Have you ever been given a descent clearance at “pilot’s discretion”? When do you start your descent? What makes an effective and comfortable descent and how do you plan for it?
Applying the same concepts I talked about for planning a visual approach (see “Tech Talk: Approaching Briefing," February 2012 Flight Training), we’re going to calculate the required rate of descent to meet a crossing restriction, and we’re going to calculate the distance prior to a fix to begin that descent. Generally, when planning a descent from a flight level, I plan for an idle power descent. If you’re making step-down descents 5,000 feet or less at a time, an idle descent may not be ideal as it will require multiple large power changes. If you’re descending more than 5,000 feet at a time, an idle descent generally will provide greater passenger comfort and fuel efficiency.
Planning for your descent is a two-step process. First we’re going to decide when to start the descent, then we’ll choose how to descend, or the rate of descent required. Often you’ll be given a crossing restriction. ATC will issue a descent such as: “Descend and maintain flight level 240, cross 30 west of ABC VOR at or below flight level 300.” Let’s determine our top of descent point (TOD). That comes from calculating the altitude we have to lose and multiplying it by 3. Assume our cruise altitude is flight level 360, or 36,000 feet. That’s 12,000 feet to the cleared altitude, and 6,000 feet to the crossing restriction.
We will find our TOD by multiplying our altitude loss requirement by 3. So 6,000 times 3, or for simplicity, 6 times 3. That will give us a distance from the fix that we should begin our descent, which is 18 miles, or 48 miles from the ABC VOR. Next we need to calculate the descent rate required to lose the specified amount of altitude in time for the crossing. This will require knowledge of your groundspeed. In cruise flight at FL 360 we’ll assume a ground speed of 500 knots. Multiply your groundspeed by 5 to determine an approximate three-degree descent angle. So 500 times 5, or for simplicity, 5 times 5. That yields a descent rate requirement of 2,500 feet per minute. I use this as a minimum.
So now I know that if I start my descent 18 miles prior to the fix at a rate of at least 2,500 feet per minute, I should cross the fix at FL 300 on my way to FL240. Even though I’ve calculated 2,500 feet per minute as the required descent, I will use 3,000 to give myself a buffer. If it looks like that is working for me, I can always slow my rate of descent as I get closer to the crossing restriction, or if it looks like I’m not going to make the crossing, I will increase the rate of descent. I know whether or not I’ll make it by continuously performing these calculations. Once I start my descent, every few thousand feet I will recalculate the required descent rate and distance required to ensure I didn’t make a mistake.
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