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So, I'm very confused. Im from a Cessna background, no FMC. So, based on my understanding, you could run the autopilot independent of the FMC right? With Speed mode, HDG SEL mode, and V/S mode, am I right or wrong?

And if you enable VNAV and LNAV, the FMC takes over right? But I heard people say you can change the V/S in VNAV mode. How is that possible? I thought the FMC has full authority, and the flight route you filed in the FMC is final? And does this mean you can also change the HDG in LNAV mode?

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I see some confusion about the Autopilot system here. First let's clear something up:

  1. The B737 has 3 guidance: speed, horizontal and vertical. These are displayed on the top of the PFD from left to right.
  2. Engaging Flight Director displays the guidance but not to follow it, engaging the Autopilot tells the plane to follow the guidance.
  3. Each guidance has several modes. E.g. Possible horizontal guidance modes include "HDG SEL", "HDG HLD", "LNAV" etc. Possible vertical guidance modes include "V/S", "ALT HLD", "VNAV" etc.

LNAV is just one of the horizontal guidance modes. The FMC decides what is LNAV. If LNAV is not engaged, then the FMC does not take part in horizontal navigation.

Your confusion likely arises from armed modes. The B737 allows you to select one active mode, plus one armed mode. For example, consider the LNAV path in FMC is heading North (360 degrees). The plane is to the East of the intended path (right side). By doing the following:

  1. Set the heading window to "340".
  2. Set "HDL SEL" as active mode.
  3. Set "LNAV" as armed mode.

The plane is on a heading to join the LNAV course. When the plane is close*1, the horizontal mode will automatically change from "HDG SEL" to "LNAV".

Similarly, "V/S" can be used to capture VNAV profile when the plane is above or below the intended altitude.

*1: The plane has to be within a certain nautical miles and the intercept angle cannot exceed a certain degree. I will update the exact number when I get reference to the B737 manuals.

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  • $\begingroup$ What do you mean by "capture the VNAV profile?" $\endgroup$
    – lpydawa
    Aug 25, 2018 at 6:04
  • $\begingroup$ @lpydawa you might want to ask another question on the site for that. $\endgroup$
    – kevin
    Aug 25, 2018 at 16:19
  • $\begingroup$ @Ipydawa, LNAV and VNAV are FMS computed mode they correspond respectively to the lateral path( FMS route) and the vertical path ( vertical restrictions you can check on the CDU). Assume you are at cruise but not respecting the FMS cruise altitude, but nevertheless you want to follow the FMS descent profile respecting all restrictions, you will arm the VNAV mode, once you cross the FMS descent profile VNAV will get active and the aircraft will start descending if the A P is active. VNAV and LNAV require to have at least an active FD $\endgroup$
    – user40476
    Jun 10, 2019 at 13:54

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