Crossing Airways
Keith Thomassen, PhD, CFII
www.avionicswest.com

Many airways now have waypoints at crossings in the event your clearance is to follow one of them and then transition to the other.  With a waypoint there you can simply program it into your GPS flight plan and the autopilot will make the transition. But if there is no defined crossing waypoint, how do you do this?  You may not think this is a practical problem because you never get such clearances, but if you do, or want to see how many ways you can do this, read on.

For this example, you’re in central California heading SE on V107 from AVE towards FIM.  This airway is the 310° radial from FIM (130° To).  As weather deteriorates you file for V107 to V12 (yup, you’re going to do it to yourself), then east on V12 to PMD, direct Apple Valley (KAPV).  V12 is the 251° radial from PMD.  (See these airways in Figure 1.)

There are lots of options for making this transition, but for starters you might display low airways on your GPS or MFD for assistance.  Now, while you can select headings (manual or autopilot) to fly the airplane and observe your progress along V107 and turn onto V12 when you get there, is this legal? No, since these maps are not your primary navigation information.  Your legal navigation information is your flight plan course displayed on a CDI/HSI.  That means you either have to be tracking a VOR or GPS course along your cleared route.  So, selecting headings to track these map lines (autopilot or not) isn’t legal.

One obvious way to do this is to track V107 using your VOR (track the 130° to FIM).  With a 2nd VOR, tune in PMD and select the 071° course to PMD and prepare to switch to that VOR as you approach the crossing. With these courses displayed on your CDI/HSI, whether flying by hand or using the autopilot, you’re now legal.

Suppose you want to do this with GPS tracking instead.  First you need to decide how to make the flight plan.  Assume your plan starts at AVE and has PMD then KAPV at the end.  What intermediate waypoints do you insert between AVE and PMD?  The last waypoint on V107 before reaching V12 is REYES and just beyond V12 is PIRUE.  Between the crossing and PMD there are no waypoints, but west on V12 is EMUXY, 76 nm from the crossing.

If you simply insert REYES, your next leg (when you get there) is direct to PMD, not on the airway.  But if you put in PIRUE, then you can go D-> PMD at the crossing.  If you do this too early you’ll have a course line that does not correspond to your clearance. Alternatively, as you approach the crossing, you can fly to PMD using the OBS mode – select the 071° GPS course to PMD.  Don’t forget to restore sequencing after you go into the OBS mode.

A slightly more devious solution (but easy to execute) is to put in PIRUE AND EMUXY.  Your flight plan now crosses V12, and then goes backwards to EMUXY to get to V12. However, there is now a leg in your flight plan from EMUXY to PMD, which is on V12. So just before you cross V12 (on the way to PIRUE) you could use Activate Leg to make a proper course to PMD. In GPSS mode the autopilot will smoothly make that transition.

Let me digress now and point out that you can search for that crossing on an iPad with ForeFlight.  Put FIM310/PMD251 into the search box, as in Figure 1 then hit SEARCH on the keyboard.  Touch the crossing point and the lat/lon of that crossing is displayed.

ipadpic

Figure 1.  An iPad search in ForeFlight for crossing radials (FIM310/PMD251) of V107 and V12.  By touching that point, the lat/lon of that crossing is displayed (34.57°N/119.06°W).

Now, you could make a User Waypoint at that lat/lon and insert it into the flight plan.  With it inserted your flight plan is your exact clearance, whereas the other techniques require you to fly the plan differently than it is programmed (using Direct or Activate Leg at the appropriate time)

 

 

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