A320 EGPWS and Weather Radar

The A320 carries two systems that look outside the aircraft for hazards the crew may not see. The enhanced ground proximity warning system (EGPWS) guards against controlled flight into terrain: five basic GPWS modes react to conditions beneath the aircraft, and database functions look ahead along the flight path. The weather radar in the nose paints precipitation and turbulence on the navigation displays (NDs), and its predictive windshear system (PWS) warns of windshear ahead at low altitude. A third alerting source, reactive windshear, is computed by the flight augmentation computers.

This article covers the A320 family installation as the Airbus operating documentation describes it. Some aircraft carry an ACSS T2CAS, which combines traffic and terrain functions in one unit, instead of the EGPWS; differences are noted where they matter. The general principles are in GPWS and TAWS and airborne weather radar.

EGPWS architecture and inputs

The EGPWS computer combines data from:

• the two radio altimeters, whose antennas are under the aft fuselage, for height above the surface;
• the ADIRS, for barometric, airspeed, temperature and inertial data;
• ILS 1, for glide slope deviation in Mode 5;
• the landing gear and slat/flap positions;
• GPS position and altitude and the FMGC navigation solution, for the enhanced functions;
• a terrain, obstacle and runway database.

From pressure altitude, radio altitude, temperature, the barometric reference, GPS altitude and its database, the computer works out a geometric altitude for the predictive functions. The enhanced functions depend on accurate position. If the FMGC detects low navigation accuracy they are deactivated, and if the EGPWS's own estimate of position accuracy is insufficient the green TERR STBY memo appears and they stay unavailable until it clears. The five basic modes, which do not use the database, keep working.

Alerts are synthetic voice messages, which cannot be canceled while their condition lasts, backed by the PULL UP-GPWS pushbutton on the instrument panel. It lights red for a PULL UP warning from Mode 1, Mode 2 or the terrain awareness function, and amber for any other GPWS alert.

Basic GPWS modes 1 to 5

The basic modes are active up to 2,500 ft radio height. Each compares one measured quantity with an envelope:

View full schematic

Enhanced functions: TAD, TCF and RFCF

The terrain awareness and display (TAD) function computes a caution envelope and a warning envelope ahead of the aircraft from its altitude, the altitude of and distance to the nearest runway threshold, ground speed and turn rate, and compares them with the database. Terrain or an obstacle in the caution envelope triggers TERRAIN AHEAD or OBSTACLE AHEAD, with solid yellow areas and a TERR AHEAD or OBST AHEAD message on the ND. In the warning envelope the alert becomes TERRAIN AHEAD, PULL UP or OBSTACLE AHEAD, PULL UP, with solid red areas.

The terrain clearance floor (TCF) is an enhanced Mode 4. It builds a terrain clearance envelope around each runway and alerts when clearance is insufficient even with the aircraft in landing configuration, and it also protects against attempts to land where there is no airfield. The database holds a TCF envelope for every runway longer than 3,500 ft that has terrain data. TCF works in takeoff, cruise and approach. Descending below the envelope triggers TOO LOW TERRAIN with a visual alert on the PFD, repeated for each further 20 % loss of radio altitude.

The runway field clearance floor (RFCF) adds an envelope for runways that sit well above the surrounding terrain. It is a circular band based on geometric altitude and runway elevation, and it also alerts TOO LOW TERRAIN.

Terrain display on the ND

A TERR ON ND pushbutton on each side of the ECAM controls the terrain display on its own side's ND. As on other EGPWS installations, terrain is painted according to its height relative to the aircraft, not above sea level, and when the crew select terrain the weather radar image is not shown. If a TAD caution or warning occurs with terrain not selected, terrain and obstacles appear automatically and the ON light of the pushbutton comes on. If position accuracy becomes insufficient, TERR STBY removes the terrain display.

The limitations are explicit: navigation must not be based on the terrain display, which is a situational awareness tool and may not be accurate enough for terrain avoidance maneuvering.

The GPWS panel and inhibitions

The GPWS panel on the overhead panel lets the crew inhibit parts of the system for a briefed reason:

• SYS OFF inhibits all basic alerts, Modes 1 to 5. If ILS 1 fails, only Mode 5 is lost, and in flight the GPWS FAULT light stays off while Modes 1 to 4 work (on the ground it comes on).
• G/S MODE OFF inhibits Mode 5, for a deliberate approach below the glide slope.
• FLAP MODE OFF inhibits the TOO LOW FLAPS alert, avoiding a nuisance alert when landing with a reduced flap setting.
• LDG FLAP 3 ON inhibits the flap mode when CONF 3 is selected, for CONF 3 landings; the landing memo then reads FLAPS … 3.
• TERR OFF inhibits TAD and TCF without affecting the basic modes, and the ECAM shows NAV GPWS TERR DET FAULT.

The predictive functions should be inhibited with TERR OFF within 15 NM of the airfield when operating to or from a runway not in the database, or on approach and departure procedures that the operator has identified as causing expected or spurious terrain alerts. The decision must not be based on crew judgment alone.

Weather radar

The antenna sits in the nose radome. The aircraft has one or two weather radars with autotilt and the PWS function, and the radar picture can appear on the ND in every mode except PLAN. The radar can be used in Multiscan, its automatic mode and the recommended setting, or in manual mode.

Autotilt sets the tilt from the EGPWS terrain database, aircraft position, altitude and the selected ND range:

• above 2,300 ft AGL, for long-range detection on the 160 and 320 NM ranges and short-range detection on 10 to 80 NM;
• below 2,300 ft AGL, for short-range detection plus PWS scanning, with alternate scanning disabled;
• on the ground, for optimum detection of weather 10,000 ft above ground level.

When the two pilots select different ranges, the radar scans alternately at two tilt settings, each suited to one range, which slows the refresh of each image. With the tilt at AUTO the radar both optimizes detection and minimizes ground clutter. If autotilt is lost, the tilt value on the ND is dashed and NO AUTO TILT appears in amber until the crew select manual tilt.

Gain and tilt in manual use follow Airbus's general weather radar guidance. The gain normally stays in its calibrated AUTO position, set to detect standard thunderstorm cells. It is reduced only to evaluate a cell: as it comes down, red turns yellow and yellow turns green, and the red areas that change last are the strongest cores. It is then reset to AUTO. Because the upper part of a storm holds ice, which reflects poorly, the tilt is lowered to scan the wet part below the freezing level; on radars without autotilt, Airbus's example settings put it at about 4° up for takeoff and approach. A cumulonimbus should be cleared by at least 20 NM laterally, preferably on its upwind side where hail is less likely, and 5,000 ft vertically.

The display modes include WX/TURB, which adds turbulence in magenta within 40 NM to the precipitation picture, and MAP, for ground mapping, with water black, ground green and cities and mountains amber. Red ND messages (WXR RT, WXR ANT, WXR CTL, WXR RNG) mean no radar image; amber ones (WXR WEAK, WXR ATT, WXR STAB) leave the image available. After landing the crew switch the radar and the PWS off so as not to irradiate people at the gate.

Predictive and reactive windshear

The PWS operates when its switch is at AUTO, even with the weather radar OFF, provided the aircraft is below 2,300 ft AGL, the transponder is on and either engine is running. It scans 5 NM ahead:

During the takeoff roll up to 100 kt, warnings and cautions cover 3 NM ahead. Alerts are inhibited on takeoff from 100 kt to 50 ft and on landing below 50 ft, and on final approach warnings are downgraded to cautions between 370 ft and 50 ft AGL. If the ND range is above 10 NM when windshear is detected, W/S SET RNG 10 NM appears; with only PWS active the ND shows PWS SCAN. The PRED W/S OFF memo is green in flight phases 2 and 6 and amber in phases 3, 4, 5, 7, 8 and 9. With two radars, a failed one's PWS is recovered by selecting the other.

Reactive windshear comes from the flight augmentation computers, which detect windshear the aircraft is already in: on approach from 1,300 ft to 50 ft radio altitude, and on takeoff from 3 s after liftoff to 1,300 ft, with at least CONF 1. Both PFDs show WINDSHEAR and the voice calls WINDSHEAR three times. See windshear and microbursts.

Alerts, priorities and crew response

PWS aural alerts take priority over TCAS, GPWS and other aural warnings, and are themselves inhibited by reactive windshear and stall warnings. GPWS, windshear and stall alerts also switch TCAS from TA/RA to TA ONLY, and in a TCAS RA the crew respect stall, GPWS and windshear warnings.

The A320 memory items set the response:

• EGPWS warnings (PULL UP, TERRAIN AHEAD PULL UP, OBSTACLE AHEAD PULL UP): simultaneously AP OFF, pitch to full backstick and hold, thrust levers TOGA, speed brakes checked retracted, wings level or bank adjusted if turning is safest; no slat, flap or gear change until clear.
• Terrain cautions (TERRAIN TERRAIN, TOO LOW TERRAIN, TERRAIN AHEAD, OBSTACLE AHEAD): the same pull-up at night or in IMC; in daylight VMC with the terrain clearly in sight, adjust the flight path.
• SINK RATE and GLIDESLOPE: adjust the flight path above 1,000 ft AAL in IMC or 500 ft in VMC; below, consider a go-around. DON'T SINK: adjust the flight path. TOO LOW GEAR or TOO LOW FLAPS: go around.
• Windshear warning: TOGA, follow SRS orders and keep the autopilot on if engaged, the opposite of the EGPWS pull-up.

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.