Over the last few years, any pilot who has flown IFR has been bombarded with this abbreviation: CDFA. But what is CDFA exactly, and how does it work? For some, it’s a technique that is so ingrained that they don’t know any better, especially with PBN peeking around the corner: CDFA will become a wider used technique.
For others (especially rotary wing pilots), it is something very new that can be quite confusing if you don’t know the finer details of the philosophy – leading to questions like ‘what is the point’?
Well, in here we will go exactly over what the point is, how to do it, and what the considerations and benefits are! Strap in!
- Precision vs Non-Precision Approaches
- What are the Risks of Dive and Drive?
- What are the benefits of CDFA?
- What are the CDFA regulations?
- How to fly a CDFA Approach?
- How to Calculate the required Rate of Descent?
Precision vs Non-Precision Approaches
So let’s start straight away with clarifying what CDFA is supposed to achieve and what it is exactly. CDFA stands for Continuous Descent Final Approach, which already suggests it has to do with the way we descent along a glide path.
For us to understand exactly what CDFA does, let’s revise the difference between the flying technique for a precision approach (PA) and a non-precision approach (NPA). Let’s start with the NPA.
The design philosophy here is to level out at the minimum descent altitude (MDA), fly to the missed approach point (MAP) while staying above the MDA, and then decide to continue or go-around based on your visual references.
The MDA is an actual altitude limitation – we are not allowed to fly below it at any point, unless we are visual with the required visual references.
There are quite a few (usually rotary) pilots out there who are very used to the old ‘dive and drive’ technique when flying a non-precision approach.
This essentially means descending to the minimum stepdown altitude along a descent profile as soon as you are legally allowed to. This will look something like this:
While this seems convenient / efficient to some, it actually presents a lot of risks that have been heavily researched for both fixed and rotary wing, and have been discussed for a long time now. We will go into these later.
If you compare this to the way a precision approach is flown, the key difference is the fact that the aircraft is always meant to be in a stable continuous descend along the glide path during a PA:
The other major difference is that the Decision Altitude (DA) is designed with the idea in mind that the aircraft will descend through it after the decision is made and the go around is initiated.
Whether you continue or go-around, the aircraft will descend through it – which it is calibrated for, unlike an MDA! To clarify, ICAO Annex 6 defines a DA as:
“The Decision Altitude (DA) or Decision Height (DH) is a specified altitude or height in the Precision Approach or approach with vertical guidance at which a Missed Approach must be initiated if the required visual reference to continue the approach has not been established.”
So to summarise: this is NOT some sort of minimum altitude for the aircraft to fly to, the DA is simply an initiation point – going beneath it is expected!
Normally the barometric altitude is used, but for CAT II and III ILS approaches, the decision point is always assessed to a height above the ground and is therefore expressed as DH (Decision Height). Keep this difference between DA and MDA in mind, as we will come back to this later!
For now, let’s zoom into what exactly the problem is with the dive and drive technique that (rotary wing) pilots seem to prefer so much.
What are the Risks of Dive and Drive?
The old way of flying a non-precision approach presents a lot of risks that are worth discussing. The elephant in the room is Controlled Flight Into Terrain (CFIT). To this day it is still the leading ‘type’ of accident in the aviation industry (as a result of human error usually).
Unstable approaches are the biggest contributor to this fact:
The level-outs along the approach cause different descent angles, varying power settings, changing pitch angles, and overall just more control inputs to achieve something that is also achievable without any of those things, while being lower to the ground than necessary.
All those changing flight variables make the whole approach a lot more unstable and demanding.
There have been plenty of accidents that can be linked to the dive and drive technique.
When trying to get down to a step down altitude ‘as quickly as possible’ it is not uncommon to encounter extremely low power settings and high rates of descends in excess of 1000 fpm.
There have been instances of throttles not being put back in time, causing a textbook CFIT accident. There is plenty of other examples and AAIB reports that pointed out the risks of dive and drive.
But even if you ignore those risks, staying at MDA for a very long time is not something most pilots would describe as particularly good time, especially in gusty or turbulent conditions, not to mention poor visibility – while flying along at 300’ AGL, yugh!
What are the benefits of CDFA?
So what exactly makes CDFA so special? Well get ready, to make things a bit clearer let’s go over the benefits that CDFA provides for flight operations.
• The entire CDFA technique facilitates and promotes the concept of stabilised approach criteria. The approach is a lot more stable as well as easier to tweak along the way. Instead of the earlier discussed pitch and roll changes, pilots can now just fly a stable 3° (on average) glide path all the way to the decision altitude.
• This makes standardisation a lot more streamlined across the industry as well, as the techniques for the PA, NPA and approaches with virtual guidance, have essentially become very similar now, easy peasy!
• Workload is the next one. The entire stabilised approach concept makes for less inputs, less corrections, and therefore more efficient flying for both single pilot and multipilot operations. The approach phase is the phase with the highest workload for both fixed wing and rotary wing pilots, so any steps that can be taken to reduce the workload should be embraced.
•Noise levels are significantly reduced over time. Anyone familiar with busy international airport operations is aware of the fact how big of a deal the noise emissions are, not to mention the problems it presents for HEMS operations and other low level endeavours. Level-outs at low altitudes, resulting in level flight at higher power settings have a lot of negative impact on the amount of noise emissions. CDFA eliminates these factors as during the entire approach, the power setting will be low + stable!
• CDFA is also more fuel efficient! Power changes, especially during a level out, are not efficient at all.
• As the last level-out and maintaining altitude at MDA is eliminated, the whole approach becomes a lot easier to fly for pilots. Just fly to the DA along a stable path and make a decision!
• Reduced risk of CFIT, due to the constant glide path which results in higher altitudes along the glide path compared to the dive and drive technique. Also the entire approach is more constant and stable, resulting in less inputs required to stay away from the ground in IMC.
Plenty of positives huh? Let’s move on to how regulations currently allow for this technique.
What are the CDFA regulations?
So what do the authorities think? Well, as mentioned before – it is a heavily researched topic, so let’s have a look what ICAO, EASA as well as the FAA have to say about it all.
Let’s start with ICAO (ICAO Doc 8168):
“Studies have shown that the risk of controlled flight into terrain (CFIT) is high on non-precision approaches. While the procedures themselves are not inherently unsafe, the use of the traditional step down descent technique for flying non-precision approaches is prone to error, and is therefore discouraged.
Operators should reduce this risk by emphasizing training and standardization in vertical path control on non-precision approach procedures. Operators typically employ one of three techniques for vertical path control on non-precision approaches.
For these, the CDFA technique is preferred. Operators should use the CDFA technique whenever possible as it adds to the safety of the approach operation by reducing pilot workload and by lessening the possibility of error in flying the approach.”
Seems pretty difinitive right? To clear things up even more, EASA has gone a step further and has even made it mandatory for fixed wing aircraft (CAT.OP.MPA.115) for quite some time now, but keep in mind though that this is written specifically for airplanes, not for helicopters (yet):
• Non-precision approaches: The continuous descent final approach (CDFA) technique shall be used for all non-precision approaches.”
And then there is the FAA of course:
“The FAA recommends CDFA for all NPAs published with a vertical descent angle (VDA) or glideslope (GS).”
While yes, there are still countries that do not specify a preference for or against CDFA, the overwhelming global trend is very obviously pro-CDFA. So now that is out of the way, let’s dive into the actual technique and practical implications!
How to fly a CDFA Approach?
So let’s zoom in to how to actually fly a NPA using the CDFA technique. To start, the CDFA start and ending point is the final approach fix to 50’ above the runway threshold. We do not fly all the way to 50’, but it is what the approach angle is calibrated for.
A very important thing to remember when flying an NPA using the CDFA technique, is that the original MDA is still a minimum altitude during the approach!
Just because you are now planning to have a decision point that you would call DA, does not mean you are exempt from the plate minima. How do we overcome this annoying but very important detail?
The operator you fly for will have to stipulate in the Operation Manual how much altitude will have to be added to the MDA for any particular approach to get the Derived DA (DDA).
Making a decision at this point should leave you with enough margin to not bust the published MDA while going around. This is what the FAA has to say about it (Advisory Circular 120-108):
Remember what we said about the DA though: we will have to go lower than it at some point during the go around, as it is a decision point! So let’s say we add 50’ to the 350’ MDA, giving us 400’ for the DA.
This allows us to treat the 400’ point as a DA, while still remaining above 350’ at all times! You should verify with your Part B, RFM or Airplane Manual what the height loss is during an autopilot-initiated go around. If that figure is more than 50’, you should increase the added margin!
The other confusing thing is that sometimes, if the plate publishes a CDFA DA and an MDA, the MDA could actually be higher than the DA! This is because the obstacle calculations are different for a DA than an MDA. For most situations though, adding a margin on top of the MDA will be sufficient.
Another big gotcha is that the go-around calibrations for the plate will be based on the assumption you have passed your MAP, so if the go-around states ‘immediate turn 090’, you cannot start turning straight away as you will still have to go the location of the MAP! Otherwise you could be turning into airspace that normally is not part of the go-around protection area.
How to Calculate the required Rate of Descent?
Next up is our required rate of descend, which is an important variable we need to fly an approach using the CDFA technique. We can do this the lazy way, or the non-lazy way.
Considering you are following a blog all about knowing things properly, we will go over both, not just the lazy one, you cheapskate!
You might be in luck depending on the plate you use, as sometimes they provide all you need to know, including glide angles, rates of descend tables and other information. We will however assume they don’t, as you won’t always find that info in the plate.
For this example we will use a helicopter starting a descend at 7 nm away from our runway threshold, at an altitude of 3000’. As discussed before, but we will assume we are flying to the exact location of the DDA, as that is what we will be flying to in real life too.
Right, so let’s stay lazy for this one. Our mission is to find the required rate of descend for us to just sit back and relax during the approach, right? All we need to know is how fast we need to come down, set up the entire aircraft, and enjoy the ride!
Deriving Rate of Descent using tables
ICAO as well as the FAA have published a handy climb/descend table, which essentially plots the required rate of descend (or climb) against the desired glide path, which will usually be shown on the plate, depending on your plate provider or AIP.
In our case it would be 4° for this particular approach. To proof that is 4°, we need to come back to basic trigonometry, yay! We need the ARCTAN of (3000/42533) which is 4°.
So all we do now is plot 4° and 100 kts GS. We get a figure inbetween some of the cells, which you will have to interpolate.
This table will spew out the answer we were looking for! To interpolate:
1) Look at the difference between the ROD for 90 kts and 120 kts (850-640=210 fpm).
2) As we want 100 kts, we need to add 10 kts to 90 kts, 10 kts is a 3rd of the difference between 90 and 120 kts.
3) We now divide 210 by 3, as we only need to add a 3rd and we get 70.
4) Now we simply add the 70 fpm to the already known 640 fpm and we get 710 fpm.
Not too bad right?
Calculating Required Rate of Descent using equations
If you are lucky in the above example, you won’t even have to interpolate anything, as the groundspeed might already be a column by itself, depending on what it is exactly and if that column exists. If you don’t have access to the table, or don’t want to use it – good!
Let’s derive the figures by ourselves. Let’s look at our main example again, an pay attention to the distance and height we need to cover:
We need to cover 7 nm while descending 3000’. If we take our known groundspeed of 100 kts, we know how long we have to descend 3000’. Back to basics, behold: the distance – speed – time triangle!
We want to know T, which is D divided by S. Making sure the units are nautical miles and kts, otherwise we’ll get nonsense as the answer.
The answer is 0.07 hours. Multiply that by 60 and we get 4.2 minutes! All we have to do know is figure out how much our rate of descend needs to be to lose 3000’ in 4.2 minutes. Back to the triangle:
There we go, we need a rate of descend of 714 fpm to cover 3000’ in 4.2 minutes, which is the time we needed to cover 7 nautical miles at 100 kts groundspeed!
This ROD will get us down from the FAF to our DDA with an angle of 4°!
If however, instead of a known distance (7 nm), you only know your required angle, but not the required distance to start the approach, simply divide the height by the TAN of your desired angle to get the distance (start of the glide).
That is CDFA in about 15 minutes! If you have questions, feel free to contact me directly or leave a comment, we will figure it out! As CDFA is still a new development in the helicopter industry, expect loads of other changes, considerations and change in regulations.
The biggest complaint amongst most rotary wing pilots currently is that CDFA is “cutting yourself short” as your decision will likely be before the original MAP. In the end it’s a big risk assessment with positives as well as negatives.
What is your perspective on it? Let me know! For now, stay safe and keep those future topic requests coming!
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