Question on tolerances for flying airways, SIDs, STARs, etc.

[brianshell]

Just a quick question on the tolerances allowed by the FAA and the controllers when flying a particular airway, SID, STAR, etc...

I've traditionally been holding my finger right on the heading button in a attempt to engage it 1.0 to 0.5 miles away from the turn, to keep the plane precisely on course... but when watching Keith's videos, I've noticed that he's a lot less picky about getting it "exactly right". (This makes me feel a lot better when I look over at Foreflight and notice I'm flying parallel to the airway, maybe a mile to the right or the left.

What is the typical tolerance allowed for these routes? If I engage the heading change too late or too soon, how far off can I be?

[Pieces]

You shouldn't ask "How far off can I be?"; instead try "How close can I get?". When flying instruments you need to be as accurate as possible. When hand flying you want to peg that CDI on the center and keep it there. When using an autopilot then you will generally use NAV mode to fly airways and radials. If you use heading mode you should adjust the heading to keep the CDI centered. You should definitely not set the heading bug to the direction indicated and fly that heading blindly (what if there is a strong crosswind?).

Now, having said that airways, SID and STAR paths, and approaches have obstacle clearance requirements. You are guaranteed to not hit any ground obstacles when you fly within those tolerances. The details of that are covered in the IFR workshops: http://www.pilotedge.net/workshops.

I read a really interesting story recently about a first officer who though his captain was way to strict in his flying. He would dead center that needle and it wouldn't move a millimeter the entire flight. The FO thought this lacked a certain finesse, no art to the flying, etc. Until they got stuck in a zero visibility situation with emergency fuel and had to fly an (CAT I) ILS and land blind. That captain could fly the plane so well that the landing was completely uneventful. Incidentally, if someone knows where to find that article, please post the link. I lost it .

[Keith Smith]

Can you find an example of where I was less picky regarding airways tolerances during straight line flight? If it's during a dog leg or some kind of turn during the enroute portion, the Baron that I fly in most of the videos is /A, so I do my best to lead the turns as needed. Generally speaking, if you have equipment that performs turn prediction, you should use it. I agree with Reece, the goal is to fly as close to center as possible. The IFR regulations also require that you maintain as close to center as your instruments allow.

Airways extend 4 miles of either side of the center line (or more in cases where the airway is more than a certain distance from the VOR), but that is to allow for inaccuracies in the equipment. That way, if you're flying 'perfectly' with equipment that is within the legal limits for course error, then you will be on the airway. If you're off center by a couple of degrees and your equipment is inaccurate, then you might violate the lateral limits of an airway or published course.

I fly less than perfectly in many of the demo videos, I'm sure.

[Pieces]

Found that story I referenced: http://www.flightgear.org/forums/viewtopic.php?p=143276. I can't say anything for the accuracy, but still a cool story.

[Keith Smith]

That's amazing, what a wonderful story. I don't I could swing that level of precision, but when I'm flying solo and focused on nothing but the flying, I strive for 0.01nm cross track error or less and about +/-5ft on the altitude. It makes the 3hr legs go by in a snap! The challenge with the Lancair is that CG is constantly moving (I run from 7 gallons down to 3 gallons in the header tank, which is well forward of the CG, then back up to 7 gallons) That's a 24lb weight shift forward of the nose. There is never a dull moment trying to keep that blasted airplane in one place

Every time I try that exercise (which is on most of the long XC flights), I'm reminded of what an amazing achievement the SR-71 was and how good those pilots had to be to fly it.

[brianshell]

This is very interesting and I appreciate the data... The fact that the airway extends 4 miles to the left and right of the line is the information I was after.... that means I'm WELL within tolerance if I'm flying a half mile off... and I feel a lot better about that. I DO strive for precision... but I'm far from perfect. =)

Keith, I'll find the segment in the video where it was clear you were off by a little (not much, but enough that made me wonder what the real tolerances were)... It's in the I5..

Really appreciate the info everyone!!

[Keith Smith]

This is very interesting and I appreciate the data... The fact that the airway extends 4 miles to the left and right of the line is the information I was after.... that means I'm WELL within tolerance if I'm flying a half mile off... and I feel a lot better about that. I DO strive for precision... but I'm far from perfect. =)

Yes and no. As an extreme example, if you are 60nm from the VOR, then each degree that you're off will cost you a mile. So, let's say you're off by one degree...but your VOR is off by 4 degs to begin with (in the same direction), that's 5nm off the center of the airway and now you might hit something I only say that as a reminder that you should strive to fly the center of the airway when you're enroute. Even if YOU are flying perfectly, the equipment itself can still put you right on the edge of the airway and still be considered usable for flight, removing your own margin for error. Does that happen often? Probably not, but it's good practice to try to keep the needles centered.

The beginning of that I-5 was pretty tricky in the JS32, I was struggling to keep up when the controller sent me to VNY rather than a shortcut onto the LAX radial. I recall that by the time I was finally intersecting the outbound radial from VNY, it was time to start the LAX intercept. Hopefully the enroute legs were on course, though!