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JSACKS

General question about SPD in VNAV....

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Am I right in thinking that VNAV will automatically adjust your SPD to what the FMC considers to be the safest level if you have entered SPD/ALT restrictions in your flightplan that the FMC considers to be unsafe? I have observed that in the NG, for e.g., if I set 210/3000 for a departure waypoint that at certain heavy weights, VNAV will accelerate me to say IAS 230 instead. Why does it do this? Because my SPD restriction was considered unsafe or unachievable in VNAV and the FMC bumped it up?Similarly, on VNAV PTH descent, the SPD seems to be "stepped down" in stages if there is a large gap in the speed restriction. I set a SPD of 170 for a waypoint at 3000 after heading down at IAS 240. Is this stepping down of speed a normal/regular VNAV feature?Thanks for any advice/info on the eternally perplexing VNAV function.Jonathan

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Yes Jonathan,Without proper flaps, VNAV will never allow anything less than clean flaps-up airspeed. This is why you'll often see the 747 climb out at more than 250, because it's so heavy that clean airspeed is above that...I'm curious as to why you'd enter a speed restriction on a departure waypoint though? The usual procedure is to get the airplane cleaned up and going 250 as quickly as possible.

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Ryan:Thanks for your reply. Appreciate the confirmation.I often enter departure SPD/ALT restrictions for the NG just for practice, to see if the FMC meets them (sometimes it does, sometimes not at all), and also because in real life, I have often taken off in both a 737 and a 747-400 and observed that departure flaps can be out for some time, especially in the 744 where I've flown at flaps 1 or 5 for the first ten minutes of the flight which I presume is because there might have been "speed limits" imposed on the aircraft. Also, now that I think about it, I am sort of surprised that the typical default flightspeed for a departure waypoint is listed as 250 in the FMC because such a speed cannot usually be attained within the first few miles of departure. So, I try to "match" the expected or projected altitude with the likely attained speed for those waypoints in the first 6 to 10 miles of liftoff. [One has to try and avoid the dreaded UNABLE NEXT ALT message in the FMC....!]Is it not normal or safe then to enter an IAS of less than 250 for departure waypoints? Should I just accept the projected departure speeds provided to me by the FMC after entering the waypoints for the route?Jonathan

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