facilities has been in use for some time.
• Accomplished via a course line computer (CLC) that creates phantom waypoints.
• Pilots navigate to and from these phantom VORs like actual VOR navigation, using DME to calculate waypoint location.
• A VORTAC or VOR/DME frequency is tuned into the computer to create a phantom VOR waypoint based on direction and distance from a VOR/DME site.
• Pilots program the RNAV computer with waypoints along the desired flight path.
• Even with direct routes, aircraft must receive usable signals from the ground site.
• Waypoints cannot take aircraft beyond the line-of-sight reception range of a VOR facility.
• RNAV doesn't extend a VORS standard service range, meaning below 15,000 ft, pilots must stay within 40 nautical miles of VOR stations for a reliable navigation signal.
• Signals from existing VOR and DME co-located beacons give range and bearing to a waypoint.
• This is specified by its range and bearing from the station necessitating continuous solving of the RNAV triangle.
• Variables defined
  • p1: distance between beacon and aircraft
  • 01: magnetic bearing from beacon to aircraft
  • p2: distance between beacon and waypoint
  • 02: magnetic bearing from beacon to waypoint
  • p3: distance between aircraft and waypoint
  • 03: magnetic bearing from aircraft to waypoint
• p1 and 01 are known from VOR/DME; P2 and 02 are pilot-entered so P3 and 03 can be found as two sides and an included angle of the RNAV triangle are known.

Inertial Navigation Systems (INS)

• INS can also be used for RNAV, as an extremely accurate self-contained navigation system.
• Pilots using INS don't need to consider ground-based radio navigation facilities.
• Any number of waypoints can be programmed.
• INS programmed waypoints navigate the aircraft on autopilot from point to point.
• Inertial navigation systems are subject to errors and can degrade by 1-2 nautical miles per hour.

Global Positioning System (GPS)

• GPS can support RNAV, as following waypoint programming, the RNAV computer calculates heading, distance, and estimated time of arrival (ETA).
• GPS ensures aircraft position and course required to intersect the next waypoint.
• RNAV for IFR flight is limited by the ATC system.
• Air traffic controllers might disapprove direct routes of flight in congested air traffic areas.
• The flight crew must reprogram the flight path and gain ATC approval to fly around storms.

Typical VOR/DME Area Navigation System

• Airborne computers made sophisticated navigation systems possible, including RNAV using VOR/DME.
• The concept shifts beacon positions to pilot-chosen waypoint locations.
• Pilots use VOR/DME instrumentation as before, but steering commands relate to remote waypoints.
• The navigation computer unit receives inputs from sensor and air data inputs.
• Using stored data and inputs, the navigation computer calculates aircraft position (latitude and longitude) and deviation from the flight path.
• Digital readouts can present position, track angle, ground speed, wind data, and cross-track error.
• Analogue presentation on the HSI shows heading, track, course, and steering commands.
• Attitude directors display pitch and roll steering commands and maps show route, beacon, and waypoint data.
• Computer require three types of stored data, accessible in three ways.
• Regularly flown routes use 'hard' data (VOR/DME location/frequency, airports, standard routes).
• Waypoint position, 'soft' data, may be entered or amended in flight on the Control and Display Unit (CDU).
• Real-time data from navigation and air data sensors are constantly available.
• Waypoint data are 'soft' but can be stored as 'hard' data and input via an Automatic Data Entry Unit (ADEU).
• Since sensor information may be analogue, analogue-to-digital conversion (AID) is needed.

RNAV System Components

• NP-2041A is a ten waypoint RNAV computer with waypoint parameters entered from a keyboard or magnetic-card reader.
• Bearing and distance to the active waypoint are found by solving the slant range and RNAV triangles.
• It manages frequency for VHF communication and navigation.
• Components
  • NP-2041A.
  • CN-2011A comm/nav unit.
  • DM-2030 DME.
  • IN-2014A electronic course deviation indicator and an encoding altimeter.
• HSI and RM are shown through an IU-2016A interface unit.
• Optionally, the above system can be complemented with an ADF and transponder to comprise a BX 2000 system.
• A weather radar interface and a magnetic-card reader are further options.
• Large business aircraft owners would be prospective customers for a full or nearly full package, but small single-engine aircraft are fitted with the basic VFR system, i.e. COMM/NAV, and an indicator.

Display and Control

• The NP-2041A front panel features seven gas-discharge seven-segment digital displays.
• The quantity shown depends on the Display Selector switch position and for some displays, the Mode Selector switch position.

Display Selector Settings

• SBY: standby waypoint parameters in FREQ, BRG/KTS, DST/TTS, EL X 100 and CRS windows.
• ACT: active waypoint parameters shown as for SBY.
• BRG/DST: bearing (BRG/KTS window) and distance (DST/TTS window) to active waypoint in RNAV or APR mode or to VOR/DME station in VOR/LOC mode.
• KTS/TTS: ground speed (BRG/KTS window) and time to station in minutes (DST/TTS window) to waypoint in RNAV or APR mode or to VOR/DME station in VOR/LOC mode.
• The SBY and ACT windows display the number (0-9) of the standby and active waypoint.
• The 'IN' legend is illuminated if course shown (CRS window) is inbound while 'OUT' legend shows course is outbound.

Mode Selector Settings

• OFF: self-explanatory.
• VOR/LOC: conventional navigation, with waypoints as stations.
• RNAV: waypoints are remote, left/right course deviation is linear within 100 nautical miles, full-scale deflection (fsd) being 5 nautical miles. From 100 nautical miles out, deviation is angular.
• APR: similar to RNAV but linear deviation up to 25 nautical miles, fsd being 1 -25 nautical miles.
• 'TEST': specifies display for satisfactory operation.
• Data is entered via keyboard or magnetic card reader.
• Of the 16 keys, 11 have dual functions.