the FMS will keep either as close as possible or be on the optimum profile, this is the conservative side.
Well unfortunately, this so called optimum profile is of no practical value to us as it (FM default profile) often :
- puts us OCTA (below CTA steps)
- puts us below DME/GPS arrival steps
- flies us at the lower end of the managed speed range (which is slower than the speed we ideally like to fly, esp if ATC want us at a fixed speed)
- might also involve a level segment at low level with a thrust increase or reversion to speed mode with some thrust (how can this be the most efficient way to descend an aircraft compared to idle thrust all the way down ?)
Therefore we usually insert constraints to keep us in CTA and above and DME/GPS arrival steps.
However, when we now insert these constraints (ie 8000' at 30nm, 5000' at 20 miles), the FM now says "TOO STEEP PATH AHEAD". Yet from experience, the aircraft is fully capable of descending this so called steep path without speed brake (in nil wind/ head wind).
That is why most pilots here use the open descent as the managed descent does not seem to work (do what we want it to do), even if you try and modify it as much as you can (create contstraints etc).
Any solution to this problem ?