B747-400 VNAV PTH Descent Speed
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!