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Guest SuperFortress

747 FMC/LEGS Altitude Question

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I am confused about something. In the LEGS section of the FMC you see each leg with its approx computed altitude. My question is this. With VNAV, LNAV, and autopilot engaged, will I be automatically taken to each altitude accordingly for each leg or is it something that I need to manually input on the MCP each time? Thank you

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superfortress,Please use your real name in forum posts. It is part of the site regulations and makes conversations so much more personal. Thanks.The FMC contains both your lateral (LNAV) and vertical (VNAV) flight profiles. With LNAV, VNAV and A/T engaged, you are NOT letting the aircraft take over navigation decisions from you. As captain, you are in sole command at all times.Because you have filed a flight plan, ATC know where you intend to navigate laterally. They may command deviations from this plan, and you are required to comply IF it is safe to do so. Otherwise it is normally safe to continue flying LNAV as filed.On the whole, your flight plan will not contain exact altitudes that you plan to fly, so your VNAV plan is incomplete. Therefore you may request of ATC permission to climb or descend, but may only do so after ATC confirm your request. Likewise, ATC may command you to change altitude, and you must comply IF it is safe to do so.With LNAV engaged, the autopilot will follow your programmed LNAV route. With VNAV engaged, the autopilot will maintain the set VNAV altitude until you dial in a new altitude on the MCP and press the altitude knob. You must do this when commanded by ATC to maintain a new altitude. During the ascent/climb phase of flight, you will set the altitude, in stages, to higher altitude settings. These settings allow the aircraft to climb to the altitude set, and no higher. During the descent phase of flight, the altitude setting is, in stages, set to lower altitude settings. These settings allow the aircraft to descend to the altitude set, and no lower. Again, you must not set the higher/lower altitude unless commanded by ATC, if on duty.I hope that this helps. Ask other questions as you need to, and I hope that they are answered to your satisfaction.Safe flying!Cheers, Richard

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Richard has given you a good account there of the way VNAV works.. I read your post and wasn't sure what exactly you meant.. so I will just add some comments here..When you enter a cruise altitude into your preflight pages, the FMC will automatically calculate a "vertical profile" for that plan, that profile will ascend to your cruise atitude, continue and then descent to your destination.If you look at your LEGS page before you have entered a cruise altitude, you will see many of your waypoints do not have altitudes against them, apart from those which are hard entered by either the pilot or as part of a SID or STAR. These are mandatory so the FMC knows what these are and enters them regardless.. It doesn't put any other values in because it doesn't know your cruise altitude and therefore hasn't calculated a "vertical profile" yet..WPT1 ---/----WPT2 ---/----WPT3 ---/3000WPT4 ---/6000WPT5 ---/6000WPT6 ---/----At this point your legs page might look like this, where WPT1 to WPT4 are an SID and WPT5 is the first point on your route. Once you tell the FMC your performance data, e.g. weights, cost index, crusie altitude etc... it can calculate your vertical profile and then it will add this data to the route on the LEGS page.. these calculations will be entered in a smaller text indicating they are FMC calcs and not pilot stipulations..WPT1 156/563 (small font)WPT2 156/2454 (small font)WPT3 184/3000 (184 in small as it's calc'd, 3000 in big as it's a stipulation)WPT4 224/6000 (224 small, 6000 large font)WPT5 245/6000 (245 small, 6000 large font)WPT6 332/FL100 (all small)WPT7 332/FL190 (all small)I hope that makes sense..The calculated values will update as the flight progresses.. the FMC will constantly update and refresh the data to ensure accuracy.. as a pilot.. the MCP is used to set MAX (if ascending) and MIN (if descending) limits.. While the aircraft is ascending or descending on the profile VNAV PTH will be displayed as the pitch mode.. if the aircraft reaches a set MAX or MIN you've set in the MCP.. it will level at that altitude and VNAV ALT will be displayed indicating an intervention.. The aircraft will not ascend or descend untill a new limit is set on the MCP and then the Altitude knob is pressed...Another bit of information for you, pressing the altitude knob during an ascent will delete the next hard entered altitude target... So.. in the case above.. if you took off.. with the MCP set to 18000... not pressing the alt knob would mean the aircraft would climb to 3000 for WPT3, then climb to 6000 for WP4 and remain at 6000 till WPT5... If you pressed the altitude knob.. 3 times after takeoff... you would delete the 3 restrictions... the restrictions entered for WP3, 4 and 5 would be deleted and the aircraft would continue to climb through WP4 and WP5 above 6000 feet till eventually it would level at FL180 before WPT7 where the pitch mode would change to VNAV ALT... indicating the profile has been interrupted again..I hope this makes sense to you.. Your MCP is to give the a/c hard limits... the indications on the LEGS page tell you if the heights are restrictions placed by you or calcuated values... These can be de-activated by pressing the altitude knob..RegardsCraigPS.. classic quote.. "If you think you understand VNAV.. you don't understand VNAV.." lol

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All that said, remember she flies on raw data as well- so should you ever exclaim, "Now why is she doing THAT?" you can always fly based on direct MCP inputs while you sort it all out (head scratching permitted).Better yet, DISCONNECT that A/P and hand fly her- she's quite a wonderful and stable girl as long as you use TRIM and that fantastic flight path symbology- brilliant!Best-Carl F. Avari-Cooper BAW0225http://online.vatsimindicators.net/980091/523.png| XP Pro SP3 with FS-GS System Unification | 2 x APC UPS | Coolermaster Stacker 830 SE | Asus P5E-Deluxe (X48) | e8500 @ 4gHz | Tuniq Tower 120 | EVGA 8800GT 512MB | Sony 40" Bravia XBR | NaturalPoint TrackIR Pro | 2 x 1 GB Corsair XMS2 | 500GB Seagate Barracuda 32MB SATA2 x2 (Acronis) | Corsair HX620W PS | PFC Jetliner Yoke and Pedals | Aerosoft 747MCP-EFIS-EICAS |

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