Ils Categories

✈️ ILS Categories — CAT I vs CAT II vs CAT III

Big Picture (remember first)

CAT I: Pilot lands visually
CAT II: Pilot lands with very late visual reference
CAT III: Aircraft can land with little or no visual reference

CAT I

Purpose

Standard precision approach

Key characteristics

Decision Height (DH): ≥ 200 ft
RVR: ≥ 550 m (typical)
Autoland: Not required
Autopilot: Optional
Pilot role: Must see runway at DH and land manually

Summary

👉 CAT I = “I must see the runway and fly the landing.”

CAT II

Purpose

Low-visibility operations

Key characteristics

DH: 100 ft ≤ DH < 200 ft
RVR: ≥ 300 m
Autoland: Usually required
Autopilot: Required to low height
Pilot role: Takes over visually below DH

Summary

👉 CAT II = “I see the runway very late, then land.”

CAT III (General)

Purpose

Very low or zero visibility operations

Key characteristics

DH: < 100 ft or no DH
RVR: < 300 m
Autoland: Mandatory
Autopilot: Multiple APs required
Alert Height (AH): Used instead of (or in addition to) DH
Pilot role: Monitor systems, not visual cues

CAT III Subcategories

CAT IIIA

DH: < 100 ft (or no DH)
RVR: ≥ 200 m
Autoland to touchdown

CAT IIIB

DH: < 50 ft or no DH
RVR: 75–200 m
Autoland to rollout

CAT IIIC (theoretical)

DH: None
RVR: None
Not operationally used

Comparison Table (Exam Gold)

| Feature | CAT I | CAT II | CAT III | |------|------|------| | Decision Height | ≥ 200 ft | 100–200 ft | < 100 ft or none | | Alert Height | ❌ | ❌ | ✅ | | Visual reference | Required early | Required late | Not required | | Autoland | ❌ | Usually | Mandatory | | Redundancy | Low | Medium | High | | Typical RVR | ≥ 550 m | ≥ 300 m | < 300 m |

Pilot vs System Emphasis

CAT I: Pilot-centric
CAT II: Shared pilot/system
CAT III: System-centric

Common Exam Traps ❌

❌ “CAT II has alert height”
❌ “CAT III always has DH”
❌ “CAT I allows autoland”

✔️ AH only exists in CAT III ✔️ CAT III may have no DH

One-Line Exam Summary

CAT I relies on pilot visual landing, CAT II allows very late visual takeover, and CAT III uses fully automatic landing with system redundancy and alert-height monitoring.

✈️ Linking CAT I / II / III to Fail-Passive & Fail-Operational

Big Picture (memorise this)

CAT I → No fail logic
CAT II → Fail-passive
CAT III → Fail-passive or Fail-operational

CAT I — No Fail Concept

Why

Pilot must see the runway at DH ≥ 200 ft
Landing is manual

Consequence

Automation failure → pilot lands visually
No need for fail-passive or fail-operational design

👉 CAT I relies on the pilot, not redundancy

CAT II — Fail-Passive Required

Why

Very late visual reference (DH 100–200 ft)
Autopilot used down to low height
Pilot still expected to take over visually

Fail logic

Fail-passive only
A single failure:
- disengages or downgrades automation
- aircraft remains stable
- pilot must land manually or go around

Typical indication

Boeing: LAND 2
Airbus: CAT 3 SINGLE (when used at CAT II minima)

👉 CAT II = automation may fail, but safely

CAT III — Fail-Passive and Fail-Operational

Why

Extremely low or zero visibility
Pilot cannot rely on vision
Landing safety depends on system redundancy

CAT III (Fail-Passive)

One failure → autoland disengages
Pilot must take over or go around
Allowed only above alert height

Examples:

Boeing: LAND 2
Airbus: CAT 3 SINGLE

Used mainly when:

redundancy is reduced
minima still permit pilot takeover

CAT III (Fail-Operational)

One failure tolerated
Autoland continues to touchdown and rollout
Required for lowest CAT III minima

Examples:

Boeing: LAND 3
Airbus: CAT 3 DUAL

👉 This is the highest level of protection

How Alert Height Fits In (Exam Favorite)

Above Alert Height
- Any critical failure → go-around
Below Alert Height
- Fail-operational: continue landing
- Fail-passive: pilot must intervene

Summary Table (Exam Gold)

CAT Level	Visibility	Pilot role	Fail-Passive	Fail-Operational
CAT I	Good	Fly visually	❌	❌
CAT II	Low	Late takeover	✅	❌
CAT IIIA	Very low	Monitor	✅	Optional
CAT IIIB	Extremely low	Monitor	❌	✅
CAT IIIC	Zero (theoretical)	None	❌	✅

One-Line Exam Answer

CAT I relies on pilot visual landing, CAT II requires fail-passive automation, and CAT III requires fail-passive or fail-operational systems depending on the visibility minima.

CAT I / II / III → fail-passive / fail-operational decision flow

✈️ ILS CAT vs Fail Logic — Decision Flow

START: What type of ILS approach is being flown?
│
├── CAT I ?
│   │
│   ├─ YES →
│   │     • Pilot must see runway at DH ≥ 200 ft
│   │     • Landing is manual
│   │     • Automation failure → pilot flies
│   │
│   │     ⇒ NO fail-passive or fail-operational concept needed
│   │
│   └─ NO →
│
├── CAT II ?
│   │
│   ├─ YES →
│   │     • Very low visibility (DH 100–200 ft)
│   │     • Autopilot used to low height
│   │     • Pilot still expected to take over visually
│   │
│   │     IF a failure occurs:
│   │        → Autoland disengages safely
│   │        → Aircraft remains controllable
│   │        → Pilot lands manually or goes around
│   │
│   │     ⇒ FAIL-PASSIVE REQUIRED
│   │
│   └─ NO →
│
├── CAT III ?
│   │
│   ├─ YES →
│   │     • Very low or zero visibility
│   │     • Pilot cannot rely on visual cues
│   │     • Autoland mandatory
│   │     • Alert Height (AH) used
│   │
│   │     FAILURE ABOVE AH ?
│   │        → Go-around (autoland not permitted)
│   │
│   │     FAILURE BELOW AH ?
│   │        │
│   │        ├─ System FAIL-OPERATIONAL ?
│   │        │     → Autoland continues to touchdown/rollout
│   │        │
│   │        └─ System FAIL-PASSIVE ?
│   │              → Pilot must take over / go-around
│   │
│   │     ⇒ CAT III can be:
│   │        • FAIL-PASSIVE (LAND 2 / CAT 3 SINGLE)
│   │        • FAIL-OPERATIONAL (LAND 3 / CAT 3 DUAL)
│   │
│   └─ END

How to Say This in an Exam (Perfect Oral Answer)

“CAT I relies on pilot visual landing and needs no fail logic. CAT II requires fail-passive systems so a failure leaves the aircraft safe for pilot takeover. CAT III uses alert height and may be fail-passive or fail-operational, depending on whether the system can tolerate a failure and still complete the autoland.”

Ultra-Short Memory Hooks

CAT I: Pilot sees → pilot lands
CAT II: System may fail → pilot takes over
CAT III: System lands → system must survive failure