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!
On the first page of the the portion of the FCOM you posted, system behaviour is described when the aircraft is
above the first speed constraint (it will allow the speed to increase up to VMO -11 to maintain path), and on the second page it describes system behaviour
below the first speed constraint ("the airplane may accelerate to 15 kts above target speed to maintain it's path). So, both are correct, depending on where you are on the descent. Once you enter a manual speed on the VNAV descent page, like .85/300, you have just entered the first speed constraint and the DRAG REQUIRED message will now be displayed once you reach 310 KIAS. So yes, you have seen the DRAG REQUIRED message at lower speeds than VMO -11. And yes, it is a very high speed for the FMC to try and maintain it's path at, so when I am starting a descent from FL370 or higher I will usually use speed intervention just before descending to bring the speed back a little to avoid it pitching down into an overspeed.