Flight Idle is different during various phases of flight. If your descending from Cruise 40 percent range sounds about right. It is not gonna go down to 21 percent like on the ground. Not sure if this is correct but I believe on descent the air getting pushed into the turbines keeps the turbines running faster, thus you idle speed is in the 40s. Hey I just thought of something. For the Op that was having problems slowing down. Is your throttle calibrated properly? If there is a problem with the throttle and it is not going all way back to idle position then that would probably make it difficult to slow down.

Airspeed has an effect on N1, but not enough to increase idle N1 to 40%. Flight Idle is controlled by the engine fuel control unit. Essentially this will compute a demanded N1 based on thrust lever position. The Idle N1 limit is computed depending on flight/ground status and aircraft configuration. This idle N1 is applied as a lower limit to the thrust lever demanded N1. If increased airspeed were to tend to increase N1 the fuel control unit would reduce fuel flow to compensate and maintain the demanded value. In the aircraft what this means is that on approach the thrust lever would have to be advanced quite a bit to begin to accelerate N1 above flight idle. In fact to approximately the angle at which flight idle N1 is obtained on ground. Depending on how PMDG have simulated idle N1 this could rule out calibration as a cause. Kevin Hall

From an aerodynamic point, why can't you use the speed brakes > 10 degrees of flaps? As a non pilot, you don't know how many times I landed the LD767 or the new 737 NGX with the speed brakes still deployed (bad pilot Pauly - but I forget to retract them all too often) -but what would happen if you landed with flaps 40 and the speed brakes deployed?

but what would happen if you landed with flaps 40 and the speed brakes deployed?

You would turn the NGX into a Space Shuttle when you went to IDLE? (Flying Brick on Approach) - Quote by Tommy Lee Jones (Space Cowboys).

From an aerodynamic point, why can't you use the speed brakes > 10 degrees of flaps? As a non pilot, you don't know how many times I landed the LD767 or the new 737 NGX with the speed brakes still deployed (bad pilot Pauly - but I forget to retract them all too often) -but what would happen if you landed with flaps 40 and the speed brakes deployed?

It would be very difficult to arrest the descent during the flare (unless you had ridiculous airspeed). A hard landing would most likely result - involving many maintenance man-hours and very large bar tabs.

Why is flight idle 40%> - is there a way to reduce this (rw)?

You probably would not want to, for two reasons. First up, the faster the N1 speed, the quicker the N1 speed will increase when you give it some juice, since the acceleration of jet engines is not linear across their range, this (and lag) is also why you push the throttles up in a couple of stages on take off, rather than ram them to the firewall, since it would risk a lot of asymmetric thrust if one throttle got ahead of the other. The average CFM 56 takes about four seconds to spool from 30 percent to 65 percent, but a large portion of that time is spent with it getting up to 40 percent rather than it being an equal spread across those four seconds, higher up the RPM range it only takes about half a second to increase N1 by 10 percent, so if you wanted to get speed on fast, then that four seconds from 30 percent N1 would seem like an eternity as you dropped under the glideslope and stared at a windshield full of approach lights. Second, the CFM 56 is still not completely immune from its potential to flame out from being swamped in heavy rainfall, it's nowhere near as susceptible as it was since the spinner was redesigned, but there is apparently still a slight risk of it at low idle speeds. I don't think you're at risk of that in FSX, but you might as well have it act like the real one is flown. Al

I know the B1900D is a different AC,but NY Ap use to have our crews fly 230 till 2 DME. If you give em an inch,they will take a mileThat being said,you should have no issues maintaining 170 till GS intercept,if you have enough flaps already extended.Just remember to do gear down at GS alive.

230 to 2DME... wow, that's really pushing a stabilized approach. Gonna trigger a FOQA event by the sounds of it.