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6.4 Surveillance & Warning Systems
Airline Transport Pilot ATP
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1 Chapter 1: Advanced Aerodynamics & High-Speed Flight
1.1 Subsonic & Transonic Aerodynamics 1.2 Swept-Wing Aerodynamics & Stability 1.3 High-Altitude Mach Phenomena & High-Speed Buffet 1.4 Stall Aerodynamics & Transport Category Recovery
2 Chapter 2: Multi-Engine Aerodynamics & Performance
2.1 Asymmetric Thrust & Critical Engine Factors 2.2 Minimum Control Speeds 2.3 Multi-Engine Inoperative Flight & Drift Down
3 Chapter 3: Turbine Engine Principles & Systems
3.1 Gas Turbine Theory & Turbofan Architecture 3.2 Engine Control, FADEC & Fuel Delivery 3.3 Engine Starting, Operating Limits & Malfunctions 3.4 Thrust Reversers & Auxiliary Power Units
4 Chapter 4: Transport Category Aircraft Systems
4.1 Hydraulic & Pneumatic Power Distribution 4.2 Electrical Power Generation & Distribution 4.3 Pressurization, Environmental Control & Oxygen Systems 4.4 Ice & Rain Protection and Fire Detection/Extinguishing
5 Chapter 5: Flight Instruments, Air Data & Primary Flight Displays
5.1 Pitot-Static Systems & High-Altitude Errors 5.2 Gyroscopic, Inertial Navigation & Attitude Reference Systems 5.3 Electronic Flight Displays & Flight Director Systems
6 Chapter 6: Advanced Navigation, Avionics & Automation
6.1 Flight Management Systems (FMS) & VNAV/LNAV 6.2 Performance-Based Navigation (PBN) & RNP/RNAV 6.3 Autoflight, Autothrottle & Automation Management 6.4 Surveillance & Warning Systems
7 Chapter 7: Airman Certification & General Operating Regulations
7.1 Part 61 ATP Certification Requirements & Privileges 7.2 Medical Standards, Pilot Impairment & Reporting 7.3 Part 91 Operating & Equipment Rules for Transport Aircraft
8 Chapter 8: Part 117 Flight and Duty Limitations & Rest Requirements
8.1 Flight Duty Period (FDP) Limits & Rest Concepts 8.2 Augmented Flightcrews & Rest Facilities 8.3 Cumulative Duty/Flight Limits & Fatigue Risk Management
9 Chapter 9: Part 121 Domestic, Flag & Supplemental Operations
9.1 Part 121 Operational Structure & Dispatch Control 9.2 Domestic & Flag Fuel Requirements 9.3 Dispatch Minimums & Alternate Airport Selection 9.4 Takeoff Minimums, Takeoff Alternates & Part 121/135 Comparisons
10 Chapter 10: Aviation Meteorology & Atmospheric Hazards
10.1 Atmospheric Structure, Pressure Systems & Fronts 10.2 Convective Hazards, Thunderstorms & Microbursts 10.3 Structural & Induction Icing 10.4 Turbulence & Mountain Wave Dynamics
11 Chapter 11: Aviation Weather Reports, Forecasts & High-Altitude Charts
11.1 METARs, SPECI, TAFs & PIREPs 11.2 Significant Weather Charts (SIGWX) & Inflight Advisories 11.3 Constant Pressure Charts, Upper-Air Winds & Tropopause Analysis
12 Chapter 12: Transport Category Aircraft Performance & Flight Planning
12.1 Transport Category Takeoff Performance & V-Speeds 12.2 Climb Segments & Obstacle Clearance 12.3 Landing Performance, Contaminated Runways & Approach Speeds 12.4 Transport Category Weight & Balance
13 Chapter 13: High-Altitude Navigation, ETOPS, CRM & Safety
13.1 High-Altitude Operations, RVSM & Oceanic Procedures 13.2 Extended-Range Twin-Engine Operations (ETOPS / EDTO) 13.3 Crew Resource Management (CRM) & Threat and Error Management (TEM) 13.4 Aeromedical Factors & High-Altitude Physiology 13.5 Safety Management Systems (SMS) & NTSB 830 Accident Reporting
6.4 Surveillance & Warning Systems
Key Takeaways Explain This Topic AI
TCAS II version 7.1 calculates time to Closest Point of Approach (Tau) via Mode S transponder interrogations, issuing Traffic Advisories (TA) at 35–48 seconds and Resolution Advisories (RA) at 20–30 seconds to CPA.
Under 14 CFR 91.123 and ICAO Doc 9863, a TCAS Resolution Advisory takes absolute regulatory and operational precedence over ATC clearances; pilots must disconnect autopilot and fly the green RA pitch zone within 5 seconds with 0.25g acceleration.
TCAS version 7.1 replaces 'ADJUST VERTICAL SPEED' with 'LEVEL OFF, LEVEL OFF' RAs and incorporates coordinated reversal advisories ('CLIMB, CLIMB NOW' / 'DESCEND, DESCEND NOW') when intruders fail to comply.
TAWS Class A / EGPWS utilizes a worldwide forward-looking terrain database and GPS positioning to provide look-ahead terrain alerts; receiving a 'TERRAIN TERRAIN PULL UP' warning requires immediate maximum thrust, autopilot disconnect, and aggressive rotation to the pitch limit indicator (PLI).
Reactive windshear alerting detects real-time energy loss via inertial versus air data comparisons, whereas Predictive Windshear (PWS) radar detects microbursts 1–5 NM ahead; escaping severe windshear requires TO/GA thrust, pitch to stick shaker, and maintaining gear/flap configuration until clear.
Last updated: August 2026
Surveillance & Warning Systems
Core Airline Transport Principle: Airborne surveillance and cockpit warning systems provide the ultimate safety barriers against mid-air collisions, Controlled Flight Into Terrain (CFIT), and low-altitude microburst windshear accidents. Transport pilots must know the system physics, alert timing thresholds, legal precedence rules, and immediate flight deck control maneuvers required during time-critical warnings.
1. Traffic Alert and Collision Avoidance System (TCAS II Version 7.1)
TCAS II version 7.1 operates independently of ground-based Air Traffic Control radar by interrogating the Mode C and Mode S transponders of nearby aircraft on 1030 MHz and receiving replies on 1090 MHz . +-----------------------------------------------------------------------------+
- | TCAS II SURVEILLANCE & TAU (t) |
- | |
- | Closure Rate (-dR/dt) |
- | Own Aircraft ==========> <========== Intruder Aircraft |
- | (Mode S) (Mode S) |
- | |<-------- Slant Range (R) ------->| |
- | |
- | Slant Range (R) |
- | Tau (t) = -------------------------------------------- |
- | Closure Rate (-dR/dt / Range Rate) |
- | |
- | * Tau defines the estimated TIME (in seconds) to the |
- | Closest Point of Approach (CPA). |
- +-----------------------------------------------------------------------------+
- TCAS Alert Thresholds & Dimensions
Rather than relying strictly on geographical distance, TCAS evaluates collision risk using Tau ( τ \tau τ ) —the time remaining to the Closest Point of Approach (CPA), supplemented by a protected distance zone (DMOD) at low closure speeds.
Alert Level Symbology on ND Aural Annunciation Tau ( τ \tau τ ) Warning Time Mandatory Pilot Action
Non-Threat Traffic Open White/Cyan Diamond None > 48 seconds > 48\text{ seconds} > 48 seconds / > 6.0 NM > 6.0\text{ NM} > 6.0 NM Situational awareness only.
Proximity Traffic Solid White/Cyan Diamond None Within ± 1 , 200 ft \pm 1,200\text{ ft} ± 1 , 200 ft and 6.0 NM 6.0\text{ NM} 6.0 NM Visual traffic scan.
Traffic Advisory (TA) Solid Amber Circle "TRAFFIC, TRAFFIC" 35 to 48 seconds to CPA Visual search; prepare for potential RA. DO NOT maneuver based on TA alone!
Resolution Advisory (RA) Solid Red Square "CLIMB, CLIMB" ; "DESCEND, DESCEND" ; "LEVEL OFF, LEVEL OFF" 20 to 30 seconds to CPA Mandatory immediate flight path maneuver. Disconnect AP and pitch into green vertical speed band.
2. Regulatory Mandate & Resolution Advisory (RA) Execution +-----------------------------------------------------------------------------+
- | RESOLUTION ADVISORY (RA) FLYING PROFILE |
- | |
- | Primary Flight Display (PFD) Vertical Speed Tape: |
- | |
- | +3000 FPM | |
- | | [ GREEN FLY-TO ZONE ] ===> Pitch smoothly into green |
- | +1500 FPM | (1,500 - 2,000 FPM) zone within 5 SECONDS! |
- | |------------------------- |
- | 0 FPM | [ RED NO-FLY ZONE ] ===> Avoid red pitch tape! |
- | | (Hazardous Climb/Desc) |
- | -1500 FPM | |
- | |
- | * Acceleration target: 0.25g load factor maneuver |
- | * Primary Objective: Establish 300 to 500 ft vertical separation at CPA |
- +-----------------------------------------------------------------------------+
- The Legal Precedence Rule (14 CFR 91.123 & ICAO Doc 9863)
Important
Absolute Regulatory Precedence: A TCAS Resolution Advisory takes absolute precedence over any conflicting ATC clearance.
Under 14 CFR 91.123 and international regulations, a pilot who maneuvers contrary to an ATC instruction in compliance with a TCAS RA is legally protected and fully authorized. Controllers cannot see the RA on standard radar displays and cannot assume separation responsibility during an event.
Flight Deck Execution & Phraseology Standard
Immediate Reaction (Initial RA): Disconnect autopilot (or ensure AP TCAS mode engages) and smoothly adjust pitch within 5 seconds , applying approximately 0.25 g 0.25g 0.25 g acceleration to achieve the commanded green vertical speed (typically 1,500 to 2,000 FPM).
Immediate Reaction (Modified/Reversal RA): React within 2.5 seconds with 0.35 g 0.35g 0.35 g acceleration (e.g., "INCREASE CLIMB" commanding 2,500 to 3,000 FPM).
ATC Notification: As soon as workload permits, advise ATC:
"[Callsign], TCAS RA."
ATC acknowledges: "[Callsign], Roger." (ATC safety alert separation is suspended).
Clear of Conflict: When TCAS announces "CLEAR OF CONFLICT" , smoothly return to the previously assigned ATC altitude and clearance, then notify ATC:
- "[Callsign], Clear of conflict, returning to FL350."
- TCAS Version 7.1 Enhancements
"LEVEL OFF, LEVEL OFF" RA: Replaced the older "ADJUST VERTICAL SPEED" command, which historically led some pilots to increase climb/descent rates inadvertently. It commands a reduction of vertical rate to zero FPM.
Coordinated Reversal Logic: TCAS units coordinate resolution advisories directly via Mode S data links (e.g., Own aircraft climbs while Intruder descends). If the intruder fails to follow its RA and climbs into own aircraft, TCAS 7.1 senses the closing geometry and issues an immediate reversal: "CLIMB, CLIMB NOW" or "DESCEND, DESCEND NOW" .
3. Terrain Awareness and Warning System (TAWS / EGPWS)
Mandated by 14 CFR 121.354 , all turbine-powered transport category aircraft must be equipped with Class A TAWS (Enhanced Ground Proximity Warning System - EGPWS). +-----------------------------------------------------------------------------+
- | CLASSIC GPWS VS. MODERN TAWS CLASS A |
- | |
- | [CLASSIC GPWS (Modes 1-6)] [TAWS CLASS A (EGPWS)] |
- | - Looks ONLY straight DOWN - Looks FORWARD, DOWN & AROUND |
- | - Measures Radio Altitude rate of change - Worldwide Terrain & Obstacle |
- | - Blind to rising sheer cliffs! - GPS + FMS Aircraft Position |
- | - Blind to landing short of runway! - Terrain Clearance Floor (TCF) |
- +-----------------------------------------------------------------------------+
- Classic GPWS Modes vs. EGPWS Look-Ahead Functions
- Mode 1: Excessive Sink Rate ( "SINK RATE, SINK RATE" → \to → "PULL UP" )
- Mode 2: Excessive Terrain Closure Rate ( "TERRAIN, TERRAIN" → \to → "PULL UP" )
- Mode 3: Altitude Loss After Takeoff or Go-Around ( "DON'T SINK, DON'T SINK" )
Mode 4: Unsafe Terrain Clearance with Gear/Flaps not in landing configuration ( "TOO LOW, GEAR" / "TOO LOW, FLAPS" )
- Mode 5: Excessive Deviation Below ILS Glideslope ( "GLIDESLOPE" )
- Mode 6: Advisory Altitude Callouts ( "500" , "MINIMUMS" ) and Bank Angle Alert ( "BANK ANGLE" )
EGPWS Forward-Looking Terrain Avoidance (FLTA): Compares GPS position, track, and vertical profile against a high-resolution worldwide digital elevation model (DEM) and obstacle database, generating look-ahead caution envelopes (40–60 seconds to impact: "CAUTION TERRAIN" ) and warning envelopes (20–30 seconds to impact: "TERRAIN TERRAIN PULL UP" ).
Terrain Clearance Floor (TCF): Enforces a virtual rising obstacle floor surrounding all certified airport runways, preventing CFIT accidents even when configured with gear down and landing flaps.
Mandatory EGPWS Terrain Warning Escape Maneuver
Upon receiving an EGPWS warning ( "PULL UP" or "TERRAIN AHEAD PULL UP" ) in IMC or at night: +-----------------------------------------------------------------------------+
- | EGPWS TERRAIN WARNING ESCAPE PROTOCOL |
- | |
- | 1. AUTOPILOT & AUTOTHROTTLE: Disconnect immediately. |
- | 2. THRUST: Advance thrust levers aggressively to MAXIMUM TO/GA RATING. |
- | 3. WINGS: Roll wings level immediately to maximize vertical lift vector. |
- | 4. PITCH: Rotate smoothly and aggressively toward the Pitch Limit |
- | Indicator (PLI) or stick shaker onset angle of attack. |
- | 5. SPEEDBRAKES: Retract speedbrakes / flight spoilers immediately. |
- | 6. CONFIGURATION: DO NOT change gear or flap position until terrain |
- | clearance is assured (gear doors opening causes transient drag spike). |
- +-----------------------------------------------------------------------------+
- 4. Airborne Windshear Detection and Alerting Systems
Low-altitude microburst windshear represents one of the most violent hazards in commercial aviation, characterized by a localized column of sinking air (downdraft up to 6,000 FPM) that spreads radially outward upon contacting the ground. +-----------------------------------------------------------------------------+
- | MICROBURST DYNAMICS & ENCOUNTER |
- | |
- | | | | | | | | |
- | | DOWNDRAFT | (Up to 6,000 FPM) |
- | v v v v v v v |
- | |
- | [1. INCREASING HEADWIND] [2. DOWNDRAFT CORE] [3. SEVERE TAILWIND] |
- | - Airspeed increases - Severe sink rate - Massive IAS drop |
- | - Aircraft balloons high - V/S collapses - Pitch collapses |
- | - Throttle pulls back (TRAP!) - High pitch required - STALL / CFIT CRASH |
- | ============================> =====================> ===================> |
- +-----------------------------------------------------------------------------+
Reactive vs. Predictive Windshear Detection Systems +-----------------------------------------------------------------------------+
- | REACTIVE VS. PREDICTIVE WINDSHEAR SYSTEMS |
- | |
- | Feature Reactive Windshear Predictive Windshear |
- | ----------------------------------------------------------------------- |
- | Sensor Physics Compares inertial accelerations Forward-looking pulse-|
- | (IRU) with aerodynamic air Doppler weather radar |
- | data (pitot-static airspeed). (Doppler shift of rain|
- | |
- | Detection Time Senses windshear AS IT OCCURS Scans 1 to 5 NM AHEAD |
- | (In situ energy loss). prior to encounter. |
- | |
- | Cockpit Alerts Red PFD Annunciation Aural: "WINDSHEAR |
- | Aural: "WINDSHEAR, WINDSHEAR, AHEAD, WINDSHEAR |
- | WINDSHEAR" AHEAD" / Red Radar Arc|
- | |
- | Operational Scope Active on takeoff/approach Active automatically |
- | from ground to 1,500 ft AGL. below 1,200 ft AGL. |
- +-----------------------------------------------------------------------------+
- Windshear Escape Guidance Protocol
Takeoff Roll (Prior to V 1 V_1 V 1 ): If windshear is sensed or PWS alerts, Reject Takeoff immediately with maximum reverse and maximum braking.
After V 1 V_1 V 1 or On Approach / Go-Around:
Press TO/GA switches and advance thrust levers to Maximum Rated Emergency/Go-Around Thrust .
Disengage autopilot and rotate wings level to the initial target pitch attitude (typically 15 ∘ 15^\circ 1 5 ∘ or follow the Flight Director Windshear Escape guidance bars).
Pitch aggressively up to the Pitch Limit Indicator (PLI) / stick shaker activation boundary to trade kinetic energy for altitude.
Configuration Rule: Do not retract gear or flaps until positive climb is established and the aircraft is completely clear of the windshear condition. Extending or retracting gear causes transient opening of landing gear doors, introducing massive parasite drag that can precipitate an immediate stall.