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If one takes off with HDG SEL armed and no vertical mode (like VNAV) armed, at what height (AGL) will the vertical mode switch from TOGA to LVL CHG, and what will be the IAS applied? Also, why isn't it possible to arm LVL CHG before takeoff, as it is with VNAV? Depending on the entered plan, there are instances when VNAV cannot be armed, even if one wanted to.

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LNAV & VNAV can be armed before takeoff; HDG SEL cannot be similarly armed.

If VNAV is armed & LNAV is not, then when VNAV engages at 400', the lateral mode switches to HDG SEL. But that's because the pilots had armed VNAV.

Taking off in TO/GA, the system won't revert on its own to LVL CHG. You can climb into the flight levels still in TOGA, although it's hard to think of a good reason to do do. If TOGA is active when the autopilot is engaged, the vertical mode will change at that point to LVL CHG, with V2 + 20 as the commanded airspeed.

As to why there is no option to arm LVL CHG on the ground, there is no need to. TOGA gives you speed guidance to bugged speeds (starting with V2 + 20) anyway, and VNAV gives you guidance to the profile speeds. Why offer another mode change that adds nothing to what you already have?

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  • $\begingroup$ "If VNAV is armed & LNAV is not, then when VNAV engages at 400', the lateral mode switches to HDG SEL." I believed HDG SEL becomes active the instant TOGA is engaged, before the takeoff. Not at 400' "You can climb into the flight levels still in TOGA" Are you sure about that? I believe TOGA changes automatically to MCP SPD when CMD is engaged. $\endgroup$ – Stamatis Vellis Jan 3 at 17:34
  • $\begingroup$ TOGA commands wings level, NOT a turn toward the heading bug (which is what HDG SEL would command. As I wrote, TOGA switches to LVL CHG when the A/P is engaged (in CMD), but you could hand fly it up to FL410 and TOGA wouldn't change to LVL CHG on its own. $\endgroup$ – Ralph J Jan 3 at 20:50
  • $\begingroup$ Well, I am not a pilot, just an enthusiast. But in the B737-800 FCOM of Boeing that I have, under Automatic Flight Takeoff Profile, it shows all the FMA stages during takeoff and it clearly shows HDG SEL for the lateral mode, starting from the stage "Push TOGA switch", before beginning the takeoff roll, until "Engage AP", after Thrust Reduction. This is what I mean by HDG SEL, the lateral mode used. If I new how to upload the page, I would. $\endgroup$ – Stamatis Vellis Jan 3 at 23:50
  • $\begingroup$ If you set the heading bug well off of runway heading & push TOGA & take off, the FD will command a wings level straight ahead track. It will only command a turn to the selected (bugged) heading when HDG SEL becomes active, either by pushing the HDG SEL (or LVL CHG, iirc) button or VNAV becoming active. Without seeing the page you're looking at, I can't address what you're seeing, but what I'm describing is how the planes that I fly do work. $\endgroup$ – Ralph J Jan 4 at 2:53

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