Updated Definition of “Established” On Course

The basic definition of established, from the Pilot/Controller Glossary, is:

ESTABLISHED−To be stable or fixed on a route, route segment, altitude, heading, etc.

The ICAO definition, which appeared in an appendix of a former edition of the Instrument Procedures Handbook, included the following language:

…an aircraft is considered established when it is “within half full scale deflection for the ILS and VOR; or within ±5 degrees of the required bearing for the NDB

A more detailed definition of established in the AIM (see below) focuses on RNAV and RNP operations. (For most general aviation pilots, that means GPS/WAAS procedures.) It’s the result of a long discussion among members of the FAA Aeronautical Charting Forum–Instrument Procedures Group. The topic (item 96-01-166) began as a discussion of “on course” but eventually narrowed its focus to “Determining Descent Point of Flyby Waypoints,” which is another way of stating the primary issue: When is it OK to descend (consistent, of course, with your last clearance from ATC) as you join the next segment of an approach or transition?

AIM 5-5-16

11. Definition of “established” for RNAV and RNP operations. An aircraft is considered to be established on-course during RNAV and RNP operations anytime it is within 1 times the required accuracy for the segment being flown. For example, while operating on a Q-Route (RNAV 2), the aircraft is considered to be established on-course when it is within 2 nm of the course centerline.

NOTE: Pilots must be aware of how their navigation system operates, along with any AFM limitations, and confirm that the aircraft’s lateral deviation display (or map display if being used as an allowed alternate means) is suitable for the accuracy of the segment being flown. Automatic scaling and alerting changes are appropriate for some operations. For example, TSO-C129 systems change within 30 miles of destination and within 2 miles of FAF to support approach operations. For some navigation systems and operations, manual selection of scaling will be necessary.

(a) Pilots flying FMS equipped aircraft with barometric vertical navigation (Baro-VNAV) may descend when the aircraft is established on-course following FMS leg transition to the next segment. Leg transition normally occurs at the turn bisector for a fly-by waypoint (reference paragraph 1-2-1 for more on waypoints). When using full automation, pilots should monitor the aircraft to ensure the aircraft is turning at appropriate lead times and descending once established on-course.

(b) Pilots flying TSO-C129 navigation system equipped aircraft [i.e., non-WAAS GPS] without full automation should use normal lead points to begin the turn. Pilots may descend when established on-course on the next segment of the approach.

The key phrase is “within 1 times the required accuracy for the segment being flown.” To know that, you must understand how your GPS box works. For example, the  Pilot’s Guide and Reference for the Garmin GNS530W notes that:

…The [default] CDI scale is set to 2.0 NM during the “en route” phase of flight. Within 31 NM of your destination airport, the CDI scale gradually ramps down to 1.0 NM (terminal area). Likewise, when leaving your departure airport the CDI scale is set to 1.0 NM and gradually ramps up to 2 NM beyond 30 NM (from the departure airport). During approach operations the CDI scale gradually transitions down to an angular CDI scale. At 2.0 NM of the final approach fix (FAF), CDI scaling is tightened from 1.0 to the angular full scale deflection (typically the angular full-scale deflection is 2.0°, but will be as defined for the approach). (p. 182)

Those scales are consistent with TSO C146a standards for WAAS-based avionics, which, for example, tighten the default CDI scale for en route operations from 5 nm (1 nm/dot) to 2.0 nm. On units like the Garmin GNS530/430 series that show 5 dots either side of the center of the built-in CDI, that scale equates to 0.4 nm/dot. (For a good discussion of this topic, see “Changing CDI Course Sensitivity” and “Enroute CDI Sensitivity with WAAS” in Chapter 13 of Rod Machado’s Instrument Pilot’s Handbook.)

If you’re curious about the various RNAV levels associated with different phases of flight and types of procedures, and the capabilities of various avionics (subject to any limitations in your AFM), see:

*To keep up with substantive changes to the AIM in each new edition, see the Explanation of Changes sections published at the AIM home page. You can find PDF versions of the AIM and updates here.

One Response to Updated Definition of “Established” On Course

  1. Pingback: Changes in February 9 2012 Edition of the AIM « BruceAir, LLC (bruceair.com)

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