A Point-to-Point through the FARs

[Gatordev]

Tbl 1-2-1 in the AIM shows required navigational performance (RNP) required for different phases of flight. Enroute is 2 NM from centerline. Terminal is 1 NM and approach is .3 to 1 NM. Also from the AIM, "For an aircraft to meet the requirements of RNAV, a specified RNAV accuracy must be met 95 percent of the flight time." (Sorry for the AIM vs FAR reference. I don't have my tabbed out and highlight FAR/AIM with me. The AIM is easier to search.)

But that centerline accuracy (my term, not a FAA term) is what is graded by the Check Airman. So there's some standard that indicates a lack of adequate precision. That's what I was getting at with the PTS. I think PTS says full deflection throughout the regime of flight, so for enroute, that goes back to your "4 NM and 1K'" quote, which makes sense, since those numbers pop up every where else. But from the terminal phase all the way through the departure phase (there's a fixed, manual RNP for proprietary departures, as well), the scale changes, so "full deflection" will be less than the total of "4 NM."

None of this is to argue against your point, just expanding on the PTS comment and why it's not a number/XTK per se but an idication (full deflection).

TACAN/VOR generally gets more precise as you get closer to the station (the "cone of confusion" at high altitude nonwithstanding), which is what enables it to be used for non-precision approaches. Many of those approaches require PTPs to reach the IAF, and a proper understanding of geometry and aircraft turn performance in order to arc and turn to radials. So yes, poor execution would be an issue- but you could argue the same for a poorly hand-flown RNAV approach. Bad piloting is bad piloting, regardless of the nav source.

But the GPS scales as you move through the approach, so it's simulating the cone of confusion in the same way, just with significantly less error and more precision than a navaid.

Or maybe I'm misunderstanding what you were getting at.

 

[Gatordev]

I think that is a lot easier than the "use your hands to make gang signs 70/110/180 right/left of the reciprocal of the inbound course and WTF does that even mean" method in the Navy pubs.

Plenty of civilizations have utilized various livestock for predictions of what will happen to them. I fail to see how the chicken claw of death should be over-shadowed by more modern means for navigational success.

 

[pilot_man]

Good points. I wasn't clear on the timing portion, I get that we emphasize one minute legs, etc. I meant the "hit the point at this exact time" timing.

That part is easy, get close enough and if you’re late, select Max AB and if you’re early, boards out and fly around at 30ish alpha for a bit.

 

[sevenhelmet]

But the GPS scales as you move through the approach, so it's simulating the cone of confusion in the same way, just with significantly less error and more precision than a navaid.

Or maybe I'm misunderstanding what you were getting at.

I was just trying to point out that RNP is N/A for TACAN and VOR-based navigation, and that pilot execution defines the outcome regardless of the navaid used. My extra-nerdery ? delves into three concepts:

-The "cone of confusion", which is the area above a TACAN or VOR where it isn't receiving the station (they have to shield the top of the navaid to avoid cross-interference). This is typically an enroute phenomenon. It's generally not an issue for approaches, because the aircraft is at relatively low altitude and not directly overflying the navaid during the final portion of an instrument approach.

-The accuracy I referred to is a separate concept due to the angular size of radials decreasing as you get closer to the station (e.g. 1 nmi wide at 60 DME, 0.5 nmi at 30 DME, etc.)

-Finally, the reason RNAV RNP values scale is to meet GPS accuracy standards for different phases of flight (enroute, terminal area, approach), not to replicate any sort of navaid from yesteryear.

Hopefully that's a little clearer than my first post. ?

 

[MIDNJAC]

Might as well train people to use the most efficient entry method while being oriented- it'll make everybody's lives easier later when they have to hold in a marshall, refueling track, CAS stack, whatever else- or fly a windline rescue pattern (a not entirely different procedure).

To be fair, there is no standard entry or pattern for a refueling track or the CAS stack. It's just where it is. For refueling, if you don't drive your AF tanker around like yellow gear, they are going to either fly directly into a thunderstorm, or continually update their track to drive straight at you until a high aspect merge occurs, negating any attempt you might have made at a smooth join.

 

[Gatordev]

I was just trying to point out that RNP is N/A for TACAN and VOR-based navigation, and that pilot execution defines the outcome regardless of the navaid used. My extra-nerdery ? delves into three concepts:

-The "cone of confusion", which is the area above a TACAN or VOR where it isn't receiving the station (they have to shield the top of the navaid to avoid cross-interference). This is typically an enroute phenomenon. It's generally not an issue for approaches, because the aircraft is at relatively low altitude and not directly overflying the navaid during the final portion of an instrument approach.

-The accuracy I referred to is a separate concept due to the angular size of radials decreasing as you get closer to the station (e.g. 1 nmi wide at 60 DME, 0.5 nmi at 30 DME, etc.)

-Finally, the reason RNAV RNP values scale is to meet GPS accuracy standards for different phases of flight (enroute, terminal area, approach), not to replicate any sort of navaid from yesteryear.

Hopefully that's a little clearer than my first post. ?

Copy all, and I'm with you. Poor wording on my part. I shouldn't have said cone of confusion. I was thinking exactly what you were saying (angular size of radials) which made my brain think "cones" and typed that.

I have a feeling much of this discussion would go much easier if it was in real-time, face to face.

-"Hey, did you hear the Army can't do PTPs?"
-"That's dumb."
-"Yeah, but apparently the FAA and AF doesn't do them anymore, either."
-"Huh, that's weird. Let me quote some FAR and AIM stuff."
-"Yeah, I get it, but that's what's happened."
-"Weird. Want another beer?"
-"Sure."

And scene.