Let's start today by watching this wonderful 'RNP' approach into Queenstown, New Zealand...then we'll get to the explanation.......
As passengers we have all looked out the window to see nothing but a shroud of cloud engulfing the aircraft as our destination grows ever closer. Still the airliner continues to descend earthward accompanied by the tell-tale sounds of flaps extending from the wings and the landing gear extending from its bays beneath our feet. The cloud breaks into patches and seconds later we are reunited with Mother Earth.
This wonderful feat has been achieved by the aircraft flying an 'instrument approach'. It is a descent along a carefully surveyed and certified flight-path that considers terrain and various other eventualities and then provides a further margin of safety. These approaches can take various forms and are known by various names and the 'Required Navigation Performance', or 'RNP', is the latest and most rapidly growing style of instrument approach.
RNP differs from its predecessors in a number of ways. Firstly, it is not dependent upon ground-based radio beacons, but draws upon its own independent airborne equipment and the satellites that circle our planet. This offers a very accurate and adaptable means to make approaches to airports that once presented significant problems.
For example, traditionally, the instrument approaches that allowed the aircraft to descend to the lowest level required a substantial final 'leg' that was aligned with the runway. If significant terrain existed in this zone, then the ability to descend was restricted as the aircraft would have to manoeuvre clear of the cloud to position for a landing and this needed to be done at a safe height. Similarly, older, less precise styles of instrument approaches were unable to offer these lower descents due their lack of precision.
The satellite derived accuracy of Global Positioning Systems, or GPS, provides the accuracy needed to descend safely and without ground-based radio beacons, the approaches can be designed to safely navigate regions that are rich in towering terrain. If approaches were once rigid funnels leading to the runway's end, then RNP is a narrow flexible hose that can be laid along the most advantageous path, safely winding through the terrain. Featured here is the RNP approach into Queenstown, New Zealand; one of the most picturesque approaches in the world and one of my personal favourites.
There are further benefits to be found in this new age of RNP approaches - they can also be friendly to the community and the environment. Their routing can avoid noise sensitive areas in ways that straight flight paths may not, while their efficient descents with the jet's engines at idle power equate to less noise and emissions. Today RNP approaches offer more reliable and safer access to airports where the weather and terrain once held the upper hand. That being said, even on fine days as you weave your way towards your destination, chances are that you could be flying the efficient path of an RNP approach.
Happy flying and click on Airbus's flight deck image below for a 360 degree tour of an A320 flight deck - similar to those which operate in Queenstown.