CDFA vs constant angle descent

[Ludolf]

I'm trying to get my head around the difference between the Continuous Descent Final Approach and constant angle descent profile.

According to Doc. 8168 you have two options when reaching the MDA/H either descent below MDA when you have the visual requirements or initiate a missed approach. No level flight to the MAPt is allowed.
The constant angle descent however allows you to descent to the MDA and continue to the MAPt where you have to decide whether to go missed or continue below for landing. Level flight at the MDA is allowed.

Let's say we have a FAF at 6000' and 8 DME at a NPA. The approach is made up of several step down procedures.
The way that I understand it is that the constant angle descent profile starts at the FAF i.e. 6000' and 8 DME (and ONLY there) whereas the CDFA can be initiated further out (for instance at 12000' and 16 DME).

Am I completely off track here?

[AtomKraft]

Slightly off topic, but what the heck was wrong with 'dive and drive' anyway?

When I started, yes- in public transport ops- this is how it was done, and I don't remember it ever causing anybody a problem.

IMHO, the CDFA is just another step in the dumbing down of flying.
This process permits ever poorer quality pilots to fly, and yet is touted as a 'good thing'.

One other advantage of dive and drive was that the GA was initiated from level flight.

The CDFA works well enough, especially on a/c with an FPA type flight director, but I liked D&D too.

[Centaurus]

especially on a/c with an FPA type flight director,

A pox on flight directors of any kind. They not only cause tunnel vision but any instrument flying scan skiils you may have once had, are shot to hell because all the concentration is on a tiny square to keep the needles centred.

Automation dependency includes flight director dependency and has been a factor in numerous IMC approach accidents as well as failure to recover from an unusual attitude. Blind faith in the following of FD needles is a serious issue that needs to be addressed in simulator training. When you have big operators such as those in in the Middle East castigating pilots for a few minutes of manual flying raw data on a sunny day, you have seen FD dependency at its worst. Big brother QAR read-outs gets you every time.

[Ludolf]

Thanks for the replies.

And yes, it's a fairly steep approach if the airport was located at sea level. My imaginary airport is located 4000' above MSL 😄.

I'm still a bit confused though.

The "old" method of dive and drive is straight forward. But I still can't seem to get my head around the difference in constant descent angle and CDFA.

Is a CDFA profile loaded in the FMS and displayed on the PFD as command bars where the constant descent angle is old fashion altitude vs distance and controlled primarily via VS and the DME read out?

[TypeIV]

Slightly off topic, but what the heck was wrong with 'dive and drive' anyway?

Many MAPts are almost at the treshold. At 300'+ agl, a touchdown 305m down the runway is difficult to ensure without applying a generous dose of cowboy airmanship.

[AtomKraft]

So?

Usually you saw the runway quite a bit before the MAP.
If you saw it, but not in a position to land, you didn't.

It wasn't really all that hard......

It was a lot less hassle than today's attempts to turn what is really a cloud break procedure, into a sort of home made ILS.


You came down to MDA, you flew along to the MAP, then you went around- unless you caught sight of the runway. Worked rather well, I thought.

Truth is "modern pilots" can't be trusted to fly straight and level, while looking out the window!

[Blantoon]

AtomKraft, the question is about the difference between CDFA and constant angle descent, not how much better flying was in the "good old days" and how terrible pilots are these days who don't know their arse from their elbow. Take your axe to grind and do it somewhere else. Lord knows this forums has enough of those threads already.

[BizJetJock]

and I don't remember it ever causing anybody a problem.
Well that just shows that you don't remember very well. The reason CDFA approaches were developed (originally using stopwatches and/or DMEs before the advent of fancy FMSs) was because of the high number of accidents involving messed up NPAs.
Apart from questions of stable approaches in principle, flying along at MDA meant that getting visual too soon and descending below the ideal approach angle to stay visual or just because you have seen the runway is almost as risky as getting visual before the MAPt but well after the ideal approach path and ending up high and fast.

[AtomKraft]

Bizjet.
I agree. And no doubt, the ****tier pilot you were, the greater the risk.

Off to grind my axe elsewhere now, as wisely advised. :uhoh:

[Jwscud]

It is in threads like this that I miss the late 411A's trenchant inputs on dive and drive.

[AtomKraft]

JW
Although 411A is not here to post his thoughts, I have little doubt that he would have posted some very straightforward, common sense stuff.

At least the guy could fly, unlike the weak types we see on here all the time.

I can't see the likes of 411A, whatever you think about his CRM skills, presiding over AF447 or The Colgan thing.

Still, what TF do I know.

[FlyingStone]

You came down to MDA, you flew along to the MAP, then you went around- unless you caught sight of the runway. Worked rather well, I thought.

What exactly can you see with (as far as typical medium jet goes) aircraft pointing into the stratosphere with straight and level flight at approach speed in landing configuration? I haven't done it in a jet so far, but my guess is not that much...

Let's say one wants to touch down exactly at the markers and the CDFA is 3.5°. What added bonus would the "dive and drive" give you if you extended the level flight beyond intersection of CDFA and MDA? Landing at the middle of runway, approach with 4°, 5°, 6° angle?

I would love to be educated on the benefits of doing "dive and drive" in a medium/heavy jet, which is your average aircraft for CAT.

[Capn Bloggs]

I haven't done it in a jet so far, but my guess is not that much...


What added bonus would the "dive and drive" give you if you extended the level flight beyond intersection of CDFA and MDA?
If you'd have spent 5 minutes thinking about it instead of going off half-cocked, you'd realise that D-D has/had some merit. The idea is you get down to the MDA before/under the 3° path, then fly into it, then go down it to land. Obviously you're not going to fly down the 3° path then level off at the MDA then try to land. :rolleyes:

In the days of no VNAV, Dive and Drive had merit (not that we ever did it that way here; we used a 300ft/nm constant descent - sounds familiar...). It had the potential to be confusing: misreading the steps. But if you got that right, getting down to the MDA early certainly made some sense. Fly into the PAPI and Bob's your uncle, especially with cloudbase AT the MDA with good vis; you'd never get in on a 3° CDA to an MDA unless you were a bit low on the profile.

But now with the advent of VNAV, better to let the FMS take you down the 3° slope and hit the button at the MDA+50 if not Visual.

As for flying level at the MDA in a Cat C jet, my FCOM still says that's the procedure for an NPA. More importantly, it's no big deal!

[megan]

Although 411A is not here to post his thoughts, I have little doubt that he would have posted some very straightforward, common sense stuff.Not our 411A, but John Deakin, who is of the same mould.

Pelican's Perch #24:<br>Sloppy, Sorry VNAV - AVweb Features Article (http://www.avweb.com/news/pelican/182091-1.html)

[westhawk]

I love those classic Avweb articles! I read many of them when they were first published and enjoyed reading this one again. Too bad Belvoir eliminated the great articles, columns and message boards in favor of news snippets. Allot of learning and discussion used to take place over there. Deakin, Durden, Brown and all the rest. Miss ya guys. Thanks for posting it megan!

As for constant descent profile approaches, they're great...until current wx is approaching mins and the location of the MAP with respect to the rwy makes getting to MDA before the MAP the better plan. NPAs with only circling mins (no straight-in mins published) when a straight-in landing is required or expected comes to mind. Especially problematic is using a CDA when the MAP is located beyond the rwy threshold! Yeah, you could always just land somewhere else but why do that when you can get in safely simply by getting to MDA a little sooner?

[Superpilot]

Hands up if you think the question has still not been answered here?

Yep me.

Difference between Continuous Descent Final Approach and constant angle descent profile.

Ludolf,

I think Jeppesen are responsible for introducing the term "Constant Angle Descent profile" and therefore causing this confusion. It is exactly the same as a CDFA in real terms. It is, as you describe, an aggregated descent angle that gets you from the FAF altitude/position to the MDA at the MAPt without having to level off in between or risk levelling off too early.

The other descent that starts well before reaching the FAF has all sorts of names including Continuous Descent Profile (CDP) and Continuous Descent Approach (CDA) and is a descent regime we should all try to stick to in order to save fuel and reduce noise over the ground. Most modern commercial jets have a FMS that calculates this for us (all the way from TOD) without much thinking on our part. Some terminal procedures explicitly call for it to be used whenever possible though in practice we remain governed by ATC descent clearances that "spoil it".

[keithl]

What a night this is! Not looked at pPrune for months, then six posts in a row. I'm sure I'll regret it in the morning.

Both CDFA and constant angle descent avoid Step Down Fixes (resulting in more stable approach) BUT CDFA treats MDA as DA rather than proceeding to MAPt.

So, using CDFA, you fly a stable descent to MDA, then land or go around. The constant angle descent gives you a stable approach to MDA, (which may not coincide with MAPt) but then a level sector to MAPt. It suits helicopters, who are still allowed to do it.

[AerocatS2A]

Especially problematic is using a CDA when the MAP is located beyond the rwy threshold! Yeah, you could always just land somewhere else but why do that when you can get in safely simply by getting to MDA a little sooner?

Why would that be problematic? Surely you don't calculate your CDA to put you at the MDA at the MAP? Don't you calculate the CDA to put you on the PAPI? In which case, yes you get to the MDA before the MAP but unless you were planning on circling to land, if you aren't visual at the MDA (not the MAP) you just go-around.

[westhawk]

Why would that be problematic?


Perhaps it shouldn't be. But it often is at one particular airport I've frequented often. Unique challenges perhaps.

[Ludolf]

Keithl & Superpilot thanks for the answers. That helped a lot.

It's now clear to me what the difference is.

Cheers.

[keithl]

You're very welcome, Ludolf.

Job done as far as the OP is concerned, but I would caution anyone wishing to take this further that previous posts appear to conflate CDA with CDFA. They are different. CDA gets you from cruise alt to instrument pattern alt. CDFA gets you from there to MDA. Ideally, they should blend into one smooth continuous descent but, just like the initial, intermediate and final segments of an instrument approach, they are different things.

[Capn Bloggs]

I would caution anyone wishing to take this further that previous posts appear to conflate CDA with CDFA.
The first post didn't actually mention anything about a CDA.

The term CDA is better described as a "Continuous Descent Arrival".

[keithl]

You are right, Capn Bloggs, it didn't.

#17 is the only one to mention CDA, and from the context he really meant CDFA. That's why I pointed it out, really - in real world discussions of this, things very quickly get chaotic if we aren't careful with our terminology.

And in real world discussions, we can smile, shrug, etc to keep things on an even keel. Online debates are trickier, aren't they?

[westhawk]

Mea Culpa on the terminology usage! While I used the acronym CDA, my remarks in the previous posts referred to a constant descent angle on the final approach segment.

Yes the terminology is important. Particularly when the terms are not used universally across the globe.