Originally Posted by
CVividasku
Except maybe a mistake or a change of plans, at least I don't see a situation where we would plan to engage VNAV during final approach.
you would use VNAV on a 3d RNP approach for starters, a visual approach, any non-ILS or even an ILS where you choose to... If you understand how the logic works and how the path is constructed it's the easiest way to fly. Set a speed restriction of Clean at 6000 then look at the last 20 miles of track and guesstimate some speeds, flap 5 turning final, flap 20 about 6 miles out etc. Its only to build a rough path and get you thinking about what's going to happen. Approaching 6000 the plane will decelerate to clean speed by itself. Now you just need to select flap and open the speed window. Flap 1 engages approach logic and will lock you onto the path* and then it's just like an ILS. You can keep LNAV and VNAV engaged the whole way so no more buttons to press.
*you don't even need to be on the path: if you're above it will default to VNAV SPD so you will need to 'drop' it onto the path; if you're below it'll fly level until it hits the path or even climb (if you're dumb enough to be at 3000' flying towards a 4000A constraint, I learned that the hard way). The classic sim scenario puts you level at 3000' doing an RNP approach: if the TRI insists that you reset the cruise alt to 3000 it's a dead giveaway that they don't understand it either 🤣
EDIT: It's not uncommon to see 'Drag Required' when slowing down on approach - it's an old system so it's smart but not that smart - the rough speeds you entered earlier will put 'flat' spots on the path for deceleration. Keep an eye on your track miles and 3x table. If you're low with 'Drag required' then you've got a flat spot coming up.