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?