Good formula / rule of thumb for CDA

[Superpilot]

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?

[Managed Descent]

Remember your 3 times table?

[Superpilot]

Yes I do. Good answer but doesn't always work when you have all of the above variables at play.

[cosmo kramer]

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.

[cosmo kramer]

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.

[Chiller]

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.

[Microburst2002]

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 :ugh:

[Denti]

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.

[TypeIV]

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 :8 just don't forget to subtract the elevation!

[cosmo kramer]

He is not asking about descend planning.

He is asking about Continuous Descend Approaches and how to plan the slowing down.

[flying apprentice]

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)

[Amadis of Gaul]

The answer is 42.

[Intruder]

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.

[cosmo kramer]

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.

[tubby linton]

There are some good points in this old thread, especially Boacs.CDA's [Archive] - PPRuNe Forums (http://www.pprune.org/archive/index.php/t-389671.html)

[Goldenrivett]

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.

[Intruder]

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...

[TypeIV]

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 :}

[cosmo kramer]

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!

[cosmo kramer]

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.

[TypeIV]

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.

This luxury is only enjoyed in few parts of the world. However it's not an excuse for not knowing your aircraft.
Surprisingly many captains from such countries who join us, have no clue on how determine whether they're on profile or not, unless they have the route accurately programmed and then read the VNAV profile. Situational awareness is key when flying.

You always have to be able to see through the clearances and keep a backup calculation on what the ATC tells you to do. Usually as a captain, you're solely responsible for what you do. I've been cleared to fly into mountains and other airplanes' paths by ATC.

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

You reduce the vertical speed to let the airplane decelerate. Very easy, I even did it during my first ever flying lesson.

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!

[cosmo kramer]

:{:{:{
...but it's all irrelevant for the topic!!! Why not start a topic about descend planning if you feel so strong about demonstrating your mighty knowledge.

[Intruder]

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).
So what?!? Do what ATC directs. After all that, VNAV will still continuously calculate the most efficient profile from your present position and altitude. That profile will most likely be a level segment at your current altitude until you intercept the CDA profile.

If you need to start a decent "NOW", use the minimum directed/legal descent rate until you intercept the VNAV profile (which will likely be very close to the traditional 300 ft/NM guideline, should you lose the VNAV profile indicator).

Slowing down on that profile is simple: Use speedbrakes, flaps, and landing gear as necessary to add drag and maintain the profile at the lower speed. Once you're below 10,000' (where the noise nazis reside), you'll be at 250 KIAS max (generally 240 or less), so adding drag should not be a problem.

I definitely cannot understand why you consider all this information "irrelevant"! If you can plan a descent from 100 mi out, why can't you plan one from 10 or 20 miles?!?

[cosmo kramer]

Ok, I'm going to try it different:

If a guy asks:
"How can I best plan a visual approach. Do you have any advise for which heigh and speed I should have a different gates? When should I start to configure?"

Is it then a good and helpful answer to explain how to best plan your TOD descend?

This is exactly the same. CDA, in this (Euro) context, is an extension of the approach. So far none of you gave any useful tips for the original poster about how to go about doing his approach.

On the contrary:
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.
It's not trial and error. There are techniques you can employ. I gave my technique in post 5. I am sure other have different ones.

Flying level is also bad advice, when ATC expects you to do a constant descend for noise abatement. And by the way, nobody is requiring the CDA to be made at idle (though of course it would be optimum). It's not a fuel saving thing, it's noise abatement. Going down with 45% N1 is perfectly acceptable, as it makes less noise than flying level with 65-75%.

What the guy is probably looking for is this:
What rate (in what height, at what distance to FAP, at what speed) should I set to enable a continuous deceleration that will ensure I reach the FAP at the prescribed altitude, at the right speed with approach flaps configured.

Such a formula won't work, as you need a variable rate, to facilitate the slowing down, according to the aerodynamic at 240-250, clean and with the different flap settings, of the plane you fly. Also the ground speed is changing, requiring a different rate to keep the desired path. Idle descend rates also differs according to the speed and configuration changes.

Hence my advice in post 5. Get a good starting position, like 5000 feet AFE at 250 knots, with 20 nm track miles to the runway (it's on the low side and won't get the original poster into trouble). Obviously you get to that point through descend planning (if ATC didn't manage your descend from 200nm out). But this is not the part the poster is asking about.

From said point onwards, it's evaluation of how the slowing down is going during the progression. And reaction to that progression. As described in my post 5.

You want to turn all that into a mental calculation?? What is needed is a good sense of TLAR, that you get from repetition practice. Or you can learn in the beginning by watching and OBSERVING how VNAV is doing it (advice post 4).

You can plan where you want to start out. The rest is reacting to the progression.

You are assuming the original poster is an idiot (thus being disrespectful), and doesn't know how to plan a descend. I am not, as he is actually asking about the part (deceleration), which is actually difficult (especially for someone who might be relatively new in the business) and requires a great deal of anticipation.


----different topic----
If you need to start a decent "NOW", use the minimum directed/legal descent rate until you intercept the VNAV profile (which will likely be very close to the traditional 300 ft/NM guideline, should you lose the VNAV profile indicator).
"ABC123, keep a rate of 2000 fpm or greater until out of FL250".

P.s.
I do feel that descend planning is important too. It just have no relevance for the topic at hand.

[Groucho]

"I do feel that descend planning is important too."

You can hear the great sigh of releief here on the forum.............all is not lost.

[Intruder]

You take great pride in your post #5, which also gives no "useful tips" by your own [implied] definition. How is Another advice:
It's a feel thing and not a mental calculation. any more useful than the 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 that you declare as "contrary" to useful?

You also give an example, but so did others...

To an extent, for the person who asked the question, it IS "trial and error"! Since he apparently does not yet have a good feel for his airplane, the best he can do is meld his FCOM procedures with any or any mix of the examples, techniques, and guidelines given here. If he is unable to start at 20 NM and 5000' (your single example), he will have to interpolate, find another guideline, and/or calculate a path using his own devices.

Getting TO a standard (or or reasonably close) profile in the "Euro" environment you describe is likely harder than staying on it. However, if you "know" that your initial VNAV rate of descent is normally around 3000 FPM at TOC and 2000 FPM nearing 10,000' (typical for the 744, but may be different for your airplane), you can easily make adjustments from those bases.

Once you're below 10,000', the picture should be much clearer. If you're lower than desired, level off, or reduce your rate of descent to 500 FPM if you cannot level. If you're higher than desired, use speedbrakes (if you think you will only need them temporarily) and/or flaps (if you think you'll need them all the way down) to help slow down and/or descend more quickly. I have flown CDA profiles from 30+ miles before CDA was even a buzzword, much less "required". Just hold 240-250 KIAS until you want (or are told) to slow down, and use thrust and/or speedbrakes to get to the 3 deg profile.

Your standard FCOM procedures will give you the rest of your guidelines (e.g., start configuring at 17-20 NM from the airport in the 744). Once you're near the ideal path, you can easily slow down by simply extending flaps and landing gear in the normal sequence while decelerating.

[Amadis of Gaul]

You can hear the great sigh of releief here on the forum.............all is not lost.

Indeed. Hope lives on.

[cosmo kramer]

Good post Intruder, not a single mention of ToD. And actually some useful advice for the topic at hand:ok:

Once you're below 10,000', the picture should be much clearer. If you're lower than desired, level off, or reduce your rate of descent to 500 FPM if you cannot level. If you're higher than desired, use speedbrakes (if you think you will only need them temporarily) and/or flaps (if you think you'll need them all the way down) to help slow down and/or descend more quickly.

I would add that it's makes most sense to start configuring if you are too high on profile and start slowing down, rather than keep speed and use speed brake when inside 25 nm distance to go.

High on profile:
1) Far away, high altitude: Increase speed and use speed brakes if necessary

2) Close up, low altitude: Slow down and configure.

If there is no possibility to get back on path in the correct energy state, it makes most sense to arrive at the FAP: Too high, but correct speed and configured. Rather that on profile, but too fast and clean.

It's easy the get a dirty aircraft to catch up with the glide slope. And because you are slow, you have more time to work the situation peacefully. It's more work to get a clean aircraft slowed down and configured once glide slope has been captured. And the latter carries a higher risk of a rushed and unstabilised approach.

This is not meant as a contradiction to what I posted in post 5 (which contained more than the one line you quoted):
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.
With the above, I must stress I meant for smaller deviations in speed, but still configured with approach flap setting. If either you or ATC made a major screw up, dirty up and catch the glide from above.

P.s.
@Groucho and Amadis of Gaul: :ugh: