I demonstrate to new F/Os to the automatics the use of VNAV, and LNAV during instrument approaches. We discuss the track miles involved when tracking a radial outbound from a VOR, the turn inbound then the final track. In the part of the world I fly, we need to cross the IAP at 185kts. Thus the speed restriction if input on the Legs Page next to the IAP. The only drawback is that the speed bug will move to the selected flap speed. Thus at heavy weights, Flaps 2 may be required resulting in a speed less than 185 knots. As the motors are at idle and “ARMED” is annunciated in the FMAs you can control the speed with the trust levers. Remember that you will see messages in the scratch pad to the effect that frequency not tuned and course not set. At the time when the plane starts it’s turn inbound, you must remember to set Frequency, Course Selectors, and Heading Bug to the inbound course and Arm the Approach. A minor problem with VNAV is that the A/P will endeavor actions to intercept the G/S thus resulting in an increase in airspeed. This is not really an issue if one is on their game.
The same can be accomplished with accomplished with LNAV and Vertical speed too.
Either way works great lasts a long time.