What is the operational advantage of starting an RNAV SID with a VI-CF leg combination?

Question

I am examining Standard Instrument Flight Procedures, mainly from FAA's charts and Coded Instrument Flight Procedures ARINC-424 file (CIFP). I am keen in understanding the operational advantages or other criteria by which a procedure designer would choose a certain leg combination over another.

As far as I could tell by searching through the aforementioned CIFP file, the following leg-combinations are mostly used for the first couple legs of an RNAV departure procedure:

• VA (Heading to Altitude) -> DF (Direct to Fix)
• VI (Heading to Intercept) -> CF (Course to Fix)

The first leg can also be DF, CF or TF, as it is stated in FAA's 8260.58A Performance Based Navigation manual, in Chapter 5. Departure Procedures, section 5-1-1. General.

For an example, take BRYCC4 Departure and its respective narrative page. Takeoff from runways 16L/R uses a VA-DF leg combination ("Climb heading 173° to 5934, then right turn direct to cross MUGBE at or below 10000..."), whereas takeoff from Runway 17L uses a VI-CF leg combination ("Climb heading 173° to intercept course 127° to cross GISTT at or above 7000...").

I can understand the need for a VA leg before a turn, since the aircraft is required to climb to a certain altitude (eg. for LNAV engagement or for obstacle clearance) before the first turn. However, I have a difficulty grasping the circumstances under which a VI leg is a more preferable choice.

Could someone point me to any source material that describe the criteria on which the decision about the leg combination during takeoff is based?

Answer 1

The PBN manual is the place to start. In Section 5 on departure procedures it includes the following reference:

b. Leg type limitations. See Order 8260.46, paragraph 3-1-5 for permissible leg types.

Also, all the TERPS requirements also apply, except where modified by the PBN manual.

There's usually no one rule that applies universally. It's about finding the best solution that meets the terrain/obstacle avoidance and meets the necessary airspace and air traffic needs. I can't say with certainty why the procedure writer made the choices they did, but I can point out a few things that would apply in the SID you reference.

The 17L/R departure is primarily constrained by the 7000 ft minimum at GISTT. It's there to keep the path within the Class B airspace. There's a cutout with a 7000' floor around Front Range airport to the southeast. GISTT is located at the edge of the cutout. It's far enough south to leave room to reach the minimum altitude at the published minimum climb gradient. The VI/CF is the solution that gets you to GISTT expeditiously. It also has an intercept angle of about 50 degrees which works well with the FMS and LNAV steering.

The 16L/R departure has two factors that push it to the VA/DF solution. First is that it departs over runway 07/25 and there's no intercept (more later), the altitude termination defines when the turn can begin. The reason the DF to MUGBE is used instead of a CF is that there is a 140 degree course change. You can't have a VI/CF combination with that large of a course change. Thus the VA/DF combo. If MUGBE (or another convenient waypoint) was better located, a VA/RF leg combo could work but RF legs are relatively 'new' in the RNP FMS world and they aren't very common.

Answer 2

Maybe I am not grasping all the nuances here, but it seems pretty obvious to me that if obstacle or terrain clearance is required you would design a departure procedure to maintain a certain heading until clearance is assured, (VA) then route from there. If obstacles or noise abatement is not an issue, then the earliest possible turn to intercept an airway, (VI) or proceed direct to a fix on the intended route is a more efficient path. This would allow the aircraft to turn on course as soon as possible.