RNAV

[Jimmy]

Something that has really been anoying me, I don't really know what RNAV is!

I see it on charts all the time, what does flying rnav involve and require?

can some one please give me a great in depth explanation

Thanks
James

[Brennanx]

http://en.wikipedia.org/wiki/RNAV

go there its air navigation thingy

[Alex]

Yeap, most of the aircraft you fly regularly have it James.

Alex

[nzav8tor]

RNAV as it applies to modern day aviation requires a flight management system which has a high level of accuracy.
RNAV approaches require two or more FMS's (for redundency and cross checking) and they have a set of position accuracy parameters which, if exceeded, mean the approach can not be flown soley with reference to the commands generated by the FMS, ie: you must revert to conventional navaids and the approach procedure for them.
(To continue an RNAV approach without the accuracy required would be like turning your headlights off when driving at night!)

The accuracy is called RNP or Required Navigation Performance and you can see this figure on the position page 3 of the boeing style FMS computers. I can't remember where its found on an Airbus FMGC.

An FMS typically gets its position information from three or more sources including inertial nav, GPS and DME information so the level of accuracy is very high, maybe to 0.1 NM.

Not to long ago Qantas trialled a new RNAV approach for Queenstown to allow them to commence approaches in worse weather than the conventional IFR approaches allowed and therefore have lower minimums. The approaches were certified and Air NZ is using them now too.
Because an RNAV approach can include turns and direct tracks between an unlimited number of fixes rather than tracking on radials from VOR's and magnetic bearings from NDB's (which are straight lines...) an approach can be designed which follows a circling track.
Perfect for somewhere like queenstown with its mountainous terrain.

The RNAV plate you are looking has these fixes on it which are not normal fixes as they aren't derived from VOR or NDB tracks.

GPS approaches (as shown in FSim) are similar as they use different fixes but the position info comes from GPS alone.

Transport aircraft saftety standards are such that they are designed to utilise as many reliable navigation and position information sources as possible without relying on one thing.
An FMS or FMGC incorporates this data and provides the nav solution for the lateral guidance computer, thats why you don't see GPS displays on modern airliner panels!

Thats about all I can think of to explain your question Jimmy, hope that helps you out.

[nzav8tor]

One more thing, regarding RNP...

Have a look at the Queenstown RNAV approach chart

Queenstown RNAV Approach Chart

Check out the box at the bottom which normally has the minimum descent altitude/height and visibility note that this now has minimums based on RNP.

Obviously the higher the accuracy of the RNAV system the lower the minima so the crew will be keeping a close eye on this info as they make their approach.
Notice the minima here with 0.10 NM RNP is 255' AGL with 2km viz, compare that to the conventional VOR/DME ALPHA approach with minimums of 3529' AGL and 5km viz! And that approach ends over Slope Hill VOR.

An RNAV approach is a precision approach, VOR/DME is Non-precision.

So if your flying in or out of Queenstown and the forecast isn't good, make sure you get a 737 or A320 flight!

[Zöltuger]

that's fascinating, I've always wondered which approach the 737's make into Queenstown. I usually use the GPS A approach into there in FS.

[Jimmy]

Thanks very much nzav8tor, great explanation very helpful

Had been really anoying me, but im prety sure I understaned now, so to use RNAV approaches and airways I need a fms ae, awesome a 763 has one of em

Thanks again
James

[nzav8tor]

Yep, make sure you have the latest airac data for it...