Remember that your SID/STAR and approach data is from MILITARY sources (DAFIF) and may not match the approach profile shown on your approach chart - so FIX IT.The autopilot will fly the appropriate path assuming you stay on top of the speed - common mistake is not being configured for landing by the final approach fix and being too fast for the final segment.However, once you reach the decision altitude (hopefully on speed), you need to land manually (use the autothrottle for speed control if you wish). To fly a LNAV/VNAV approach, one should use the approach chart and modify the fixes and speed/altitude constraints accordingly. The advantage of flying an LNAV/VNAV approach is a constant glide path instead of the "dive and drive" path that may result by using VS or LVL CHG.As others have noted, when "soft" constraints are used the VNAV performance is variable. WAAS may be required for LPV approaches (which give you ILS-like tolerances to a very low decision altitude), but for LNAV/VNAV approaches all that's required is RNP-0.3 performance, which the 737NG does easily (RNP-0.1 is common on NG models). RAIM equipped aircraft are IFR legal as long as it’s TSO certified. Now, non-RAIM and WAAS GPS they’re not legal to fly IFR and they’re much better used for VFR flights. An example of this is when you use GPS to fly a VOR approach. Pilots equipped with WAAS can flight plan their alternate airport based on LNAV and Baro-VNAV lines of minimum versus legacy instrument approaches that rely on ground-based navigational aids. However some 737NG with some airlines>are capable of flying really special RNP-RNAV aproaches>(Alaska Airlines pioneered them) and these are truly>remarkable approaches. WAAS equipped aircraft are also a substitute means of navigation for ground based Nav Aids. Because WAAS is permitted as a sole-means navigation system, general aviation reliance on ground-based navigational aids for instrument flight is reduced. VNAV in 737 can get you at most>to standard MDA on non-precision approaches but this is just a>level-off step-down aproach so whether one uses VNAV or LVLCH>is of little importance. To fly those>semi-precision and precision LNAV/VNAV approaches you need>WAAS certified GPS receivers. >I don't think VNAV as implemented in Boeings was ever meant>for LNAV/VNAV approaches of today world.
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