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zackman6240

AP crzing alt

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When i engage the V/S the nose pitches way down even though i have it set to 1800 usaly, and i have to sit there and maualy trim it back to get it to go up. I know this is pilot error so i was wondering if anyone knew what i am doing wrong? thanks!

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Zack, before we can help you we need a bit more info. Do you have AP engaged? FD engaged? Are you pitching down from level cruise flight? Have you set a target altitude in the AP Alt window? Do you have a speed value set in ASI? Are you manually setting power or do you have AT armed/engaged? Depending on what your answers to these settings could determine whether the a/c is simply acting according to your inputs. VSI is not always the dominant command depending on what mode the AP is in.

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ok, before i take off i set my A/T on and my F/D on and i have a value set in my ASI and in my CRZing ALT. when im climbing out i push my speed hold and then i press my Lnav then i put my autopilot master cmd on, after that i push V/S and set it to 1800 or so.

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How have you set up your FMC? Did you set a Cruise Altitude in the Perf page confirming it in the Climb page and Cruise page? You know that if you arm Vnav before takeoff and apply full Takeoff power <CTRL>-G then Vnav will activate above 400 agl and will automatically command (or pitch if your engage AP at this point ) to maintain V2+20 until flap retraction altitude of 1500 ft ( unless you have set different value in FMC Setup options) and then reduce pitch to accelerate for flap retraction. Above 800 ft agl takeoff power will reduce to N1 climb power automatically if AT is armed. With Vnav active the FMC is controlling power and pitch to maintain best rate of climb to target altitude (unless restrictions have been input into the FMC). Once flaps are retracted and the plane has accelerated to 240K ( or whatever airspeed restriction has been preset in FMC) it will again pitch up ( assuming AP is engaged, if not then you must manually follow the FD bars on the PFD) to whatever rate is necessary to maintain airspeed and N1 power. Any reduction in ASI (SPD INTV) setting at this point will cause the AP to INCREASE climb-rate. However, at this point, if you enter a specific climb-rate into the VSI, the AP will REDUCE power to maintain both the desired climb-rate AND the ASI speed setting until the target alitude is reached which is what I ascertain is your desired result. Try this method and you should not have problems. I still am not sure why it is not working for you. May be the order in which you activate the various modes or that you are selecting these too early in the climbout and aircraft is still in Takeoff Mode and is using full takeoff power and pitching to maintain V2 + 20, ignoring your intervention.

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