Originally Posted by
Sliceback
That doesn't save fuel. The biggest fuel savings is cruising at you most efficient altitude until you can do an idle descent at L/D. That's typically about minimum clean speed or up to 10 kts faster. 250 kts closer enough. Zero out the K factor (727 PDCS - Performance Data Computer System????) or a modern FMC cost index and it typically defaults to 250 kts but the real best fuel speed is even slower. But the time goes up a LOT so the total cost goes up. However if you're trying to 'go fast' the most efficient way to save fuel/time is...leave early, taxi quickly, be ready, get short cuts. Inflight high speed descent is best, followed by a bit faster climb speed, with a higher cruise speed being the least efficient fuel burn to save time.
If the F/E had descended the cabin faster manually the problem could be avoided. Takes a bit of head work ahead of the descent to realize the programmed setting won't hack it. Either way, manually increasing the rate of descent or falling behind will result in a higher cabin descent rate. Due to the lower pressure differential it's better off doing the higher cabin descent rate at altitude. That's how the modern pressurization systems handle it.
Barberpole had pretty good fuel statistics with his methods. He even thought the 72 wing could be put on a "step" at high speed. Back then the pressurization was manual and yes, ears could take a higher descent at upper cabin altitudes. Which we did. At idle some of the birds wouldn't descend the cabin. If you asked nicely Barberpole would give you an inch or so on a pod thrust lever. I'm not espousing Barberpole's methods and agree with the book methods.