"Descent at your discretion"

Until a few years ago, Canada and the US differed on the meaning of the phrase in the title. Now, I believe the books read the same way.

When issued a climb or descent at pilot's discretion, the pilot has the option of starting the altitude change when he wants to. Also, a pilot is allowed to stop the climb or descent at an interim altitude. The further descent or climb clearance is also still valid, so they can continue climb/descent without further approval from ATC (unless the clearance is not subsequently cancelled, that is). The pilot must climb or descend at a rate that is consistent with the aircraft type's performance when climbing or descending. This means that "descend at your discretion" does not constitute approval for a "cruise descent". If a pilot wants a slow descent rate, that should be asked for separately to keep ATC in the picture. Additionally, once an altitude is vacated, the aircraft cannot return to that altitude. If a pilot may wish to climb and descend at will, perhaps skimming tops of cloud or such but wishing to remain out of them, then he should request a "block altitude". I'll write more on that in an upcoming post.