Descend at pilots discretion

If we were going to issue multiple calls at once - for example if I want a pilot to descend using pilots discretion to FL180, how would I do that?

Do I simply make two calls (first to descend, then the second to call altitude at your discretion) or should I only make the one single call to descend and allow the pilot to decide how quickly or slowly to do so?

The latter of the options you provided is correct. Assigning an altitude to an aircraft does not indicate to them how quickly they should complete the instruction. They are expected to complete it in a safe and professional manner.

“Altitude at your discretion” would allow the pilot to maintain their current altitude or descend/ascend at their discretion. Using the command after an altitude assignment would effectively counter the previous instruction completely.

It is assumed that the pilot’s vertical speed is at their discretion when they are first assigned an altitude. If you’d like them to descend/ascend faster, there’s a command for that—“Expedite altitude change.”

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I understand that its necessary at times to request an expedited descent, but is there a standard for rate of descend when being vectored? -Paul

There’s no standard rate of altitude change from what I know. Climb or descend at a rate that’s best for you and your aircraft and while keeping safety, efficiency, and effectiveness in mind. In the real world, unless I’m mistaken, pilots don’t read back an altitude change instruction and then check for surrounding aircraft to decide on the best vertical speed; instead, they operate based on what aircraft type, weight, speed, etc. Do that and radar controllers will let you know if they’d like something more expeditious.

Speaking from real world experience, for unpressurized aircraft, a descent rate of more than 1000 feet/min is really uncomfortable for the inner ear because of the rapid pressure changes. And for pressurized aircraft, its rather difficult to maintain a rate of descent of greater than 2500 feet/min without gaining excessive amounts of airspeed.

I typically aim for 700 feet/min in unpressurized and 2000 feet/min in pressurized aircraft. Do keep in mind, however, that this is just based on the aircraft that I personally fly (Cessna Caravan and Citation II).

Hope this helped somehow!
Cheers, Pjpilot99

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