Digital RNAV equipt

[topcat450]

Many IFR equipped a/c for sale these days, often list/mention that they have digital RNAV.

When exactly would this be used, is it convenient and easy to get to grips with?

Is it solely used for IFR - or does it have uses when travelling VFR too?

[IO540]

There's not enough information to say.

"RNAV" means "area navigation" which means being able to navigate direct to a point which is not necessarily a navaid (a navaid in this context is a VOR or an NDB).

In conventional radio navigation you can track direct to/from a navaid but not to some other point.

Years ago, there was a Bendix-King product called KNS80 or KNS81 which would receive one VOR and its co-located DME and which would synthesise a virtual VOR/DME at some other location. You could then track towards this virtual point, as if it was a real VOR/DME.

A KNS80 is a waste of space because it isn't FM immune. Also one has to be within reception of the real VOR/DME for the entire leg; if you are flying airways and ATC give you a DCT to some point which is 150nm away then there is little chance of having reception of the real VOR/DME for that distance. It also doesn't work in e.g. France whose VORs rarely have a DME in them.

The modern way of doing RNAV is with an IFR approved GPS.

You can use it for VFR too, if you plan your routes on airways intersections. This is a very handy way to do VFR route planning because all these points are in the GPS database. It is years since I had to enter the lat/long coordinates of some point. ATC quite like it too because it sounds like you know what you are doing :) Some countries, e.g. Greece, like VFR pilots to route via airways intersections... this effectively implies a GPS.

[S-Works]

Many IFR equipped a/c for sale these days, often list/mention that they have digital RNAV.
When exactly would this be used, is it convenient and easy to get to grips with?
Is it solely used for IFR - or does it have uses when travelling VFR too?


In a shorter version of IO540'a answer it generally means the aircraft is GPS equipped if you are looking in terms of the GA fleet. The term refers to allowing you to enter a flight plan to airways intersections. When flying IFR I generally only put in the first couple of intersections and the destination as you rarely get the filed route. After that you just add in the direct to intersections that ATC will givew you. All very civilised.

[topcat450]

It was the Bendix-King items I was thiking of and that I'd seen mentioned a few times. I've just never flown with them and hadn't really grasped when you would want to/need to synthesise a 'ghost' VOR.

I suppose if the a/c has a Garmin 430 in it, that'll do all of that plus more, as you said, for flight planning via airways intersections, the 430 would have them all too.

Cheers anyway.

[IO540]

This is getting rather distant from the original Q :) but I find that entering the entire flight planned route is very useful.

The IFR sector controllers do have your filed route in their system and they will generally give you DCTs to waypoints that lie on that route.

With any GPS, it's a lot quicker to get the cursor up and select a waypoint already there and press DCT, than to enter one from scratch. It's also a lot easier to identify an unfamiliar intersection/navaid name from a list than to ask ATC to spell everything.

Personally I find that - apart from general weather avoidance tactics - locating unfamiliar intersections that ATC throw at you is the major workload in IFR flying.

I would not say that "RNAV" automatically means GPS. Bendix-King make both the KNS80 and various IFR GPS receivers. It indeed should mean GPS these days, but there is a lot of old planes around with a KNS80, which is practically useless.

[S-Works]

thread drift? you Peter? Never........


I seem to recall that you can get the KNS80 BRNAV approved and FM Immune.

When I did my IR it was virtually all done with the KNS80. The problem came when we did Annecy as a training flight and got a waypoint that was miles in front and not as filed and we did not know where it was. The trusty 296 sat on the dash saved the day, we used that for a direct to and to locate it and then about 20 minutes later when in range for the KNS80 went back to that.




This is getting rather distant from the original Q :) but I find that entering the entire flight planned route is very useful.
The IFR sector controllers do have your filed route in their system and they will generally give you DCTs to waypoints that lie on that route.
With any GPS, it's a lot quicker to get the cursor up and select a waypoint already there and press DCT, than to enter one from scratch. It's also a lot easier to identify an unfamiliar intersection/navaid name from a list than to ask ATC to spell everything.
Personally I find that - apart from general weather avoidance tactics - locating unfamiliar intersections that ATC throw at you is the major workload in IFR flying.
I would not say that "RNAV" automatically means GPS. Bendix-King make both the KNS80 and various IFR GPS receivers. It indeed should mean GPS these days, but there is a lot of old planes around with a KNS80, which is practically useless.

[IO540]

You can get a KNS80 FM Immune (with antenna lead filters) and you can even get the installation BRNAV certified. I know a man (this sounds like the yellow pages advert) who has got one of these.

Unfortunately it is close to useless, because ATC routinely give you DCT legs way outside the DOC of the navaid defining the waypoint. I get such a DCT on perhaps 50% of IFR flights. And the rest of the time I ask for one, to get a shortcut :) On Saturday I got a DCT across much of France, to some place in the Channel (very nice).

Obviously there is a way to hack such a situation with a KNS80, tuned into a series of nearby navaids, one after the other, but it escapes me how you could do it in a hurry. You would have to assume that you will fly the filed route in entirety (never the case) and you would have to pre-plan all the RNAV waypoints in between. This has to be done right because if ATC gives you a DCT XXX where XXX is 150nm away, and when they see (say 30 secs later) that your track to XXX is 10 degrees off, they will be on top of you. This is bad enough when I am 10 degrees off at the start of a leg because the autopilot is slow to adjust the wind offset...

That's why I think a KNS80 is a good doorstop and little else.