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is a 3-to-1 formula. It takes 3NM to descend 1,000 feet. Multiply the al- titude to lose in thousands by 3. (22 thousand feet X 3 = 66). Start your descent 66 miles out. To determine your ROD, divide your ground speed by 2, then multiply by 10 by adding a 0. (500 knots/2 = 250 and add a “0” = 2,500 feet per minute rate of descent). Some adjustments may be necessary for a tailwind or decreasing speed to 250 KIAS descending through 10,000 feet. A simple rule is to add 1 NM for each 10 knots of airspeed you need to lose. Some pilots add 2 NM for each 10 knots of tailwind and subtract 2 NM for each 10 knots of headwind. I just use ground speed values and skip this last computation.
More advanced avionic systems, such as the devices by GARMIN, Rockwell Collins, and Honeywell, all have VNAV or Vertical Naviga- tion functions. These more advanced units even have auto-throttles to manage speed and thrust settings. Although VNAV is great, we still see errors with these modern avionics. When setting up for the descent, pi- lots need to verify that the automaton is going to do what we are asking it to do. Anytime you are using automa- tion, you need to make the request by pushing buttons on the Flight Control Panel (FCP). You then need to verify that the request is going to happen by confirming the button pressing with the display on the Flight Mode An- nunciator (FMA). I hear some pilots refer to the FMA as the “scoreboard.” Always verify!
But when using the VNAV function there are a few more steps to make sure the action is going to happen. Verify that the Top of Descent (TOC) point is ahead of you. Sometimes a late descent from ATC may require a steeper than normal descent. Ac- tivate the VNAV button and verify that “VPATH” or equivalent wording is displayed on the FMA. Verify that the altitude selector is set for the bot- tom altitude you are descending to, or the descent will not occur. I also review the default setting of the de- scent path angle on my VNAV page. It is usually set to 3.0 degrees, but
May 2024 / TWIN & TURBINE • 5