Let us assume you are flying a B737NG and you are cleared for an RNAV (RNP) approach which has several legs with different altitude crossing restrictions.

  1. What do you enter in the MCP altitude window? The first, highest crossing restriction, and then keep repeating by entering the next restriction or does one enter the last and lowest one and let VNAV manage the step descents automatically?

  2. If during this gradual VNAV descent procedure you have to use SPD INTV to alter an FMS commanded airspeed, does this "cancel" the VNAV profile, or will it adjust automatically to suit the new manually selected airspeed?


1 Answer 1

  1. This usually depends on the airline SOPs. The safest thing to do when cleared for the approach is to set the MCP altitude window to the lowest altitude restriction because that way you won't forget to lower it again. As long as the aircraft is descending in VNAV PTH mode, it will follow all altitude crossing restrictions.

    When cleared for final approach, one can also set the DA (Decision Altitude) for the approach instead of a crossing restriction into the window. This will allow VNAV to descend further and is required unless IAN with G/P is used:

    The Integrated Approach Navigation (IAN) modes are armed/engaged by selecting the APP button on the AFDS Mode Control Panel (MCP). Once armed, the autopilot/flight director will capture and track the localizer/final approach course and glide slope/glide path.

    (Boeing 737 NG FCOMv2 - 4.20.20 - Automatic Flight - System Description)

    Once the aircraft is descending on final approach, one can set go-around altitude into the window:

    When the FMC is “on approach”, the following features are available:

    • when preparing for a missed approach and the MCP altitude is set at least 300 feet above the current airplane altitude, VNAV will continue to command a descent

    (Boeing 737 NG FCOMv2 - 11.31.37 - Flight Management, Navigation - Flight Management System Operation)

  2. Using SPD INTV will usually cause the vertical mode to switch from VNAV PTH mode to VNAV SPD mode, which does not care guarantee to comply with crossing restrictions:

    If speed intervention is engaged:

    • during a path descent with flaps up on an idle leg, VNAV switches to VNAV SPD
    • with flaps down, VNAV remains in VNAV PTH

    (Boeing 737 NG FCOMv2 - 11.31.38 - Flight Management, Navigation - Flight Management System Operation)

    So if you already have flaps selected, VNAV PTH will remain active. During VNAV SPD operation, it is the pilots duty to ensure all crossing restrictions are met. Once the aircraft goes back on the path, VNAV PTH mode will engage automatically again (assuming you are on a STAR or approach):

    A speed descent cannot automatically revert to a path descent, except during STAR, approach transition, or approach leg with a vertical angle.

    (Boeing 737 NG FCOMv2 - 11.31.33 - Flight Management, Navigation - Flight Management System Operation)

    Changing the target speed in the FMS will compute a new descent profile. This usually results in the aircraft no longer being on profile and therefore the vertical mode will switch to VNAV SPD as well.

    Note that VNAV SPD will not violate altitude restrictions, if possible:

    The descent attempts to comply with waypoint altitude restrictions, and will not violate these restrictions. The VNAV speed descent will not, however, guarantee the airplane reaches an altitude restriction at the required point.

    (Boeing 737 NG FCOMv2 - 11.31.33 - Flight Management, Navigation - Flight Management System Operation)

  • $\begingroup$ Problem is that the FMS decreases the target speed way far away from the airport and one has to start using flaps and slow down from way out which is not practical or "traffic flow" efficient. So, SPD INTV becomes a necessity, to bring speed up again. Which as you have explained switches from VNAV PTH to VNAV SPD and "distorts" the pre-determined descent path, unless flaps are already extended. $\endgroup$ Commented Jul 2, 2020 at 11:34
  • $\begingroup$ But if you already know that the FMS speed at various waypoints is too slow for you, you can just manually insert different speeds with the CDU before you even start the descent. That way you get an optimal descent profile without the need to intervene later. By the way, please use the comment link under answers for comments, not the answer field ;) $\endgroup$
    – Bianfable
    Commented Jul 2, 2020 at 11:38
  • $\begingroup$ So when using LNAV/VNAV for an RNAV approach as above, will VNAV get you down to the MDA (DA) on Autopilot or will it level off the aircraft at the last waypoint (FAF), before the G/P begins? $\endgroup$ Commented Sep 11, 2020 at 19:44
  • $\begingroup$ @StamatisVellis VNAV will let you fly down the G/P until MDA (assuming you have the MDA set in the MCP). $\endgroup$
    – Bianfable
    Commented Sep 13, 2020 at 15:16

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