QUOTE(Ranger @ Jan 12 2008, 10:52 PM)
3X the altitude that you want to lose plus 1 mile for each 10 knots of airspeed that you might want to lose. Example:
Level at FL 310 and cleared to cross a fix at 9,000 feet. 31,000' minus 9,000'= 22. 22 X 3= 66 miles. If we assume that we're in the U.S., we need to slow to 250K before we descend below 10,000'. If we are descending at 300K then we need to lose 50K before we descend below 10,000'. That's another 5 miles. Sooooo, 66 + 5 miles= 71 miles. Even with the FMS this mental exercise sould ALWAYS be done. Why? Because occasionally the automation reaches back and screws you.