I find myself in basic agreement with
john_t but I think the analysis of engine/airframe combination needs to consider the fuel used in the climb, in the first instance. The question posed in the opening post is probably the best aviation-related example of the age-old question :- which came first, the chicken or the egg?
I also think it's a case of "horses for courses". Sure, we all try for the simplest, most fuel-efficient descent that doesn't upset the residents under the approach path too much. Where there's no obstacle/airspace limitations, I've always gone for a standard 3-times descent profile in a TP with P&W engines - this profile requires
some power but, as long as there's no ATC hold-ups, I've been able to maintain the profile in the lower levels, through configuration changes that result in a bit more power being used.
The only time I ever executed a full FI descent was one time when ATC held me up (under procedural control) to 10,000 feet at 10 DME! Even then, the thing in my favour was a fairly strong headwind.
Had it not been for that, fuel savings would've been history - and, of course, being held at 10,000 feet didn't actually help the fuel economy very much so I doubt that, overall, any real savings eventuated.
