Originally Posted by
cf6-80c2b5f
Hello,
My company's FCOM states that prior to reaching the first speed constraint on descent, the aircraft will descend in VNAV PTH and pitch down to as high as 354 knots (VMO/MMO - 11) to stay on path (at 349 knots the scratchpad will display DRAG REQUIRED). When even 354 knots won't keep it within 150' of the path, speed reversion occurs, the pitch mode goes from VNAV PTH to VNAV SPD, VNAV resets the target speed to 349 knots, and the scratchpad message DRAG REQUIRED displays again.
This seems like a really high speed to be flying to maintain the path and to give warning that drag is required. Was there a revision to the software at some point that allowed this increase, or am I reading this wrong? The Big Boeing Book mentions it will only go up to the FMC target speed + 15 in this situation and the DRAG REQUIRED will illimunate at that point. But it doesn't state whether this applies to the B744. Thoughts?
Thanks!
I believe it will create a path to target whatever speed you have entered into the VNAV descent page. So if for example, you manually enter an unusually high speed such as 345 knots as the desired speed(or your cost index is of such a value that this speed is used, if that can happen), I suppose if the winds are not as forecast, you will see at 354 a change to VNAV SPD as overspeeding prevention becomes a higher priority than following the path and the aircraft goes above the path in order to maintain a margin from maximum operating speed.