Recently, while flying the LUCKI 1 RNAV ARRIVAL with the Parker Transition into San Diego, CA (KSAN) Airport, I heard the air traffic controller say, “Advise when ready to copy a phone number for a possible pilot deviation.” An airplane ahead of us had missed a crossing restriction on the arrival and was now likely more concerned about filling out NASA Reports or ASAP Reports than flying the airplane. Although most arrivals are relatively easy to fly, some planning is necessary to ensure a safe and accurate descent profile.
At larger airports, SIDs and STARs are a way of life. You may attempt to write “No SIDs/No STARs” on your IFR flight plan as you were taught when you were working on your instrument rating, but good luck with that, especially if you are flying into larger airports with higher-performing aircraft. STARs can be assigned without requesting them. Some are relatively simple ones, but others are very complex arrivals with many altitude and speed restrictions. Of course, the intent of Standard Terminal Arrival Routes (STARs) is to transition the airplane from the enroute environment to the approach phase. The STAR will typically end in one of three ways: flying directly to an Initial Approach Fix (IAF) from the STAR, vectors to an instrument approach, or a visual arrival.
Descent planning involves some calculations, either with mental math or the VNAV calculator on your airplane. The FAA offers a recommended rule of thumb to determine when you need to start your descent and how to determine a rate of descent (ROD) for a three-degree path. Divide the altitude needed to be lost by 300. For example, if you are in cruise flight at FL320 (~32,000 feet) and need to descend to 10,000 feet: (22,000/300=73). Start your descent 73 miles before your planned level-off at 10,000 feet. To determine your ROD, multiply your groundspeed by 5. 500 knots X 5 = 2,500 feet per minute rate of descent will be needed.
Here is a similar technique I learned in my early jet days that can be done quickly with mental math while flying a jet. The general rule is a 3-to-1 formula. It takes 3NM to descend 1,000 feet. Multiply the altitude to lose in thousands by 3. (22 thousand feet X 3 = 66). Start your descent 66 miles out. To determine your ROD, divide your ground speed by 2, then multiply by 10 by adding a 0. (500 knots/2 = 250 and add a “0” = 2,500 feet per minute rate of descent). Some adjustments may be necessary for a tailwind or decreasing speed to 250 KIAS descending through 10,000 feet. A simple rule is to add 1 NM for each 10 knots of airspeed you need to lose. Some pilots add 2 NM for each 10 knots of tailwind and subtract 2 NM for each 10 knots of headwind. I just use ground speed values and skip this last computation.
More advanced avionic systems, such as the devices by GARMIN, Rockwell Collins, and Honeywell, all have VNAV or Vertical Navigation functions. These more advanced units even have auto-throttles to manage speed and thrust settings. Although VNAV is great, we still see errors with these modern avionics. When setting up for the descent, pilots need to verify that the automaton is going to do what we are asking it to do. Anytime you are using automation, you need to make the request by pushing buttons on the Flight Control Panel (FCP). You then need to verify that the request is going to happen by confirming the button pressing with the display on the Flight Mode Annunciator (FMA). I hear some pilots refer to the FMA as the “scoreboard.” Always verify!
But when using the VNAV function there are a few more steps to make sure the action is going to happen. Verify that the Top of Descent (TOC) point is ahead of you. Sometimes a late descent from ATC may require a steeper than normal descent. Activate the VNAV button and verify that “VPATH” or equivalent wording is displayed on the FMA. Verify that the altitude selector is set for the bottom altitude you are descending to, or the descent will not occur. I also review the default setting of the descent path angle on my VNAV page. It is usually set to 3.0 degrees, but last week I found that someone had changed it to 2.5 degrees. Not a problem if I was planning on descending at that rate, but it could have been a problem if I was expecting a steeper descent and did not get it. Having the “Banana,” or Range to Altitude, is also a good feature to have turned on. And then crosscheck that the fixes, altitudes, and speeds in the FMS match the STAR.
As you see above, there are many ways to error in a descent using automation. The most common way to err and miss a crossing restriction when not using VNAV automation is just to miss or be late on the descent point or planned TOD. This is easy to do if ATC gives a descent with a crossing restriction somewhere ahead of your present position.
An early descent will result in more flight time at lower altitudes, with increased fuel consumption. A late descent will result in difficulty controlling airspeeds and descent rates. The two types of VNAV descents that are mostly invisible to pilots are the performance path and the geometric path. Performance path provides a more efficient (at least for jets) descent at or near idle power to the first constrained waypoint. A geometric path provides a computed 3-D point-to-point path between two constrained waypoints and is a shallower descent, typically not at idle power.
ATC may issue a clearance to “descend at pilots’ discretion” where the pilot may begin the descent whenever they choose. During our descent on the LUCKI 1 RNAV ARRIVAL into San Diego, the bottom altitude on the arrival at LYNDI Intersection was charted at 5,000 feet and 210 kts; however, ATC advised us to descend via the arrival, except maintain 12,000 feet. At 12,000 feet ATC later assigned us lower altitudes, different than the published altitudes, but kept us on the lateral portion of the arrival. ATC may modify the arrival altitudes as well as delete or assign speed limits. They may also vector you off the arrival. But the more challenging issue is that they sometimes take you off an arrival, then have you rejoin it. This could catch you by surprise if you thought you were being vectored for the approach, and you deleted the arrival.
As we were headed back from California to Midway Airport (KMDW) we were assigned the MOTIF 6 ARRIVAL with the IRK Transition. Note that this arrival is based on conventional NAVAIDs (no RNAV or GPS equipment required), and the altitudes on the arrival (KORTT Turbojets Expect 17,000, MOTIF Expect 10,000, MINOK Expect 6,000) are all listed as “EXPECT.” These altitudes are not part of the arrival unless ATC specifically assigns them, such as “cross MOTIF at 10,000 feet.” Furthermore, these altitudes are not displayed on the Foreflight Map or on the airplane FMS. (see Figures 1 and 2).
As we neared our destination, ATC stated “advise when ready to copy a reroute.” ATC changed our arrival to the ENDEE 6 RNAV ARRIVAL but kept the IRK Transition. This is an RNAV arrival, so GPS or RNAV equipment is required. The ENDEE 6 arrival has mandatory altitude and airspeed restrictions. This arrival also displays the altitude and airspeeds on the Foreflight Map and on the airplane FMS. (see Figures 3, 4, and 5).
“Cleared LUCKI 1 RNAV ARRIVAL, descend and maintain 12,000 feet.” This is clearance to descend and maintain 12,000 feet. Maintain this altitude till cleared for further descent.
“Descend Via the LUCKI 1 RNAV ARRIVAL.” This is clearance to follow the lateral guidance and vertical altitude restrictions on the chart (as well as airspeeds) all the way down to the bottom altitude of 5,000 feet.
“Descend Via the LUCKI 1 RNAV ARRIVAL, except maintain 12,000 feet.” This is a clearance to follow the lateral and vertical guidance until reaching TRIXI and then maintain 12,000 feet until cleared to a lower altitude by ATC.
STARs may serve multiple airports or multiple runways. Sometimes it is necessary to enter the instrument approach into the FMS to populate the full routing of the STAR. Review the STAR before you get busy in the descent. Also, listen carefully to the descent clearance as they can have many nuances.
