The RNAV SIDs and STARs have been around for a few years with more popping up every day. In fact, they’ve become such an integral part of the ATC system that you hardly see any new SIDs or STARs that aren’t RNAV. Although we’ve published a few articles that cover certain aspects of these procedures, we still get a lot of questions about them. It’s time for a thorough analysis of these procedures.
What Are They?
Although possibly daunting with all their restrictions and assignments, these are still the SIDs and STARs we’ve known for a long time. The only difference from one designed a couple decades ago is that these follow RNAV paths. In addition to simpler navigation—OK, simpler if you’ve got the procedure in your database—these SIDs and STARs also usually include altitude and speed restrictions at some of the waypoints.
Those altitude restrictions are typically in the range of a few thousand feet above the airport up to the flight levels, and the speeds today might range between 180 and 280 knots. Thus, these procedures are seldom encountered by aircraft burning gasoline. Nonetheless, these procedures are becoming so popular that we’d be wise to expect to see procedures suitable for piston GA at some point.
But, let’s take a step back a moment and look at the real purpose of SIDs and STARs. I can’t provide an overview any better than the FAA did in the Instrument Procedures Handbook. In talking about SIDS, it says: “While obstacle protection is always considered in SID routing, the primary goal is to reduce ATC/pilot workload while providing seamless transitions to the enroute structure. SIDs also provide additional benefits to both the airspace capacity and the airspace users by reducing radio congestion, allowing more efficient airspace use, and simplifying departure clearances. All of the benefits combine to provide effective, efficient terminal operations, thereby increasing the overall capacity of the national airspace system.”
You could substitute STARs and arrivals for SIDs and departures in the above and it would be equally valid, but let’s dig a bit deeper. These routes were originally developed using VORs, but didn’t necessarily follow airways. By creating a route from an enroute VOR to or from an airport, the FAA got what would sometimes be a complex route that required airways, radials and distances that they could assign with a single name, saving a lot of time in issuing a clearance and vectoring aircraft to and from an airport.
But they weren’t perfect. While the lateral path was carefully prescribed and notes about speeds and altitudes to expect were provided, the biggest terminal’s controllers still found themselves quite busy assigning those speeds and altitudes. Plus, of course, the limitations of ground-based navaids in busy terminal areas often meant that more direct or desirable routes couldn’t be constructed. More work was needed.
With the dominance of area navigation (GPS for those of us who lack rho-theta and inertial-navigation FMS boxes), the restrictions of ground-based navaids could be overcome. About the same time, the airspace around some of the busiest airports began to get stupid-crowded. Something needed to be done to increase capacity and to give the controllers some breathing room.
RNAV-based SIDs and STARs are little different in theory than those built on terrestrial navaids. They still follow a specific path over the ground between the airport and the enroute structure. But rather than a few expected altitudes and speeds, the RNAV versions of these procedures introduced absolute speeds and altitudes. This way, dozens of ATC instructions to an arriving aircraft, for instance, could be reduced to a simple, “Descend via the ARIVL One.”
I used to fly into Chicago O’Hare a lot. Before RNAV SIDs and STARs, ATC frequencies as far away as even a couple hundred miles would be going nuts with all the radio calls assigning new altitudes and issuing speed restrictions to organize the dense flow of aircraft into that monster airport.
Now, using RNAV procedures, the radio is still busy simply from the volume of traffic, but it’s lost the frenetic pace of a few years ago. You can actually contact a controller and expect a response on your first call. The RNAV procedures work. But, they’re still not always perfect.
Vectors, Speeds and Altitudes
At most large airports, RNAV procedures allow the traffic to follow the course of the SID or STAR with little more attention than frequency changes. Some STARs even directly feed an approach. At Los Angeles International, for example, traffic still 100 miles out to the east on one of the STARs will often be cleared, “Descend via the SEAVU Two arrival. At SEAVU, cleared for the ILS Runway 25L.” You might not get anything more than frequency changes until the tower clears you to land. This is depicted in a combined graphic on the previous page. (Note that neither the STAR nor the approach portions are to scale.)
I said, “Might not.” That means you might. The letters of agreement between centers and approach-control facilities typically specify that the center will feed traffic onto the active STAR with everybody neatly spaced out. However, like approaches have multiple feeder routes, STARs often have multiple transitions to allow traffic from different directions to eventually enter the same stream. So, combining traffic from those transitions often means that further work is necessary to get everybody funneled into that single stream.
The tools available to do that are still the same speed, altitude and vectors. So, that nice orderly STAR can still require the controllers to get creative and that’s when the frequencies get congested with speed and altitude restrictions and even vectors off the procedure for spacing. The language required to make those adjustments in an arena of fixed, assigned routes with required speed and altitudes can get confusing.
The phraseology used to vary from the published speeds and altitudes is the same for SIDs and for STARs, with only the context of climbing/departing and descending/arriving changing. See the chart for an explanation common ATC phraseology. As always, though, the best practice is to ask if you’re not certain.
When you’re on an RNAV STAR you start at your assigned altitude and speed, then just follow the speed and altitude restrictions on the procedure. Some procedures can be quite daunting with a lot of restrictions, but once you get used to them they’re not difficult to fly—as long as you carefully brief them. The only gotcha is that there might be a small note about initial speeds on the procedure that might say something like “Descend via Mach number until intercepting 280 knots. Maintain 280 knots until slowed by STAR.” See the example at the top of the facing page.
This can be a gotcha because you’re used to looking for speed restrictions at the fixes, not in the chart notes. The indicated airspeed for a given Mach number gets higher as you descend, so the idea is to descend maintaining your desired Mach speed until you reach 280 knots, then simply maintain 280 knots.
SIDs have a similar gotcha in that the clearance is usually to “Climb via the SID.” Climb to what altitude? Only restrictions are listed by the waypoints on the SID; the expected top altitude is listed in the notes that might even be over on a second page with the text. Rarely, no top altitude is published, in which case you’ll be given one as part of your clearance. Be sure to check.
Similarly, there might be a general speed restriction in the notes such as, “Do not exceed [speed] until [altitude/fix].” Since the listed fix might be several fixes into the SID or there might be no other speed restrictions, you might not notice the speed restriction until it’s too late and you say, “Oh, ASRS!” The lower chart to the right shows an excellent example of this.
Finally, once you get a speed restriction, continue to comply with it until you get a different one or until the restriction is relaxed. Say you cross a fix on a STAR with a crossing restriction of 230 knots and the next fix requires 210 knots, maintain 230 knots until you need to slow to make the 210 restriction.
Even the most complex RNAV SID or STAR isn’t difficult to fly, but it does require you to pay attention. No longer can you simply let the controller do all the planning to get you onto an approach. Instead, that workload is now transferred to you. Ease the load with a careful briefing in advance, by closely following the restrictions, flying conservatively and always planning ahead. That kinda just sounds like good technique for all your flying, eh?
Since he’s studied these RNAV procedures closely, Frank Bowlin now exclaims, “Oh ASRS!” far less frequently.