What is a Derived Decision Altitude (DDA) on a non-precision approach?

A Derived Decision Altitude is a pilot- or operator-calculated altitude, set typically 50 feet above the published MDA, that serves as a go/no-go decision point during a Continuous Descent Final Approach. At the DDA, the pilot must either have the runway environment in sight and be able to make a normal landing, or immediately execute a missed approach. It functions like a precision approach Decision Altitude but is derived rather than published, and AC 120-108 directs that it be established with a margin so it does not result in descent below the MDA.

How do you calculate the required descent rate for a CDFA on a non-precision approach?

Multiply the published vertical descent angle gradient (in feet per nautical mile) by your groundspeed expressed in nautical miles per minute (groundspeed in knots divided by 60). For a standard 3° path (approximately 318 ft/nm, often rounded to 320 ft/nm) at 120 knots groundspeed, the required rate is 320 × 2 = 640 fpm. Always adjust for actual groundspeed and factor in wind — a tailwind increases your groundspeed and therefore increases the required descent rate.

Is the CDFA technique required for all IFR flights, and what FAA document governs it?

CDFA is not universally required for all IFR flights. AC 120-108 directs air carriers operating under 14 CFR Parts 121 and 135 to use the CDFA technique on non-precision approaches, while Part 91 operators are encouraged but not legally required to adopt it. The primary FAA governing document is Advisory Circular AC 120-108, Continuous Descent Final Approach, which establishes the standards for applying CDFA and deriving the DDA.

Continuous Descent Final Approach (CDFA) Technique and Derived Decision Altitude

The Continuous Descent Final Approach (CDFA) technique transforms the way pilots fly non-precision instrument approaches. Rather than descending to the Minimum Descent Altitude (MDA) and then flying level until reaching the Missed Approach Point (MAP), CDFA allows a pilot to fly a constant-angle, stabilized descent all the way from the Final Approach Fix (FAF) to the vicinity of the runway threshold or the MAP. The technique closely mirrors the method used on precision approaches such as the ILS, making training and standardization more straightforward and dramatically reducing the risk of Controlled Flight Into Terrain (CFIT).

AC 120-108, Continuous Descent Final Approach, is the primary FAA advisory circular governing this technique. It applies to air carrier and commercial operations conducted under 14 CFR Parts 121 and 135 and provides a framework that Part 91 operators can adopt voluntarily. Understanding CDFA is not just a written-test requirement for ATP candidates — it is a core proficiency standard in modern airline operations.

How CDFA Works

A conventional non-precision approach (VOR, LNAV, LOC, NDB, etc.) historically used a "dive and drive" method: the pilot descends to the MDA, then levels off and continues flying at that altitude until either the runway environment comes into sight or the MAP is reached. This level segment at MDA introduces significant risk. If the pilot descends even slightly below MDA during the level-off, or if obstacle clearance assumptions are violated, the consequences can be catastrophic. The level flight segment also makes it harder to maintain a stabilized approach profile.

CDFA eliminates the level segment entirely. The pilot calculates — or retrieves from the approach chart or avionics — a Vertical Descent Angle (VDA) or published Vertical Path Angle (VPA), typically around 3.00°, and maintains that constant angle from the FAF all the way down. The aircraft's descent rate is managed to track that angle, just as a glideslope is tracked on an ILS. When the runway environment is in sight and the pilot judges that a normal landing can be made, descent continues. If not in sight at the appropriate point, a missed approach is executed immediately.

Derived Decision Altitude (DDA)

Because CDFA converts the non-precision approach into a continuous-descent profile, the concept of a Derived Decision Altitude (DDA) is introduced. The DDA is an altitude (expressed in MSL) used in place of the MDA as the decision point for a CDFA, calculated by adding a buffer — commonly 50 feet — to the published MDA. This buffer accounts for the fact that a pilot descending on a constant angle will be passing through MDA while still in motion downward; without the buffer, momentary descent below MDA before initiating a missed approach is almost certain.

For example: if the LNAV MDA is 1,200 feet MSL, the DDA would typically be set at 1,250 feet MSL. When the aircraft reaches 1,250 feet on the constant-angle descent, the pilot must either have the runway environment in sight and be in a position to make a normal landing, or immediately initiate a missed approach. The DDA functions operationally like a Decision Altitude (DA) on a precision approach — it is a go/no-go decision point, not a floor to level off at.

The specific buffer value (typically 50 feet but potentially adjusted by the operator's operations specifications) must not cause the aircraft to descend below the published MDA during the missed approach maneuver. AC 120-108 directs that the DDA be established by adding this margin to the MDA so that a missed approach initiated at the DDA does not result in descent below the published MDA.

Calculating the Descent Gradient and Rate

To fly a CDFA accurately, the pilot needs to know the required descent gradient in feet per nautical mile and convert it to a vertical speed in feet per minute. The standard 3° glidepath equates to approximately 318 feet per nautical mile (sometimes rounded to 320 ft/nm for planning purposes, though other sources round to 300 ft/nm for quick estimation). If the published VDA differs from 3°, the chart will provide the corresponding gradient.

The formula to convert gradient to vertical speed is straightforward:

Vertical speed (fpm) = Gradient (ft/nm) × Groundspeed (nm/min)
Groundspeed in nm/min = Groundspeed (knots) ÷ 60
Example: 320 ft/nm × (120 knots ÷ 60) = 320 × 2 = 640 fpm

Pilots and dispatchers precompute these values at several expected groundspeeds. Many FMS and glass-panel avionics compute the required vertical speed automatically once the VDA and current groundspeed are entered or sensed. Wind corrections must be applied: a headwind reduces groundspeed and thus required descent rate; a tailwind increases both.

Why CDFA Matters: Safety and Operational Benefits

The safety case for CDFA is compelling. CFIT accidents have disproportionately occurred on non-precision approaches, particularly during the level flight segment at MDA when pilots inadvertently descend below the minimum altitude. CDFA reduces this risk because the constant descent angle keeps the aircraft above the obstacle clearance surface until the pilot deliberately continues below the DDA after gaining visual reference — or climbs away on a missed approach.

Stabilized approach criteria are far easier to meet on a CDFA. Many operators define stabilized approach gates — commonly cited as 1,000 feet AGL in IMC and 500 feet AGL in VMC — that require the aircraft to be on speed, on glidepath, properly configured, and with thrust stabilized; these gate altitudes are industry-standard guidance (originating from sources such as the Flight Safety Foundation) rather than fixed FAA regulatory numbers, and specific values vary by operator. The dive-and-drive technique made it structurally impossible to be stabilized in the conventional sense at these gates because the aircraft was still transitioning between descent and level flight. CDFA eliminates that problem entirely.

From a training and checking perspective, CDFA standardizes the non-precision approach skill set. Pilots trained on CDFA carry the same scan and control technique from ILS approaches directly to LNAV, VOR, LOC, and even NDB approaches. This reduces cognitive workload and procedural variation, both of which are known risk factors in crew resource management.

Key Numbers and Rules

Typical VDA/VPA: 3.00° (approximately 318 ft/nm, sometimes rounded to 320 ft/nm); published on the approach chart when available.
DDA buffer: Typically 50 feet above MDA (operator-specific; defined in OpSpecs or company manuals).
DDA established relative to MDA: AC 120-108 directs that the DDA be set by adding a margin to the MDA so that a missed approach initiated at the DDA does not result in descent below the published MDA.
Decision point: At the DDA, the pilot must decide to continue (runway environment in sight, normal landing assured) or immediately execute the missed approach — no level flight is permitted.
Applicability: AC 120-108 is directed at Part 121 and 135 air carrier operations; Part 91 operators may adopt the technique voluntarily but are not required to.
Missed approach initiation: If the missed approach is initiated at or above the DDA, the aircraft will not descend below the published MDA, provided normal missed approach climb gradients are achieved.
Vertical speed calculation: fpm = (ft/nm gradient) × (groundspeed in knots ÷ 60).

Common Test Traps

Confusing DDA with DA: The DDA is derived from a non-precision approach MDA; it is not a published DA. However, it functions like a DA operationally. Precision approach DAs are published; DDAs are calculated by the operator.
Setting DDA below MDA: Exam questions sometimes suggest using the MDA itself as the decision point without a buffer. AC 120-108 directs that the DDA be established with a margin so the aircraft does not penetrate MDA during the missed approach pull-up.
Forgetting the groundspeed variable: Required descent rate changes with groundspeed. A fixed 640 fpm answer is only correct at one specific groundspeed for a given angle. Always recalculate for actual groundspeed, including wind effects.
Believing CDFA eliminates MDA: The MDA remains the published regulatory minimum. The DDA is an operational tool placed above MDA; the MDA is still the hard floor for obstacle clearance.
Assuming CDFA is mandatory for all operators: AC 120-108 targets Part 121 and 135. Part 91 operators are encouraged but not required. Know your regulatory category on the exam.