Why do the airlines, 135 operators and the FAA use and recommend CDFA? Controlled Flight Into Terrain (CFIT) is one of the leading causes of fatal accidents, and an un-stabilized approach can lead to CFIT. Based on this, all major airlines and most Part 135 On Demand Operators use Continuous Descent Final Approach (CDFA) procedures for non-precision approaches rather than the old dive-and-drive method. It makes them safer. So, why not learn from this and do it in your privately flown aircraft as well?
This isn’t something new, I was taught CDFA procedures at my first airline more than twenty years ago, but we still see accidents that might have been avoided using CDFA best practices. If you need just a little more motivation, the FAA recommends adopting CDFA as standard operating procedures for ALL operators in their Advisory Circular AC 120-108A.
In just one example, on August 14, 2013, an Airbus A300 was conducting a localizer-only approach (the glide slope was out of service) to Runway 18 at Birmingham, Alabama (KBHM). The airline’s normal procedure is to use CDFA, but on this day, the crew mismanaged the final approach descent, set the vertical descent rate to 1500 fpm, switching to the dive-and-drive technique without briefing the change, and flew the airplane into the ground. (See NTSB Report AAR 1402 and the “video companion” for details).
A CDFA approach path might have more appropriately allowed the pilots of this aircraft to fly a continuous descent to minimums and then continue to a landing if the runway environment was in sight or execute a go-around and fly the missed approach needed instead of having a descent, a level-off, then re-establishing a descent if the runway environment were seen.
The main point is that a CDFA approach increases the probability of a stabilized approach. The FAA states that a “CDFA is a technique for flying the final approach segment of a non-precision approach as a continuous descent. The technique is consistent with stabilized approach procedures and has no level-off”.
This definition also agrees with ICAO and the European Aviation Safety Agency (EASA), which has been a leader in using CDFA approaches for years. They even highlight this in their approach procedures when applicable. When reviewing the approach procedures for the VOR Rwy 36C Approach at EHAM/AMS (Amsterdam, Netherlands), you will see published DME altitudes for each mile on final approach and published “CDFA” minimums.
The advantages of CDFA include stabilized approach criteria, standardized procedures, improved situational awareness, reduced workload, improved fuel efficiency, reduced noise level, and safety. It also reduces the probability of infringement on obstacles, something even the least risk-averse pilot would agree is a good thing to avoid. Precision approaches provide vertical guidance and have a significantly better safety record, while non-precision approaches were originally designed without vertical guidance and sacrifice some of this safety margin. Flying a CDFA approach emulates some of the benefits of a precision approach and will provide a more stabilized descent.
The best part is that CDFA only requires what is necessary to fly the non-precision approach. Any FAR Part 91 operator may also adopt CDFA without any FAA approval process. Part 135 operators do not require FAA Operations Specifications for CDFA approval, but the procedure should be added to their manuals and training program.
Now, you may be thinking, how do I conduct a CDFA approach?
When flying a localizer, LDA, or back-course (LOC, LDA, or BC) approach, the pilots need to determine the vertical descent rate required for the approach from the table on the approach chart (Jeppesen Charts). (If you use U.S. Government Charts, you must determine the descent rate from the published Climb/Descent Table. You will then need to use Vertical Speed (VS) to descend. This sounds confusing if you have never done it, but I promise it is easy when you see how it works.
Let’s look at an example.
To conduct an approach with a 3-degree glide path and an approach speed of 134 knots, look at the descent rate table and choose the next higher approach speed of 140 knots. Then observe that your target VS will be a descent rate of 743 fpm. You should be fully configured as you cross the Final Approach Fix (FAF). At the FAF, start your descent using VS. This should put you close to a 3-degree glide path, approximating an ILS Glide Slope. As you break out of the weather and see the runway or PAPI, you should make any necessary adjustments visually to your descent rate.
How you enter this in your avionics package might vary. If we considered the Rockwell Collins Proline 21, a pilot would select the lateral mode to “APPR mode” for navigation transfer from FMS to the localizer, then select NAV mode to prohibit capturing any glide slope. You
would select “BC” for a Back-Course approach. For the vertical mode, you would just need to select the vertical speed (VS) and adjust the descent rate.
You can do this for NDB or VOR approaches using the abovementioned procedures.
It’s even easier if you have overlay options, such as in the Rockwell Collins Proline 21 avionics package, to fly an NDB or VOR approach. The system allows you to receive a computer-generated “Advisory Glide Path.” This glide path provides the appearance of an RNAV LPV or RNAV LNAV/VNAV Glide Path but is advisory in nature. VOR and NDB approaches may be flown with the Flight Management System (FMS) if they can be selected from the FMS database. The advantages should be obvious with more stabilized approach guidance and a computer-generated advisory glide path.
CDFA procedures get even easier when flying GPS-based approaches such as an LNAV with an MDA.
The same procedures and calculations above certainly apply, but you may not even need to calculate these in some systems that depict a computer-generated “Advisory Glide Path.” By selecting the Approach mode and the VNAV mode on the flight control panel, you will have vertical guidance as well as lateral guidance for the approach. Most modern GPS approach capable systems, such as the Proline 21 and the Garmin 5000, do this as a native function and facilitate easy implementation of CDFA approach benefits on GPS-based approaches that do not have LPV glide slopes.
While technically not a “precision approach,” many modern avionics packages can also receive, depict, and operate LPV approaches. In approach terms, this is the latest and greatest addition to the RNAV approach category. The Localizer Performance with Vertical Guidance (LPV) approach generates a vertical Glide Path using a WAAS-generated glide path. This system provides a Glide Path to a Decision Altitude (DA) and is considered “precision-like.” They offer minimums comparable to traditional ILS minimums. This is possible due to some help from the Wide Area Augmentation System (WAAS) that improves the accuracy, integrity, and availability of the GPS signals. LPV minimums can be as low as 200 feet AGL, and 1800 RVR.
A couple of challenges to keep in mind.
Some approaches still have step-down fixes along their path.
The FAA cautions that pilots are still responsible for crossing any step-down fixes. If the FMS provides an advisory glide path and the step-down fix is in the database, the glide path should clear the step-down fix. But confirm with your avionics manufacturer to be sure.
Another is to learn the technique of “Predicted Monitoring.” With a 3-degree glide path, the airplane will descend 300 feet per mile. The Pilot Monitoring (PM) should crosscheck the airplane altitude one mile before the step-down fix. If the airplane is 300 feet or more above the step-down altitude, and you are using the proper descent rate, the airplane will cross at or above the step-down fix altitude.
Oh, and this only applies if the approach path is straight. Pilots may only conduct a CDFA approach when it is a straight-in approach (no circling). Like any approach, you must also have a local altimeter setting.
This methodology only applies when the approach chart displays the descent angle and the glide path is within established restrictions. For example, the glide path angle for a category C airplane must be within 2.75 to 3.77 degrees. You cannot do this with a 2000fpm descent rate on a 6-degree glide path.
All right, enough details. The simple fact is that there is a reason that professional passenger-carrying operations such as airlines and Part 135 carriers have adopted CDFA procedures for their flight operations and that EASA and the FAA recommend adoption. Adding Continuous Descent Final Approach (CDFA) to your standard operating procedures will significantly reduce your chance of a Controlled Flight Into Terrain (CFIT) accident while conducting a non-precision approach. It’s just safer to have a more stabilized approach. If you have questions, make them a part of your next recurrent training and figure out how to best implement them in your own flying.
Excellent article on CDFA!