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Good formula / rule of thumb for CDA

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Good formula / rule of thumb for CDA

Old 8th Jul 2015, 11:48
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Good formula / rule of thumb for CDA

Variables:

1.) Track miles to go
2.) Aircraft deceleration capabilities (if level, idle and decelerating, is it ok?)
3.) Wind conditions

We fly heavy A321s which are pain to go down and slow down. This means we often level off in order to slow down enough to get the flaps out. Although idle when slowing down (and thus not as noisy as some might think), some airport stats don't give a damn and simply want to see what they want to see, a CDA all the way down.

Using the above three variables, does anybody have a good rule of thumb for a CDA down to platform altitude that works?
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Old 8th Jul 2015, 15:36
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Remember your 3 times table?
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Old 8th Jul 2015, 16:12
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Yes I do. Good answer but doesn't always work when you have all of the above variables at play.
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Old 8th Jul 2015, 16:25
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I can't answer for airbus, and I know it's not what you look for but the answer is this: Experience.

But I can give you this advice:
If you are still new to the type, observe how the automatics is doing it. Leave it in VNAV (or whatever the Airbus equivalent is called). Don't just let the auto fly it - really OBSERVE and make mental notes of when the auto slows down etc.

Do it some times and then replicate it while hand flying.
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Old 8th Jul 2015, 16:39
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Another advice:
It's a feel thing and not a mental calculation.

I'll give an example, for Boeing.
At 20 nm to go, aim to be in e.g. 5000 feet AFE at 250 knots. With 3x height you now have 1000 feet leeway to slow down. Plus you most likely have an intercept turn as well. You can be higher if you have strong headwind or a sharp turn.

Break the RoD to start a deceleration. Now it's just a matter of adjusting RoD to facilitate the extension of the flaps. The experience comes into play in that, you decide when to extended the flaps (thus increasing drag), according to the progression.

Goal is of course to arrive at approach flap setting and according speed at the FAP, and at the correct height.

If it doesn't fit correctly, you can either arrive at a wrong height or wrong speed. Always arrive at the right height, and choose speed as your variable.

1)
Should you be slightly slow, add a little thrust, while maintaining and during the continuous descend, before reaching the FAP

2)
If you arrive at the FAP a little too fast, extend the gear a little earlier.

Easy peasy... Don't make a craftsmanship into rocket science.
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Old 8th Jul 2015, 17:35
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Descent planning

Cosmos's advice is excellent: Watch the gadget do it. If you're learning to fly a new airplane, take a lesson from the autopilot. My recollection is that the VNAV feature on the generation of equipment that I flew (767) could get you in trouble on some arrivals if you didn't watch it close (I've been retired for 10 years). I remember the automation computing a TOD a little closer than I would do it manually with the 3X linear rule of thumb. Maybe the VNAV knew it could get more descent rate initially in thinner air but it seemed to get behind the schedule in the lower altitudes. If you're heavy; within say 20% of your max landing weight, add about 10 miles to your plan. If you have an above average tailwind, add another 5 miles and of course the appropriate mileage for mandatory and assigned speed reductions; usually 1nm/10kts. At any rate, on average, the 4X=1500fpm, 3X=2500fpm, 2X=3500fpm rule of thumb works well under nominal conditions. The point here; say you're using the 3X rule (1,000'/3miles) and you're not seeing a nominal 2500 fpm you're probably getting behind your plan. Of course all the above assumes you're descending at cruise Mach to some predetermined IAS (310-320+kts). At the lower altitudes and at 250kts you should see around 1800'/minunte. This is typically where you'll need to add some drag. In the terminal areas and on vectors, the controllers in Germany were pretty good at giving you a planned mileage to the platform altitude, but then they started messing with your speed. If I could, I'd have the final approach fix up on the "legs" page (Honeywell FMS) to have, at least, a linear mileage from which to plan.

Hope this does you some good. I'm sure everyone has a personal technique that has served them well.
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Old 8th Jul 2015, 19:59
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It took me a while to get my rules of thumb in the types I have flown.

Then I use those and also I monitor my actual gradient, by looking at my rate and both GS and TAS. In this manner I see if things are going well, because sometimes your descent is just steeper than others, and who knows why… Atmosphere is complex, air goes up and down, you know.

In the 320, FLx3+6 was accurate below 10,000, but some models had a higher idle and in these FLx3+10 was more like it. A321 I can't remember, too long since I flew it. A330 is FLx4 plus plus, it is a glider!

But don't worry, just make your descent and if you get low, reduce V/S as required, even if it is 200 fpm. Then you can boast that you did a CDA because you didn't fly level
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Old 8th Jul 2015, 20:43
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Quite honestly, the A320 series is a flying brick house and easy to get down, even the A321. But then, that is coming from the 738 short field performance.

Apart from that, the usual 3 times table is conservative and keeps you on the safe side in my experience. Of course the usual adjustments for strong tailwinds apply. And the donut can help of course as well in energy management. Just download an up to date wind profile before you start your descent.
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Old 8th Jul 2015, 20:59
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Some useful tools I use:

GSx5 (or divide by half and add a zero ie 400kts = 200 0 = 2000) gives appr. a 3 degree profile.
It's very useful when you're using the 3-rule. If you're 1000' too high for instance, then, increase the V/S to your 3 degree rate and add another 200fpm for 5mins (or as req) and you'll get back on profile.

Learn to use the DME and track miles to go before you start depending on the automatics. On the boeing we have the progress page, if things are setup correctly it's very useful for perfect descents.

For a rough ToD wind calculation: divide the wind by three and use as a correction if above FL300. Works pretty accurately with low cost indexes on the 737NG, I don't know how slippery the bus is in comparison. For example: A headwind of 60kts = postpone your descent by 20nm. (60/3=20) It gives pretty much the same ToD as the one VNAV calculates. Interpolating works pretty good but VNAV does a slightly better job if you're still far out.

For slowing down, it's trial and error. It depends alot on the wind and the grossweight with the 737NG but I assume it's the same with the bus. In the beginning, try to slow down in level flight to get to know the energy management of the aircraft, note the distances and times needed, to make some mental rules of thumb and then start combining the slowing down while descending at the same time as you get more confident.

Comparing the DME/trackmiles with the metric altitude is also a fool proof method for us lazy people just don't forget to subtract the elevation!
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Old 9th Jul 2015, 00:41
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He is not asking about descend planning.

He is asking about Continuous Descend Approaches and how to plan the slowing down.
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Old 9th Jul 2015, 03:31
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GSx5 (or divide by half and add a zero ie 400kts = 200 0 = 2000)
Small but important correction....you mean divide by 2.

(Divide by half is the same as multiplying by 2)
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Old 9th Jul 2015, 03:33
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The answer is 42.
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Old 9th Jul 2015, 04:26
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He is not asking about descend planning.

He is asking about Continuous Descend Approaches and how to plan the slowing down.
I always thought that slowing down was part of the descent planning...

All the ideas presented so far -- both simple and complex -- are valid. use any combination of them that works for you. If it look like the prior planning isn't working, either add thrust or drag (speedbrakes and/or flaps) to correct.
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Old 9th Jul 2015, 09:58
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Intruder, it's a Eurothing, for noise abatement. How to plan your TOD is irrelevant for the question.

Think of it as extending the ILS 20-30 nm, that you start out with 250 knots, and you have to configure without a level segment. That was what Superpilot was asking about:
We fly heavy A321s which are pain to go down and slow down. This means we often level off in order to slow down enough to get the flaps out. Although idle when slowing down (and thus not as noisy as some might think), some airport stats don't give a damn and simply want to see what they want to see, a CDA all the way down.
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Old 9th Jul 2015, 10:30
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There are some good points in this old thread, especially Boacs.CDA's [Archive] - PPRuNe Forums
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Old 9th Jul 2015, 11:47
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We fly heavy A321s which are pain to go down and slow down. This means we often level off in order to slow down enough to get the flaps out.
Try using VS -300 ft per min and idle thrust instead of flying level. You'll keep everyone happy then.
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Old 9th Jul 2015, 12:20
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Intruder, it's a Eurothing, for noise abatement. How to plan your TOD is irrelevant for the question.

Think of it as extending the ILS 20-30 nm, that you start out with 250 knots, and you have to configure without a level segment.
Not irrelevant at all. VNAV on the 744 and 748 ideally does exactly that. Think of it as extending the ILS 100-130 nm, that you start out with Mach 0.82, and you configure without a level segment.

Does Airbus not do the same thing? Seems they oughtta, since they live right in the middle of the Eurothing...
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Old 9th Jul 2015, 17:33
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He is not asking about descend planning.

He is asking about Continuous Descend Approaches and how to plan the slowing down.
What's the difference?

If you want a CDA, just select a descend mode and let the aircraft descend.

However, if you want to have a continuous descent and reach a point at a certain altitude, configuration and speed without adding more thrust than necessary, well... then you have to do some planning!

If there are other ways I would love to know
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Old 9th Jul 2015, 18:13
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Not irrelevant at all. VNAV on the 744 and 748 ideally does exactly that. Think of it as extending the ILS 100-130 nm, that you start out with Mach 0.82, and you configure without a level segment.
Yes your ToD becomes irrelevant, when ATC takes you down 200 nm prior to you destination, gives you 10 step descends and controls your rate in between (all due to airspaces structures).

Welcome to euro land!
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Old 9th Jul 2015, 18:16
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However, if you want to have a continuous descent and reach a point at a certain altitude, configuration and speed without adding more thrust than necessary, well... then you have to do some planning!
ATC will take you down to a certain level, like FL070. Absolutely no planning required.
They will keep you there to a certain point, where they are allowed to let you descend further.

What the original poster is asking about, is how to manage his deceleration / configuration - while continuing to go down.
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