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VNAV explained!

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Not sure why VNAV causes so many problems for some and generates so many questions - I find it actually quite elementary.Michael J.

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Have you ever tried flying an Airbus?Its just a question what you are used to!Most 767/757 pilots transitioned from older electro-mechanical cockpits, without any computers that told you when to descend...And now suddenly along comes an entire computer, coupled with the autopilot telling you how to fly. Why is it you are descending through 14000ft, MCP target set to 4000 and as you approach a waypoint you level off without touching the autopilot?Reason: waypoint in the FMS with a 14000A altitude restriction.Easy to grasp?On modern airbuses you no longer have a V/S, Hdg sel or hdg hold button.You just have knobs you can pull/push!Quite straightforward, ain't it ;-)Mark

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My autopilot can track a CDI to some degree of accuracy and hold an altitude if you hit it in just the right way. When I shoot a VOR/DME I descend fairly rapidly to make sure I'm down to the altitude for the next fix... I don't mind leveling off for a mile. I'm not quite nerdy enough to calculate the feet per mile I have to loose between each fix, convert that to FPMinute, and then watch the VSI during my entire approach. Now if a computer had all the calculations done for me and volunteered to hold the descent rate without any input on my part, great! Is it practical for my Piper Archer? Ohio U's avionics research department thinks so: supposedly they're setting up 4d precision approaches with the help of the FAA. So now the GPS will not only take us direct to the IAF and fly us down to 200', but will also sequence my approach with every other GPS-equipped plane. Great stuff. I think they're going to kick us out of the cockpits pretty soon too. Hopefully they'll leave forward looking windows in the planes. :)SP

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Guest GrantM

Yes, I second that Michal. What is the mystery?Grant

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Guest D17S

Who's got a link back to the old forum? I remember a post linking to a study. It was by a human factors study group that tested FMS rated crews on their in-depth knowledge of Vnav principles. They didn't do so good! Vnav's the hardest part of a crew's transition to their first FMS airplane. Remember, the initial FMS airplanes (757s/767s) were certified to use Vnav, but the Feds did NoT let those first crews use that Vnav function for actual navigation for for years. Lnav was fine. They had been using "Lnav" with their old INSs forever. We think the FMS was all new? No way! The INS has always had this Lnav function . . .it was just never called that. No pretty pictures in the old days. Just a course deviation bar in the HSI. Remember they would only hold 10 waypoints? Boy I do!. That was the flight mechs job on long flights . . so the flight crew could be napping. I've punched in a LoT of waypoints grinding away over Africa in an old blaster of one kind or 'nother. But Vertical navigation? "Now, What? It does what?!!" they asked. This Vnav stuff was something VerY new and it took a while to sink in, even for 10,000 hour crews. The PMDG airplane is a very good representation of what our youngest and smartest crews of the future . . . our video game generation . . . will have to "work through" to sort this function out. Better here than there. But the time they're "there," it'll be old hat. So, I'd defend that nobody's born with it. Makes perfect sense to me too . . . now. It's just that sometimes, it's (real!) hard to remember back to when common sense wasn't so common!

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Guest D17S

Woah, that's a blast from the past . . . Thanks John. Here's that old post:

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