A320 Descend Calculations
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A320 Descend Calculations
hi,
I used to use the following calculations during descend,please if some one wants to add any information I appreciate it.
ILS-I start by subtracting the FAF altitude from current cruising altitiude,then calculate the tail or head wind based on 10 knots equal 2 miles,so for example if I got 20 knots tail I just add 4 miles on the descend distance if headt wind I subtract ,plust 10 miles for deceleration,taking into consideration the a/c weight,in case of radar vectors also I see where am I with regard to the runway and I add the distance to go.
VOR-I usually put the ENTRY point -FAF- in porg page ,then I do the same calculation but I add 5 to 6 miles on top to be stabalized at the FAF.
a friend captain told me, also it is good idea to take two references at 30 miles from runway be 9000 feet and at 60 miles be 19000 feet.
sometimes I see how many miles I need to loose altitude with the current v/s and I adjust accordingly.
I would appreciate any hents and recommendations.
I used to use the following calculations during descend,please if some one wants to add any information I appreciate it.
ILS-I start by subtracting the FAF altitude from current cruising altitiude,then calculate the tail or head wind based on 10 knots equal 2 miles,so for example if I got 20 knots tail I just add 4 miles on the descend distance if headt wind I subtract ,plust 10 miles for deceleration,taking into consideration the a/c weight,in case of radar vectors also I see where am I with regard to the runway and I add the distance to go.
VOR-I usually put the ENTRY point -FAF- in porg page ,then I do the same calculation but I add 5 to 6 miles on top to be stabalized at the FAF.
a friend captain told me, also it is good idea to take two references at 30 miles from runway be 9000 feet and at 60 miles be 19000 feet.
sometimes I see how many miles I need to loose altitude with the current v/s and I adjust accordingly.
I would appreciate any hents and recommendations.
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Sounds fair enough to me. We use 3 x height + 5 nm for a 60T GW and nil wind starting point. i.e. FL370 = 116nm required. Corrections for wind we use +/3nm for every 10kts (I find this a bit too conservative personally) and +/- 1nm for every 1T above/below 60T. The important thing is to keep the maths simple enough that you can monitor the progress regularly during the descent while still doing all the other stuff! ie if your brain is exploding just use the 3 x profile plus a "bit" to slow down and a "bit" for wind and weight and if possible maintain idle thrust for as much of it as you can, ATC permitting, by adjusting IAS to maintain your optimum profile. If you are not on radar headings then of course you can use managed descent, but its not nearly so much fun 
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Depending on descent speed. In general 3 x altitude +10 to start. Cross check with 10,000' = 40 track miles give or take. I always reduce to 250 kts reaching 5000'. from then its VS mode to use the level off arrow on ND to "hit" tip over at GS intercept + additional for slowing down . Wind correction as necessary on the descent + if not in NAV mode, use Energy circle on ND as rough X check.
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The [VDIFF FT/100 divided by 3] formula should be applied just to stay on a "perfect" 3° path: nevertheless to make things easier by avoiding divisions, the following formula can be used: [VDIFF FT/1000 X 3 + 10%]. Then a correction factor may be added to allow for minor wind shifts (GS), but it changes from SOP's to SOP's.
Cheers,
fredgrav
Cheers,
fredgrav
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It also depends on your CI, and on your engines. Some have a very high idle.
The FLX3 + 10 is a good rule of thumb, plus/minus a 5% per each 20 kt average TW or HW. Below FL100 at 250, FLx3 + 6. At S speed, FLx3.
I rarely use DES. V/S and OPEN can be used more efficiently. If well computed, you will descend at constant speed, thrust in idle, while the "optimum" profile goes up and down around you, until you add thrust at about 1,500 ft.
The closer you are, the less effect the "corrections" have (wind, weight, etc) on the result, but the harder it is to correct a deviation.
Anyway, the greate uncertainty about descents is how many miles you are actually going to fly...
I am trying (just for the fun of it) during idle descents, to find a relationship between the AoA (which we can find in the AIDS, ALPHA PARAM, entering "AOA") and the speed difference between actual speed and green dot speed. And then, a relationship between this difference and the descent angle. Using a rule of thumb based on that relationship would make it easy to know the required miles for altitude for any weight and speed. We would only have to allow for wind.
So far I have no conclusions, since I am not the chief mad scientist in the deck-lab. And I have realised that I also have to take into account the effect of MN, which complicates matters somewhat, I guess.
The FLX3 + 10 is a good rule of thumb, plus/minus a 5% per each 20 kt average TW or HW. Below FL100 at 250, FLx3 + 6. At S speed, FLx3.
I rarely use DES. V/S and OPEN can be used more efficiently. If well computed, you will descend at constant speed, thrust in idle, while the "optimum" profile goes up and down around you, until you add thrust at about 1,500 ft.
The closer you are, the less effect the "corrections" have (wind, weight, etc) on the result, but the harder it is to correct a deviation.
Anyway, the greate uncertainty about descents is how many miles you are actually going to fly...
I am trying (just for the fun of it) during idle descents, to find a relationship between the AoA (which we can find in the AIDS, ALPHA PARAM, entering "AOA") and the speed difference between actual speed and green dot speed. And then, a relationship between this difference and the descent angle. Using a rule of thumb based on that relationship would make it easy to know the required miles for altitude for any weight and speed. We would only have to allow for wind.
So far I have no conclusions, since I am not the chief mad scientist in the deck-lab. And I have realised that I also have to take into account the effect of MN, which complicates matters somewhat, I guess.
... you boys and girls have way to much time on your hands Moderator
(a) the more one practises the better one gets at the task.
(b) any number of external constraints on the day (ATC, cup of coffee, etc) can conspire to upset the planning and execution
(c) (b) can be addressed by superior skill or, as a fall back, the PECL.
.. or do I have a needlessly simplistic view of things ?
(b) any number of external constraints on the day (ATC, cup of coffee, etc) can conspire to upset the planning and execution
(c) (b) can be addressed by superior skill or, as a fall back, the PECL.
.. or do I have a needlessly simplistic view of things ?
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Why bother with all of this tortuous mathematics. Next time you fly you wil notice that you will have a groundspeed indication somewhere. Let's say that you have 30000' to lose to your platform height and your aircraft loses altitude at 3000' p.m. Ergo it wil take 10 miutes to descend. If your groundspeed indicates 8 miles per minute then you need 80 miles to the FAF if it's 6 miles per minute you will need 60 miles. Your TAS wil reduce in the descent, therefore so will your groundspeed, thus building in a fudge factor for decelleration. If winds change in the descent therefore affecting your groundspeed then this is instantly displayed and you can make adjustments as necessary. KIS. I originally put in KISS but that would be rude.
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I use the TOD mark then push the little knob on the panel just below the windshiled....
try it, it works!
try it, it works!
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[QUOTE](a) the more one practises the better one gets at the task.
(b) any number of external constraints on the day (ATC, cup of coffee, etc) can conspire to upset the planning and execution
(c) (b) can be addressed by superior skill or, as a fall back, the PECL.
.. or do I have a needlessly simplistic view of things ?/QUOTE]
No you do not, or at least two of us are needlessly simplistic!!
Cheers
Colocolo
(b) any number of external constraints on the day (ATC, cup of coffee, etc) can conspire to upset the planning and execution
(c) (b) can be addressed by superior skill or, as a fall back, the PECL.
.. or do I have a needlessly simplistic view of things ?/QUOTE]
No you do not, or at least two of us are needlessly simplistic!!
Cheers
Colocolo
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flyhelico,
the managed DES sometimes sucks (even when the pilot bothers to introduce winds). Specially when you are being vectored. I have seen so many times people flying the dona when it makes no sense at all, being actually like 4000 or 5000 low and even extending speedbrakes. Or actually being very high and maintaining 1,000 fpm. All due to overreliance on computers.
Gusting 45,
both rate of descend and GS change significantly during idle escents. I think it is better to find a NM per FL rule, since the gradient changes less. I do make G/S versus rate calculations for clearances such as "when ready descend to FL 290 to be level 20 miles before HUMEL". And then I use V/S, for a rate of, say 2000 fpm, or whatever I deem appropriate, and depending on the NM per min, plus a margin, I fly nicely till 1,000 above the target, then reduce to 1000 fmp, level off and one mile ahead is HUMEL. If you introduce an ALT CSTR and let the DES do, it will be so unconfortable and ugly...
The "mad scientist" calculations I do are just for fun. But the truth is that all my "research" on the vertical navigation subject has made me a quite good vertical navigator, so the third dimension never claims too much of my limited brain power.
the managed DES sometimes sucks (even when the pilot bothers to introduce winds). Specially when you are being vectored. I have seen so many times people flying the dona when it makes no sense at all, being actually like 4000 or 5000 low and even extending speedbrakes. Or actually being very high and maintaining 1,000 fpm. All due to overreliance on computers.
Gusting 45,
both rate of descend and GS change significantly during idle escents. I think it is better to find a NM per FL rule, since the gradient changes less. I do make G/S versus rate calculations for clearances such as "when ready descend to FL 290 to be level 20 miles before HUMEL". And then I use V/S, for a rate of, say 2000 fpm, or whatever I deem appropriate, and depending on the NM per min, plus a margin, I fly nicely till 1,000 above the target, then reduce to 1000 fmp, level off and one mile ahead is HUMEL. If you introduce an ALT CSTR and let the DES do, it will be so unconfortable and ugly...
The "mad scientist" calculations I do are just for fun. But the truth is that all my "research" on the vertical navigation subject has made me a quite good vertical navigator, so the third dimension never claims too much of my limited brain power.
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Actually, the correct calculation to do is:
Angle of descend=Variation in Altitude / Distance to Go
As the required angle is, generally, 3º, we can rewrite the formula as:
Distance to start a descend to destination=Altitude of Aircraft / 3º
Now, dividing by 3 or multiplying by 3 gives approximately the same result. For instance,
FL 300 -> 300/3=100 and 300*3=900 (which we figure out to be 90). It's easier to multiply by 3 than it is to divide by 3, so that is what we do, and then add 10%, which would be 9 and the final result is 99 Nm, a good approximation.
But we still have to add or subtract the final altitude we want to be at the end of the descend. So, If we started at FL330 and wanted to be at 3000' by the end of our descend, we would do the following math: 33000'-3000'=30000'; 30000/3=100 Nm or 30000*3=90Nm+10%(9)->99Nm.
On top of this, we have to add or subtract the wind and if the aircraft is very heavy, the glide being at a constant speed whatever the weight (fixed cost index), we have to start our descend a bit earlier. If the descend was at the optimum glide speed of L/D Max, as when you have to glide after a total engine failure, the descend point would be at exactly the same position regardless of the weight. (Two aircrafts, exactly alike are in a formation flight. One fully loaded and the other with no payload. If they both experience a total engine failure, which one would arrive first to the ground? Answer: The Heavier one, but they would crash-land exactly at the same position).
During descend we reverse the calculation. "Am I high or Low?" Easy. Multiply the distance to go by 3 and add the final altitude. At 50 Nm I must be at approx. 15000 above the final altitude. The difference from my actual altitude is the result of the wind and weight. If we multiplied by 3 to calculate our Top of Descend, the correct math would be now to divide by 3 but now we multiply by 3 again and no 10% is necessary to be added. Also, we have to take into account the required distance to decelerate from the descend speed (~280Kts) to the zero flap optimum speed (~200kts), so we subtract about 10 nm from the distance we are at.
After a couple thousand hours all this become second nature. But it is all still very fallible. On a context of fuel economy, Flight Management Computers do all this math to the Nm and foot. We just follow up.
Happy flights
Luis R.
A330/A340 Jockey
Angle of descend=Variation in Altitude / Distance to Go
As the required angle is, generally, 3º, we can rewrite the formula as:
Distance to start a descend to destination=Altitude of Aircraft / 3º
Now, dividing by 3 or multiplying by 3 gives approximately the same result. For instance,
FL 300 -> 300/3=100 and 300*3=900 (which we figure out to be 90). It's easier to multiply by 3 than it is to divide by 3, so that is what we do, and then add 10%, which would be 9 and the final result is 99 Nm, a good approximation.
But we still have to add or subtract the final altitude we want to be at the end of the descend. So, If we started at FL330 and wanted to be at 3000' by the end of our descend, we would do the following math: 33000'-3000'=30000'; 30000/3=100 Nm or 30000*3=90Nm+10%(9)->99Nm.
On top of this, we have to add or subtract the wind and if the aircraft is very heavy, the glide being at a constant speed whatever the weight (fixed cost index), we have to start our descend a bit earlier. If the descend was at the optimum glide speed of L/D Max, as when you have to glide after a total engine failure, the descend point would be at exactly the same position regardless of the weight. (Two aircrafts, exactly alike are in a formation flight. One fully loaded and the other with no payload. If they both experience a total engine failure, which one would arrive first to the ground? Answer: The Heavier one, but they would crash-land exactly at the same position).
During descend we reverse the calculation. "Am I high or Low?" Easy. Multiply the distance to go by 3 and add the final altitude. At 50 Nm I must be at approx. 15000 above the final altitude. The difference from my actual altitude is the result of the wind and weight. If we multiplied by 3 to calculate our Top of Descend, the correct math would be now to divide by 3 but now we multiply by 3 again and no 10% is necessary to be added. Also, we have to take into account the required distance to decelerate from the descend speed (~280Kts) to the zero flap optimum speed (~200kts), so we subtract about 10 nm from the distance we are at.
After a couple thousand hours all this become second nature. But it is all still very fallible. On a context of fuel economy, Flight Management Computers do all this math to the Nm and foot. We just follow up.
Happy flights
Luis R.
A330/A340 Jockey
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Wow, great post! You helped me realize that one of my rules-of-thumb is more accurate if I add 10% instead of the method I have been using (adding 100fpm below 250 and adding 200fpm above 250). Thanks for that!
I'm only an instructor in 4 seat "bug smasher" Diamond DA-40/DA42, but ten years ago I was asking around on some forum (it may have been a flight simulator forum) about how to calculate my descent using the MS flight simulator 737. Someone who said he was an airline pilot responded that (in addition to the divide by 3 add 10% rule), if you want to calculate your decent rate multiply your mach by 3: .72 x 3= 2100fpm
Is that true?
He also said that your mach number x 10 is your miles per minute.
His examples were .80 = 8 miles per minute
.72= 7.2 miles per minute
so 300'/NM x 7NM/Min = 2100fpm
Is all of the above accurate? If it is maybe you guys will find it helpful. I have no reason to believe that my source wasn't a real airline pilot because flight sim nerds don't have that kind of advanced knowledge!
Back when I had an airline interview date approaching in 2008 with a regional airline that furloughed 69 pilots immediately after I interviewed, I wanted to finally figure out how to accurately calculate descent rates at typical turboprop speeds. The data I came up with applies to any speed...
I based my calculations on the fact that when we fly a 3 degree ILS we calculate fpm: 140kts/2=700fpm (however I did recently notice that jepp charts often say 753fpm)... anyway... I knew that airline pilots multiply feet to lose by 3 to get a mileage (I thought that you added 10 miles to slow and configure- from the previous post I now realize that it is to get the 3 degree path), but after getting out some graph paper I realized that rule gives you a 3.33 degree glideslope, since multiplying 1 thousand feet to lose by 3 gives you 3 miles to lose 1,000'- that equates to 333' per mile.
So if you do want to use the 3 to 1 rule (without adding the 10%) in order to get a steeper 3.33 degree path the way to calculate your descent rate is to take half of 140 to get 700fpm, same as an ILS, but then add 10% to that number.
The easiest way to visualize it without getting out a pad of paper is to think in reference to 180kts (I actually was laying in bed thinking about all this- once I thought about it at 180kts the answer it became obvious because 180kts is 3 miles per minute... at 3 miles per minute you need to lose 900 fpm on a 3' GS (180/2=900 fpm for a glideslope), but if you calculated a 3.33' with the 3-to-1 rule you have 999' to lose (333'x3 miles)... at three miles per minute you need a 999fpm rate. The difference of 99' can be made up by adding 10%.
At 420 kts GS: 420/2= 2100fpm + 10% (210)= 2310fpm [Note: if you draw it out on graph paper the desired descent rate at 420 is actually 2,333fpm, but I think this is close enough]!
For comparison (confirmation), here are the actual distances from a Boeing 737-400 flight manual:
FL370: 114nm, 23 minutes (37x3= 111)
FL350: 109nm, 22 minutes (35x3= 105)
FL330: 103nm, 21 minutes (33x3= 99)
FL310: 97nm, 20 minutes (31x3= 93)
I'm only an instructor in 4 seat "bug smasher" Diamond DA-40/DA42, but ten years ago I was asking around on some forum (it may have been a flight simulator forum) about how to calculate my descent using the MS flight simulator 737. Someone who said he was an airline pilot responded that (in addition to the divide by 3 add 10% rule), if you want to calculate your decent rate multiply your mach by 3: .72 x 3= 2100fpm
Is that true?
He also said that your mach number x 10 is your miles per minute.
His examples were .80 = 8 miles per minute
.72= 7.2 miles per minute
so 300'/NM x 7NM/Min = 2100fpm
Is all of the above accurate? If it is maybe you guys will find it helpful. I have no reason to believe that my source wasn't a real airline pilot because flight sim nerds don't have that kind of advanced knowledge!
Back when I had an airline interview date approaching in 2008 with a regional airline that furloughed 69 pilots immediately after I interviewed, I wanted to finally figure out how to accurately calculate descent rates at typical turboprop speeds. The data I came up with applies to any speed...
I based my calculations on the fact that when we fly a 3 degree ILS we calculate fpm: 140kts/2=700fpm (however I did recently notice that jepp charts often say 753fpm)... anyway... I knew that airline pilots multiply feet to lose by 3 to get a mileage (I thought that you added 10 miles to slow and configure- from the previous post I now realize that it is to get the 3 degree path), but after getting out some graph paper I realized that rule gives you a 3.33 degree glideslope, since multiplying 1 thousand feet to lose by 3 gives you 3 miles to lose 1,000'- that equates to 333' per mile.
So if you do want to use the 3 to 1 rule (without adding the 10%) in order to get a steeper 3.33 degree path the way to calculate your descent rate is to take half of 140 to get 700fpm, same as an ILS, but then add 10% to that number.
The easiest way to visualize it without getting out a pad of paper is to think in reference to 180kts (I actually was laying in bed thinking about all this- once I thought about it at 180kts the answer it became obvious because 180kts is 3 miles per minute... at 3 miles per minute you need to lose 900 fpm on a 3' GS (180/2=900 fpm for a glideslope), but if you calculated a 3.33' with the 3-to-1 rule you have 999' to lose (333'x3 miles)... at three miles per minute you need a 999fpm rate. The difference of 99' can be made up by adding 10%.
At 420 kts GS: 420/2= 2100fpm + 10% (210)= 2310fpm [Note: if you draw it out on graph paper the desired descent rate at 420 is actually 2,333fpm, but I think this is close enough]!
For comparison (confirmation), here are the actual distances from a Boeing 737-400 flight manual:
FL370: 114nm, 23 minutes (37x3= 111)
FL350: 109nm, 22 minutes (35x3= 105)
FL330: 103nm, 21 minutes (33x3= 99)
FL310: 97nm, 20 minutes (31x3= 93)
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Calculation?
I use the TOD mark then push the little knob on the panel just below the windshiled....
try it, it works!
try it, it works!
If the flight plan inserted goes along with the expected flight mileage - push button.
If it doesn't - modify or add a constraint and push the button again.
You're flying an Airbus - use it!
Crosscheck with raw data - but use your capacity for FLYING - not math!
W.
