Boeing VNAV logic question

  • Hello,

    What is the specific logic of VNAV SPD and VNAV PTH? In what circumstances will one be chosen?

    Sometimes VNAV SPD gets me way higher on profile, can I manually choose one over the other?

    Also, sometimes the FMA gets into a seizure switching between the two special times a second, is it a bug or am I simply on the edge of the two?


  • In VNAV SPD the autopilot pitches for the VNAV target airspeed or Mach number with the auto-throttle targeting either climb thrust or idle thrust. During VNAV PTH the autopilot pitches up and down to stay on the computed vertical profile and the auto-throttle maintains the airspeed. There are a lot of conditions for each but in general the path mode is only selected when you are close to the computed path, at least right now. I know that in the real world the aircraft is capable of diving down a lot more in VNAV.

    During the approach phase the target airspeed switches from a fixed speed target to an adaptive mode, where the airspeed for the next flap extension is targeted. If you then extend flaps the target airspeed drops down to the next higher flap speed. This may currently trigger a switch from path to speed until the airspeed is reduced and then back to path when you are back on profile.