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How to calculate if asked "to be level in X miles"

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Hi all How is the formula to calculate when asked to be level in a certain amount of miles so you can descend at the last moment with the engines at idle? Thanks!


Peter Aerts

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Hi, below are not my words but a good explantion of the 3x descent rule. Take the altitude you need to lose and multiply it by 3, then drop the last three zeros. That gives you the distance at which you need to start down. Then, take your groundspeed, divide it in half, then multiply that by 10 to get the rate at which you'll need to descend at.So, for example:You're told to cross XYZ VOR at 5,000 ft. You're currently at 9,000 ft with a ground speed of 150 knots. You have 4,000 feet to lose, so you'll need to start down at 12 DME prior to the VOR (9000-5000 = 4000, 4000 x 3 = 12000, 12000/1000 = 12). You'll need to descend at about 800 FPM (150/2 = 75, 75 x 10 = 750FPM).If you were told to cross 10 before XYZ, you'd need to start down at 22 DME (12 + 10).Most people will add in 1 or 2 DME as a fudge factor to the distance at which you need to start down.


Rob Prest

 

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Rob gives great advice, but keep in mind the fudge factor as your calculation will never be perfect, even the best FMCs can't get it right all the time.


Noah Bryant
 

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When to start down:

  1. Your altitude to lose. ex: 41,000 to lets say 12,000 (typical crossing altitude).
  2. drop the zeros and subtract, then multiply the answer by 3. ex. 41 - 12 = 29 then x 3 = 87
  3. Start down approximately 87 nm out.

How fast to descend:

  1. Take your ground speed and half it then add a zero.
  2. 450/2 = 225 add zero 2250 fpm
  3. This one is a ballpark figure to get the descent started, you'll probably find you need to descend faster than that to make a crossing and a speed restriction.
  4. For the above example you'll probably need more like 2500 fpm to make the crossing.

For speed reduction. ex. 250 kias at xxxxx fix.:

  1. 1 nm for every 10 knots of airspeed needed to lose in level flight.
  2. ex. 320 kias to 250 kias = 7 nm
  3. Again, ballpark figure, you'll probably need more like 10 nm in level flight slow down, the last 10 knots of airspeed are the hardest to lose and take the longest,

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The real fun comes in when youflying an old propliner (www.calclassic.com) and do not have a dme.Then you need time! Johan

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Hi. If you are told, 'Start down now please: I need you level in XX miles or less' you have a different problem. TOD and BOD are already fixed and you must determine a suitable vertical speed... You know the distance you have in which to descend and you know your GS. Distance divided by ground speed will give you the number of hours in which to complete the descent. Multiply by 60 to find the number of minutes you have available. You know the number of feet through which you must descend. Divide that by the number of minutes to find the required vertical speed. You will need to adjust flight controls to maintain GS and VS. You often can't do it at flight idle, and if you have been kept high until the last possible moment by a sadistic controller you will need to apply any braking you can find to maintain the appropriate speeds, especially in a light or a slick plane. I'm currently using the Flight1 C441 and often need to drop flaps and gear to comply (partly because the flight model is a bit off- I don't imagine such a thing would be necessary in the real world). Regards,D

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