Berbe

CDA means Continuous Descent Approach.About the descent rate. I read some of you saying it would be 1000 ft/min, 2000 ft/min, etc.You're all right, partially.The main thing to consider for your descent is the angle. Pilots use x° descent anlge, and follow it.Why don't they use a descent rate rather than a descent path? Because your decent rate is linked to your speed (not the IAS - Indicated Air Speed, but the GS - Ground Speed), which changes when you climb/descend or with winds, even if your IAS is the same.The main reason for all that is letting the passengers remain comfortable during the whole flightA quick example: a plane climbing/descending at a rate of 2000 ft/min. If it is in cruise flight (~ M.78), there is no problem at all. Imagine the same descent rate when the plane is in initial approach on an airport. Strangely all passengers will fill a little dizzy... in the best case.Because the descent rate isn't constant, it would be a nightmare (even impossible) to calculate any TOD - Top Of Descent. With the descent angle, it's only some basic trigonometry (need to climb xxxxx ft down with a y° angle, which means I need zzz nm).Your TOD has been manually calculated, now the problem is: how to follow the descent path?Simple: Descent rate (ft/min) = Descent slope (%) * Ground Speed (kt) = Descent angle (°) / 60 * Ground Speed (kt).Uh wait... Descent slope (%) = Descent angle (°) / 60??Let's draw a rectangular ABC, our angle is Â.tan  = BC (altitude difference, ft) / AC (distance, ft) is our slope (%), but  is small, so  is close to tan Â. Thus  (rad) = BC (ft) / AC (ft) = Slope (%)But our angle  is in radians, not in degrees. Pi radians = 180°, so  (°) =  (rad) * 180 / Pi. Let's approximate Pi with 3:  (°) =  (rad) * 180 / 3 =  (rad) * 60.Then, since we said  (rad) = tan  (rad) = BC (ft) / AC (ft) = Slope (%), we have both calculations:-  (°) = Slope (%) * 60- Slope (%) =  (°) / 60Note: BC & AC must be comparable value, i.e. with the same unit. Typically, you'll get AC distance in nm and BC height in ft, and since a feet is smaller than a nautical mile, it's better to convert nm in ft.You've got your TOD and your descent path. The last step, in the cockpit, is to monitor your speed and vary your descent rate accordingly. Prepare some Excel calculation... Or let VNAV do the job, which is maybe more realistic but lots less fun!Remember to add some safety distance. If you are to respect the 250 kt under FL100 restriction, remember that you'll need to anticipate by reducing speed or by stopping/reducing your descent dramatically just before reaching FL100, which means some distance flown above the descent path... Maybe start descending sooner to make sure being on/under the descent path.