What is the difference between an MDA and a DA on a VOR or NDB approach?

The Minimum Descent Altitude (MDA) is used on non-precision approaches such as VOR and NDB procedures, where the pilot descends to that altitude and then flies level until the runway environment is in sight or the missed approach point is reached. A Decision Altitude (DA) is used on precision approaches, where the pilot must immediately execute a missed approach if visual references are not established at that altitude. Unlike a DA, the MDA is not a decision point for an immediate climb — you may continue at the MDA but may not descend below it unless the required visual references are clearly established. This distinction is defined in 14 CFR Part 91.175 and reinforced throughout the Instrument Flying Handbook.

How do you read the visibility minimums on a VOR or NDB approach chart?

Visibility minimums on a non-precision approach chart are listed in the minimums section alongside the MDA and are expressed either in statute miles or runway visual range (RVR) in hundreds of feet. The minimums vary by aircraft category (A, B, C, or D) based on approach speed, so pilots must apply the correct column for their aircraft. According to 14 CFR Part 91.175, a pilot may not operate below the MDA or continue an approach below published visibility minimums unless the aircraft is continuously in a position to make a normal landing and the required visual references are clearly visible. The Aeronautical Information Manual further clarifies what constitutes the required runway environment for legal descent below the MDA.

Why do NDB approaches typically have higher minimums than VOR approaches?

NDB approaches generally have higher MDAs and greater visibility requirements than VOR approaches because ADF/NDB navigation is inherently less precise, more susceptible to interference from thunderstorms and terrain, and provides only bearing information without distance guidance. The wider course sensitivity of NDB guidance results in larger obstacle clearance areas and therefore higher obstruction clearance altitudes reflected in the minimums. VOR receivers provide more stable and accurate lateral guidance, allowing procedure designers to apply tighter tolerances and lower obstruction clearance surfaces. These principles are rooted in FAA approach design criteria established by the Instrument Flying Handbook and reflected in the published minimums on each respective approach chart.

VOR and NDB Non-Precision Approach Minimums

When clouds hang low and visibility drops, instrument pilots rely on published approach procedures to descend safely toward a runway environment. Two of the oldest and most widely used procedures are the VOR approach (using a VHF Omnidirectional Range station) and the NDB approach (using a Non-Directional Beacon). Both are classified as non-precision approaches (NPAs) because they provide lateral course guidance but no electronic vertical guidance — unlike ILS or LPV approaches, which broadcast a glideslope. Understanding how NPA minimums are structured, what the chart numbers mean, and how to apply them in real flight is essential for the instrument rating knowledge test and, more critically, for safe instrument operations.

This article focuses specifically on how VOR and NDB approach minimums are determined, how to read them from a Jeppesen or FAA approach chart, and what the regulatory and practical implications are for descending to the Minimum Descent Altitude (MDA) and executing a missed approach.

How Non-Precision Approach Minimums Work

Unlike a precision approach (ILS/PAR), a non-precision approach has no electronic glidepath. Instead, the pilot descends to a Minimum Descent Altitude (MDA) — a hard floor — and then flies level at that altitude while looking for the runway environment. The MDA is expressed as an MSL altitude. The pilot may not descend below the MDA unless the runway environment is in sight and a normal descent to landing can be made.

The MDA is calculated by the FAA using obstacle clearance criteria. For most VOR and NDB approaches, the standard obstacle clearance requirement is 300 feet above the highest obstacle within the primary obstacle clearance area (which extends laterally to defined widths on either side of the final approach course). Secondary areas receive progressively less protection. This is why the MDA can vary widely from airport to airport — terrain and obstacles near the approach corridor drive the number up or down.

The descent to the MDA must be accomplished within the final approach segment, which begins at the Final Approach Fix (FAF) — typically depicted as a Maltese cross on the profile view of the chart. On a VOR approach, the FAF is usually the VOR station itself or a crossing radial/DME fix. On an NDB approach, the FAF is often the NDB beacon or a designated fix using the ADF bearing. After the FAF, the pilot descends at an appropriate rate to reach the MDA before arriving at the Missed Approach Point (MAP).

The Missed Approach Point

On a non-precision approach, the MAP is not defined by reaching a glidepath intercept with the ground — because there is no glidepath. Instead, the MAP is defined by one of the following: crossing a specific navaid (such as flying over the VOR station), reaching a published DME distance, or expiration of a time from the FAF. Time-based MAPs are especially common on NDB approaches where DME may not be available. The chart profile view will state the time at various groundspeeds (e.g., 90 KTAS, 120 KTAS) to reach the MAP. At or before the MAP, the pilot must either have the runway environment in sight or begin the published missed approach procedure.

Reading the Minimums Section of the Chart

The minimums box on an FAA Terminal Procedures Publication (TPP) chart is organized in rows by aircraft approach category (A, B, C, D) and in columns by type of equipment or procedure. For a straight-in VOR approach you will see an entry labeled S-VOR XX (where XX is the runway number). For circling minimums, a single row applies to all runways at the airport. Key items in each cell include:

MDA (MSL) — the altitude floor, e.g., "1,260" feet MSL. You may not descend below this unless visual contact with the runway environment is established.
HAT (Height Above Touchdown) — shown in parentheses next to the MDA, e.g., "(347)". This tells you how far above the touchdown zone elevation the MDA sits. HAT is used for straight-in minima.
HAA (Height Above Airport) — shown in parentheses for circling minimums; it references the airport elevation rather than touchdown zone elevation.
Visibility — stated in statute miles (e.g., 1 SM) or in runway visual range (RVR) in hundreds of feet. Visibility is the controlling legal minimum under 14 CFR 91.175 for whether you may initiate the approach and whether you may continue past the MDA to land.

Aircraft approach categories are based on 1.3 times the aircraft's stall speed in the landing configuration (Vso) at maximum certificated gross weight. Category A is speeds less than 91 knots, Category B is 91 knots or more but less than 121 knots, Category C is 121 knots or more but less than 141 knots, and Category D is 141 knots or more but less than 166 knots. Higher categories typically face higher minimums because they need more room to maneuver and stop, and obstacle clearance areas are defined differently for faster aircraft.

VOR vs. NDB: Key Differences in Flying the Approach

While the charted minimums section looks similar for both approach types, the navigation equipment and technique differ significantly. A VOR approach uses the CDI (Course Deviation Indicator) or HSI to track inbound on a specific radial. VOR accuracy is generally better than NDB accuracy, and the signal is less susceptible to interference. NDB approaches use an ADF (Automatic Direction Finder) to track inbound on a magnetic bearing to the beacon. NDB signals are vulnerable to electrical storms, coastal refraction, and precipitation static — factors that can introduce significant bearing errors. Because NDB approaches are inherently less accurate, their MDAs and visibility requirements are often slightly higher than the VOR approach at the same airport, when both exist.

Importantly, some airports publish a VOR or GPS approach, meaning the procedure is designed with VOR signal geometry but may also be flown with an IFR-certified GPS receiver that meets the required RAIM performance. The minimums lines on such charts will include separate entries for each authorized equipment type.

Why It Matters: Safety at the MDA

The MDA is a hard floor — it is not a target to fly through on the way down. A critical safety discipline of non-precision approaches is the "dive and drive" technique (which the FAA now supplements with the preferred Continuous Descent Final Approach, or CDFA technique). In the traditional dive-and-drive, the pilot descends steeply from the FAF to the MDA and then levels off, potentially flying at or just above the MDA for an extended period looking for the runway. This creates risk: a momentary distraction or altimeter error can allow descent below the MDA into terrain or obstacles.

The CDFA technique, endorsed by the FAA Instrument Flying Handbook (FAA-H-8083-15), has the pilot compute a constant descent angle (typically 3 degrees) from the FAF altitude to the touchdown zone, similar to flying an ILS glidepath, but treating the MDA as the decision point. If the runway environment is not visible when the aircraft reaches MDA (or a calculated add-on height above MDA to allow for a missed approach climb), the pilot immediately executes the missed approach rather than leveling off and continuing at MDA. CDFA substantially reduces the risk of controlled flight into terrain (CFIT) on NPAs.

Key Numbers and Rules

MDA vs. DA: Non-precision approaches use MDA (a floor); precision approaches use DA (a decision point on a glidepath). At MDA you fly level; at DA you are simultaneously at a height and on a glidepath.
14 CFR 91.175: You may not operate below MDA unless: (1) the aircraft is continuously in a position to make a normal landing, (2) flight visibility meets the published minimum, and (3) at least one of ten listed runway environment features is distinctly visible (approach lights, threshold, touchdown zone, runway markings, etc.).
Standard obstacle clearance: 300 feet above the highest obstacle in the primary area on NPAs (compare to 200 feet for ILS).
Aircraft categories: Category A (less than 91 kt), B (91 kt or more but less than 121 kt), C (121 kt or more but less than 141 kt), D (141 kt or more but less than 166 kt). Flying faster than your category requires you to use the next higher category's minimums.
Circling minimums: Apply when landing on a runway not aligned within 30 degrees of the final approach course. Circling MDAs are always higher than straight-in MDAs. During circling, visibility minimums are in statute miles, and the pilot must keep the runway environment in sight throughout the maneuver.
Time to MAP: If using a time-based MAP, start the timer at the FAF crossing — not when you intercept the approach course. Missing this step is a frequent error.
NDB signal caution: During thunderstorm activity or heavy precipitation, ADF needles can deflect toward the electrical activity ("thunderstorm effect"), making NDB approaches unreliable.

Common Test Traps

Confusing MDA with DA: The FAA knowledge test frequently asks whether a pilot may descend below MDA. The answer is no, unless all three conditions of 14 CFR 91.175 are met — and "runway environment in sight" alone is not sufficient; flight visibility must also meet or exceed the published minimum.
Wrong aircraft category: If you fly faster than your published category speed on the approach (for example, flying a Category B aircraft at 125 knots approach speed), you must use Category C minimums, not Category B. Many test questions present a scenario where the airspeed bumps into the next category.
Circling vs. straight-in: If the runway is aligned within 30 degrees of the final approach course AND the descent gradient from FAF to touchdown zone is not excessive, straight-in minimums apply. Simply being able to see the runway doesn't mean you can use straight-in minimums — the course alignment requirement governs which row of the minimums box applies.
Forgetting to start the timer at the FAF: On time-based MAP approaches, many students start the timer when they begin the final approach course segment, not at FAF crossing. Starting early makes your MAP appear to arrive sooner than it actually does, potentially leading to a premature missed approach or — worse — continuing past the true MAP.
Treating the MDA as a target, not a floor: Some test questions describe a scenario where a pilot descends to MDA and then continues descending slightly while looking for the runway. This is a violation of 91.175 and a serious CFIT risk regardless of visibility conditions.