Don't want to talk too much about the captain that day, needless to say I wasn't the only one in the crew that seemed to think his behavior was odd, but thats another story.
Coming back to CDAs, I guess my real question is, if you are in the last stages of a descent (below 10,000ft, thats usually the time we start playing with V/S) and you chose to descend at low V/S as opposed to descending faster and then leveling off is there much of a fuel savings? Is the extra workload worth it, especially if noise is not an issue (the approach into my home airport is entirely over the sea)? Are we absolutely sure of the benefits in terms of fuel burn.
Let me give you an example where CDA won't save any extra fuel. Assume a 30NM leg at a constant ground speed of 300kts (6 minute leg). We are instructed to descend and an open descent will leave us level after 15NM. A CDA will be at a v/s of half of open descent.
Hypothetically assume a fuel flow of 20KG/Min at idle, 55KG/Min at a V/S of half of open descent and 80KG/Min at level flight.
If we go for a CDA (at half of open descent) then over 30NM we will burn 330kg (55kg/min for 6min)
If we choose an open descent followed by a level off we will burn 300kg (20kg/min x 3 min + 80kg/min x 3min)
These numbers are entirely made up, but what I'm trying to say is that maybe we have made it a fashion in wanting to always see a negative V/S even if its only 200-300 fpm? Is it adding unnecessary workload?
In writing this post I went online to see if anyone has done any significant empirical research in this field and found this
http://web.ics.purdue.edu/~dsun/pubs/jac13.pdf
It is a paper by some guys at Purdue in Indiana, and they do say that although CDAs do reduce fuel burn it is not always the case, and that flying at optimal speeds is also important. In fact maintaining a higher speed in terminal airspace (i.e. 250kts) may have a greater impact on fuel savings than the descent profile.