From the Instrument Procedures Handbook
ATC may ask the pilot to descend to and maintain a specific altitude. Generally, this clearance is for en route traffic separation purposes, and pilots need to respond to it promptly. Descend at the optimum rate for the aircraft being flown until 1,000 feet above the assigned altitude, then descend at a rate between 500 and 1,500 fpm to the assigned altitude. If at any time, other than when slowing to 250 KIAS at 10,000 feet MSL, the pilot cannot descend at a rate of at least 500 fpm, advise ATC.
The second type of clearance allows the pilot to descend “… at pilot’s discretion.” When ATC issues a clearance to descend at pilot’s discretion, pilots may begin the descent whenever they choose and at any rate of their choosing. Pilots are also authorized to level off, temporarily, at any intermediate altitude during the descent. However, once the aircraft leaves an altitude, it may not return to that altitude.
A descent clearance may also include a segment where the descent is at the pilots’ discretion—such as “cross the Joliet VOR at or above 12,000, descend and maintain 5,000.”
This clearance authorizes pilots to descend from their current altitude whenever they choose,
as long as they cross the Joliet VOR at or above 12,000 feet MSL [note how the word "pilot discretion" is not used, but implied by context
]. After that, they are expected to descend at a normal rate until they reach the assigned altitude of 5,000 feet MSL.
Clearances to descend at pilots’ discretion are not just an option for ATC. Pilots may also request this type of clearance so that they can operate more efficiently. For example, if a pilot was en route above an overcast layer, he or she might ask for a descent at his or her discretion to allow the aircraft to remain above the clouds for as long as possible. This might be particularly important if the atmosphere is conducive to icing and the aircraft’s icing protection is limited. The pilot’s request permits the aircraft to stay at its cruising altitude longer to conserve fuel or to avoid prolonged IFR flight in icing conditions. This type of descent can also help to minimize the time spent in turbulence by allowing pilots to level off at an altitude where the air is smoother.
See AIM 4-4-10
Here are some examples:
- You are 30 miles south of IRONS intersection at 8,000 feet. ATC says, "N69SM, cross IRONS at and maintain 4000 feet." You think--instantly--that you want to stay at 8,000 feet till 15 nm from IRONS and you can reply in any of these ways:
- "N69SM, cross IRONS at and maintain 4,000." .... then at 15nm you call again and say, "Potomac Approach, N69SM vacating 8,000 feet for 4,000 feet."
- "N69SM, we'll stay at 8,000 for now and cross IRONS at and maintain 4,000." ... then at 15nm you call again and say, "Potomac Approach, N69SM vacating 8,000 feet for 4,000 feet."
- "N69SM, we'll start down 15 miles from IRONS for at and maintain 4,000."
- You are 30 miles south of IRONS at 8,000 feet. It is bumpy below 5,000 feet. ATC says, "N69SM, descend and maintain 4,000 feet." You reply:
- "Potomac, N69SM, how about pilot's discretion to 4,000 feet, it's a bit bumpy lower."
- Potomac may reply:
- "N69SM Descend now and maintain 4,000 feet" --- DENIED!
- Your response: "Okay, leaving 8,000 feet for 4,000 feet, N69SM."
- "N69SM Pilot's discretion to 4,000 feet." --- unlikely, but no crossing restriction.
- Your response: "Pilot's Discretion for 4,000 feet, thanks. We'll call you back leaving." (then you call them back leaving 8,000 feet).
- "N69SM cross IRONS at and maintain 4,000 ft."
- This brings you back to the first example where ATC issued you a crossing restriction right off the bat (which, according to AIM 4-4-10, is the same as pilot's discretion.)