So here we are at 39000' and the ever vigilant FO brings to your attention the cabin vsi. The cabin alt is just over 8000' but has a roc of 200'/min, the outflow valve appears more open than normal, checklists are followed with no appreciable effect so that, after 5 minutes, the cabin is now at 9000'. You have no choice, you have to descend, but you DO NOT yet have an emergency.
Asking ATC for descent since you have a pressurisation issue is the sensible thing to do, it would be foolish to get into the more marginal procedure of an emergency descent if there is no need to. Why wait for the inevitable and place oneself in the more dangerous situation simply to satisfy ATC.
As has been previously stated, the commander of the aircraft can override ANY ATC instruction if he believes that instruction compromises the safety of his aircraft, common sense please, if a pilot states that he has a pressurisation problem why compound that problem by forcing him to declare a MAYDAY?
Last edited by Bengerman; 24th Sep 2013 at 21:27.