향상된 지상 접근 경고 시스템 (EGPWS)

Overview

The Enhanced Ground Proximity Warning System (EGPWS), also known as the Terrain Awareness and Warning System (TAWS) in ICAO terminology, is a safety-critical avionics system that warns flight crews when the aircraft is at risk of Controlled Flight Into Terrain (CFIT). CFIT — where an airworthy aircraft under crew control inadvertently flies into terrain, water, or obstacles — was the leading cause of fatal commercial aviation accidents for decades. EGPWS addresses this by combining traditional reactive Ground Proximity Warning System (GPWS) modes with proactive terrain awareness using a worldwide digital terrain/obstacle database and GPS position.

The original GPWS, mandated in the US from 1975 following the 1974 Eastern Air Lines crash in the Great Dismal Swamp, used only radio altimeter, airspeed, configuration, and sink rate data to generate warnings. It was purely reactive — warnings came only after the aircraft was already in a dangerous situation. Honeywell developed EGPWS in the mid-1990s, adding the terrain database and GPS lookup to provide look-ahead warnings of terrain ahead of the aircraft, even when the terrain is not yet detectable by radio altimeter. ICAO mandated Class A TAWS (EGPWS equivalent) for turbojet and turboprop aircraft above 5,700 kg from 2003.

How It Works

EGPWS continuously compares the aircraft's GPS position and altitude against a worldwide terrain and obstacle database (updated periodically) to generate a projected terrain clearance picture ahead. This "look-ahead" envelope is defined by the aircraft's current track, speed, and performance envelope. If the terrain database shows terrain penetrating this envelope within approximately 60 seconds of flight time, the system generates a visual terrain display overlay (color-coded from green through yellow to red) on the navigation display, and audio/visual caution or warning alerts.

In addition to the look-ahead function, EGPWS retains the six original GPWS reactive modes: Mode 1 (excessive sink rate), Mode 2 (excessive terrain closure rate), Mode 3 (altitude loss after takeoff), Mode 4 (unsafe terrain clearance in landing configuration), Mode 5 (below glideslope), and Mode 6 (altitude callouts). A seventh windshear detection mode uses inertial data to detect velocity divergence. The terrain and obstacle database must be updated regularly (typically annually) to capture new obstacles such as towers and tall structures.

Key Components

• EGPWS Computer: The central processor containing the terrain/obstacle database and running the look-ahead algorithm. Honeywell's MK V, MK VI, MK VII, and MK VIII are the dominant commercial aviation systems. The computer interfaces with radio altimeter, air data, GPS, and FMS.
• Worldwide Terrain Database: A 30 arc-second (approximately 1 km) or finer resolution digital elevation model covering the entire Earth, stored in non-volatile memory. Includes obstacle database with man-made structures above 200 ft AGL.
• Radio Altimeter: Provides precise height above terrain (0–2,500 ft) for the reactive GPWS modes and for final approach terrain display calibration.
• Terrain Display (ND Overlay): Color-coded terrain picture on the navigation display showing relative terrain elevation: green (more than 2,000 ft below), yellow (500–2,000 ft below), and red (less than 500 ft below or above).
• Audio Warning Outputs: Synthesized voice callouts ("TERRAIN, TERRAIN, PULL UP" or "CAUTION TERRAIN") generated by the EGPWS and broadcast on the flight deck speakers and headsets.

Aircraft Applications

• Boeing 737-800: Honeywell MK V EGPWS standard on all production aircraft; terrain display shown on the ND in terrain overlay mode, with RAAS (Runway Awareness and Advisory System) callouts at equipped airports.
• Airbus A320-200: Honeywell MK V or Thales TAWS integrated with ECAM alerting system; terrain display on either ND, with aural "TERRAIN AHEAD, PULL UP" warnings for CFIT threats.
• Boeing 777-300ER: Honeywell MK VII EGPWS with enhanced terrain display resolution and Smart Terrain function, which improves terrain rendering near airports with challenging approaches (e.g., Innsbruck, Kathmandu).
• Boeing 787-9: Honeywell MK VIII EGPWS integrated with the Common Core System and ADS-B, using GPS-based look-ahead terrain with an updated global database. The RAAS system provides runway identification callouts during taxi and approach.

Advantages and Limitations

EGPWS/TAWS has been one of the most effective aviation safety interventions of the past three decades. The CFIT accident rate for commercial jets equipped with Class A TAWS dropped by more than 90% following its widespread adoption. The look-ahead function provides warnings up to 60 seconds before terrain impact, giving crews enough time to execute an escape maneuver in most scenarios. The terrain display provides continuous situational awareness in mountainous terrain even without an active warning.

Limitations include the database resolution — at 30 arc-seconds, features smaller than approximately 1 km may be missed. New obstacles (towers, wind turbines) may not appear in the database until the next update cycle, creating a lag. Terrain database errors, although rare, have produced nuisance warnings and in a few cases masked the severity of actual threats. The system cannot protect against sudden unexpected terrain features not in the database, or against steep canyon walls below the lateral boundaries of the look-ahead envelope. Runway awareness functions (RAAS) require a separate airport database and do not substitute for crew vigilance on unfamiliar airport layouts.