Tom Haines

When an ATC change isn’t really a change at all

April 29, 2009 by Thomas B. Haines, Editor in Chief

The latest edition of the NASA ASRS Callback newsletter points out an annoyance surfaced by airline pilots but shared by those of us who don’t fly for hire–ATC route clearance changes that aren’t really changes at all. You file one thing and clearance delivery clears you for something that sounds quite different, yet when you dig out a stack of charts and plot out the new route, it’s the same or nearly the same as what you filed, but described differently.

Sometimes the subtle differences can lead to annoyance, but there is the potential for more serious issues, especially if the pilot doesn’t notice the subtlety. Here’s one example from the newsletter.

“A pet peeve of some ASRS reporters is PDC’s [pre-departure clearances] that contain apparent route revisions (amendments), when the amendment doesn’t actually change the filed routing. We included an example of this in the March 2009 CALLBACK (Clearance Clarity). Here is an excerpt from that report:

…I have many times encountered an ATC clearance problem that just simply does not have to exist. We are often given a clearance that reads something like, ‘You are cleared direct ABCDE intersection, direct FGHIJ intersection, XXX VOR 123 degree radial to KLMNO intersection, then flight plan route.’…We are forced to dig out charts that we might not normally have out, then try to find the VOR in question and trace out the radial, only to find that the given radial is a direct route from FGHIJ to KLMNO. If we have the equipment to proceed direct to the first two intersections, we obviously have the equipment to proceed directly to the third. Why not just give us direct to all three? Why confuse the issue by throwing in a VOR and radial, when both are completely unnecessary and serve only to create confusion?”

ASRS proposes several ways to improve the situation, including one very logical one:

Discontinue the ATC practice of amending the filed route of flight with fixes that do not represent an actual change of routing. It is time-consuming for pilots to verify that a routing “revision” does not change the filed route of flight. In some cases, pilot confusion may result in track deviations and loss of separation events.

ASRS is a terrific safety program administered by NASA on behalf of the FAA. For more on it and to sign up for their free newsletter that compiles pilot and controller safety concerns, see the Web site (http://asrs.arc.nasa.gov/).

Meanwhile, listen carefully to those ATC clearances.

19 Responses to “When an ATC change isn’t really a change at all”

  1. Brian Veazey Says:

    While the difference is subtle, X direct Y is not the same as X X123R Y. Depending upon a host of variables–wind, airspeed, nav performance, pilot competence–the lateral differences in the routes could be up to several miles. A properly detail-oriented airman already understands this.

  2. Marc Clemente Says:

    “While the difference is subtle…”

    And what exactly is this subtle difference? Can you tell me how airspeed, windspeed and direction affect the location of a magenta line on my display? The shortest distance between two points is a straight line (or great circle arc). VOR radials are also straight lines because light and radio waves travel in straight lines.

  3. David Heileman Says:

    I consider myself a detail-oriented airman and I don’t understand it. If Y intersection is on the 123 radial from X, how can X direct Y be any different than X X123R Y? At first I thought you were talking about GPS navigation vs VOR navigation, and waypoints vs intersections, but you’re not. So please explain the difference!

  4. Brian Veazey Says:

    The difference is that if you are cleared via the radial, you are bound to stay on the straight line between the two points. If you are cleared point-to-point, you are likely to be off to one side, especially if VOR is the method. This is because 1) airplanes don’t turn square corners, and 2) given a direct clearance, some pilots will home to the fix instead of adhering to a specific track.

    Yes, it is better with GPS, and the differences won’t hurt anything 99.9% of the time. I am acquainted with the 0.1%, however.

  5. David Heileman Says:

    Can you point to the section of the FAR or AIM that substantiates what you are saying? I don’t believe the regs or the ATC system expects airplanes to turn instantaneously, and no decent pilot would ever home to a fix. When using VOR navigation, a direct clearance from X to Y means that you must fly within 4NM either side of a straight line (great circle route) from X to Y. If Y is an intersection on the 123 radial from X, then the straight line from X to Y is defined by the 123 radial. These are fundamental principles of VOR navigation! If a pilot can’t turn from one course centerline to another within 4NM, they need to go back to school. You seem to have had a bad experience (0.1%?) that you think is relevant to this discussion. Please share the details so we can understand.

  6. Caleb Newville Says:

    This happens commonly with a cleared altitude that is close to minimum altitudes for a given area. Go to SkyVector.com and look up PACD (Cold Bay, Alaska), and switch to IFR Chart AL-2.

    If you want to fly direct from Sand Point (PASD) to Cold Bay, and you have filed Direct at 10,000 feet, the controller will clear you via G12 because the MEA on G12 is 10,000 feet. If you look at this: http://www.internetwizardry.net/0902251f%20(Sec%209).pdf chart, you can see there is an area along G12 where the Minimum IFR Altitude is 10,400.

    A controller can only clear you to an altitude at or above the Minimum IFR Altitude for an area for a direct routing. If you want to be cleared ‘direct’ you would have to be issued at least 10,400, so the controller just issues you the airway instead of changing your altitude to 12,000 (next available, right for direction).

    The airway has been flown by Flight Standards and is guaranteed at 10,000. If you are flying direct, you can’t be guaranteed to not hit whatever is making the Minimum IFR altitude go up that extra 400 feet.

    Please let me know if I can help clarify this any more.
    -=Caleb Newville=-

  7. W.H.Darby Says:

    It is the NAS computer “spitting” out what it considers the “logical” way to travel the route we think we want.
    Having spent 25 years “on the boards” I have seen some really strange “stuff” come out of the computer, and yes it is often the exact same route we filed, but the folks that program the computer no longer are required to UNDERSTAND what the controllers and pilots need and or expect, so they just program what they consider “logical”. It bugs the heck out of me as a pilot and I often ask the CD to “supress the PDR or PDAR” so it will then let him clear me as filed. Some of the newer controllers don’t know how to do that and there are some Towers or Tracons that have an internal order prohibiting it … so I don’t always get it “my way”.

  8. Joe Shugart Says:

    Basic ATC separation comes in only three forms; lateral, longitudinal and vertical. Direct routings do not provide lateral or longitudinal separation; only vertical remains. In other words, no other IFR aircraft can be cleared through the first aircraft’s altitude. In addition, direct routings can’t proctect from terrain. Radar and Visual separation techniques can reduce the minima but, due to inherent limitations, are often unavailable for use between departures and en route aircraft. Finally, adjacent ATC facilities (e.g.: a TRACON and an ARTCC) may have a Letter of Agreement that requires the controller to issue a specific clearance defined by airway or published procedure. Think of it this way: if you were issued “Direct” and smacked a hilltop, antenna or another aircraft, your attorney (or the one hired by your survivors) would grill the controller as to why a direct routing wasn’t issued – the controller’s only defense would be, according to David Heileman’s perspective, “That pilot should have been a better navigator.”

  9. Steven Dale Says:

    The real issue here is that many pilots do not try to learn or use preferential departure routes. They just file any old way that suits them. If you learn the PDR then you can file them and you won’t have this problem. Most, if not all, PDR’s are published. It just takes the time and effort to find them and use them. VOILA, no more route changes. If a PDR is not published for an airport but they issue you one….make a complaint and get them to publish it so you can file it.
    The programming it takes to be able to adapt every individual pilots filings would take a lot bigger computer system than is available today.
    As an ATCS I have seen many very lax flight plans…mostly because the pilot was too lazy to file a correct routing. An example is filing a direct route from a busy airport without regard to Special Use Airspace (Yep that flight plan takes them right through the prohibited area), or through busy airspace, and through terrain. They expect ATC to use their time to look up a good route and amend the flight plan (I know that does not address the above issue, but is an example of poor planning that can lead to that issue).

  10. Brian Veazey Says:

    David Heileman–

    I’ve been a big-city controller for 27 years now and have had more than one unpleasant experience with this issue. A airplane two miles south of where he’s expected to be can be a problem.

    A couple of points:

    1. I should have said it’s been better with FMS, not necessarily GPS.
    2. Where in the heck did you get the four miles of latitude you cite above? You’re
    expected to be on the route centerline.
    3. You sort of answered the question for yourself: “…no decent pilot would ever home to a fix.” Whoever wrote the original ASRS complaint above sure as heck isn’t a decent pilot. He gets a clearance and then complains about having to get a chart out to check it. Give me a break. A decent pilot checks the SID, route, and STAR against the FMS (or whatever)every trip. That FAR is 91.103.
    4. Mr. Darby above is quite correct. NAS computers do spit out loads of superfluous PDRs, etc., and ATC is remiss to issue them in clearances.

  11. Elliott Schiffman Says:

    Stop making excuses about .01% when it is computer programming. I often file through the NY airspace over JFK where V1, V16 and V229 overlap. It is common, regardless of how I file, to get “Standby for route change, advise when ready to copy” and the change will be simply changing one of the above for another when all three are collocated.

  12. David Heileman Says:

    Brian Veazey, it helps me to know that you are a controller; I am a pilot. Since VOR equipment on aircraft is only required to be accurate +/- 6 degrees, and since a VOR can be misaligned (relative to true north) by as much as 2 degrees before the FAA will re-align it, the whole system is inherently “fuzzy”, and is designed to be safe despite the fact that it is fuzzy. The AIM (section 5-3-5) addresses the reality that airplanes do not turn instantaneously, and also talks about the 4nm of “latitude” on each side of an airway/route centerline. I still don’t understand how, when one is talking only about VOR navigation, a clearance of “X direct Y” (where Y is an intersection on the 123 radial of X) is any different than a clearance to go “X X123R Y”. If these clearances really have different meanings, I would love to understand exactly how they are different, because I was taught to treat them exactly the same.

    To Joe Shugart’s comment, are you really saying that a direct clearance from X VOR to Y intersection (where Y is defined by a radial of X) does not provide traffic separation or terrain clearance!?!? I hope you are incorrect! It certainly contradicts my understanding…

    I gave up filing a specific route shortly after getting my instrument rating because it seemed that my filed route was always ignored by the system anyway. For the two routes I fly most frequently, the system always gives me the same clearance no matter what I file, and I don’t think I’ve actually flown the entire clearance even once — I always get amended clearances in the air — although usually the amendments are vectors, or allow me to shortcut (i.e., direct to MXE VOR when the original clearance was EMI V419 MXE).

    I do have a couple of pet peeves with ATC clearances (please don’t take this personally).

    #1 peeve is getting clearance direct to an intersection when I am not RNAV-capable and I am not on one of the radials defining the intersection. This happens to me all the time. Hello! Isn’t this why I specified slant alpha when I filed the flight plan?

    #2 peeve is getting clearance to an intersection or waypoint that is not depicted on the low altitude chart. This has only happened to me once. I had already accepted the clearance before discovering that I had no way to fly it. Fortunately my radios did not pick that moment to fail, so I was able to request an amended clearance. But shouldn’t a pilot be able to assume that any fix given in a clearance will be charted?

  13. Joe Shugart Says:

    Yes, David, from an ATC separation perspective, a direct clearance from X VOR to Y intersection is not the same as one from X VOR V123 to Y intersection. The reason is that a direct clearance has no defined dimension upon which to base separation from terrain or other aircraft. In contrast, airways, as you mentioned, are defined as plus and minus four miles (up to 10 at 51nm from the navaid). It helps to remember that the clearance issued prior to departure is primarily a non-radar routing. Even when using radar separation, controllers must know the heading of the “direct” aircraft in order to assign a sufficiently divergent heading to another IFR aircraft before discontinuing vertical separation.

  14. Joe Shugart Says:

    Correction on the airway dimensions: the protected airspace extends beyond 4nm when 51nm from the navaid on a 45 degree angle until, at 130nm, it reaches its maximum distance of 10nm from the centerline.

  15. Jean Paul Says:

    After reading the comments from Joe Shugart and Brian Veazy I now understand where the problem lies.

  16. Paul Goforth Says:

    “Pilots and controllers” are much like “Labor and Management” they have been gravelling for years and somehow just tolerate each other!.

  17. Joel Blue Says:

    The biggest problem is the “old” center computers, each center computer only knows all the fixes in it’s own airspace, and some of the bigger fixes in all the other airspace. If you file fixes (i.e. GPT..LAX) the computer computes a fix close to the center boundaries and adds them to the flight plan information then you get a FRC, because the computer needs a fix to figure the next sector to get the flight progress strip sent to. But if you file departure airport fix direct to a Lat/Long (i.e. GPT..Lat/Long..LAX & add remarks Lat/Long is LAX) the computer somehow knows how to send the flight progress strips to the right sectors, & never adds an unnecessary fix (at least I’ve never seen an added fix, but I’ve only been a controller since 1977).

  18. Joel Blue Says:

    GPT 33-56-33N/118-24-25W LAX

  19. satrap Says:

    great post. i just cant get my head around the idea of not taking this oppertunities to get what you want online.

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