Selecting low V/S for CDA

[Airmann]

Hi,

Flew with a captain recently who got quite annoyed when I selected around -500 V/S in terminal airspace in order to maintain a continuous descent. The other option would have been to go Open Descent (Airbus) and to level off.

Obviously the best possible way to descend for fuel savings and noise is to have the thrust at idle from TOD until established on final (a true continuous descent), however, this requires planning on the part of ATC, and at the airport we were landing at they do not do this. So almost everyone I know selects V/S in order to try to not level off at any point.

Question: Are we just being stupid by selecting V/S and going down at-500 ft/min. Is that captain right? Does this really save any fuel compared to just going down at idle and flying level for a while? Do they amount to the same thing at the end of the day? Is fuel savings only realised on a proper continuous descent from TOD i.e. at idle.

[Kennytheking]

Seems a perfectly sensible way to do a CDA. In fact, I'm not sure there is any other way. Just make sure the VS is appropriate wrt ATC requirements(many have a min 500'/minute requirement, although UK defines it along the lines of altitude lost per mile). Also making sure the VS doesn't override the speed required.

Maybe he was just having a bad day..............

In terms of fuel saving, I suspect a CDA does save a drop of fuel and they then multiply that by number of flights to make it more substantial. It's the kind of thing that works nicely in theory but is not really practical given the volumes of traffic that ATC have to deal with. Sort of like the optimum levels in a flight plan.......

[RAT 5]

It's the kind of thing that works nicely in theory but is not really practical given the volumes of traffic that ATC have to deal with

CDA's are encouraged by airlines to save fuel. CDA's are encouraged by airports to reduce noise. There are many airports in Europe where it is mandatory and the ATC arrivals are designed to achieve a CDA. Either they put speed/Alt at the apt's on the STAR or they have an IAF with speed/ALT and then radar vectors with distance to touchdown info and they expect a CDA. It works at busy airports who have designed their airspace for such techniques. The volume of traffic does not affect it.

[casablanca]

While CDA is often great , sometimes it is not practical when you consider there are a hundred airplanes sharing the airspace....sometimes separation from other traffic is more important than saving 6 dollars in fuel.
I recently saw someone do 300 fpm descent into JFK....Atc said I need your best rate to 4000... Then said expedite to 4000.... So they went to 400 fpm.
The 3rd call from Atc wasn't so pleasant!
If you get violated by Atc and are invited for tea and biscuits.....will your company back you up cause you saved few bucks?

[sharpshooter41]

I have seen pilots select V/S, activate approach phase and then wonder why the speed is not reducing to Green Dot. The resultant increase in workload including at times, non standard lowering of gears (before taking Flaps-2), is not really comfortable.


In today's high traffic environment, better to keep a good SA and forget about saving a few pounds of fuel.


My two cents

[RAT 5]

CDA's were called Low drag, low noise, low fuel approaches when I was first taught in the 80's. It was the company standard long before noise sensitive airports e.g. Germany, published them as their SOP. In many UK airports it is now a requirement.
If you know how your a/c works it is not a problem to find the easiest technique. Not understanding your a/c systems is not an excuse for not complying.

[Mikehotel152]

CDAs are a simple idea, complicated by ATC demands and misinformation: a surprise shortened 25 track miles, instigating a dirty dive, regularly lengthens to become 40 track miles and 200 fpm V/S... Gotta love STN...

One has to use all descent modes (B737) to ensure you can adapt to whatever is thrown at you. If that involves a period at -500 fpm, fair enough.

As for fuel burn, of course a continuous descent from cruise to flaps will be most cost effective, but this is rarely possible in a busy TMA. I'd guess this is seen as one of the operating costs, similar to the many kgs we burn while holding off from the stand waiting for our useless ground crew to turn up...

[felixthecat]

Don't see any problem with it at all. The days of dive and drive have long gone :)

[FlightDetent]

Airmann, what would your colleague feel like if you did managed DES, and assuming you were below the VNAV profile, the FMGS/FD did -500 fpm? That's how the VNAV is re-designed on FMS2 that has been the manufactured standard for more than 10 years already :)

I share the sentiment of posters above. UK, where mandated, stipulates CDA below TL as one of the reasons to use less than 500 fpm, which is the de-facto minimum required rate in their airspace.

[Cough]

Airmann,

At the briefing stage did you highlight your intention to do a CDA? As others have noted, some airspaces are suitable for it but others aren't - If your skipper felt your destination wasn't suitable for a CDA then that should have been sorted prior to TOD.

That said...

500fpm is entirely suitable for CDA purposes, as others have noted some airfields in the UK also allow rates less than that below TA (6000ft generally in the UK)...

[gatbusdriver]

I believe the requirement in the UK was for a CDA to be performed from 6000' although I believe LGW now suggest from FL070.

You used to qualify as long as your V/S was 200fpm or greater with a level segment of no more than 2.5 nm (or something like that).

Regards,

GBD

[RAT 5]

Like I said, years ago it was called a low fuel, low drag, low noise approach. That usually meant a CDA from FL350 with engines at idle until 1500/OM or you bought the beers. Good game. Good game.
It could also be achieved by 300kts to 2500' downwind and a level wider circuit to slow it down and still be idle until 1500' on finals.It was all about energy management and idle thrust. Then the political correct aviation H&S guys came along & spoilt all the fun.

[FlightDetent]

Oh, of course you owned the airspace then. :ugh:

[Uplinker]

Unless it has changed recently, I understand the definition of a CDA to be:

No level flight for longer than 2.5nm below 6000'

Therefore a V/S of -100'/min falls within this definition.

At our base we are often held high then have to dive down at the last minute to get under the G/S, making a CDA less than easy to achieve.



As for the Captain getting annoyed, there is rarely a good reason for this, but it might have been:

1. He was annoyed at ATC/late cabin secure call/etc, not you.
2. Your flight selections were innappropriate for some reason*
3. He was having a bad day.
4. He has a problem with you for whatever reason (unlikely).
4. He is a ****/control freak (much less likely these days).

*Ask him in the crew room "Was there anything about my approach you would have done differently?"

When I brief an arrival, we both know our base wants CDAs, so that bit is taken as read. However, what I usually say at the end of any brief is "I will follow SOPs but if I have missed something or am beyond any limits, just remind me - I won't be offended".


That last bit is the duty of PM of course, but even so; saying this tells the other guy that you won't bite their head off or get moody, and they almost always reply "yeah, same goes for me too" So now you both know that you are both open to appropriate comments and are going to have a good day.


.

[Airmann]

Don't want to talk too much about the captain that day, needless to say I wasn't the only one in the crew that seemed to think his behavior was odd, but thats another story.

Coming back to CDAs, I guess my real question is, if you are in the last stages of a descent (below 10,000ft, thats usually the time we start playing with V/S) and you chose to descend at low V/S as opposed to descending faster and then leveling off is there much of a fuel savings? Is the extra workload worth it, especially if noise is not an issue (the approach into my home airport is entirely over the sea)? Are we absolutely sure of the benefits in terms of fuel burn.

Let me give you an example where CDA won't save any extra fuel. Assume a 30NM leg at a constant ground speed of 300kts (6 minute leg). We are instructed to descend and an open descent will leave us level after 15NM. A CDA will be at a v/s of half of open descent.

Hypothetically assume a fuel flow of 20KG/Min at idle, 55KG/Min at a V/S of half of open descent and 80KG/Min at level flight.

If we go for a CDA (at half of open descent) then over 30NM we will burn 330kg (55kg/min for 6min)
If we choose an open descent followed by a level off we will burn 300kg (20kg/min x 3 min + 80kg/min x 3min)

These numbers are entirely made up, but what I'm trying to say is that maybe we have made it a fashion in wanting to always see a negative V/S even if its only 200-300 fpm? Is it adding unnecessary workload?

In writing this post I went online to see if anyone has done any significant empirical research in this field and found this

http://web.ics.purdue.edu/~dsun/pubs/jac13.pdf

It is a paper by some guys at Purdue in Indiana, and they do say that although CDAs do reduce fuel burn it is not always the case, and that flying at optimal speeds is also important. In fact maintaining a higher speed in terminal airspace (i.e. 250kts) may have a greater impact on fuel savings than the descent profile.

[sharpshooter41]

Airmann: You seem to have answered it yourself.


For me the atmosphere in the cockpit (read CRM) is more important. If the skipper wants it a certain way, just do it. You will have your chance when you move seats. A few pounds of fuel saving is not that important.

[RAT 5]

Oh, of course you owned the airspace then.

With no radar around the Greek rocks it was just us & the birds. No big brother watching inside or outside. And no, it was't cowboy. Place & time.

Surely a CDA as the norm, from CRZ, is a more satisfying achievement than just stepping it down. It is a finessing of the profile and it is certainly more comfortable down the back. You start a quite descent from CRZ and the next thing is a gentle bump on the runway with very little disturbance. Apllause.

[gatbusdriver]

And then came Andravida.....!!!!!!!!!

How I dream of just the birds.

Nothing like taking up the hold for 15 minutes because someone else is on the approach into KEF some 25 miles ahead of you.

[Ivan aromer]

In my day it was called airmanship. Keeping a 3d picture in your head and workin around all the obstacles to do the best you could!!!

[DaveReidUK]

It's the kind of thing that works nicely in theory but is not really practical given the volumes of traffic that ATC have to deal with. Sort of like the optimum levels in a flight plan.......

Heathrow achieves around 550 CDAs per day, on average, out of 650 daily arrivals.

[misd-agin]

If you're far enough below the path/VTI you'll either level off, and break the required minimum (no level off) or could use v/s, or advance the throttles while in FLCH(Boeing), to shallow out the descent rate.

[RAT 5]

All this chat is 'how to make an easy job difficult'. Paralysis by analysis.;)