Peter, RF is a GPS DME Arc.
I can’t help thinking there is a lot of job creation in all this, since GPS did to the navigation business what the CD did to the vinyl record business
I can’t help it but I was thinking the same all the time after learning more about the topic and flying GPS up and down during my training in the US. Interesting times …
RF is a GPS DME Arc
I recall a discussion, perhaps close to 10 years ago, of people flying DME arcs (parts of holding patterns, back then) using equipment available back then.
I don’t know how it was done, but a sequence of waypoints would be easy to generate. After all, the tolerance is something like 0.5nm or 1nm, which is a huge distance.
But, yes, what is the big deal in flying an arc? How many approaches have GPS arcs?
Peter, you are right it is trivial even if hand flown. But it is not allowed for now.
So you are allowed to fly a DME based DME arc in the usual way but are not allowed to fly a GPS based DME arc without the special approval for the equipment?
Is that all there is to it?
Yep.
There are differences between DME arcs and RF leg arcs, although they are both a radius. A DME arc is based on a VOR being collocated with the center of the arc radius and may be used to determine the current desired course tangent. A DME arc has a minimum radius of 7 NM and a maximum of 30 NM. An RF leg has a 3 NM minimum radius. At the smaller radius of turn, bank angle required acts to limit the speed that the leg may be flown at, so there are speed limits on some RF legs. The TERPS criteria for a DME arc is enormous, the primary ROC width is +/- 4 NM with another 2 NM of secondary area on each side or 12 NM across. RF legs on RNP procedures have an ROC width of +/- 2 RNP and there is no secondary area provided which mandates that the course deviation be maintained +/- 1 RNP on a 95% basis. Depending on the location of the RF leg, they can use RNP down to 0.3 NM. A DME arc has a DME value to a know location that is charted and can be flown as a series of short straight legs. The RF turn is a fixed radius to an unmarked point. To hold the course centerline, a constant bank angle is needed to fly the RF leg (no wind). Back to back RF legs with different directions or different radius of turn are permitted, as opposed to a single constant distance to the DME location.
All this said, Garmin’s evaluation indicated that pilots of varying experience were up to the task with some training and simple CDI guidance although a moving map display, a Flight Director, and/or a roll steering autopilot were very helpful.
@NCYankee, of course the tolerances are different but it is just an arc. Any decent roll steering autopilot can do it easily as can a hand flown pilot assuming they have a FD (and probably without). And whether it is an arc to an aid or a GPS waypoint is irrelevant to a modern system.
On the speeds, a GA aircraft will be flying it far slower than the design speeds for a light jet or bigger. Hence bank angles should not be a problem.
A DME arc is based on a VOR being collocated with the center of the arc radius and may be used to determine the current desired course tangent.
Is that necessarily true? Can’t you have an arc around a stand-alone DME? (I realise it would be more difficult to fly.)
Is that necessarily true? Can’t you have an arc around a stand-alone DME? (I realise it would be more difficult to fly.)
No, how would you determine where to enter and leave?
