In the Boeing 737 NG series, can the usual 3 degrees Path angle used for VNAV Descent be modified through the FMC to another angle?
In the 737 NG, the descent angle is never entered by the pilot into the FMC. One enters a Cost Index, or alternatively a descent speed that's desired, and the FMC, considering forecast winds, computes the Top of Descent point and the angles that it expects to fly in order to achieve that speed.
But, it never tells the pilot what descent angle it's planning to use (although you can see the present vertical angle to a specified waypoint/altitude). Whatever the angle it's planning, you can steepen it or shallow it by increasing or decreasing (respectively) the planned descent speed or Cost Index.
But, unlike some other FMC's, the 737's never explicitly tells you the planned descent angle for enroute operations.
(On an approach, it's a different story; there your gradient path is coded with the waypoints, and it is displayed on the LEGS page. But the user can't change values in the database like that. Speeds, yes; GP angles, no. A pure visual approach, with only a runway-end waypoint is the ONLY time you can specify whatever angle you wish... but that's only to the runway itself, and not at all usable on any other leg.)
I am type-rated on the 737-800 and I have never used the FMC to program a specific angle that I want it to fly. The most economical way to descent is to use your VNAV profile that has been calculated (provided that it has necessary information like Descent Winds, speed / altitude restrictions and E/D point). So by selecting an instrument arrival procedure, it automatically generates an E/D point.
As from the FCOM 11.31.29 , a vertical angle can be assigned to approaches, approach transitions and STARs.
Ralph is right on the time when you can only change it with a visual approach. The only time I have ever used it to set a particular Flight Path Angle is during a visual circuit maneuver where I would enter the FPA of 3 degrees in the approach procedure page when I enter the procedure preference in the DEP/ARR page.So example, when you pick a particular runway for the visual maneuver, it has some blanks for you to fill in the FPA and the runway ext. point.