Drag required warning came on the FMC, relayed to the captain who said no need for speed break because so close to descent path. For previous experience I did not see a problem. I then entered some descent information on the FMC. Looked up and saw a rapidly (very rapidly) increasing green speed trend arrow. Which resulted in a Mach over-speed warning.
Were you in VNAV speed, or VNAV path on your FMA?
Using VNAV is objectively tricky to use.
Many pilots never use it during the descent, because even with a good understanding of the VNAV mode, many incident happens every year due to the use of VNAV during the descent (and approach...). VNAV has to deal with the flight path, speed, pitch, rate of descent, thrust all at the same time and autonomously as the situation evolves. Wrong wind input or altitude constraint will automatically result in the pilot adjusting the thrust (speed decreasing) or the speed break (speed increasing). As a result choosing VNAV for the descent should imply to take very seriouly a FMC warning "drag required", and a very close speed monitoring.
There's a mention of an abrupt change in tailwind as a contributing factor.
Could that have been accompanied by an abrupt decrease in OAT?
That would cause a drop in the speed of sound, hence an increase in Mach number for the same airspeed.
Very interesting indeed...
Cheerio InChina
good luck.