Continuous descent approaches

[ATCOJ30]

AS par of our recent airspace change proposal (EGGD), we are aspiring to introduce continuous descent approaches whenever possible, if environmental benefits can be identified for local communities accordingly, as we suspect will be the case.

I've never applied CDAs before in all my 30 years + in the job so can anyone from the Terminal units in the UK who regularly control using CDA techniques give me a quick tutorial as to what's involved from the ATC aspect?

I think I understand the objective - minimal use of power by a/c from leaving the stack to touchdown but there's more to it than that, I'm sure! Any specific phraseology? When do you start the CDA (range or alt)? Speed control? What if you need to tactically extend/shorten the routeing to the LLZ? Separation considerations?

For info, we rarely hold (so far...) except in LVPs and traffic will come to us on tactical headings from Cardiff/S23, typically descending to FL70/80 (from EGFF) and FL120 (S23), at approx 20-30 miles out.

Many thanks indeed - pilot comments about CDAs most welcome too.

[Warped Factor]

Well for Heathrow and Gatwick standard speeds are used...220kts off the stack, 180kts in the base leg area and 160kts on final to 4dme.

Descent from FL70 or 80 is usually when the CDA starts and all that's really needed is an accurate track miles estimate for the pilot who then manages his ROD so that in theory there's no period of level flight before landing.

"Shuttle Seven Alpha, twenty four miles two seven right, descend to altitude four thousand feet etc etc".

On first contact with the final director another track miles check should be given so that the driver can adjust ROD as required, and of course the track miles should be updated if they change significantly at any time.

All ATC really have to do is issue accurate track mileages when giving the descent from the point the CDA begins and then updating as required. Works quite well with a high percentage of arrivals making CDAs most of the time. Rates of descent may be lower than you're used to and sometimes CDAs will not work as you need to hurry traffic down for separation purposes.

WF.

[javelin]

CDA's don't work at LGW or MAN, you end up either high or low.

Why ?

Because we have lost the cooperation between flightdeck and ATC. We need to know what you are doing, you need to know what our aeroplane does.

Problem is for ATC people, they get rostered for fam flights on days off and vice versa. Well everyone - go the extra mile and take the trip, it makes for a better life. I have been to MAN on a day off and enjoyed the ATC sim - couldn't do it but hey, whatever !

CDA's sound great but a Boeing CDA is different to an Airbus CDA is different to a Leyland 146 CDA.

[Gary Lager]

Quite valid points above...CDA's work well in the TMA because the crews know what is expected of them, they have practice, and the ATCOs assist them by providing an accurate track mileage estimate at an early stage of the approach, and facilitating descent clearances without needing prompting. If pilots are having to nag ATC for 'track miles' and 'further descent' it gets hard work, next time we are more likely to fly a more conservative profile ahich ensures we don't end up high or fast, but which will probably negate the CDA.

Pilots do the most of the work on CDAs - but ATCOs need to provide them with adequate airspace and info to do so.

Success comes about through crews knowing what to expect from an ATC unit, and being able to trust the 'shared mental model' approach plan - either from charts (AMS) or from experience.

Arrange a meeting in the function room of the local, invite crews from airlines, put a few quid behind the bar and bring an OHP/laptop...you will probably be surprised how quickly the word gets out!

And just to re-iterate the most important points:
1) Tell us the track miles to run
2) Make it accurate!

[flower]

Gary,
it was one of the items that was discussed at last weeks ATCO Pilot forum at Cardiff.

I have worked CDA's, range checks are the most important issue, other than that once you are used to them they are no problem at all.

[ATCOJ30]

Thanks for comments to date, all helpful.

[Giles Wembley-Hogg]

The key to CDAs is predictability. I think it is fair to say the Heathrow are the experts at CDAs and I believe this is due to the following reasons:

1. We follow more or less the same track every day. Any variations in track (for traffic/spacing etc) are of a magnitude such that the alteration in rate of descent required is within the normal operating envelope of modern aircraft.

2. One person (usually the ATCO) controls both speed and track distance. This leaves the pilot just in charge of the vertical profile.

3. The speed control applied is both predictable and complies (roughly) with the flap schedule of most of the airport users.

4. Any changes in the arrival route/speed control etc from the norm are usually passed well up the chain of controllers (eg WCO snatches, high speed approaches etc).

Places like CC and PH make it trickier to judge a CDA, because the speed control is often left in whole or part to the pilot. This is (in my opinion) not best practice unless the ATCO also allows the pilot to self position on to the approach or gives VERY accurate ranges from touchdown and does not alter the track he/she has planned, unless the pilot so requests.

At LL because speed and distance are controlled by the ATCO all the pilot has to consider is "are we high/low/on profile". At PH (for example) the pilot has to consider "how far out am I being taken/if I slow down now will they reduce the distance to run/are they going to suddenly impose speed control on me/where am I being taken next... THEN... are we high/low/on profile".

I (personally) disagree with Mike when it comes to FMS transitions. I've flown the Schipol one a few times and I think that it brings the aircraft down too soon and makes it fly too slowly too far out. Although a CDA results, I think that if I self positioned or was given Heathrow style vectors the approach would be made with the thrust levers at idle for longer and at any given point on the intermediate approach the aircraft would be higher up and thus quieter to those on the ground.

Just my views.

G W-H

[Daysleeper]

If you want efficient CDAs then publish what you like, just dont change it on the night.
I've lost track of all the wasted time setting up to fly the required vert profile to then have vectors which stuffs the whole thing.
Often the vectors just end up following the approximate published path anyhow or involve a 5 or 10 mile track shortening..the time difference is only a couple of minutes but in energy management terms its a mare. We end up hanging off the speedbrake swearing (off mike) at the controller. Then having bled off all that energy you get vectored to 10 miles rather than the published 8 and you end up with a 2 mile level segment anyhow.
Why not just clear the approach at 25 miles and shut up.

[Scott Voigt]

For those pilots out there (I am one too) who think that you can design one arrival/approach that everyone can fly the same will work at any busy airport with more than one runway is sadly lacking in knowledge of how we sequence and fill gaps to the airport. We indeed could just allow the aircraft to flow into the airport at a leisurly rate to allow for hands off decents. To get to that point though the flow rates would go down and the capacity of the airport would suffer, thus generating even more delays.

I hate to be the one to break the news to you, but pilots do NOT fly thier aircraft the same and we have to take that into account in the sequence. You would be amazed at what we see on a daily basis on differing profiles of airlines. Not just different airlines, or aircraft, but the same airline and same aircraft.

regards

Scott

[Gary Lager]

flower -

yeah I know - I was disappointed to miss it (lates), looking forward to the next one!

GL