6.4 Surveillance & Warning Systems

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.

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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 ($\tau$)—the time remaining to the Closest Point of Approach (CPA), supplemented by a protected distance zone (DMOD) at low closure speeds.

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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

1. Immediate Reaction (Initial RA): Disconnect autopilot (or ensure AP TCAS mode engages) and smoothly adjust pitch within 5 seconds, applying approximately $0.25g$ acceleration to achieve the commanded green vertical speed (typically 1,500 to 2,000 FPM).
2. Immediate Reaction (Modified/Reversal RA): React within 2.5 seconds with $0.35g$ acceleration (e.g., "INCREASE CLIMB" commanding 2,500 to 3,000 FPM).
3. ATC Notification: As soon as workload permits, advise ATC:
   "[Callsign], TCAS RA."
   ATC acknowledges: "[Callsign], Roger." (ATC safety alert separation is suspended).
4. 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".

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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). |
+-----------------------------------------------------------------------------+

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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

1. Takeoff Roll (Prior to $V_1$): If windshear is sensed or PWS alerts, Reject Takeoff immediately with maximum reverse and maximum braking.
2. After $V_1$ 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 $15^\circ$ 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.