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Guest Andrea

Cannot engage VNAV after takeoff

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Hi PMDG,I'm trying to fly with an FMC flight plan from Heathrow EGLL to Brussels EBBR complete of SID and STAR procedures. I followed the intructions by the PMDG Flight Tutorial (from KDEN to KLAX) adapting them to my course. Everything went OK but a noisy alarm during takeoff after pressing TO/GA.After takeoff, gear up, press AP and have it engaged, press LNAV and have it engaged, press VNAV but haven't it engaged and the FMC continue blinking a message saying that I have VNAV not active... so the 737 does follow the flight plan route but climbs direcly to the target FL altitude.What am I missing? Is there a 737 V1.2 up to date Flight Tutorial?Andrea

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Did you check your FMC for errors? Usually it will show an error message when it disengages VNAV. The most common reason is because the MCP altitude is set too low. The aircraft will not climb higher than the current MCP altitude setting when in VNAV mode, and will disengage VNAV when it reaches that altitude, or is higher than the MCP altitude when you attempt to engage VNAV.

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Two most common mistakes, AFAIK:- some input missing in the pre-flight pages- or, to quote from the Boeing AOM:"If the airplane is between the FMC target altitude (depicted on the RTE LEGS page for the active waypoint) and the manually selected MCP target altitude, VNAV will not engage. To enable VNAV, adjust the FMC or MCP target altitude as appropriate."So take your choice :-)Leo Bakker

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Hi all,I have had the same problem! I think you can only turn on VNAV when you passed 3000ft. This is said in the manual of SU1.1 'After 3000ft AGL turn on VNAV'Regards,Steven van Leeuwen

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Andrea: Yes, there is an updated version of Fred's tutorial in the library.sj MantelKEWR

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Hi Steven,No, that's not it. You can turn on VNAV at 400 feet.However, I believe that in the Classic VNAV was not entirely dependable during flap retraction or something along those lines, so for the Classic the usual procedure is to use VNAV only after flap retraction. For commonality, you quite frequently see the same procedure for the NG as well.Leo Bakker

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After take-off you would generally maintain the nose-up attitude, and accelerate to V2+15 kts. At 400 feet RA you may select a roll mode. Select climb thrust on the MCP at the flap retraction altitude, then increase the speed selector on the MCP to the flaps up maneuvering speed. Above 800 fett RA you may engage an autopilot in CMD. As Leo noted, it is most common to use VNAV after fully flap retracion. Greetings,Zoltan Hegedus

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