Although the concepts are easy to understand, the inter-relationship between systems can become blurred when the various types of approaches and departures are incorporated into the navigational system.
LEFT: Collins Mode Control Panel (MCP) showing illuminated LNAV annunciation (click to enlarge).
This post will not provide an in-depth review of these systems; such a review would be lengthy, confusing and counterproductive to a new virtual flyer. Rather, this post will be a ‘grass-roots’ introduction to the concept of RNAV, RNP, LNAV and VNAV. I will also touch on the concept of Performance Based Navigation (PBN).
In the Beginning there was RNAV
RNAV is is an acronym for Area Navigation (aRea NAVigation).
Prior to complex computers, pilots were required to use established on-the-ground navigational aids and would fly directly over the navaid. Such a navaid may be a VOR, NDB or similar device. Flying over the various navaids was to ensure that the flight was on the correct route. Often this entailed a zigzag course as navaids could not be perfectly aligned with each other in a straight line - airport to airport.
When computers entered the aviation world it became possible for the computer to 'create' an imaginary navigation aid based on a direction and distance from a ground-based navaid. Therefore, a straight line could be virtually drawn from your origin to destination and several waypoints could be generated along this line. The waypoints were calculated by the computer based on ground VORs and positioned in such a way to ensure more or less straight-line navigation.
In essence, RNAV can be loosely defined as any 'straight line' navigation method similar to GPS that allows the aircraft to fly on any desired path within the coverage of referenced NAVAIDS.
Required Navigation Performance (RNP) and Performance Based Navigation (PBN)
Simply explained, Required Navigation Performance (RNP) is a term that encompasses the practical application of advanced RNAV concepts using Global Navigation Satellite Systems (GNSS).
However, there is a slight difference between RNP and RNAV although the principles of both systems are very similar.
RNAV airspace generally mandates a certain level of equipment and assumes you have a 95% chance of keeping to a stated level of navigation accuracy. On the other hand, RNP is performance based and requires a level of on-board performance monitoring and alerting. This concept is called Performance Based Navigation (PBN).
RNAV and RNP both state a 0.95 probability of staying within 1 nm of course. But RNP (through PBN) will let you know when the probability of you staying within 2 nm of that position goes below 0.99999. In essence, RNP and PBN enable an aircraft to fly through airspace with a higher degree of positional accuracy for a consistently greater period of time.
To achieve this level of accuracy a selection of navigation sensors and equipment is used to meet the performance requirements. A further enhancement of this concept is the use of RNP/ANP (Required Navigation Performance and Actual Navigation Performance. Advanced RNAV concepts use this comparative analysis to determine the level or error between the required navigation (the expected path of the aircraft) and the actual navigation (what path the aircraft is flying.) This information is then displayed to the flight crew.
LNAV and VNAV
LNAV and VNAV are parts of the Flight Guidance System, and are acronyms for Lateral Navigation and Vertical Navigation'. Both these functions form part of the automation package that the B737NG is fitted with.
LNAV is the route you fly over the ground. The plane may be using VORs, GPS, DME, or any combination of the above. It's all transparent to the pilot, as the route specified in the clearance and flight plan is loaded into the Flight Management System (FMS), of which the Flight Management Computer (FMC) is the interface.
The route shows up as a magenta line on the Navigation Display (ND), and as long as the LNAV mode on the Mode Control Panel (MCP) is engaged and the autopilot activated, the aircraft will follow that line across the ground. LNAV however, does not tell the plane what altitude to fly, VNAV does this.
VNAV is where the specified altitudes at particular waypoints are entered into the FMS, and the computer determines the best way to accomplish what you want. The inputs from VNAV are followed whenever the autopilot is engaged (assuming VNAV is also engaged).
The flight crew can, if necessary alter the VNAV constraints by changing the descent speed and the altitude that the aircraft will cross a particular waypoint, and the computer will re-calculate where to bring the throttles to idle thrust and begin the descent, to allow the aircraft to cross the waypoint, usually in the most economical manner.
VNAV will also function in climb and take into account airspeed restrictions at various altitudes and will fly the aircraft at the desired power setting and angle (angle of attack) to achieve the speed (and efficiency) desired.
There is not a fast rule to whether a flight crew will fly with LNAV and VNAV engaged or not; however, with LNAV and VNAV engaged and the autopilot not engaged, LNAV and VNAV will send their signals to the Flight Director (F/D) allowing the crew to follow the F/D cue display and hand fly the aircraft the way the autopilot would if it were engaged.
Reliance on MCP Annunciators
LEFT: Flight Mode Annunciator (FMA) showing LNAV and VNAV Path Mode engaged. The Flight Director provides a visual cue to the attitude of the aircraft while the speed is controlled by the the FMC. CMD indicates that the autopilot is engaged (ProSim737 avionics suite).
However, reliance on the MCP annunciators to inform you of a mode’s status is not recommended. Rather, the Flight Mode Annunciator (FMA) which forms part of the upper area of the Primary Flight Display (PFD) should be used to determine which modes are engaged. Using the FMA will eliminate any confusion to whether VNAV (or any other function) is engaged or not.
This post explains the Flight Mode Annunciators (FMA) in more detail.
In summary, RNAV is a method of area navigation that was derived from the use of VOR, NDBs and other navaids. RNP through it use of GNSS systems has enabled Area Navigation to evolve to include LNAV and VNAV which are sub-systems of the Flight Guidance System - LNAV is the course across the ground, and VNAV is the flight path vertically.
Historically, navigation has been achieved successfully by other methods, however, the computer can almost always do things better, smoother and a little easier – this translates to less workload on a flight crew.
In my next post, we will discuss RNAV approaches and how they relate to what has been discussed above.
The information for this article came from an online reference for real-world pilots.
Acronyms and Glossary
Annunciator – Often called a korry, it is a light that illuminates when a specific condition is met
DME – Distance Measuring Equipment
FMA - Flight Mode Annunciator
FMC – Flight Management Computer
FMS – Flight Management System
GPS – Global Positioning System
GNSS - Global Navigation Satellite System
LNAV – Lateral Navigation
MCP – Mode Control Panel
ND – Navigation Display
NPA - Non Precision Approach
PBN - Performance-based Navigation
RNAV – Area Navigation
RNP - Required Navigation Performance
VNAV – Vertical Navigation
VNAV PTH – Vertical Navigation Path
VNAV SPD – Vertical Navigation Speed
VOR – VHF Omni Directional Radio Range