Loading Procedures from Databases

Most IFR pilots who use GNSS (GPS) are aware that they must load instrument approach procedures (IAPs) by name from the unit’s database. But there’s some confusion in IFR land about flying instrument departure procedures, arrivals, and other routes.

For more information about database currency, see Database Currency for IFR Operations here at BruceAir.

AIM 1−1−17. Global Positioning System (GPS), 2. IFR Use of GPS, includes the following paragraph about IAPs:

(3) All approach procedures to be flown must be retrievable from the current airborne navigation database supplied by the equipment manufacturer or other FAA−approved source. The system must be able to retrieve the procedure by name from the aircraft navigation database, not just as a manually entered series of waypoints. Manual entry of waypoints using latitude/longitude or place/bearing is not permitted for approach procedures. (p. 1−1−20)

That language specifically address IAPs, but it doesn’t mention DPs, STARs, or airways.

If you use an IFR-approved GNSS (see AIM 1−1−17. Global Positioning System (GPS), 2. IFR Use of GPS for the details), you should check the Aircraft Flight Manual Supplement or the AFM (if you fly an aircraft with an IFR-approved GNSS installed as original equipment) for the limitations associated with the unit(s) in your aircraft.

For example, the AFM supplement (a required document for a unit installed under an STC) for the Garmin GNS 530W includes the following language:

2.5 Flight Planning

Whenever possible, RNP and RNAV routes including Standard Instrument Departures (SIDs), and Standard Terminal Arrival (STAR), routes should be loaded into the flight plan from the database in their entirety, rather than loading route waypoints from the database into the flight plan individually. Selecting and inserting individual named fixes from the database is permitted, provided all fixes along the published route to be flown are inserted. Manual entry of waypoints using latitude/longitude or place/bearing is prohibited. (Garmin document 190-00357-03_F)

Similar language appears in the AFM supplements for the GNS 430, GTN 750, and GTN 650 units. For example:

2.4 Flight Planning

Whenever possible, RNP and RNAV routes including Standard Instrument Departures (SIDs), Standard Terminal Arrival (STAR), and enroute RNAV “Q” and RNAV “T” routes should be loaded into the flight plan from the database in their entirety, rather than loading route waypoints from the database into the flight plan individually. Selecting and inserting individual named fixes from the database is permitted, provided all fixes along the published route to be flown are inserted. Manual entry of waypoints using latitude/longitude or place/bearing is prohibited. (AFMS, Garmin GTN GPS/SBAS System, 190-01007-A2 Rev. 8)

You should take care, however, when entering a departure procedure as a series of fixes rather than by name from the database. A DP is more than a series of points defined by LAT/LON. A DP typically contains several types of legs, and you must ensure that you understand how each leg works and how the GNSS in your aircraft handles different leg types and interfaces with your autopilot.

You can find more information about leg types in Avoiding Confusion when Flying GPS Legs here at BruceAir. See also Chapter 6 of the Instrument Procedures Handbook.

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Impact of Magnetic Variation on PBN Systems

If you use an IFR-approved GNSS (GPS), you’ve probably noticed that the courses shown on the GNSS usually don’t match the numerical values printed on charts, if only by a few degrees. For example, as you fly an instrument procedure, a WAAS GNSS prompts you at each turn point to turn to a new course, but the number displayed on the screen is probably 2-3 degrees different from the number on the chart.

These differences are explained in AIM 1−1−17. Global Positioning System (GPS) , paragraph k. Impact of Magnetic Variation on PBN Systems (p. 1-1-27):

(1) Differences may exist between PBN systems and the charted magnetic courses on ground−based NAVAID instrument flight procedures (IFP), enroute charts, approach charts, and Standard Instrument Departure/Standard Terminal Arrival (SID/STAR) charts. These differences are due to the magnetic variance used to calculate the magnetic course. Every leg of an instrument procedure is first computed along a desired ground track with reference to true north. A magnetic variation correction is then applied to the true course in order to calculate a magnetic course for publication. The type of procedure will determine what magnetic variation value is added to the true course. A ground−based NAVAID IFP applies the facility magnetic variation of record to the true course to get the charted magnetic course. Magnetic courses on PBN procedures are calculated two different ways. SID/STAR procedures use the airport magnetic variation of record, while IFR enroute charts use magnetic reference bearing. PBN systems make a correction to true north by adding a magnetic variation calculated with an algorithm based on aircraft position, or by adding the magnetic variation coded in their navigational database. This may result in the PBN system and the procedure designer using a different magnetic variation, which causes the magnetic course displayed by the PBN system and the magnetic course charted on the IFP plate to be different. It is important to understand, however, that PBN systems, (with the exception of VOR/DME RNAV equipment) navigate by reference to true north and display magnetic course only for pilot reference. As such, a properly functioning PBN system, containing a current and accurate navigational database, should fly the correct ground track for any loaded instrument procedure, despite differences in displayed magnetic course that may be attributed to magnetic variation application. Should significant differences between the approach chart and the PBN system avionics’ application of the navigation database arise, the published approach chart, supplemented by NOTAMs, holds precedence.

The key text is:

a properly functioning PBN system, containing a current and accurate navigational database, should fly the correct ground track for any loaded instrument procedure, despite differences in displayed magnetic course that may be attributed to magnetic variation application.

Paragraph 2 in that section of the AIM also notes that:

(2) The course into a waypoint may not always be 180 degrees different from the course leaving the previous waypoint, due to the PBN system avionics’ computation of geodesic paths, distance between waypoints, and differences in magnetic variation application. Variations in distances may also occur since PBN system distance−to−waypoint values are ATDs [along-track distances] computed to the next waypoint and the DME values published on underlying procedures are slant−ranged istances measured to the station. This difference increases with aircraft altitude and proximity to the NAVAID.

A similar question also arises when comparing DME and GNSS-derived distances, such as when flying holding patterns.

AIM 5−3−8. Holding provides further details on this issue in:

  • 5. Distance Measuring Equipment (DME)/GPS Along−Track Distance (ATD)
  • 6. Use of RNAV Distance in lieu of DME Distance

Substitution of RNAV computed distance to or from a NAVAID in place of DME distance is  permitted when holding. However, the actual holding location and pattern flown will be further from the NAVAID than designed due to the lack of slant range in the position solution (see FIG 5−3−7). This may result in a slight difference between RNAV distance readout in reference to the NAVAID and the DME readout, especially at higher altitudes. When used solely for DME substitution, the difference between RNAV distance to/from a fix and DME slant range distance can be considered negligible and no pilot action is required.

For more information about using GNSS to substitute for or to complement conventional navaids, see Use of GPS on Conventional Approaches (Update) and Use of Approved GPS (RNAV) Systems on Conventional Procedures and Routes here at BruceAir.

For more information about how the FAA assigns magnetic variation to VORs and other navaids, see FAA Order 8260.19H.

Jeppesen also discussed this issue in one of its Chart Clinic series: The Chart Clinic – Database Series.