I think our systems work the same, but why worry about reaching up to make the switch? Activate HDG, arm NAV and let the system keep you on runway heading to 400' and make the switch for you?
For a few reasons, and these are just screwy issues with a poorly designed autopilot and FMS.
First, we could do what you're saying, but as soon as you're on the runway and the HSI needle centers up, it'll capture the nav track. The autopilot/FMS doesn't know what altitude your at, or maybe I should say, it doesn't care what altitude you're at. As soon as that needle comes within maybe a dot of being centered, it'll capture and try to fly you to it.
Second, we COULD just take off in nav, and in fact it's been discussed. The problem with this plane is that if you select nav at the gate, and if you THEN taxi close enough to another fix, it'll cycle to the next waypoint. A good example of this would probably be in Newark, where we park at the A gates, and then taxi down to the 22's for departure. We're so close to the end of runway 22, that the FMS may will cycle onto the next waypoint on the departure, thinking that we've gotten close enough to it.
It's frustrating, to tell you the truth. As "modern" as this airplane is when it comes to systems, it has a toy autopilot/FMS combo, and forces us to setup a lot of the automation manually, if that makes sense. Our first solution was to do LNAV at 400' manually, but that was kind of a mess of button pressing with how our after takeoff flows work. Now we're doing LNAV when the gear comes up, and it'll likely work in most engine out situations because we're going to end up flying to a heading almost right away, so in the rare event of an engine failure, we'll just punch up HDG and dial in whatever we need for the engine failure path.
And don't get me wrong, what you're saying sounds like a superior way of doing things, we've just got some systems limitations that prevent us from doing it.
