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Eliminating Dive and Drive

Eliminating Dive and Drive

Unraveling Continuous Descent Final Approach, VNAV Using DA in Lieu of MDA, and Derived Decision Altitudes

Continuous Descent Final Approach (CDFA), VNAV using a DA in lieu of an MDA, and Derived Decision Altitudes (DDA) are confusing terms. This article will de-mystify CDFA and help determine how to apply CDFA to operations more safely and easily, making non-precision approaches more like those with a glideslope and eliminating the need to “dive and drive.”

What Is CDFA?

Continuous Descent Final Approach is the broad term that has several methods for flying a non-precision approach using a constant angle from the final approach fix (FAF) (or shortly after) that places the aircraft at the minimum descent altitude (MDA) in a position to make a normal landing using normal maneuvers without the need to level off and/or “dive and drive.”

According to the FAA:
“CDFA requires no specific equipment other than that specified by the title of the approach procedure. Pilots can safely fly suitable non-precision approaches with CDFA using basic piloting techniques, aircraft flight management systems (FMS), and RNAV systems. Pilots can use points defined by a DME fix, crossing radial, GPS distance from the runway, etc. on the approach plate to track their progress along both the lateral and vertical approach paths to the MAP.” (Federal Aviation Administration, Advisory Circular 120-108, 2011)

The easiest way to do CDFA is in an aircraft with a modern cockpit that includes Vertical Navigation (VNAV) capability and preferably the Vertical Glide Path (VGP) submode. However, there are numerous ways of legally performing CDFA using basic VNAV (VPATH), Vertical Speed (VS), or Flight Path Angle (FPA). Be sure to reference the aircraft flight manual (AFM) for any limitations regarding use of vertical modes during the Final Approach Segment (FAS).

CDFA makes a non-precision approach look and feel like a precision approach where, while flying a constant angle descent, the crew either sees the airport and lands using a stabilized approach or reaches a pre-determined altitude and treats it like a Decision Altitude, executing the missed approach from that point. Even the standard operating procedures for a precision approach are used.

A stabilized approach is characterized by maintaining a stable approach speed, descent rate, vertical flight path, and configuration down to the touchdown point. Guidance for CDFA is explained in AC 120-108: Continuous Descent Final Approach:

“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. A CDFA starts from an altitude/height at or above the FAF and proceeds to an altitude/height approximately 50 feet above the landing runway threshold or to a point where the flare maneuver should begin for the type of aircraft being flown. This definition harmonizes with the ICAO and the European Aviation Safety Agency (EASA).” (Federal Aviation Administration, Advisory Circular 120-108, 2011)

CDFA is more common (even required) in other countries, but has been adopted in the U.S. at a much slower rate. CDFA consists of techniques for flying a non-precision approach using a pseudo glide path, thereby making them similar to precision-like approaches (i.e. having a glide path). When non-precision approaches are flown conventionally, they are inherently unstable and increase the risk of controlled flight into terrain (CFIT), a leading cause of aviation accidents throughout history.

Although EGPWS has done a lot to mitigate CFITs, conventional non-precision approaches still have a high margin of risk. Levelling at step downs and the MDA both require pitch and power changes, destabilizing the approach. And even once the runway environment is in sight, leaving the MDA too early or too late can also be problematic. Unfortunately, many pilots do non-precision approaches today the same way they were trained to do them during their instrument training. Combining this with the fact that they do them somewhat infrequently, makes for a large margin of risk.

Benefits of CDFA Over Conventional Non-Precision Approach Methods

CDFA provides several benefits over conventional methods of shooting non-precision approaches. CDFA:

  • Stabilizes non-precision approaches
  • Reduces pilot workload and improves situational awareness
  • Improves fuel efficiency by reducing power adjustments during level offs
  • Reduces the likelihood of penetrating the required obstacle clearance during the FAS

Possible Errors From Conventional Dive and Drive 
James Albrite, Code 7700 - Likeness Reproduced by Permission

The FAA recommends CDFA for any non-precision approaches that have a published MDA, such as VOR, NDB, RNAV (GPS) with LNAV or LP minimums, LOC Only, etc. Localizer or LOC/BC can be a little tricky. Although the procedure works with both, a localizer-based procedure cannot be flown using FMS guidance inside the FAF. Therefore VGP is not an available vertical mode. They can still be done using CDFA, but they require use of an alternate vertical mode such as Flight Path Angle, Vertical Speed, etc.

From a regulatory standpoint (FAA AC 120-108 and the FAA 8900), CDFA can be executed in two ways:

  1. VNAV using DA in lieu of an MDA
  2. Derived Decision Altitude

VNAV Using DA in Lieu of an MDA

With VNAV using DA in lieu of an MDA, operators have probably seen the note on the profile view of approach plates that says “only approved operators may use VNAV DA in lieu of MDA.”

VNAV DA in Lieu of MDA 
Chart Copyright Jeppesen, Inc. – Used by Permission – Not for Navigation

This is a specific type of CDFA that requires either C073 OpSpec approval or a Letter of Authorization. It also requires specific training and has several requirements that the approach must meet in order for the crew to be able to shoot the approach and treat the MDA as a DA (allowing for the dip below MDA like a precision approach). The requirements can be found in FAA Order 8900, Volume 3, Chapter 18, Section 5 and Notice 8900.248. There are multiple requirements that are covered in the training and documentation, but once completed and an OpSpec or LOA is issued, operators can use CDFA to the MDA and treat it like a DA, meaning they are allowed all the way to the MDA before deciding to land or execute the missed approach.

Derived Decision Altitude

Derived Decision Altitude works very similar to the VNAV to a DA in lieu of MDA procedure, except that there are no authorizations required for it. The FAA encourages operators to train and use this method of non-precision approach procedure.

AC 120-108 states: “Pilots must not descend below the MDA when executing a missed approach from a CDFA. Operators should instruct their pilots to initiate the go-around at an altitude above the MDA (sometimes referred to as a DDA) to ensure the aircraft does not descend below the published MDA.” (Federal Aviation Administration, Advisory Circular 120-108, 2011)

By using the FMS’s VNAV capability to fly the approach to an altitude above the MDA that is raised to allow for a missed approach and altitude loss without penetrating the MDA, the crew has performed a DDA. This is often the maximum demonstrated altitude loss during coupled approach referenced from the AFM, but a higher value can be used. At the risk of sounding redundant, the key is to ensure the MDA is not violated during the go around. Once this number is determined, it can be added to the MDA and becomes a “Derived Decision Altitude" (DDA). No special approvals are required for DDAs and the FAA encourages them.

What makes it a DDA is adding a “buffer” to the MDA to allow it to be used as a DA without violating the MDA during execution of the go around maneuver. Once calculated by raising the MDA, any non-precision approach (except circling or sidestep) can be performed using this technique. As a reminder, only non-precision approaches (those with a published MDA) can use this technique. Approaches with a published Glideslope or Flight Path Angle (RNAV (GPS) to LPV or LNAV/VNAV minimums) are already charted and flown with a DA.

One additional point about CDFA to a Derived Decision Altitude: The FAA Advisory Circular states that an optimum vertical descent angle is 3.0 degrees. It goes on to say descent angles are found in the following range when the optimum VDA is not possible:

  • 2.75º–3.77º for category C and below
  • 2.75º–3.50º for categories D and E

If a descent angle is not published, has the note “Descent Angle NA,” or is beyond the above limits, neither type of CDFA should be attempted.

The following example describes the procedure for an airplane with VGP capability. There are multiple ways it can be performed without VGP, but VGP is the easiest and looks most like an ILS. The procedure is retrieved and loaded from the NAV database. Once loaded it can be flown using the appropriate lateral and vertical modes that arm the Vertical Glide Path (VGP). The procedure is now functionally similar to an ILS. The crew descends down the VGP until they reach their Derived Decision Altitude. At this point, they have two choices:

  1. They either see the airport environment and continue to the visual segment, or
  2. They execute the missed approach.

There is no level segment using CDFA. If the runway becomes visible after the missed approach has begun, it is too late to attempt to land. The airplane is no longer stabilized and has passed the point where a normal landing can be made using normal maneuvers.

It's All About the MDA

The biggest trap operators can fall into when performing CDFA is assuming, like an ILS, the vertical path can be flown all the way to the runway. Pilots have a habit of assuming if they’re on a path, it is clear all the way to the runway. That’s only true if the approach has a Glideslope or Flight Path Angle. Vertical Descent Angles aren’t anything more than an angle from the FAF to the runway threshold. Although it may look like an ILS and fly like an ILS, only the ILS (or LNAV/VNAV and LPV) has a guaranteed obstacle-free path to the runway.

It can be easy to get lulled into a false sense of security using DDA because of the functional similarities to precision approaches, but understanding the three types of vertical profiles on plates will prevent running into things when below the MDA. As mentioned above, the three profile types are:

Charts Copyright Jeppesen, Inc. – Used by Permission – Not for Navigation

Remember, the MDA is meant to provide protected airspace above the controlling obstacle to a point the runway can be located, then the pilot must see and avoid obstacles in the visual segment or be on a Visual Approach Slope Indicator (VASI) or a Precision Approach Path Indicator (PAPI). From the Aeronautical Information Manual (AIM):

Obstacles may penetrate the visual segment of an IAP that has a published VDA. Pilots that follow the previously published descent angle below the MDA on procedures with this note may encounter obstacles in the visual segment. (Federal Aviation Administration, Aeronautical Information Manual, 2019)

Despite the challenges of getting from the MDA to the runway, there are several flags that should alert pilots to be particularly vigilant when operating below the MDA:

> The FAA surveys several clearance planes when evaluating an approach. Obstacle penetrations may cause them to increase restrictions to the approach such as higher visibility or prohibit operations to a runway at night. These are charted in the profile view as “34:1 is not clear.”

> The note “Visual Segment Obstacles” may be charted when obstacles penetrate the obstacle surface in the visual segment of the approach where descent from the published VDA does not guarantee obstacle clearance. From the Aeronautical Information Manual (AIM):

“Obstacles may penetrate the visual segment of an IAP that has a published VDA. Pilots that follow the previously published descent angle below the MDA on procedures with this note may encounter obstacles in the visual segment.” (Federal Aviation Administration, Aeronautical Information Manual, 2019)

Visual Segment – Obstacles
Chart Copyright Jeppesen, Inc. – Used by Permission – Not for Navigation

> Finally, the note in the remarks section, “VGSI and descent angles not coincident,” should be an alert. This can happen when the angle set by the VASI or PAPI to clear obstacles in the visual segment happens to be different from the Vertical Descent Angle to get from the FAF to the runway.

VGSI and Descent Angles Not Coincident
Chart Copyright Jeppesen, Inc. – Used by Permission – Not for Navigation

Hopefully, the information in this article helps to clarify Continuous Descent Final Approach and provides the takeaway that there are essentially two methods of CDFA: VNAV to a DA in lieu of an MDA and Derived Decision Altitudes. It should also raise awareness of the importance of being vigilant when operating below the MDA. Operating using CDFA is a safer, lower workload-way to execute non-precision approaches but, as with many things in aviation, it’s best to start with proper training before using them in the operational environment.

Program Pilot David Rogers supports Gulfstream, Cessna, and NG FMS. He can be reached via email at David.Rogers@Honeywell.com.