Cruising along at altitude, air traffic control clears you to “descend at pilot’s discretion to 10,000 feet, contact Atlanta Center on 125.8.” Being a fuel conscious pilot, you decide to remain at cruise altitude a bit longer when the new controller tells you to descend at maintain 13,000. Here’s the dilemma: are you still cleared down to ten thousand feet, or do you have to stay put at thirteen?
The thing to keep in mind is that any new ATC clearance effectively cancels the preceding clearance. Despite the fact that center had previously issued a discretionary clearance to a lower altitude, the new clearance to descend now to 13,000 feet takes precedence. The pilot should descend to thirteen thousand and remain there until a new altitude is issued by ATC.
Most of the time, this is a non-issue as ATC typically needs you to descend anyway. At some point in the descent, before reaching your issued altitude, controllers will likely issue a clearance to the original discretionary altitude (or lower). Other times, controllers are mindful of the original clearance and will reissue it with your new clearance.
This sounds something like:
“Descend now and maintain one three thousand, pilot’s discretion to one zero thousand.”
This descent clearance is a no brainer: start down to thirteen, then drop to ten thousand at your leisure.
Remember, a new clearance always cancels the preceding clearance. When in doubt ask the controller for clarification (they don’t bite!).