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VNAV VOR approach help in 737

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Hi allI was just shooting a VOR/DME approach onto RWY30 at YMHB in the 738. I was attempting to use VNAV down until minimums, or at least on initial approach.I have entered a few DME waypoints and their alitudes for the imaginary glidepath down to the runwa- one at 7 DME Of 2395ft and one at 5DME at 1755ft, so VNAV has a glidepath to follow.However, it follows it very badly - it drops engines to idle, and starts to descend rapidly (about 2000ft/min) just as it would if it were dropping from cruise alitude at TOD.I understand VNAV is used to conduct these sort of approaches until minimums fairly regularly - surely the real 737 doesn't behave like this? Or perhaps I am doing something wrong?Any suggestions would be very much appreciated!CheersRudy


Rudy Fidao

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I don't recommend VNAV on approach for many reasons... in fact I don't use it much at all for descents - only climbs. Easiest way to shoot a non-precision is to use the MCP Speed and V/S controls. A 800 fpm descent rate usually works, which at 160 kts is 300 ft/nm (800 / (160/60)) no wind.VNAV responds to an altitude constraint as if it were a "level off" altitude rather than a "descend through" altitude, and it often hangs-up after crossing a fix with a constraint (you'll see a magenta bug on the VSI tape). As you mentioned, you're going to hand fly below minimums anyway so you might as well get into the control loop throuhout the approach.


Dan Downs KCRP

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