11.3 Instrument Approach Procedures & Minimums

11.3 Instrument Approach Procedures & Minimums

The instrument approach is the critical operational link connecting the terminal arrival phase to touchdown. For the Part 121 aircraft dispatcher, approach procedures govern airport eligibility, dispatch weather minimums, takeoff and destination alternate requirements under 14 CFR Part 121 Subpart U, and in-flight reanalysis when conditions deteriorate below minimums. Understanding the precise architectural differences between precision, APV, and non-precision approaches—as well as the legal threshold requirements of 14 CFR § 91.175—is paramount for operational control.

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The Five Segments of an Instrument Approach Procedure

Under FAA TERPS (Order 8260.3), every standard instrument approach procedure (IAP) is divided into up to five distinct segments, each with specific obstacle clearance criteria, lateral widths, and operational objectives:

Instrument Approach Segments Architecture:

[En Route Airway] ---> [FEEDER ROUTE]
                             |
                             v
                     [INITIAL SEGMENT]  (Begins at IAF - Initial Approach Fix)
                             |
                             v
                  [INTERMEDIATE SEGMENT] (Begins at IF - Intermediate Fix)
                             |
                             v
                     [FINAL SEGMENT]     (Begins at FAF - Final Approach Fix)
                       /           \       • Precision: Glideslope intercept (lightning bolt)
                      /             \      • Non-Precision: Maltese cross
                     v               v
                [Touchdown]    [MISSED APPROACH SEGMENT] (Begins at MAP: Missed Approach Point)

1. Feeder Route: An en route track connecting an en route fix or airway to the Initial Approach Fix (IAF). Feeder routes publish minimum altitudes providing 1,000 feet of obstacle clearance (2,000 feet in designated mountainous areas).
2. Initial Approach Segment: Begins at the Initial Approach Fix (IAF) and ends at the Intermediate Fix (IF). It transitions the aircraft from the en route environment to the approach course, accommodating procedure turns, holding in lieu of procedure turns, or DME arcs. Minimum obstacle clearance is 1,000 feet in the primary area.
3. Intermediate Segment: Begins at the Intermediate Fix (IF) and terminates at the Final Approach Fix (FAF). This segment aligns the aircraft with the final approach course and permits speed reduction and aircraft configuration (flaps and gear). Obstacle clearance tapers from 1,000 feet down to 500 feet.
4. Final Approach Segment (FAS): Begins at the Final Approach Fix (FAF) or Final Approach Point (FAP) and ends at the Missed Approach Point (MAP) or touchdown. This segment provides vertical descent to visual alignment with the runway:
   • Non-Precision Chart Symbol: Represented on profile views by a Maltese Cross.
   • Precision Chart Symbol: Represented by a Lightning Bolt indicating electronic glideslope intercept at the specified altitude.
5. Missed Approach Segment: Begins at the Missed Approach Point (MAP) and terminates at an en route fix, NAVAID, or designated holding fix. It guarantees obstacle clearance during a go-around, provided the missed approach climb begins at or above the MAP and maintains at least 200 ft/NM (or published climb gradient).

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Precision Approaches: ILS Categories and Minimums

A precision approach is an instrument approach based on a facility using navigation systems that provide both lateral course guidance (localizer/azimuth) and vertical glidepath guidance (glideslope/elevation) conforming to ICAO Category I, II, or III standards. The minimum altitude on a precision approach is designated as a Decision Altitude (DA) in feet MSL, or Decision Height (DH) in feet AGL (measured above the Touchdown Zone Elevation, TDZE, via radio altimeter).

Instrument Landing System (ILS) Categories

Decision Altitude (DA) vs. Minimum Descent Altitude (MDA): At a Decision Altitude (DA) on a precision approach, the decision to land or execute a missed approach must be made at that exact point. The aircraft is permitted to dip momentarily below the DA during the initiation of the missed approach go-around. In stark contrast, an aircraft may never descend below a Minimum Descent Altitude (MDA) under any circumstances until the required visual references are established!

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Non-Precision Approaches (NPA) and APV

Non-Precision Approaches (MDA)

Approaches providing lateral guidance only without electronic vertical guidance:

• Types: Localizer (LOC), VOR, NDB, and RNAV (GPS) approaches flown to LNAV-only minimums.
• Vertical Floor: Governed by a Minimum Descent Altitude (MDA). The pilot descends along a step-down profile to the MDA, leveling off until reaching the Missed Approach Point (MAP).
• HAT vs. HAA:
  • Height Above Touchdown (HAT): The height of the MDA (or DA) above the Touchdown Zone Elevation (TDZE). Published for straight-in approaches.
  • Height Above Airport (HAA): The height of the MDA above the published airport elevation. Published for circling approaches.

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

Traditional "dive-and-drive" step-down descents on non-precision approaches historically led to elevated Controlled Flight Into Terrain (CFIT) accidents. Modern Part 121 air carriers mandate Continuous Descent Final Approach (CDFA) techniques:

• The aircraft flies a continuous, stabilized 3-degree descent from the FAF to the runway threshold.
• Derived Decision Altitude (DDA): Because an aircraft executing a go-around from a continuous descent will experience momentary downward inertia, Part 121 carriers add an operational buffer (typically 50 feet) to the charted MDA:
  $\text{DDA} = \text{Published MDA} + 50 \text{ feet}$
• The flight crew treats the DDA as a Decision Altitude: if visual references are not sighted upon reaching the DDA, the missed approach is initiated immediately, guaranteeing the aircraft never penetrates the hard MDA floor.

Approaches with Vertical Guidance (APV)

APV procedures provide lateral course guidance and electronic vertical glideslope guidance, but do not meet full ICAO precision approach standards:

• LNAV/VNAV (Lateral Navigation / Vertical Navigation): Relies on barometric vertical navigation (Baro-VNAV). Critical Dispatch Limitation: Highly sensitive to non-standard temperatures. At cold temperatures, true altitude is significantly lower than indicated altitude, requiring cold temperature altitude corrections or causing uncompensated Baro-VNAV to become unavailable.
• LPV (Localizer Performance with Vertical Guidance): Utilizes Space-Based Augmentation Systems (SBAS / WAAS) to provide extremely precise lateral angular guidance and vertical glidepaths down to minimums as low as 200 feet HAT, behaving operationally like an ILS.

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Straight-in vs. Circling Approaches

Straight-in Minimums Criteria

An instrument approach qualifies for published straight-in minimums only when:

1. The final approach course aligns within 30 degrees of the runway centerline (within 15 degrees for GPS/RNAV approaches).
2. The descent gradient from the FAF to the runway threshold allows a normal rate of descent (typically not exceeding 400 ft/NM).

Circling Approaches

If the final approach course exceeds 30 degrees of runway alignment, or if a steep descent gradient is required, only Circling Minimums are published. A circling approach is an IFR approach to one runway followed by visual maneuvering to land on another runway (or the opposite end of the same runway).

Aircraft Approach Categories (14 CFR § 97.3)

Approach categories are based on 1.3 times the stalling speed in the landing configuration at maximum certified landing weight ($1.3 V_{so}$) or reference landing speed ($V_{ref}$):

Circling Protected Airspace: Standard vs. Expanded TERPS

Circling protected airspace provides 300 feet of Required Obstacle Clearance (ROC) over all obstacles within a specified radius drawn from the threshold of all runways at the airport:

• Standard TERPS Radii (Pre-2012): Constant radius regardless of airport elevation:
  • Cat A: 1.3 NM | Cat B: 1.5 NM | Cat C: 1.7 NM | Cat D: 2.3 NM | Cat E: 4.5 NM
• Expanded TERPS Circling Radii (Negative "C" Icon): Depicted on approach charts by a white letter "C" inside a black box. Recognizes that as airport elevation increases, higher true airspeed (TAS) requires wider bank angles and larger turn radiuses. Protected airspace expands dynamically with airport elevation:
  • Example Cat C Expanded: 2.7 NM (sea level) up to 3.3 NM (high elevation).
  • Example Cat D Expanded: 3.6 NM (sea level) up to 4.5 NM (high elevation).

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Visual References for Descent Below DA/MDA (14 CFR § 91.175(c))

Under 14 CFR § 91.175(c), no pilot may operate an aircraft below the authorized MDA or continue an approach below the authorized DA/DH unless three cumulative conditions are met:

1. Continuous Normal Maneuvers: 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 will touchdown within the touchdown zone.
2. Flight Visibility: The flight visibility is not less than the visibility prescribed in the standard instrument approach procedure being flown.
3. Visual Reference to the Runway Environment: At least one of the following visual references for the intended runway is distinctly visible and identifiable to the pilot:

The 10 Recognized Visual References (§ 91.175(c)(3))

1. The approach light system (ALS).
2. The threshold.
3. The threshold markings.
4. The threshold lights.
5. The runway end identifier lights (REIL).
6. The visual glideslope indicator (VASI or PAPI).
7. The touchdown zone or touchdown zone markings.
8. The touchdown zone lights.
9. The runway or runway markings.
10. The runway lights.

The Critical Approach Light System Rule (§ 91.175(c)(3)(i))

If the flight crew sights the Approach Light System (ALS), they are legally permitted to descend below DA/MDA, BUT with a strict legal ceiling:

The pilot may NOT descend below 100 feet above the Touchdown Zone Elevation (TDZE) using the approach lights as the sole visual reference, UNLESS the red terminating bars or the red side row bars are also distinctly visible and identifiable (found on ALSF-1 and ALSF-2 high-intensity systems).

Once the red terminating or side row bars—or any of the other 9 runway references—are distinctly sighted, the aircraft may descend below 100 ft TDZE to touchdown.