13.3 Non-Precision Approaches: VOR, LOC, RNAV (LNAV/LP)

Non-Precision Approaches: VOR, LOC, RNAV (LNAV/LP)

Quick Answer: A Non-Precision Approach (NPA) provides lateral course guidance only without electronic vertical guidance. Pilots descend through published step-down fixes to a Minimum Descent Altitude (MDA), which acts as a hard regulatory floor until the Missed Approach Point (MAP). Localizers (LOC) are aligned within $3^\circ$ of the runway centerline, whereas Localizer Directional Aids (LDA) are offset by $> 3^\circ$ (only circling minimums apply if offset $> 30^\circ$). Simplified Directional Facilities (SDF) have fixed $6^\circ$ or $12^\circ$ course widths. Modern operations replace legacy "dive and drive" profiles with Continuous Descent Final Approach (CDFA / CANPA) techniques using a Derived Decision Altitude (DDA = MDA + 50 ft).

While precision approaches like the ILS guide aircraft down an electronic glidepath, thousands of public airports lack precision ground infrastructure. At these locations, pilots rely on Non-Precision Approaches (NPAs) utilizing ground-based radio facilities (VOR, LOC, LDA, SDF) or space-based satellite navigation (RNAV GPS LNAV and LP lines of minima).

Safely flying an NPA requires understanding lateral sensor characteristics, step-down obstacle clearance boundaries, timing tables, and modern constant-angle descent techniques.

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Anatomy of a Non-Precision Approach

+-----------------------------------------------------------------------------------------+
|                         NON-PRECISION APPROACH PROFILE ANATOMY                          |
|                                                                                         |
|   Intermediate Fix (IF)                                                                 |
|   2,500 ft MSL          Final Approach Fix (FAF)                                        |
|      │                  Maltese Cross (✠)                                               |
|      ▼                  1,800 ft MSL                                                    |
|   ───●───────────────────✠───────────────────────── Step-Down Fix                       |
|                          │                          1,200 ft MSL                        |
|                          │                              │                               |
|                          │   Continuous Descent         ▼          Missed Approach Point|
|                          │   Path (3.00°) ───┐     ───●──┐                 (MAP)        |
|                          │                   ▼           │                   │          |
|                          │                               ▼                   ▼          |
|                          └───────────────────────────────■═══════════════════●          |
|                                                    MDA: 820 ft MSL (Hard Floor)         |
|                                                                                         |
|   ══════════════════════════════════════════════════════════════════════════■════════   |
|                                                                       Runway Threshold  |
+-----------------------------------------------------------------------------------------+

1. Lateral Guidance Only

• NPAs provide no electronic glidepath guidance. The pilot controls vertical descent using barometric altimeters and step-down minimum crossing altitudes.

2. Final Approach Fix (FAF): The Maltese Cross (✠)

• Charted as a bold Maltese Cross (✠) on the profile view of non-precision approaches.
• The FAF marks the point where the final approach descent commences and where the pilot initiates descent from the intermediate altitude to the first step-down fix or MDA.

3. Step-Down Fixes

• Intermediate fixes between the FAF and MAP published to provide obstacle clearance over specific high terrain or towers. Descent below each step-down altitude is prohibited until crossing the associated fix (identified via DME, GPS waypoint, or cross radial).

4. Minimum Descent Altitude (MDA)

• An absolute hard floor altitude (in feet MSL). Unlike a Decision Altitude (DA), the aircraft must NEVER descend below the MDA unless all three visual requirements of 14 CFR § 91.175 are satisfied.

5. Missed Approach Point (MAP)

• The point along the final approach course where the missed approach procedure must be initiated if the required visual references are not established. Identified via:
  • Timing: Calculated from FAF to MAP based on aircraft groundspeed.
  • DME: Specific distance readout from a paired NAVAID.
  • Fly-Over Waypoint: GPS waypoint marked with a circle around the star over the runway threshold.
  • NAVAID Station Passage: Flying directly over an on-airport VOR/NDB.

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VOR Approaches: On-Airport vs. Off-Airport

+-----------------------------------------------------------------------------------------+
|                        ON-AIRPORT vs. OFF-AIRPORT VOR APPROACHES                        |
|                                                                                         |
|  Feature              | On-Airport VOR                    | Off-Airport VOR             |
|  -------------------- | --------------------------------- | --------------------------- |
|  NAVAID Location      | Located on the airport grounds    | Located 3 to 15 NM away     |
|  Final Approach Fix   | Station passage or No FAF charted | Maltese Cross (✠) charted   |
|  Missed Approach Point| Overflying the VOR station        | DME fix, intersection, timing|
|  Profile Character    | Descent begins after passing VOR  | Descent begins at FAF cross |
+-----------------------------------------------------------------------------------------+

Timing Tables from FAF to MAP

When DME or GPS is not available on a VOR or LOC approach, the pilot must start a timer (stopwatch or clock) when crossing the FAF (Maltese cross). Approach plates publish a groundspeed-to-time conversion table in the aerodrome sketch:

+-----------------------------------------------------------------------------------------+
|                     FAF TO MAP TIMING CONVERSION TABLE (EXAMPLE: 4.2 NM)                |
|                                                                                         |
|   Groundspeed (Knots)   | 60 KT   | 90 KT   | 120 KT  | 150 KT  | 180 KT                |
|   --------------------- | ------- | ------- | ------- | ------- | -------               |
|   Time (Minutes:Seconds)| 4 min 12| 2 min 48| 2 min 06| 1 min 41| 1 min 24              |
+-----------------------------------------------------------------------------------------+

Operating Rule: If visual contact is not established when the timer expires, the pilot must immediately execute the missed approach. Timing must be adjusted based on actual groundspeed, not indicated airspeed!

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Localizer (LOC), LDA, and SDF Approaches

When runways lack a full ILS, lateral directional beams provide highly accurate non-precision guidance:

+-----------------------------------------------------------------------------------------+
|                        DIRECTIONAL FACILITY COMPARISON MATRIX                           |
|                                                                                         |
|  Facility     | Course Width      | Runway Alignment      | Minima Types Published      |
|  ------------ | ----------------- | --------------------- | --------------------------- |
|  Localizer    | 3° to 6° (700 ft) | Aligned within 3°     | Straight-in & Circling      |
|  (LOC)        | (Very Sensitive)  | of Runway Centerline  |                             |
|  ------------ | ----------------- | --------------------- | --------------------------- |
|  LDA          | 3° to 6° (700 ft) | Offset > 3° from      | Straight-in (if offset ≤30°)|
|  (Loc Dir Aid)| (Identical to LOC)| Runway Centerline     | Circling only (if offset >30°)|
|  ------------ | ----------------- | --------------------- | --------------------------- |
|  SDF          | Fixed 6° or 12°   | May or may not be     | Straight-in (if aligned) or |
|  (Simp Dir Fac)| (Does not narrow) | aligned with runway   | Circling                     |
+-----------------------------------------------------------------------------------------+

1. Localizer (LOC)

• Course width is adjusted to provide 700 feet width at the runway threshold (typically $3^\circ$ to $6^\circ$ beam).
• Aligned within $3^\circ$ of the physical runway centerline.
• Flown with four-times the sensitivity of a standard VOR ($2.5^\circ$ full-scale needle deflection).

2. Localizer Directional Aid (LDA)

• Uses the exact same transmitting equipment and signal format as a standard localizer, with an identical $3^\circ$ to $6^\circ$ beam width.
• Distinguishing Feature: The course is offset from the runway centerline by more than $3^\circ$ due to surrounding terrain, obstacles, or airspace constraints.
• Straight-In vs. Circling Criteria:
  • If the offset angle is $\le 30^\circ$, the FAA publishes Straight-in minimums (e.g., LDA RWY 19). Upon breaking out, the pilot performs a slight turn to align with the runway.
  • If the offset angle is $> 30^\circ$, the FAA publishes Circling-only minimums (e.g., LDA-A).
  • Some LDAs incorporate an electronic glide slope (e.g., LDA w/ GS RWY 26L at Roanoke, VA), which is flown to a DA but classified as an APV approach.

3. Simplified Directional Facility (SDF)

• A legacy directional transmitter providing lateral guidance.
• Distinguishing Features: Course width is fixed at either $6^\circ$ or $12^\circ$ (does not narrow to 700 ft at threshold). The transmitter may or may not be aligned with runway centerline, and it does not provide glide slope signals.

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RNAV (GPS) Approaches: LNAV vs. LP Lines of Minima

Modern RNAV (GPS) approach charts publish multiple lines of minima based on aircraft avionics capability:

+-----------------------------------------------------------------------------------------+
|                        RNAV NON-PRECISION MINIMA: LNAV vs. LP                           |
|                                                                                         |
|  Feature              | LNAV (Lateral Navigation)         | LP (Localizer Performance)  |
|  -------------------- | --------------------------------- | --------------------------- |
|  Receiver Requirement | Basic IFR GPS (TSO-C129 / C196)    | WAAS / SBAS (TSO-C145/C146) |
|  CDI Scaling          | Linear: ±0.3 NM full-scale         | Angular: Narrows to threshold|
|  Vertical Guidance    | None (MDA floor)                  | None (MDA floor)            |
|  Application          | Standard GPS approach             | Runways with obstacle issues|
|  Minimums Type        | MDA                               | MDA (Lower than LNAV)       |
+-----------------------------------------------------------------------------------------+

1. LNAV (Lateral Navigation)

• The standard non-precision line of minima for basic IFR-certified GPS receivers (TSO-C129/C196).
• Employs linear CDI lateral scaling of $\pm 0.3\text{ NM}$ full-scale from the FAF to the MAP.
• Flown to a published MDA.

2. LP (Localizer Performance)

• Published on RNAV charts for runways where terrain or obstacles prevent publishing vertical guidance (such as LPV or LNAV/VNAV).
• Requires WAAS (TSO-C145/C146) equipage.
• Generates angular lateral scaling identical to a ground-based localizer, narrowing to $\pm 40\text{ meters}$ at the threshold.
• Flown to an MDA, but typically offers lower minimums and greater precision than standard LNAV.

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Continuous Descent Final Approach (CDFA / CANPA) vs. "Dive and Drive"

Historically, pilots flew non-precision approaches using the "dive and drive" technique: pitching down aggressively after the FAF at 1,000+ FPM to reach the MDA as rapidly as possible, leveling off, and motoring horizontally near the ground until reaching the MAP or spotting the runway.

+-----------------------------------------------------------------------------------------+
|                    "DIVE AND DRIVE" vs. CONTINUOUS DESCENT (CDFA)                       |
|                                                                                         |
|  LEGACY "DIVE AND DRIVE" (High CFIT Risk)                                               |
|  FAF (✠)                                                                                |
|  ───●──────┐                                                                            |
|            │ Rapid High Descent Rate (>1,000 FPM)                                       |
|            ▼                                                                            |
|            └───────────────────────── Level Flight at MDA ──────────────► MAP           |
|                                                                                         |
|  CONTINUOUS DESCENT FINAL APPROACH (CDFA / CANPA) (Stabilized & Safe)                   |
|  FAF (✠)                                                                                |
|  ───●                                                                                   |
|      \ Continuous Stabilized Descent (FPM = GS x 5)                                     |
|       \                                                                                 |
|        \                                     Derived Decision Altitude (DDA)            |
|         \                                    [DDA = MDA + 50 ft]                        |
|          \                                           │                                  |
|           ▼──────────────────────────────────────────■                                  |
|                                           MDA (Hard Floor)                              |
+-----------------------------------------------------------------------------------------+

Hazards of "Dive and Drive"

1. High pilot workload during step-down level-offs in low-visibility conditions.
2. Increased power changes and unstabilized airspeed/configuration.
3. Prolonged flight close to obstacles at MDA, significantly increasing Controlled Flight Into Terrain (CFIT) accidents.

Continuous Descent Final Approach (CDFA) & Derived Decision Altitude (DDA)

• CDFA / CANPA: The aircraft flies a continuous, stabilized descent from the FAF at a constant flight path angle (typically $3.00^\circ$) matching precision approach descent rates ($\text{FPM} = \text{Groundspeed} \times 5$).
• Derived Decision Altitude (DDA): Because non-precision approaches do not provide TERPS obstacle clearance buffers for sinking below an MDA, operators add an operational buffer—typically $50\text{ feet}$—to the published MDA (e.g., if $\text{MDA} = 820\text{ ft}$, $\text{DDA} = 870\text{ ft}$). Upon reaching DDA without visual references, the pilot initiates an immediate go-around, ensuring the aircraft never sinks below the legal MDA floor.