DESCENDING FROM THE EN ROUTE ALTITUDE

As you near your destination, ATC issues a descent clearance so that you arrive in approach control’s airspace at an appropriate altitude. In general, ATC issues either of two basic kinds of descent clearances.

• ATC may ask you to descend to and maintain a specific altitude. Generally, this clearance is for en route traffic separation purposes, and you need to respond to it promptly. Descend at the optimum rate for your aircraft until 1,000 feet above the assigned altitude, then descend at a rate between 500 and 1,500 feet per minute (FPM) to the assigned altitude. If at any time, other than when slowing to 250 KIAS at 10,000 feet MSL, you cannot descend at a rate of at least 500 FPM, advise ATC.
• The second type of clearance allows you to descend “… at pilot’s discretion.” When ATC issues a clearance to descend at pilot’s discretion, you may begin the descent whenever you choose and at any rate you choose. You also are authorized to level off, temporarily, at any intermediate altitude during the descent. However, once you leave an altitude, you may not return to it.

A descent clearance may also include a segment where the descent is at your discretion—such as “cross the Joliet VOR at or above 12,000, descend and maintain 5,000.” This clearance authorizes you to descend from your current altitude whenever you choose, as long as you cross the Joliet VOR at or above 12,000 feet MSL. After that, you should descend at a normal rate until you reach the assigned altitude of 5,000 feet MSL.

Clearances to descend at pilot’s discretion are not just an option for ATC. You may also request this type of clearance so that you can operate more efficiently. For example, if you are en route above an overcast layer, you might ask for a descent at your discretion to allow you to remain above the clouds for as long as possible. This might be particularly important if the atmosphere is conducive to icing and your aircraft’s icing protection is limited. Your request permits you to stay at your 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 you to level off at an altitude where the air is smoother.