Ground Proximity Warning System (GPWS) and Enhanced GPWS (EGPWS)

Introduction: The CFIT Problem

Controlled Flight Into Terrain (CFIT) was once the leading cause of fatal airline accidents worldwide. CFIT occurs when an airworthy aircraft under full crew control is inadvertently flown into terrain, water, or an obstacle — often with no warning whatsoever. The aircraft is functioning normally; it is the crew's situational awareness that has failed. To address this catastrophic pattern, the FAA mandated Ground Proximity Warning Systems (GPWS) and, later, the far more capable Enhanced GPWS (EGPWS). Understanding both systems — how they work, what they warn, and their limitations — is essential knowledge for any ATP-certificate candidate and for every crew member operating in the transport environment.

GPWS: How the Original System Works

The original GPWS, introduced in the 1970s following a series of devastating CFIT accidents, is a reactive system. It monitors a set of specific flight-parameter inputs and generates warnings when those parameters exceed defined thresholds. The system does not have any knowledge of what terrain actually lies ahead — it only knows what the aircraft is doing right now and extrapolates danger from that behavior.

Required Inputs

A classic GPWS typically receives and continuously monitors the following data streams:

• Radio altimeter — measures actual height above ground (AGL) up to approximately 2,500 feet AGL, providing the primary height-above-terrain reference.
• Barometric altitude and altitude rate — determines how quickly the aircraft is descending.
• Indicated airspeed — used to assess approach configuration context.
• Landing gear position — distinguishes approach configuration from cruise.
• Flap position — also used for configuration context, particularly to suppress nuisance alerts during intentional steep approaches.
• Instrument Landing System (ILS) glideslope deviation — used for Mode 5 glideslope alerting.

The Seven GPWS Alert Modes

The FAA and ICAO define seven standard GPWS warning modes. Each mode monitors a specific dangerous condition and triggers both an aural warning (a synthesized voice or tone) and a visual alert (a warning light or annunciator):

1. Mode 1 — Excessive Descent Rate: Triggers when the aircraft descends too rapidly for its height AGL. The aural alert is "SINK RATE" followed by "PULL UP" if conditions worsen. A descent rate exceeding roughly 1,000 feet per minute at low altitude can trigger Mode 1.
2. Mode 2 — Excessive Terrain Closure Rate: Activates when the aircraft is closing on rising terrain at an unsafe rate. Alert: "TERRAIN, TERRAIN" followed by "PULL UP." This mode is split into Mode 2A (gear up) and Mode 2B (gear down, flaps in landing configuration).
3. Mode 3 — Altitude Loss After Takeoff or Go-Around: Detects significant altitude loss after initial climb. Alert: "DON'T SINK." This is particularly important for catching inadvertent descent after rotation or a missed approach climb.
4. Mode 4 — Unsafe Terrain Clearance: Covers situations where the aircraft is too close to the ground without being in a proper landing configuration. Mode 4A triggers when gear is up and airspeed is low ("TOO LOW GEAR"); Mode 4B when flaps are not in landing position ("TOO LOW FLAPS"); and Mode 4C monitors terrain clearance during the initial climb.
5. Mode 5 — Below Glideslope: Generates a "GLIDESLOPE" alert when the aircraft descends significantly below the ILS glideslope on an instrument approach. This mode is inhibited when the crew deliberately flies a non-precision approach and is also suppressible.
6. Mode 6 — Descent Below Decision Height: Provides callouts at decision altitude/height (DA/DH) and selected minimums. The specific aural callouts vary by operator and aircraft type but commonly include "MINIMUMS" or "DECISION HEIGHT."
7. Mode 7 — Windshear Warning: Detects the airspeed and energy changes characteristic of microburst windshear. Alert: "WINDSHEAR, WINDSHEAR, WINDSHEAR." Not all GPWS installations include Mode 7; some aircraft have a separate dedicated windshear detection system.

The Critical Limitation of Classic GPWS: No Lookahead Capability

Here lies the most important limitation of the original GPWS: it has no terrain database and no forward-looking capability. It can only react to what is happening directly beneath and around the aircraft at that instant. If an aircraft is flying level at a constant airspeed directly toward a steep cliff or a gradually rising plateau, the radio altimeter may read a comfortable 1,500 feet AGL right up until the moment of impact. The aircraft is not descending, so Modes 1 through 3 are silent. The gear and flaps may be up for cruise, so Mode 4 is also quiet. Classic GPWS provides no warning in this scenario — and this exact failure mode killed hundreds of people in mountainous or gradually rising terrain environments.

EGPWS: The Enhanced System and Terrain Awareness

Enhanced GPWS, developed by Honeywell (originally as the Mark V and Mark VII series) and mandated by the FAA for most transport-category turbine-powered aircraft, addresses the classic GPWS blind spot by adding a worldwide digital terrain database and a forward-looking terrain avoidance (FLTA) function. EGPWS is sometimes called a Terrain Awareness and Warning System (TAWS) — the FAA's generic regulatory term found in 14 CFR Part 91 and Part 121 for this class of equipment.

How EGPWS Adds Situational Awareness

The EGPWS continuously combines GPS-derived aircraft position (latitude, longitude, altitude) with its onboard terrain and obstacle database. It projects the aircraft's current flight path forward in time and compares that projected path to the terrain database. This gives the system true lookahead capability — typically projecting 60 seconds ahead for caution alerts and a shorter interval for warning alerts. The crew sees a color-coded terrain display, often overlaid on the navigation display (ND) of the Electronic Flight Instrument System (EFIS):

• Green — terrain well below the aircraft, no threat.
• Yellow/Amber — terrain within the caution envelope; "CAUTION TERRAIN" aural alert.
• Red — terrain within the warning envelope; "TERRAIN, TERRAIN, PULL UP" aural alert.

The system also incorporates an obstacle database (towers, antennas) and an airport database that helps suppress nuisance alerts during normal steep approaches to airports surrounded by terrain (a known weakness of classic GPWS Mode 2 and Mode 4).

TAWS Classes Under FAA Regulations

14 CFR defines two classes of TAWS equipment:

• TAWS Class A — required for turbine-powered aircraft with ten or more passenger seats operating under Parts 91, 121, and 135 (certain operations). Must provide forward-looking terrain alerting, all classic GPWS modes, and a cockpit terrain display. This is essentially EGPWS capability.
• TAWS Class B — a less comprehensive standard for smaller turbine aircraft (six to nine passenger seats). Must include at minimum Modes 1–4 plus a forward-looking function, but a terrain display is not mandatory.

Crew Response to GPWS and EGPWS Alerts

The required crew response to a "PULL UP" or "TERRAIN, TERRAIN, PULL UP" warning is immediate and standardized. Standard operating procedures (SOPs) across virtually all transport operators specify:

1. Disconnect autopilot (if not already disconnected by the maneuver).
2. Apply maximum thrust simultaneously.
3. Pitch aggressively to the manufacturer-specified escape pitch attitude — typically in the range of 15° to 20° nose-up, depending on aircraft type.
4. Retract speed brakes/spoilers if extended.
5. Do not reduce bank angle if terrain avoidance requires a turn — in EGPWS escape maneuvers, level wings is the default but a turn may be required if displayed terrain dictates.
6. Do not descend to verify terrain — maintain the escape maneuver until the warning ceases and safe terrain clearance is confirmed.

The fundamental principle is that a GPWS or EGPWS "PULL UP" warning is never to be ignored or treated as spurious during the maneuver. If the alert is subsequently determined to be a nuisance alert after safe altitude is confirmed, that is addressed in debriefing — not during the event.

Why EGPWS Dramatically Reduces CFIT

Studies and accident data consistently show that EGPWS, when properly used, provides crews with terrain awareness that classic GPWS simply cannot. The forward-looking alerting gives a typical warning time of 60 seconds or more in many scenarios, compared to the 5–15 seconds that classic GPWS might provide for the same threat. This added time converts a potential CFIT accident into a survivable escape maneuver. ICAO and the FAA credit the widespread adoption of TAWS/EGPWS as a primary factor in the dramatic reduction in transport-category CFIT accidents since the late 1990s.

Common Test Traps

• GPWS vs. EGPWS confusion: Classic GPWS is reactive and has no terrain database or lookahead. EGPWS/TAWS adds a digital terrain database and forward-looking alerting — this distinction is heavily tested.
• Mode 5 suppression: Mode 5 (glideslope) can be inhibited or suppressed by the crew during non-precision approaches. Test questions may ask which mode can be manually suppressed — Mode 5 is the correct answer, not the warning modes.
• "PULL UP" is mandatory: The correct response to a GPWS "PULL UP" warning is always immediate maximum performance escape — never a slow or tentative response, and never ignored because the crew believes it is a false alarm.
• Radio altimeter range: The radio altimeter, which feeds GPWS, is effective only to approximately 2,500 feet AGL. Above that height, GPWS modes that rely on radio altitude are not active — a fact sometimes tested in the context of system limitations.
• TAWS Class A vs. Class B: Class A requires a cockpit terrain display; Class B does not. Class A is required for the larger transport-category turbine operations under Parts 121 and 135.