Controlled descent approaches

[Julian Hensey]

Non pilot here.

What is the workload on doing a proper CDA approach compared to a step down by ATC approach? How do you "set up" a CDA approach and what necessary steps do you need to programme into the Autopilot to achieve it?

I am asking this because these still seem to be a rareity rather than a usual thing and trying to sus the reasons - is it because it is a lot more workload?

J

[BOAC]

Julian - not TOO sure I have the question right, but if you refer to Continuous DAs, they are quite easy to fly. With/without Autopilot, a vertical speed is set (nearly always printed on the approach chart for the given speed) but easily worked out if not, and down she goes. Checks are made to ensure we are not below check heights and descent rate adjusted accordingly. Step downs are slightly more work-intensive, as if flown as per the approach plate there are power/trim changes as we level at each height. HOWEVER - there is really no requirement to descend IMMEDIATELY to the next check height so we often fly these as CDAs.

[Julian Hensey]

Thanks that is helpful. I was just watching two approaches from far out today, one was clearly a CDA as the aircraft was much higher on the approach way out whereas another one was flying on the base of the airspace all the way in. I just wonder why they are not done all the time if it is that easy at airports that have the traffic level to enable it to be done.

[Rainboe]

I think sometimes foreigners or non-London pilots are not aware of the requirement. Inexperienced pilots also tend to disregard it, and I find I have to watch it closely and remind them of the requirement not to fly level It can get exasperating pointing out that at 15 miles out and flying downwind, maybe we shouldn't be descending through 5000' at idle power. I think it is a question of habit- I reminded a pilot the other day that it wasn't such a good idea to wake the citizens near Stockholm Arlanda early by flying level at 2500' on an extended base turn. I think over the next few years it will become a firm requirement and practice for all approaches. It's just not concentrated enough on as yet.

[extreme P]

CDA's never seem to have caught on with full use of automatics for some reason. From a Boeing perspective (757/767) LNAV and VNAV will fly it fine provided there is some understanding of the limitations of the airplane's somewhat limited thinking ability. Gently easing the airplane into the approach is key. HDG SEL and V/S seem to increase the work load when simplicity is what I think we are trying to achieve.

[DooblerChina]

CDA's are the norm in my lot, it has been incorported into the ops manual that we should always try and fly them (except obvious exceptions) the reasons are for the noise and fuel efficiency.

I find them quite straight forward, following VNAV and the 3 times table and you can't go wrong.

If I flew level for 5 miles before intercepting the glide Id probably get an inquisitive glance from the other seat, I suppose theres an element of pride thats crept in.... who can fly the best one.

Not sure I agree with the above post, maybe there are cultural differences, in the UK noise is a big issue but more importantly the price of fuel. If I can do my bit and save a couple of hundred quid every sector just by not being lazy and making an effort to fly efficiently then thats fine by me....... however some airfields (AMS) and US ATC dont seem to like it, complicates things for them I suppose.

Doobler.

[Julian Hensey]

So where is the pressure on to do CDA's? From ATC or from your chief pilot? If it really can be calculated as saving £££ of fuel then surely the airline will be pressing for it to happen as much as possible.

Another aspect of CDA's is potentially it might save a small amount of airproxes - if you are coming in high rather than low for long track miles you are unlikely to meet stray GA stuff compared to the long and low approach....?!

[TACHO]

I was under the impression that CDA's were used primarily for noise reduction as oppsoed to cost reduction, IE not blatting around at 2500' with the flaps out and the engines fighting the drag. Not sure it has anything to do with airproxes, the aim isnt to be high on the approach, but to be at the correct height for the correct distance. Just out of interest what techniques do people on here use to judge the required ROD for a CDA? me, I tend to use a little chart I made about 3 months ago that has track miles and the ROD for a given altitude.

Tacho

[Rainboe]

I just use 3 x dist to go but make allowances- if you are heading downwind, then back descent right off and keep high above the profile, if on a base, less so, but be ready for an early turn on. You are always under radar headings, don't listen too carefully to controller supplied distance to go, I find it inaccurate, but more a general indication of how far out he is intending to take you. It works, and it's the noise suppression that is the driving force. For instance, the westerly approach at LHR takes you right over the city centre, then at about 1500' over Mick Jagger's house. We don't want him to lose his beauty sleep, do we? Even though I guess he didn't sleep much before....like

[Chilli Monster]

don't listen too carefully to controller supplied distance to go, I find it inaccurate,

Ummm - would you like to expand on that?

For example - there's a UK regional airport where CDA's are the NORM. Because of that the distances you get are going to be right probably 90-95% of the time. Listen to the distance, look at your speed, fly the apropriate ROD. If we get it wrong we'll adjust the mileage. Far better that than you going out the bottom of CAS at 2500fpm.

(A CDA is defined as a descent where the aircraft has to fly no more than 2.5 miles in level flight - it really pi$$es off the neighbours when you get it so wrong that you end up dragging it round the countryside for 10-12 miles at 2500ft!)

[Rainboe]

I found from years of flying into LHR on CDAs from the beginning, the controller's DTG tend to be inaccurate and an underestimation. I found I was relying more on ILS DME distance and an estimation of turning distance plus whatever allowance on top I thought was likely. I found it worked out different and more accurate to the figure given over the radio. But that was from years of practical experience.

[Chilli Monster]

Thankfully we're a bit more accurate - it's been a case of having to be due to the environmental pressures.

[fireflybob]

I find that one of the key parameters to watch when flying a CDA is the groundspeed - once you have more or less intercepted the correct glidepath its just a question of multiplying the groundspeed by 5 (or divide by 2 plus an appropriate amount of zeros if you find that easier!) and selecting the required rate of descent. This is especially important when there is a significant wind blowing and the track is being changed because vectoring etc.

It depends a bit on the location but in conjunction with VNAV I also have a "window" where I want the "energy" to be correct. Once the window has been established with the correct energy (speed/height etc) the CDA becomes a lot easier. As well as updating VNAV on a regular basis I also have my own mental computer going - before FMCs it was the only way you could do it and it became a matter of pride on the B737-200 to close the thrust levers at F350 and the next time you moved them was circa 4 miles out as you stabilised the approach. You need also to have a good idea of how many miles you need to slow up from, say 300 to 210 kts - if you are heavy you need a few miles extra.

I agree with the comments above that CDAs are becoming very much the norm everywhere and what is more they can be very satisfying to fly correctly.

At the airports where I operate I have always found track miles to go to be accurate - thanks Chilli Monster and Co!

[DooblerChina]

Have to admit, MAN track miles can be hit and miss. The problem seems to lie in the radar controller giving the miles to go before handing over to director. Then this person can often have a different model of what he/she is trying to achieve.