Navigational accuracy required by ATC

FAR 121.121 states that aircraft must be navigated to the degree of accuracy required for air traffic control. What is the degree of accuracy required? Is there a hard number or is it the opinion of whomever is operating the radar scope?

If I file /India, and use a King 80 to fly direct using phantom VOR's, how close to a perfectly straight course line to I need to be?

VOR to VOR, Victor airways and STARS (possibly J Routes?) is 4 miles either side of center line IIRC. I wouldn't really sweat it, you'd have to try to do worse than a /Q and even with those guys its usually not all that bad, maybe a mile or two off. I don't know about any degree of accuracy required for the use of RNAV or NDB.

In actual practice, if you're flopping around headings trying to stay on course like a fish out of water and I'm slow, I'll just use altitude on your traffic and give it some extra room. If I'm busy I'll just give you a hard heading "for traffic".

As long as your within a few miles of your route of flight, your fine with me. As Genot said, if I need it I'll lock you on a heading for "traffic" or I'll give you a heading then "When able direct your next fix" if you seem too lost.

So it is the opinion of the controller then? I just want to know at what point would a pilot be violating FAR 121.121. I can't find a hard fast number. I didn't know if ATC had any guidelines on this.

Well, we don't set or enforce the F.A.R.'s. The enforcement side of the FAA is completely separate from us. We can only say what we'd expect.

It's kind of like these Chautauqua flights recently that are flying 230 KTS indicated in cruise and even descent without telling us. I know it's in the AIM, but not sure about the FAR's and quite frankly not my concern as long as separation isn't lost.

If your speed is way different than what you filed, I like to know so I can update your flight strip so your route line can stay up to date.

Did they file for a slower than normal TAS, and we're they in the ball park?

Were

iPad autocorrect had different thoughts.

As was said, a controller probably isn't going to be out to violate you, unless somehow separation was lost.

For each type of route you are on (Victor Airway, Jet Airway, Q route, STAR, SID, GPS/ILS/VOR/TACAN/Whatever approach etc) there are prescribed Required Navigation Performance values that are associated. You can generally find them in the AIM.

My Canadian interpretation of that rule... and it's just that an interpretation.

Would be defined by whatever type of airspace/airway rules you're operating under.

MNPS airspace Vs. RNPC airspace

RNP4 Vs RNP10 etc....

As has been said, its semantics. We aren't sky police (save a blatant violation that causes a loss of separation). Anyone on an STAR for example, there are airspace structures. Turn 2 miles early, I'll have diverging headings on that departure (or a in no way can they catch each other) before I think about climbing them through your altitude. If you do manage to mangle the navigation that bad, the other controller was in my ear before it ever became an issue for me. You're FAR more likely to (again STAR example) turn at the wrong fix that to blow any navigation accuracy standards out the window unless you're flying a first year Cessna 150 with the OEM VOR that hasn't seen maintenance in the last two decades.

Do RNP standards apply to aircraft that are not certified for RNP? I don't think they apply unless you filed as /R