Another curved ball has been thrown into the GTN vs IFD debate. The pun is intended 
The GTN is now approved for RF legs (provided you have an EHSI set to autoslew) whereas it seems unlikely that Avidyne will prioritise such approval, as RF legs are (not yet) common in the US.
There are already a few RF legs in RNP approaches and transitions in Europe, but it seems likely that the numbers are set to increase. GAINS is going to start RF leg trials shortly.
What exactly happens if RF legs are not supported? Is the entire procedure not loadable? If so, are there any examples of these, applicable to GA (i.e. no special crew authorisation required)?
The practical difference in the flight path, versus flight path tolerance, is nil in most real world scenarios.
Peter wrote:
What exactly happens if RF legs are not supported? Is the entire procedure not loadable?
Yes, exactly so. I have set this up on my sim and you literally switch certain procedures on and off when you switch RF legs on and off.
Peter wrote:
If so, are there any examples of these, applicable to GA (i.e. no special crew authorisation required)?
A small, but growing, number. The GAINS project is investigating making them more commonplace. I don’t know whether the locations they are working on are “commercial in confidence” so I will leave that question to @bookworm.
Peter wrote:
The practical difference in the flight path, versus flight path tolerance, is nil in most real world scenarios.
I don’t know what that means, but I have flown some RF legs on the sim under the most trying circumstances (crossing a very strong wind half way round the procedure, for example) and have had no difficulties whatsoever. An RF leg is not practically different to a base turn in an overlay procedure.
As so, so often with these things, the regulators are making a mountain out of a molehill, probably due to a deep lack of understanding (cf LNAV/VNAV approaches in the UK.)
As ever, my door is open to anyone who wants to come play with RF legs on my sim in Epsom.
Are there any current examples of such IAPs?
A typical GPS navigator box flies every waypoint (other than the very last one) as a fly-by waypoint, anyway. Even the KLN94 does that.
Peter wrote:
A typical GPS navigator box flies every waypoint (other than the very last one) as a fly-by waypoint, anyway. Even the KLN94 does that.
An RF leg is very different to a fly-by waypoint. It is closer to a DME arc. It needs to be flown to the same precision as any other RNP 1 leg. Fly-by waypoints track over the ground is affected by groundspeed, which RF legs are not.
This is all explained in the PBN Manual, which I am about to update with information about GTN approval.
I’ve moved these posts to what looks like a good previous thread on same topic.
I wonder if there is anywhere where
Peter wrote:
I wonder if there is anywhere whereflying the RF leg as a DCT between the two waypoints would compromise obstacle clearance, and
the procedure doesn’t require special crew authorisation
That is not a risk worth taking in my opinion.
IFR procedures are there for very good reason, and to guess that flying something different will probably be ok is not sensible or acceptable.
You may as well forget the whole procedure and work out your own approach on a moving map. That is doable, of course, but you are removing so many protective slices of cheese that it is not wise.
What I was getting at is whether the procedure designers are doing this because airliners can all fly these already, so “why not”?
Peter wrote:
What I was getting at is whether the procedure designers are doing this because airliners can all fly these already, so “why not”?
No. Many, many airliners cannot fly them, which is causing a certain amount of disgruntlement in some airlines. It’s the same chicken’n’egg thing as the introduction of most new technology.
But the main point is that if they are not available and switched on in the navigator, they may as well not exist. It’s not like on a TSO129 box, where the legs that it can draw are drawn and the rest are not (like the outbound and inbound track on an overlay teardrop procedure, but no base turn.) It’ll simply not be there on the list. Think of it like the Lydd ILS overlay procedure, or the Cranfield NDB overlays; they just ain’t listed.
There are two hurdles.
To see them at all, you must have a GTN with recent software (6.41 from memory, maybe 6.21) and the config switch must be on (as in my photo above). This will physically show the procedure on the moving map and move the CDI on the HSI or OBS. This enough to make them possible to fly, though not approved.
To fly them approved, you additionally must have an autoslew EHSI, various annunciations in the pilot’s field of view and a supplement in the AFMS.
