A320: "Thrust Idle"
Sorry in advance if this material has been previously discussed.
My concern is fuel savings. CFM engines.
When on descent profile, the FMA reads "thrust idle" yet N1 is well above idle, approximately 45% through 30,000'. When I pull for "open descent" N1 drops to about 30%, fuel flow reduces by almost half. So I start figuring it saves more fuel to fly beyond the top of descent by 30 miles or so and open descent in an attempt at constant idle descent as close to final as possible. I've been flying this way for a few weeks now, but there are too many variables for me to know if I'm really saving anything.
When I asked a knowledgeable instructor, he told me our fleet is configured for N1 at 45% when on profile managed descent in case engine and wing anti-ice is selected on the N1 simply drops to real idle and we don't end up above profile. Kinda makes sense, but I'm not entirely convinced. What I suspect is that real thrust idle is actually a negative thrust condition, EPR less than 1, creating some drag like props not feathered. Maybe N1 at 45% is actually what the engineers calculated to be 0 thrust, which renders my current attempts at open descent all the way to finals futile as far as fuel savings is concerned. Any input?