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Guest FRI737

737NG / RNAV Approach

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Guest FRI737

Hi there, I was just trying an alternative approach technique by flying the RNAV(GPS) RWY 19R into KSNA with the PMDG 737-700 and using VNAV for descent. Unfortunately the VNAV part didn

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Hello Stefan,The reason your aircraft won't descend is because your altitude is set to 3000FT. Yes, you're right about setting it to the missed approach altitude, however, that will prevent your descent in this situation.The PMDG 737 is remarquably capable of conducting a VNAV approach if your speed and timing is correct. In the LEGS page of the FMC, you have SPEED/ALTITUDE constraints, set them adequatly for a final approach (you can use charts for this) and set your altitude the MCP accordingly, the A/P will bring you down to 50FT.As a matter of fact, I do this for VOR/DME approaches, I fly them manually of course but the purple magenta on the ND helps as a descent profile indicator.Oh, BTW, if you experience any problems while descending in VNAV, in order to continue your desecent be sure to switch off and on VNAV mode, at each altitude constraint the aircraft will level off.There are very minor bugs in the VNAV system which have been debated a lot of times here but I believe it's pretty well simulated once you get the grips.Best regards

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<>Exactly.I really like the KSNA south approaches, try out the ones I include in my SIDSTAR collection found in the AVSIM library. I have connected the STARS to the approaches to simulate vectors to final.Back to topic: The current FMS generation does not provide true LPV coupled approaches (coupled glide-slope for RNAV). In this specific case, the altitude constraint at VEGYU at or above 1420 will result in the FMS commanding a level off at 1420 until past the fix, which is not correct. The LPV function should provide a constant descent rate of 318 ft/nm (3 deg) and cross over VEGYU descending through 1420. As it turns out, the RNAV (RNP) approaches work best with the FMS because they do not have intermediate fixes between the FAF and the Runway so you can fly a constant descent rate on final course.Therefore, on RNAV approaches I always use LNAV, MCP SPD and V/S until the MDA, then hand fly the landing.I have high hopes that the next generation FMS coming in the 737NGX will do a better job and we will have a true RNP 0.1 capability.


Dan Downs KCRP

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