yeah i couldn't agree more. i dont see any reason why wny airline whould restrict usage of open descent BELOW 10000. unless the elevation of the airfield is 8000 ft as stated above...haha
of course...perhaps maybe yr airline has imposed this rule due heavy and dense height constraints into most of yr destinations and there have been a number of times that yr pilots may have busted these altitudes or the "star steps". therefore...to stop this from happening then yr airline may have enforced the usage of the managed DES to adhere to the height constraints. not good of course as they should probably train them better rather than enforce such a ruling
this of course is not an accusation pleaSE remember.
this is merely an assumption to add colour to this grey area
