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Guest BAW393

747 FMC

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Guest BAW393

Need some helpAfter intering cruise ALT in the FMC the leg page will have different ALT's, its very strange, never use to do this.ThanksDavid

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Guest D17S

How about a test flight? With Vnav engaged during climb, Vnav will always climb in "Vnav Spd." This mode sets a fixed thrust then maintains speed with the pitch of the airplane (speed on pitch). If you have Vnav engaged, this is the Vnav mode that will exist prior to a Vnav altitude capture event during climb. First get a Vnav climb going and check to see that Vnav Spd is displayed in the PFD's pitch window. With Vnav still engaged, at altitude capture, you will transition from "Vnav Spd" to one of two Vnav altitude capture modes, "Vnav Alt" or "Vnav Path" (There are only these 3 Vnav pitch modes). "Vnav Alt" means that you have captured the altitude set in the MCP prior to reaching a Vnav target (Vnav cruise or a Legs page Vnav altitude restriction). "Vnav Path" means that an altitude has been captured by the FMC's Vnav function (restriction or cruise). So take a look which pitch mode is annunciated in your PFD's pitch mode window when the airplane captures an altitude. The system will capture the Lowest of the MCP set altitude with "Vnav Alt" displayed . . . or an FMC set Vnav altitude with "Vnav Path" displayed (restriction or cruise). Does the PFD's pitch mode annunciation make sense with the altitude you have captured?This is to prevent the airplane from (unintentionally) climbing to the moon. De-rate away, then try a light TO. Just try to keep this thing to V2+0 prior to Vnav capture. Have you noticed? This machine is a rocket ship on steroids.Additionally, any change in cruise altitude on the Vnav page or target/ restriction on the legs page can cause the FMC to re-calculate its Vnav 'course.' If you select a higher cruise altitude, the FMC will re-calculate altitudes at waypoints along the way (to the new cruise altitude) on the basis of its prediction of how the climb will go. The legs page waypoint altitude predictions will change. Same in decent except in reverse.

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