7.2 Non-Precision Approaches, RNAV & Decision Minimums (MDA/DA)

7.2 Non-Precision Approaches, RNAV & Decision Minimums (MDA/DA)

Quick Summary: Non-precision and Area Navigation (RNAV) approaches form the core of modern airport arrival infrastructure. Unlike precision ILS, non-precision approaches lack ground-based electronic glide slopes and terminate at a Minimum Descent Altitude (MDA). Modern air carriers bridge this operational gap through Continuous Descent Final Approach (CDFA) techniques, Derived Decision Altitudes (DDA), satellite-based augmentation (WAAS/LPV), and RNP AR curved procedures. Aircraft dispatchers must evaluate Baro-VNAV temperature cutoffs, approach lighting outages, and the strict visual descent requirements of 14 CFR 121.651.

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1. Precision vs. Non-Precision Approaches & Altitude Reference Architecture

The fundamental operational dividing line in instrument approach procedures lies between precision and non-precision navigation architectures:

• Precision Approach (PA): An instrument approach procedure based on navigation systems that provide continuous electronic lateral guidance and vertical glidepath guidance (e.g., ILS, MLS, and military PAR). Precision approaches culminate at a Decision Altitude (DA) referenced to mean sea level, or Decision Height (DH) referenced to the touchdown zone elevation.
• Non-Precision Approach (NPA): An instrument approach procedure based on navigation systems that provide lateral course guidance only, without approved electronic vertical guidance (e.g., VOR, NDB, LOC, SDF, LDA without glide slope, and basic RNAV LNAV). Non-precision approaches culminate at a Minimum Descent Altitude (MDA).
• Approach with Vertical Guidance (APV): A satellite-based instrument approach that provides lateral and vertical guidance but does not meet international precision standards (e.g., LNAV/VNAV and LPV). Operationally, APV approaches are flown to a Decision Altitude (DA).

Decision Altitude (DA) vs. Minimum Descent Altitude (MDA)

The regulatory distinction between a DA and an MDA is critical to flight safety:

• Decision Altitude (DA / DH): A specified altitude on a precision or APV approach at which a missed approach must be initiated if the required visual references to continue the approach have not been established. Crucially, the aircraft is permitted to momentarily descend slightly below the DA during the go-around transition maneuver as the flight crew arrests the downward momentum of the transport aircraft.
• Minimum Descent Altitude (MDA): A specified altitude on a non-precision approach below which descent is not made until the visual references for the intended runway are distinctly visible. The MDA is a hard, inviolable regulatory floor. Penetrating the MDA by even a single foot without required visual references constitutes an immediate violation of 14 CFR 91.175 and 14 CFR 121.651.

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

Historically, non-precision approaches were flown using the "dive-and-drive" technique: upon crossing the Final Approach Fix (FAF), the pilot executed a steep, rapid descent toward the published MDA, leveled off at low altitude (often 400 to 800 feet above obstacles), and flew horizontally through instrument conditions until reaching the Missed Approach Point (MAP). Dive-and-drive produced high cockpit workload, unstabilized approaches, increased fuel consumption, and was identified by the NTSB and FAA as a primary cause of Controlled Flight Into Terrain (CFIT) accidents.

The CDFA Technique

Under FAA Advisory Circular AC 120-108A, Part 121 air carriers are mandated to conduct non-precision approaches using the Continuous Descent Final Approach (CDFA) technique. CDFA employs a constant, stabilized vertical descent angle (nominally 3.0°, matching standard ILS descent profiles) from the FAF directly to a point approximately 50 feet above the runway threshold.

The Derived Decision Altitude (DDA) Solution

Because CDFA requires a continuous descent, an aircraft reaching the published MDA cannot instantly level off without destabilizing the approach. However, if the pilot waits until reaching the MDA to initiate a go-around, the physical inertia of a 150,000-pound transport jet will cause the aircraft to sink 30 to 50 feet below the MDA during engine spool-up and attitude rotation, violating federal law.

To solve this dilemma, Part 121 air carriers establish an operational buffer known as a Derived Decision Altitude (DDA):

$$\text{DDA} = \text{Published MDA} + 50\text{ feet}$$

• Flight crews treat the DDA exactly like a precision Decision Altitude (DA).
• Upon reaching the DDA, if the runway visual references are not established, the flight crew immediately initiates a missed approach.
• Any approved additive or derived decision-altitude method must account for the go-around height loss and the actual operation. A generic 50-foot additive does not guarantee that every airplane remains above MDA in every condition.

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3. Conventional Non-Precision Approach Profiles: LOC, VOR & NDB

Conventional terrestrial non-precision approaches rely on ground-based radio beacons transmitting radial or directional signals:

• Localizer (LOC) Approaches: Utilizes the runway localizer transmitter without glide slope guidance. It provides an exceptionally sensitive lateral course (3° to 6° total angular width). Because of this tight signal coincidence with the runway centerline, LOC approaches feature the lowest MDAs among conventional non-precision procedures (often 350 to 500 feet HAT).
• VOR Approaches: Radials diverge at a rate of 1 nautical mile lateral width for every 60 nautical miles from the station. Consequently, VOR approaches located off-field provide wider obstacle clearance corridors and higher MDAs. Straight-in VOR minimums require the final approach course to pass within 30 degrees of the runway centerline; if the angular offset exceeds 30 degrees, only circling minimums are published.
• Visual Descent Point (VDP): A defined point on the final approach course of a straight-in non-precision approach from which normal descent from the MDA to the runway touchdown point may begin. If an aircraft acquires visual references after crossing the VDP, the flight crew cannot descend at a normal 3° angle without executing an excessively steep descent rate; a missed approach must be executed.

$$\text{VDP Distance from Threshold (NM)} \approx \frac{\text{HAT (feet)}}{300}$$

Example: An approach with an MDA of 600 feet HAT establishes its VDP at approximately $600 / 300 = 2.0\text{ NM}$ from the runway threshold.

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4. RNAV (GPS) Approach Minima Lines: LNAV, LNAV/VNAV, LP & LPV

Modern Performance-Based Navigation (PBN) utilizes satellite-based Global Navigation Satellite Systems (GNSS) to publish RNAV (GPS) approach charts featuring up to four distinct lines of landing minimums:

The Critical Baro-VNAV Cold Temperature Trap

LNAV/VNAV approaches calculate vertical descent paths using the aircraft's barometric altimetry system. However, pressure altimeters are calibrated to the International Standard Atmosphere (ISA). In cold weather, atmospheric density increases, compressing the vertical distance between isobaric surfaces:

• The altimeter overreads in cold air: it indicates the aircraft is higher than its true physical altitude above ground.
• If an aircraft flies an uncompensated Baro-VNAV glidepath in extreme cold, its true altitude will be dangerously close to obstacles and terrain.
• Consequently, approach charts publish mandatory Cold Temperature Limitations on the chart header (e.g., "Baro-VNAV NA below -15°C (-4°F) or above 48°C (118°F)").
• If the surface temperature is below the published chart limit, Part 121 flight crews are prohibited from flying to LNAV/VNAV DA minimums unless the aircraft is equipped with an approved automated temperature-compensating Flight Management System. The crew must revert to the LNAV MDA.

LP vs. LPV Approaches

• Localizer Performance (LP): Utilizes WAAS differential corrections to generate an angular lateral beam that narrows toward the threshold (matching a localizer's sensitivity), but provides no vertical guidance. Flown to an MDA.
• Localizer Performance with Vertical Guidance (LPV): Utilizes WAAS for high-precision lateral and vertical guidance. Provides minimums down to a DA of 200 feet HAT and RVR 2,400 feet (matching Category I ILS). Although classified under ICAO as an APV procedure, LPV delivers precision-equivalent operational utility without ground-based ILS transmitters.

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5. Required Navigation Performance Authorization Required (RNP AR)

Governed by FAA Advisory Circular AC 90-101A and Operations Specifications OpSpec C384, RNP AR represents the pinnacle of modern area navigation. RNP AR procedures are designed for airports surrounded by complex, mountainous terrain or congested airspace (e.g., Aspen, Palm Springs, Queenstown):

• Radius-to-Fix (RF) Legs: Authorizes curved flight paths with defined turn radiuses in both the intermediate, final, and missed approach segments, keeping aircraft tightly contained within narrow mountain valleys.
• Tight Lateral Containment: RNP specifies navigation accuracy for at least 95% of flight time together with the applicable monitoring and alerting requirements. RNP AR procedures can use values such as 0.30 or 0.10 NM when specifically approved. Read the actual aircraft, procedure, and authorization criteria rather than asserting one universal 99.999% containment guarantee for every installation.

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6. Approach Lighting Systems & Visual References for Descent (14 CFR 121.651 & 91.175)

Under federal aviation regulations, crossing the DA or MDA does not authorize an automatic landing. The approach represents an instrument procedure designed to position the aircraft where the pilot can complete the landing visually.

The Three Inviolable Descent Requirements (14 CFR 121.651(c))

No pilot may operate an aircraft below the authorized DA or MDA unless all three of the following conditions are simultaneously met:

1. Continuous Position to Land: The aircraft is continuously in a position from which a descent to a landing on the intended runway can be made at a normal rate of descent using normal maneuvers, and the descent rate will allow touchdown to occur within the touchdown zone.
2. Flight Visibility: The flight visibility is not less than the visibility prescribed in the standard instrument approach procedure being used.
3. Visual Reference for the Runway: At least one of the following visual references for the intended runway is distinctly visible and identifiable to the pilot:
   • The approach light system (ALS).
   • The threshold, threshold markings, or threshold lights.
   • The runway end identifier lights (REIL).
   • The visual approach slope indicator (VASI / PAPI).
   • The touchdown zone, touchdown zone markings, or touchdown zone lights.
   • The runway, runway markings, or runway lights.

The Critical 100-Foot Approach Light System Limitation

Under 14 CFR 91.175(c)(1) and 14 CFR 121.651, if the pilot identifies the approach light system (ALS) as the sole visual reference, the pilot is authorized to descend below the DA or MDA, but may NOT descend below 100 feet above the touchdown zone elevation unless:

$$\text{Descent Below 100 ft HAT Requires: The Red Terminating Bars OR The Red Side Row Bars are distinctly visible!}$$

• Red Terminating Bars: Installed on ALSF-1 systems across the outer edge of the 100-foot light bar.
• Red Side Row Bars: Installed on ALSF-2 systems on either side of the centerline along the inner 1,000 feet of the approach lights.
• If the runway is equipped with a MALSR or ODALS (which have no red side row or terminating bars), the pilot cannot descend below 100 feet HAT using the approach lights alone; the pilot must see the runway threshold, touchdown zone, or runway pavement.

Dispatch Analysis: Approach Light System Outages

When an Approach Lighting System (ALS) is inoperative via NOTAM, the impact on approach minimums is published in the Inoperative Components Table of the US Terminal Procedures Publication:

• An out-of-service ALSF-2 or MALSR typically increases straight-in visibility minimums by 1/4 to 1/2 statute mile or 1,000 to 2,000 feet RVR.
• For a flight dispatched to an airport where published minimums are RVR 2,400 ft, an ALS outage raises the legal requirement to RVR 4,000 ft. If forecast visibility is RVR 3,000 ft, the flight cannot legally be dispatched without an amended release or additional fuel reserves.

Official References

Source check: 2026-10-12. FAA instrument-procedure resources; General operating and instrument rules; FAA AIM clearances and holding. For operational use, apply current issued authorizations and the assigned aircraft’s approved data.