CAT III Approach and Autoland Procedure (Boeing 737)

General Definition

A Category III (CAT III) approach is a high-precision instrument landing procedure used in conditions of very low visibility. With the aid of Autoland systems, the aircraft can safely land on the runway when visual cues are insufficient for human pilots.

CAT III Subcategories

• CAT IIIa: Minimum Runway Visual Range (RVR) of 200 m; Decision Height (DH) approximately 50 ft.

• CAT IIIb: RVR down to 75 m; DH may be zero.

• CAT IIIc: Theoretical zero visibility and zero DH; not used in practice.

Operational Conditions

• Applied when visibility is significantly restricted.

• Aircraft, crew, airport, and ILS system must be certified for CAT III operation.

• Minimum RVR: 200 meters

• Maximum wind limits:

  • Headwind: 25 kt

  • Crosswind: 20 kt

  • Tailwind: 10 kt

• No reported or forecast windshear or moderate/severe turbulence.

Technical Requirements

• Dual autopilot engagement (fail-passive system) is required.

• Crew and aircraft must hold appropriate CAT III certification.

• Radio altimeter-based Decision Height is used instead of barometric minima (typically set to 50 ft RA).

Procedural Steps

1. Eligibility CheckPrior to a CAT III approach, the specific checklist in the QRH must be completed. This checklist verifies the qualification of the aircraft, crew, airport, and ILS system.

   
   

2. Approach Briefing and SetupSimilar to a standard ILS briefing, with special attention to taxi phase risks due to limited visibility. Key differences from CAT I procedures should be clearly outlined at the end of the briefing.

   
   

3. Autopilot ConfigurationFor CAT III approaches, both autopilots (CMD A and CMD B) must be engaged. This must be done once on the intercept heading and cleared for the approach. If the second autopilot is not engaged before descending below 800 ft RA, the approach must be discontinued.

   
   

4. Descent and Final Approach

   
   

   • Upon localizer and glide slope capture, missed approach altitude is set.

   • At 500 ft RA, verify “FLARE ARMED” on the Flight Mode Annunciator (FMA).

   • Below 500 ft, aircraft begins flare prep (trim adjustments).

   • The monitoring pilot observes visual references while also checking for system integrity.

   
   

5. Minimums and Decision Point

   
   

   • Standard GPWS callouts must be acknowledged (e.g., “plus hundred,” “minimums”).

   • At decision height, a clear call must be made: “Land” or “Go Around.”

   • If “Land” is called, the aircraft will enter flare mode at ~50 ft, reduce thrust at ~27 ft, touch down, and disconnect autopilot/autothrottle after rollout.

   
   

6. Go-Around Procedure

   
   

   • Upon the “Go Around” command:

     
     

     • TO/GA is pressed.

     • “Go around, flaps 15, set go-around thrust” is called.

     • Aircraft pitches up automatically.

     • Upon positive rate of climb, “Gear up” is commanded and gear is retracted.

     • At 400 ft, appropriate roll mode is selected (LNAV or HDG SEL).

     • Flaps are retracted step-by-step as speed increases.

     • Upon reaching missed approach altitude and entering altitude acquire mode, the second autopilot disconnects.

System Feature – Fail-Passive

The Boeing 737’s fail-passive system involves both autopilots independently interpreting ILS signals. In case of discrepancy, both autopilots disengage without making abrupt control inputs. Continued CAT III approach is not permitted after autopilot disconnection.