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Guest PalmerFan4Life

I have a question about the VNAV.

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I have a question about the VNAV.1.) once you properly setup the FMC and engage the VNAV does it automatically climb/decend you to your desired altitudes you specifed? Or do you still have to enter the altitudes in the autopilot while the VNAV automatically sets your cimb and decent rate speeds?Thanks,Ryan

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>I have a question about the VNAV.>>1.) once you properly setup the FMC and engage the VNAV does>it automatically climb/decend you to your desired altitudes>you specifed? Or do you still have to enter the altitudes in>the autopilot while the VNAV automatically sets your cimb and>decent rate speeds?>>Thanks,>>RyanRyan,That depends on what you enter into the altitude window of the MCP. If VNAV is engaged and you programmed the FMC to cruise at 33,000 feet, and you dial 33000 in the altitude window of the MCP, it will continue to climb until you've reached that altitude. If, for example, you dialed 19000 into the MCP, it will level off at 19,000 feet. It will remain at 19,000 feet until you tell it to climb to a higher altitude. You do that by entering the new altitude into the MCP and press the center button. This will release it from its currect altitude and you will climb to the new altitude entered into the MCP. In the real world, pilots usually receive altutude clearence from a departure and enroute traffic control centers. If the pilot is instructed by ATC to climb to FL210, the pilot will enter 21000 into the MCP and press the center button. You'll noticed that as you level off at 21,000 feet, you'll get VNAV ALT on the FMA, telling you that it's captured the altitude. When ATC clears you to climb to FL330, you enter 33000 in the MCP. When you reach your assigned cruising altitude of FL330, you will get VNAV PATH, indicating this is the altitude programmed in the FMC. Hope this answers your question.Ken.

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Understood but if you would be so kind, could you alaborate on the down-side (pun intended) of VNAV descent? How come we don't have to push the MCP ALT center-button after setting a lower intermediate-altitude and having passed the T/D? It seems that I have been just dialing-down the MCP altitude (without pushing the center-button) and VNAV captures it fine. What is it that pressing the MCP altitude center-button really does for you? Thanks..

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>Understood but if you would be so kind, could you alaborate>on the down-side (pun intended) of VNAV descent? How come we>don't have to push the MCP ALT center-button after setting a>lower intermediate-altitude and having passed the T/D? It>seems that I have been just dialing-down the MCP altitude>(without pushing the center-button) and VNAV captures it fine.> What is it that pressing the MCP altitude center-button>really does for you? Thanks..jordanal,You're correct but let me explain more. If you're at 33,000 feet and ATC clears you to decend and maintain FL210, you would enter 21000 in the MCP. When you have reached your T/D, the airplane will start it's decent down to 21000 feet without you having to press the center button. Remember, the FMC is effectively flying the airplane since you're in LNAV and VNAV mode. Let's say you reach 21000 and level off. You would now be in altitude hold and you'll noticed the altitude hold switch light will be illuminated. You will also noticed that the magenta bug on the PFD will start to move down because you will be above the FMC's flight path if you continue to maintain your present altitude. If you're level at 21000 feet and ATC clears you to decend to FL110, you would enter 11000 into the MCP and press the center button to release it from it's current altitude of 21000 feet and start the decent down to 11000 feet. But normally, you would continue your decent so that you'll be at the correct altitude when you approach the airport, so there would be no need to press the center button as long as you don't level off. The reason you press the center button is to release it from it's current altitude when you're in altitude hold. I hope I made it clearer for you. Also download the TRC 4 by Robert Randazzo. It's a very good course explaining VNAV and the FMC. Ken.

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OK, that makes sense and I much appreciate the reply. RCv4 ofter gives me intermediate altitudes on my VNAV descent profile but before I reach those intermediate altitudes, RCv4 gives me another lower intermedidate altitude which I again, immediatly dial-down. I guess ATL Hold hasn't shown up becuase I haven't actaully leveled off at the first intermediate altitude. Guess I'll think of the ALT center-button as a "just do it" button whereby I want VNAV to resume the descent if I was in an ALT Hold step. Again, thanks for the informative reply.:-beerchug

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