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Best way to calculate descent point.

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Best way to calculate descent point.

Old 8th Oct 2011, 15:05
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Best way to calculate descent point.

If someone could please give me an example, and how it is solved.
I asked a few of my friends (First Officers) and they said all they do is put it in the MCDU

For a certain Ground Speed, maintaining a certain altitude, and ATC asks you to descend and BE at another altitude lower than the one you are at, at a certain waypoint.

How exactly would you calculate your descent point?
Add any number that you like.

Thank you in advance ....
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Old 8th Oct 2011, 16:36
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oh sorry. You asked for TOD. Flightlevel to descend/ 3 = NM to descend should work for idle descent.

here are good thumb rules:

http://www.b737mrg.net/downloads/b737mrg_thumbrules.pdf

Last edited by Da-20 monkey; 8th Oct 2011 at 18:06. Reason: wrong question
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Old 8th Oct 2011, 21:22
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Okay for example, FL410, GS 420.
ATC asks you to be at FL 250 at a certain waypoint.

Take into consideration ROD = 1000 FPM
You're going at about 7 mile/minute

That makes it 13 minutes to lose the 13,000 feet, and 7 miles per minute
13 x 7 = 91 NM
You start your descent 91 NM beforehand.

When numbers get a little bit confusing, is there an easier way to do this?
Does the MCDU usually do this for you, or are you required to manually do this?

Cheers
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Old 9th Oct 2011, 01:30
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Time to go back to third grade maths...

FL410 - FL250 = 16,000 ft

Also consider that your TAS will reduce with the lower altitude, so your GS will change (not taking different winds into account).


So using Monkey's info of 3 miles per 1000ft, try 16 x 3 = 48 miles.

Add a few miles to allow for entry to descent, and recovery to S&L.
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Old 9th Oct 2011, 09:33
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Runaway gun,

Sorry that was my mistake
Thanks for the response, and I have a few questions.
Where do we assume the 3 miles per 1000 feet?
Is this an average for commercial aircrafts?
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Old 9th Oct 2011, 16:27
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The basic techinique for me is to use 3 x the altitude to be lost. Add/subtract 1 mile for each 10kts of TW/HW. If making a straight-in, add another 10 miles for slow-down until you get used to your particular airplane.

On the Whale, I also made a correction for weight @ TOD. My plan was to never use the speedbrakes during the descent...unlike most of the children of the magenta line who use the speedbrake as a crutch to poor descent planning. As pilots, we need to plan and interface to avoid having to make a reactive correction, but maybe that's just dinosaur-me. My license still says Airline Transport Pilot, not Airline Transport Dog Watching Television.

Pride, professionalism and performance prevents piss-poor performance. Which 3-P's are you? Judging by your question, perhaps the former and not the latter. Good for you.
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Old 9th Oct 2011, 19:00
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Which aircraft? What speed profile? What type of descent?

PM
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Old 9th Oct 2011, 20:23
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It's an MS2010 I suspect.
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Old 10th Oct 2011, 00:19
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VNAV ..
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Old 10th Oct 2011, 04:57
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Forum blur thread drift divided by TAS multiplied by HAT

I think this:

Best way to calculate descent point.

If someone could please give me an example, and how it is solved.
really belongs someplace like here: Professional Pilot Training (includes ground studies) - PPRuNe Forums

Professional Pilot Training (includes ground studies)A forum for those on the steep path to that coveted professional licence. Whether studying for the written exams, training for the flight tests or building experience here's where you can hang out.
.............. But I hope I can cobble together some plan of action before my next flight that requires me to have thousands of kilos moving close to the speed of sound arrive at a specific point in space at a exact time and altitude.............

Last edited by Northbeach; 10th Oct 2011 at 18:29.
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