Surveillance: The Aircraft's Electronic Eyes and Protective Systems

Every flight depends on your ability to see and avoid hazards—from weather and terrain to other aircraft and runway overruns. But what happens when visibility is poor, terrain rises unexpectedly, or another aircraft poses a collision risk? The surveillance systems solve these challenges by providing multiple layers of electronic awareness that extend far beyond what your eyes can see. These systems don't just display information—they actively monitor threats and provide warnings that can save your flight.

How Weather Radar Works: Your Window Through the Storm

Your weather radar operates like a sophisticated lighthouse, sending radio waves ahead of your aircraft and interpreting what bounces back. The system continuously scans up to 320 NM ahead, painting a picture of precipitation, turbulence, and even windshear on your Navigation Display (ND).

The display mode selector gives you different views of the same weather picture. In WX mode, you see precipitation intensity through color coding—black represents the lowest intensity, progressing through green and amber to red for the most severe conditions. This isn't just pretty colors; each level tells you something critical about what lies ahead. Green might mean light rain you can penetrate, while red indicates severe precipitation that demands avoidance.

WX+T mode adds turbulence detection, showing dangerous air in magenta within 40 NM. This becomes crucial because turbulence often exists without visible precipitation—clear air turbulence that radar normally can't detect. The TURB mode focuses solely on these turbulence areas, helping you find smooth air when passengers' and crew's safety is paramount.

The Multiscan function represents the radar's intelligence. In AUTO mode, it automatically adjusts the antenna tilt and combines scans at various angles to give you the most complete weather picture while minimizing ground clutter. Think of it as having an experienced radar operator constantly optimizing your view. When set to MAN, you control the tilt manually using the TILT knob, with zero position aligned to your horizon reference from the IRS.

Managing Weather Detection and Ground Clutter

Ground clutter—those unwanted radar returns from terrain, buildings, and other surface features—can mask real weather threats. The Ground Clutter Suppression (GCS) switch in AUTO position intelligently filters these returns when Multiscan is active, but displays them in manual mode regardless of switch position. This design ensures you always know what the radar is doing.

The GAIN knob in CAL position automatically adjusts radar sensitivity based on your altitude, location, season, and time of day. This isn't random—the system knows that weather appears differently at different altitudes and locations and compensates accordingly. Manual gain adjustment becomes necessary in heavy rain when you need to pinpoint areas of maximum intensity that automatic settings might smooth over.

Predictive Windshear: Your Early Warning System

Windshear—sudden changes in wind speed or direction—poses one of aviation's most serious threats. Your Predictive Windshear System (PWS) scans up to 5 NM ahead, detecting windshear before you encounter it. This forward-looking capability gives you precious seconds to react.

The system provides three alert levels based on threat severity. Warnings during approach trigger the red "W/S AHEAD" message and the critical "GO AROUND WINDSHEAR AHEAD" aural alert. During takeoff, you'll hear "WINDSHEAR AHEAD" repeated twice. Cautions display amber "W/S AHEAD" with "MONITOR RADAR DISPLAY," while advisories show only the windshear icon on your ND without Primary Flight Display (PFD) messages.

Understanding the system's operational envelope is crucial. During takeoff, alerts are active up to 100 kt within 3 NM, then inhibited until 50 ft AGL. On approach, warnings transition to cautions between 370 and 50 ft AGL, with detection range narrowing to between 1.5 and 0.5 NM. Below 50 ft, all alerts are inhibited—you’re committed to landing.

Ground Proximity Warning System: Your Terrain Guardian

The GPWS monitors five basic modes that protect you from terrain collision up to 2500 ft AGL. Each mode addresses specific threat scenarios that have historically caused accidents.

Mode 1 watches for excessive descent rates, alerting "SINK RATE, SINK RATE" and escalating to "PULL UP" if conditions worsen.

Mode 2 detects rapid terrain closure with two sub-modes: 2A during climb and cruise when not configured for landing, and 2B during approach or after takeoff when flaps are set for landing. The "TERRAIN, TERRAIN" alert gives you immediate awareness of rising ground.

Mode 3 catches altitude loss after takeoff or go-around—those critical moments when you should be climbing but aren't. "DON'T SINK, DON'T SINK" reminds you to arrest the descent.

Mode 4 monitors terrain clearance based on your configuration, alerting "TOO LOW TERRAIN," "TOO LOW GEAR," or "TOO LOW FLAPS" depending on the specific threat.

Mode 5 keeps you on the glideslope with repeated "GLIDESLOPE" alerts when you descend below the approach path. Each mode uses different combinations of radio altitude, barometric altitude, descent rate, and aircraft configuration to determine threats.

Predictive GPWS: Enhanced Terrain Awareness

Predictive GPWS uses a terrain database to look ahead of your flight path, providing warnings before basic GPWS modes would activate. This forward-looking capability displays terrain on your ND, with brightness controlled by the weather radar brightness knob—a design that ensures terrain visibility matches your weather radar settings.

The system calculates geometric altitude using pressure altitude, radio altitude, temperature, and barometric references, then compares this with GPS altitude and database information. This multi-source approach ensures accuracy even when individual sensors fail.

The TERR pushbutton allows you to inhibit predictive modes while keeping basic GPWS modes 1 to 5 active. This becomes important near airports where terrain features might trigger nuisance alerts, but remember—inhibiting predictive GPWS disables RNP AR operations and ROW/ROP functions.

Runway Overrun Prevention: Your Landing Safety Net

ROPS prevents runway overruns through two complementary functions. ROW (Runway Overrun Warning) activates at 400 ft AGL and monitors your approach, calculating whether you can stop within the available runway length. ROP (Runway Overrun Protection) engages when you start braking, continuously updating stopping distance calculations.

The system uses runway condition settings and compares the calculated landing distance with the runway length from the TAWS database. If your calculated landing distance exceeds the available runway length, ROPS alerts activate, demanding immediate action. However, ROPS cannot prevent overruns caused by performance issues, unavailable thrust reversers, or incorrect runway condition settings—it's only as good as the information you provide.

Traffic Collision Avoidance: TCAS and ATSAW

Your TCAS system processes traffic data from other aircraft transponders and ATC ground stations, providing collision avoidance through Traffic Advisories (TA) and Resolution Advisories (RA). The system automatically responds to interrogations from ATC and other TCAS-equipped aircraft, transmitting your altitude, speed, heading, and other critical data.

ATSAW enhances traffic awareness by processing ADS-B messages from nearby aircraft and displaying detailed information, including flight identification, position, altitude trends, and wake vortex categories. Unlike TCAS, ATSAW doesn't issue collision avoidance alerts—it's designed for situational awareness and visual acquisition, not separation.

The ND shows up to 9 aircraft using different symbols based on data sources: TCAS only, TCAS + ADS-B, or ADS-B only. Understanding these symbols helps you quickly assess traffic threats and their relative positions.

What This Means for Your Operations

These surveillance systems work together to create layers of protection, but they require your understanding to be effective. Weather radar shows you where not to go, but you must interpret the colors and understand that weak echoes might hide dangerous conditions. GPWS alerts demand immediate response—"PULL UP" means pull up now, not after you finish what you're doing.

Predictive windshear warnings during approach mean go around immediately, while cautions require heightened awareness and readiness to execute a go-around. ROPS alerts indicate you cannot stop on the available runway with current conditions—continuing the approach risks an overrun.

Remember that these systems enhance but don't replace good airmanship. Weather radar can't detect clear air turbulence, GPWS databases might not include the latest obstacles, and TCAS requires other aircraft to have functioning transponders. Your surveillance systems provide incredible capability, but they work best when you understand their strengths, limitations, and proper use.

A320 surveillance systems explained – weather radar, GPWS, predictive windshear, TCAS, and runway overrun protection, and how they work as a layered defense.