# Descent Calculations-

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Could someone please provide the formula for planning your descent, or some software that you plug in your variables and get your information. I have been simming with a lot of GA planes lately and have been just "winging" it. Thank you,JohnnyKTUS

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Usually for jets you can do something like: if you are level at FL20 and the airport is at 1,000 MSL then you do:20,000 - 1,000 = 19,000; 19,000 / 1,000 = 19; 19 * 3 = 57 nm.So you should start your descent 57 nm from the airport. It works ok for GA planes (the worst that can happen is you level off and fly for some minutes before reaching the glideslope).RgdsMarcelo

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I recall a small executable program that sits on your desktop and performs the calculations for you. I deleted it because I did not like constantly minimizing FS to go to my desktop to get my figures. Otherwise it worked well. I seem to recall it was called descent1.zip or maybe descenti.zip. It should be available either at the AVSIM file library or Flightsim.com file library.

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I use this one from Avsim called : DescentRateCalculator.zip

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Marcelo said ...Usually for jets you can do something like: if you are level at FL200 and the airport is at 1,000 MSL then you do: 20,000 - 1,000 = 19,000; 19,000 / 1,000 = 19; 19 * 3 = 57 nm. So you should start your descent 57 nm from the airport. It works ok for GA planes (the worst that can happen is you level off and fly for some minutes before reaching the glideslope). If you're using GPS it's even easier ...20,000 - 1,000 = 19,000; 19,000 / 1,000 = 19 minutesMost GPS units have time to destination, so if I'm using the GPS I usually plan my descent based on time rather than distance. In this example, I would start a 1,000 fpm descent when the GPS shows 19 minutes to go on the flight.If you're flying average GA single or twin you should probably use a 500 fpm descent rate instead of 1000 fpm .. which just doubles your time.If you plan for distance rather than time, the "3" in Marcelo's formula is miles per minute (which is the equivilent of 180 kts ground speed). For most GA singles you should use a value of 2 (or 120 kts).In the real world, I plan my descents to arrive at 1,000' AGL about 3 miles out from the airport .. which is the "standard" traffic pattern altitude. This allows you to slow the plane down to initial approach speed well before you hit the airport fence.There's nothing magic about descent planning ... it's really just some simple math. The trick is to start thinking about it well before it's time to start down :-)

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It is in the flight manual for most aircraft. Is this the sort of thing you are looking for?http://195.35.145.44/marcbrodbeck/perf.htmSorry I can't cut and paste a sample as it is protected .pdfIan

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The "Rule of Three" is probably the most useful if you fly an aircraft that can vary greatly in speed. Depending on winds, a jet can be travelling over the ground anywhere between 300-600kts at normal cruise speed.However, because of that, the Rule of Three cannot work alone. You need to couple it with the "Rule of Five" for descent rate calculation. Multiple your groundspeed by five to obtain a rate of descent that approximates the three degree slope that the Rule of Three is based on.Example from FL330 to sea level, with a groundspeed of 400kts. 33*3=99miles out for the ideal 3 degree top of descent. 400*5=2000fpm descent to stay on that slope initially. Periodically through the descent, recompute the ROD since your speed changes with altitude.For a typical GA plane, your groundspeed won't be all that variable. It is probably more likely that you will just always plan a 1000fpm descent for simplicity. In that case, work backwards.Example from 7500' to sea level, with a groundspeed of 120kts. At 1000fpm, it will take 7.5 minutes to descend. 120kts is 2 miles per minute. 7.5*2=15 miles out for the ideal top of descent. A little more difficult, but manageable with normal head math skills if you keep the speed used to those which are multiples or half multiples of 60, such as 90, 120, 150, 180, etc.

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Thanks to all who replied! I will go home tonight and give all this info a shot. JohnnyKTUS

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As a matter of fact, I'm currently working on a freeware Descent Rate Calculator gauge (in the style of my Pushback gauge), that lets you input various variables like airspeed, descent rate, wind, new altitude, etc.You will be able to use it to calculate the time/distance to a new flightlevel (given airspeed and descent rate) etc. etc. Plus: - optionally drives the autopilot.- Gives you the option to calculate a descent rate/heading given your current position and a new set of coordinates (so you can calculate a descent to e.g. an airport).If you want to know more: just be patient :-)Rob

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For those who are interested, a real world aviation site that has alot of "rules of thumb" (descents, etc...) is http://www.flightinfo.comRegards,ScottCFII/KJMS

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If you're looking for help with FS2002 and flying, in addition to the good stuff already mentioned you could try here: http://www.tooby.demon.co.uk/FS2002/FS2002_Assistant.htmlThis also has a section on rules of thumb, but lots of other stuff you might find useful as well. Try the section "Flight Procedures" for example, or take a look at the freeware Pilot's Assistant Toolset.Have fun!--Brian ToobyTCA Pilot #2658Pilot's Assistant Home Page:http://www.tooby.demon.co.uk/P_Assist_Home.html

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Hey there,Here is a method that I have used flying the MD-88 for DAL. Some of this you have heard already and some may be new. I'll just give you the whole picture as I see/use it.Say that you are descending from FL350 to destination. First, we plan to be at 250kts and 10,000ft 30 miles from the airport. So, in this example we subtract FL350 from 10,000. Take the result of 25000 drop the zeros and times by 3. 75 miles is your answer. Now, what descent rate?For a 3 degree descent rate take your current ground speed and divide by 2. For example, say GS is 400kts the answer would be 200. Now multiply by 10. 2000 would be your rate of descent for the 3:1 that you figured out above. So, it should take you 75 miles at 2000fpm to descend from FL350 to 10,000.Now at 10,000 you have to slow to 250kts. I figure 1 mile per 10kts. That rule of thumb works well.So, you would have to leave FL350 105 miles prior to the airport for descent to 10,000 and then allow yourself time to slow from your indicated.Another tip. You can use that 3:1 ground speed rule for ILS descent rates and for Non Precision approaches as well.Grady Boyce

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Thanks Grady. The part of your tip relating to slowing to 250 knots is new to me, and I'd like to understand it. For some reason my brain refuses to function this morning, could you expand that bit a little?I can't see where the extra 30 miles (75 to 105 miles) comes from. At 1 mile per 10 knots, that extra distance corresponds to 300 knots, right? I guess what I am asking is, is it 1 mile per 10 knots of your original cruise speed in KIAS before descending, and if not, what kind of speed?MTIA!Best regards,--Brian ToobyTCA Pilot #2658Pilot's Assistant Home Page:http://www.tooby.demon.co.uk/P_Assist_Home.html

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His 75 miles will get you down from FL350 to 10000 ft.His 105 miles will get you down from FL350 to the ground, assuming the ground is at Mean Sea Level elevation :-)However, he really needs slightly more that 105 miles if he also needs to slow down from say 280 kts to 250 kts when at 10000 ft.With his 1 mile for 10 kts rule of thumb he would need an extra 3 miles for slow down.Stamatis

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Ah so!Thanks, I think I've got it now...Regards,--Brian ToobyTCA Pilot #2658Pilot's Assistant Home Page:http://www.tooby.demon.co.uk/P_Assist_Home.html

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Brian,The 30 miles is a planning point only for smaller airports ( a target rule of thumb so to speak). Thank you, I should have clairified that. At larger airports ATC will controll you all the time. If you are descending into CHS for example and ATC says, "Pilot's descretion descend and maintain 10,000" I can figure that I want to achieve this by at least 30nm from CHS. This is good planning since below 10,000 life can get real busy.Now when ATC assigns you a restriction to meet at a waypoint you have to plan for that requirement. For example, when ATC says, "Cross X Intersection at 10,000 and 250KTS" the slow down 10kts per 1 mile comes in handy. Typically when you level off at 10,000ft your IAS will be approx 320kts. So for planning you can figure 7 miles to slow down. So you should plan to be level at least 7 miles prior to X Intersection and use the remaining 7 miles to slow.I hope that helps more. Grady Boyce

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I still like to plan for 500 fpm with passengers in an un-pressurized GA aircraft, and base calculations on that figure. 750 fpm is workable, but some people are sensitive to it.........ears as an example.1000 fpm as suggested elsewhere in this thread, can get downright un-comfortable for some.L.Adamson

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I appreciate the info, Grady. It's always good to hear how the real people do it, and thanks for taking the time to clarify.Best regards,--Brian ToobyTCA Pilot #2658Pilot's Assistant Home Page:http://www.tooby.demon.co.uk/P_Assist_Home.html

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