Courtesy of Andrew Ayers

The enroute part of IFR flying isn't tremendously complicated. We'll be spending most of our time tracking airways and intercepting new ones. Occasionally we'll be handed off to a new Center controller and in rare circumstances, we'll be asked to hold. Let's first talk about airways tracking

As we learned before, airways are simply imaginary paths which are based on radials to and from VORs. We try to fly as directly as possible, but VORs aren't always going to be in straight lines between where we're starting from and where we're going. So, we're going to have to piece together the airways which will take us to our destination. Along the way, we'll have to be diligent to maintain our course and be aware of when our next changeover is coming. Now, airways are protected 4NM either side of the centerline, so we have a little room to play with before getting ourselves into danger of obstruction interference, but that doesn't mean we should get sloppy with our tracking skills.

So, let's take a look at a sample flight from Vero Beach to West Palm Beach International (PBI). Near the top, you'll see Vero Beach. West Palm Beach is off the chart to the southeast. If you look at "D", you'll see that it says "Palm Beach 115.7". This indicates that the airways which are going off the map at this point are based on the Palm Beach VOR (which is on the field at PBI). So, if you look back up at Vero Beach, you'll see that V3 runs in a straight line directly from the Vero Beach VOR to the Palm Beach VOR. This seems like the logical choice for us to use then right? Well, sure it is, but you're not getting it. Unless you're flying high, there's too much VFR traffic coming up and down the coast in that area, so ATC isn't going to want to keep and eye out for you the whole time. Instead they'll bring you out and then back in when you get near Palm Beach.

So, let's assume that we were given this clearance: "9246F is cleared to the West Palm Beach International airport via Victor 159 TBIRD Victor 531..." Looking on our chart at "A", we see that V159 extends south along the 182 radial FROM Vero Beach. At 31DME, it passes through TBIRD intersection ("B"). From there, we see that V531 ("C") extends southeast bound 35DME to the Palm Beach VOR via radial 327 FROM.

Alright, now that we know how we're supposed to proceed to PBI, let's talk a bit about altitudes. You'll remember that going on a magnetic course of 180-359 degrees (see the altitude chart for details), we've got to fly even thousand feet intervals. VOR radials are magnetic, so along V159, we're going to be going on a magnetic course of 182 degrees. We see that the MEA along that segment of V159 is 2500ft, so we'll have to go at least 4000ft. That'll be fine for us, so we'll plan on that. However, once we cross TBIRD, we'll be proceeding along a magnetic course of 147 degrees(the reciprocal of 327 FROM). So, now we've got to fly an odd thousand foot altitude. The MEA along this segment of V531 is 3000ft, which will work fine for us, so we'll plan to make a descent to 3000ft after passing TBIRD.

This brings me to an important point which applies more to climbs, but also to descents. If you are proceeding along an airway and you're about to cross into a section with an MEA higher than your present altitude, you are expected to start your climb to your new altitude UPON crossing into that segment. Some intersections have Minimum Crossing Altitudes (MCAs) which indicates that you should climb to the higher altitude BEFORE crossing into the new segment. You'll see an "X" with an altitude by it at those intersections, but they're not all that common. OK, let's go flying!

We'll assume that, like our "Departures" example, we've gotten radar vectors to join our assigned course. Now we're established on V159 and we're tuning our radios to intercept V531. We know that TBIRD is 31DME from VRB, so we don't actually need a second NAV radio to define TBIRD. We could simply fly until 31DME, switch to the Palm Beach VOR frequency, tune the new radial, and turn on course. But, for the sake of backup and practice, we'll also use dual NAVs to fly our courses. So, established on V159, we're proceeding along course with NAV2 set to VRB radial 182 FROM. We've set NAV1 to PBI radial 147 TO (reciprocal of 327 FROM).

Figure 2: NAV2 shown on the HSI indicates heading 182 with CDI centered. NAV1 OBS set to 147 and we are left of that course (V531). Note the DME in the top right shows 22.5 NM from VRB.

The panel shown here is from an early version of X-Plane. NAV2 is shown on the HSI instrument (follow arrow to the left of "A"). NAV1 is the VOR instrument shown (follow arrow to the right of "A").

Once the needle on NAV1 centers and the DME (set to NAV2) reads 31, we know that we've reached TBIRD. At that point, we'll turn to track NAV1 and continue inbound towards PBI. Also, at this point, we need to start our descent down to 3000ft. ATC expects that we know the rules and will be expecting us to make our descent. However, we still need clearance to vacate any assigned altitude. So, we could call Center and ask for clearance, something like "Miami 9246F would like a descent to 3000" to which they would reply "9246F descend and maintain 3000ft". We would then report leaving 4000 for 3000.

Figure 3: 31 DME from VRB we are starting the turn to intercept V531. Note the CDI on NAV1 is centering, attitude indicator (ADI) shows bank left (about 15 degrees of bank), turn coordinator shows aircraft in bank (less than standard rate turn). Starting to lose a bit of altitiude, better pull back on the stick a little!

I should also note that any ATC clearance supercedes the course/altitude rule. If ATC tells us to maintain 4000ft, even though we're now on a magnetic course that dictates we fly odd thousands, we do it. Now that we're established on V531, I'd begin the arrival procedure which we will discuss in the next lesson. I'd set NAV2 to the ILS frequency in preparation for the approach and continue inbound.

Figure 4: CDI on NAV1 centered indicating we are tracking V531 now. Straight and level, NAV2 has been retuned for the ILS frequency in preparation for the approach phase of the flight.

So that's about all there is to enroute flying. We won't discuss holding procedures at this point. That will be a subject of a future lesson. Thanks and enjoy!


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