EGPWS: Terrain Awareness and Warning

Controlled flight into terrain (CFIT) was for decades the number-one killer of jet transports; the GPWS → EGPWS thirty years is the thirty years that pushed it into the statistical tail. This article has two faces: the five basic modes — reactive "veterans" that consult no database, going on radio height and configuration alone and calling only when terrain closes in; and the predictive functions (TAD/TCF/RFCF) — "prophets" carrying a global terrain, airport, and obstacle database, painting yellow 60 s ahead and red 30 s ahead.

1. The five basic modes — veterans without a map

Per FCOM DSC-34-20-40-10:

The purpose of the Ground Proximity Warning System (GPWS), also named Terrain Awareness Warning System (TAWS), is to warn the flight crew of potentially hazardous situations, such as a collision with terrain. It detects terrain collision threats and triggers applicable aural and visual indications.

All five basic modes play on the radio-height stage (< 2500 ft), consulting no database, blind to what is ahead — they measure what is "underfoot" and its trend.

Mode 1 | excessive rate of descent (the only always-active mode). Per FCOM DSC-34-20-40-20:

Mode 1 triggers aural and visual alerts about excessive rates of descent, based on the radio height, and the rate of descent of the aircraft.

Mode 1 is active for all phases of the flight.

Caution "SINK RATE, SINK RATE" (amber GPWS light), escalating to "PULL UP" (red). It cares nothing for configuration — low to the ground and dropping fast, it calls.

Mode 2 | excessive terrain closure rate (the cliff mode). Per FCOM DSC-34-20-40-20:

Mode 2 triggers aural and visual alerts, based on the landing gear/flaps configuration of the aircraft, the radio height, and the RA rate of change.

There are two types of Mode 2 alerts, Mode 2A (active during climb, cruise, and initial approach), and Mode 2B (active during approach and 60 s after takeoff).

Note it measures not the aircraft's descent rate but the rate of change of the RA reading — level flight toward a cliff has V/S = 0 but the RA plunging, and mode 2 still calls. Non-landing configuration: caution "TERRAIN, TERRAIN" → warning "PULL UP"; it also has a "grudge" — after leaving the warning condition the red stays and it changes to "TERRAIN" until you have climbed enough.

Mode 3 | altitude loss after take-off / go-around. Per FCOM DSC-34-20-40-20:

Mode 3 triggers aural and visual alerts when the altitude significantly decreases after takeoff, and go-arounds with landing gear or flaps not in landing configuration.

Calls "DON'T SINK" (caution only). It treats "losing height unknowingly" early after take-off or in a go-around.

Mode 4 | unsafe terrain clearance out of landing configuration (three siblings). Per FCOM DSC-34-20-40-20:

There are three types of Mode 4 alerts, Mode 4A and Mode 4B (both active during cruise and approach), and Mode 4C (active during takeoff).*

4A cures "gear not down when it should be" (TOO LOW GEAR / TOO LOW TERRAIN), 4B cures "flaps not fully out" (TOO LOW FLAPS / TOO LOW TERRAIN), 4C cures "terrain rising after take-off" (TOO LOW TERRAIN). Speed participates: faster, it calls TOO LOW TERRAIN earlier — fast means you are not in a normal approach.

Mode 5 | excessive deviation below glideslope. Per FCOM DSC-34-20-40-20:

Mode 5 triggers aural and visual alerts, when the aircraft descends below the glide slope on ILS, SLS and GLS approaches.

Calls "GLIDESLOPE" (soft and hard, louder the lower), caution only. The data source is ILS 1's G/S deviation — the seed of a famous knock-on: on the ground, an ILS 1 failure inhibits only mode 5, with no FAULT light (§6).

2. Geometric altitude — the floor against "wrong altimeter setting"

The basic modes eat RA, but the predictive functions need altitude above sea level — and barometric altitude is trapped by temperature and the human hand. The FCOM recipe. Per FCOM DSC-34-20-40-10:

The GPWS computes the geometric altitude of the aircraft by using: ‐ Pressure altitude ‐ Radio-altitude ‐ Temperature ‐ Barometric references

The AMM states the purpose plainly. Per AMM 34-48-00:

Geometric Altitude uses an improved pressure altitude calculation, GPS altitude, radio altitude, and terrain and runway elevation data to reduce or eliminate errors potentially induced in corrected barometric altitude by temperature extremes, non standard altitude conditions and, altimeters miss-sets.

Per FCOM DSC-34-20-40-30:

The Geometric Altitude function can protect against certain BARO setting errors, provided the components used to compute the Geometric Altitude are valid and accurate enough.

A 10 hPa QNH error = a 280 ft altitude error, which corrupts barometric altitude across the board — but geometric altitude is not fooled, and TAD's terrain comparison uses this independent "true altitude," the mechanism by which EGPWS stops a "wrong-setting fly-into-terrain." With the QFE option fitted, geometric altitude has a QFE-specific temper. Per AMM 34-48-00:

The MCDU message prompts for input when < 180 NM from destination.

TERR STBY will be displayed if the QNH value is not entered when the aircraft is more than 180 NM from the takeoff runway (Geometric Altitude not activated).

In QFE operation the EGPWS cannot get a sea-level reference and asks you to feed it a QNH; feed it nothing and it surrenders (TERR STBY).

3. TAD — 60 s of yellow, 30 s of red, looking ahead

Per FCOM DSC-34-20-40-30:

The Terrain Awareness and Display (TAD) function computes a caution and a warning envelope ahead of the aircraft, depending on the aircraft altitude, the nearest runway altitude, the range to the nearest runway threshold, the ground speed, and the turn rate. When the boundary of these envelopes conflicts with the terrain, or with an obstacle memorized in the database, the system generates the relevant alert:

Warning and caution distances change with ground speed and turn rate. Warning distance is approximately 30 s. Caution distance is approximately 60 s.

A time geometry: the envelope is not a fixed few miles but "the patch of sky swept in the next 60/30 s at the current ground speed" — a fast aircraft is warned farther, and the envelope bends with a turn. Vertically there is a "terrain floor," hugging the ground near the airport (else approach would always alert) and rising farther away. And it gives you no "I didn't have the display on" excuse. Per FCOM DSC-34-20-40-30:

If an alert is generated (caution or warning) and TERR ON ND is not selected, the terrain is automatically displayed, and the ON light of the TERR ON ND pb comes on.

The manual-display conditions and an identification tip. Per FCOM DSC-34-20-40-40:

The terrain and obstacles image appears on the ND if: ‐ The aircraft is within 2 000 ft above the terrain, and ‐ The TERR ON ND pb is pressed, or automatically, in the case of terrain caution(warning) alerts.

To be able to identify the difference between the terrain and the weather display, the terrain display goes from the center outward to both sides of the ND.

This configuration has the Peaks function. Per FCOM DSC-34-20-40-40:

Depending on the capacity of the TAWS system installed (e.g. with Eleview or Peaks functions), the terrain can display the highest and lowest geographic terrain elevations for the selected ND range.

Cruising high, the terrain display is no longer all black — green density layers plus two elevation figures read "how high is the highest below me" at a glance. Note the TERR image and the weather image are mutually exclusive (selecting TERR ON ND removes the radar image), each ND selected independently.

4. TCF and RFCF — catching the landing traps mode 4 cannot reach

Mode 4 has a congenital gap: it falls silent in the landing configuration — but "landing fully configured toward the wrong place" is a classic CFIT script. TCF fills it. Per FCOM DSC-34-20-40-30:

The TCF function computes a terrain clearance envelope around the airport runway. This function triggers an alert if the terrain clearance is not sufficient (even when the aircraft is in landing configuration). This function provides an enhanced GPWS mode 4.

The TCF function also protects against an attempt to land where there is no airfield.

The TAWS database includes the applicable TCF envelope for each airport runway that has terrain data, and that is longer than 3 500 ft.

Enter the envelope and it calls "TOO LOW TERRAIN" + amber; not recovering, it repeats to a rhythm. Per FCOM DSC-34-20-40-30:

If the aircraft continues to descend below the envelope, the alert is triggered one time for every 20 % degradation in radio altitude

RFCF treats another landform — a table-top airport (a runway far above its surroundings): clearance over the "valley terrain" is ample, but relative to the runway plane the aircraft is already too low, and TCF's terrain datum cannot catch it. Per FCOM DSC-34-20-40-30:

The Runway Field Clearance Floor (RFCF) function provides an additional protection envelope for runways that are at an elevation that is significantly more than the surrounding terrain.

The envelope is a circular band that starts within a specific distance of the runway. The width of this circular band depends on the runway. The envelope is based on both the geometric altitude and the runway elevation.

Note the datum changes to geometric altitude vs runway elevation: too low relative to the runway, and it still calls "TOO LOW TERRAIN."

5. Position sources and self-inhibition — even the prophet goes blind

Everything predictive is premised on "where am I." The position-source degradation chain. Per AMM 34-48-00:

When available on board, Enhanced GPWS uses preferably GPS, then IRS latitude and longitude data as valid position source and, if these positions are downgraded, then Flight Management System (FMS) position will be used.

And if none of the three is accurate enough? It switches itself off. Per AMM 34-48-00:

The Enhanced GPWS uses GPS, corrected IRS and FMS as valid position sources and will inhibit itself automatically if all the FOMs for these sources exceed the current Enhanced GPWS required accuracy.

The FCOM crew-language version. Per FCOM DSC-34-20-40-30:

In case of low accuracy of the aircraft position computed by the EGPWS, the enhanced modes of the EGPWS are automatically deactivated. The five GPWS modes remain active.

Per FCOM DSC-34-20-40-40:

In flight, the TERR STBY memo appears in green when the aircraft position accuracy (computed by the EGPWS) is insufficient to allow the enhanced TCF and TAD modes to operate. These modes are not available until the TERR STBY memo disappears. If selected, the terrain data display on the ND is automatically deselected when the TERR STBY memo is triggered.

Better blind than misdirecting — painting a terrain map on an inaccurate position is more dangerous than painting none (the standard companion of GPS-interference scenarios). And a big signpost buried in the wiring — the EGPWC's inertial/air data connect only to ADIRU 1. Per AMM 34-48-00:

Additionally, aircraft ground track and ground speed data are received from the IR portion of the ADIRU 1 (IRS). The aircraft altitude MSL is received from the air data portion of the ADIRU 1.

There is no channel-2/3 backup line — an ADR 1 (or IR 1) death takes the whole GPWS with it (ECAM writes GPWS into the ADR 1 FAULT INOP list). This is the most painful footnote in the chapter to "side 1 is the lifeline," and the taproot of the abnormal-chapter knock-on network.

6. The panel — four keys and their knock-on accounts

The guarded SYS key (this group's text). Per FCOM DSC-34-20-40-40:

All basic GPWS alerts (Mode 1 to 5) are inhibited. If OFF is selected, the ECAM caution NAV GPWS FAULT is displayed.

If ILS 1 fails, only mode 5 is inhibited. Consequently, the FAULT light does not come on, and the NAV GPWS FAULT alert is not triggered.

The latter is the frontal confirmation of the knock-on network: an ILS 1 death quietly removes glideslope protection, with no ECAM (so learn to read "GPWS G/S" in the STATUS INOP list). G/S MODE key: manually inhibits mode 5 — the QRH-authorised use is "a deliberate approach below glideslope." FLAP MODE key: inhibits TOO LOW FLAPS, with an automation. Per FCOM DSC-34-20-40-40:

If LDG CONF 3 is selected on MCDU, the flap mode will be automatically inhibited when FLAPS 3 position is reached.

The TERR key — this turns off the prophet, with a three-item bill. Per FCOM DSC-34-20-40-40:

The predictive modes of the GPWS are inhibited. The basic GPWS modes 1 to 5 are operative.

The inhibition of predictive GPWS modes may lead to nuisance alerts of the basic GPWS mode 2 alerts on approaches with sudden radio altitude changes (e.g. cliff on the approach path).

RNP AR operations are not permitted.

(The third item also inhibits ROW/ROP, not fitted here.) Why does turning off the prophet make mode 2 nag? Because in normal operation the EGPWS uses the terrain database to modulate mode 2's envelope down against nuisance — the prophet out of a job, the veteran reverts to its thirty-years-ago temper. The memo family: TERR OFF (green in phases 2/6; amber in 3/4/5/7/8/9 or on a T.O CONFIG test), GPWS FLP OFF (green), TERR STBY (green, §5).

7. Envelope modulation — the "airport allow-list" in the database

Some airports' normal procedures inherently graze the GPWS envelope (steep approaches, terrain rising near the threshold). The EGPWS answer is not global desensitisation but per-airport tuning. Per AMM 34-48-00:

The envelope modulation feature provides improved alert/warning protection at some key locations throughout the world, while improving nuisance margins at others.

Four modulation types: mode 1 boundary shifted right (allowing a steeper descent — steep-approach airports); mode 2A/2B upper limit lowered (mountainous approaches against nuisance); mode 4 upper limit lowered; mode 5 upper limit raised (at a table-top airport an ILS approach spends most of its length above 1000 ft RA where old GPWS could not protect it — modulation lets GLIDESLOPE trigger higher and removes the gear-down requirement). The guard is the point — before modulating, it must cross-verify "you really are in a normal approach." Per AMM 34-48-00:

For ILS approaches, the glide slope deviation is used to establish that adequate terrain clearance exists (i.e. a "normal" approach). Consequently, errors in altitude data do not enable envelope modulation during an unsafe condition

Without ILS it uses a "snapshot" comparison; if it does not match, no modulation. An aircraft already low never gets desensitised.

8. Audio priority and RAAS — the queueing school of one mouth

The EGPWC has one mouth, and alerts queue by severity (AMM priority table, summarised): the PULL UP family highest (mode 1 PULL UP > mode 2 PULL UP > the TERRAIN-TERRAIN preface > the TAD/obstacle warning family) → mode 2 TERRAIN → TAD/obstacle caution → TOO LOW TERRAIN (mode 4/TCF) → TOO LOW GEAR/FLAPS → SINK RATE → DON'T SINK → GLIDESLOPE lowest → RAAS (ground only). The four queue rules. Per AMM 34-48-00:

- only one message at a time is output, - higher priority messages interrupt lower priority messages except if interleaving is allowed (Ref. table above), - a triggered message is completed even when warning condition ceases, - a 0.75 second pause exists between warning messages.

The external pecking order was established in the radar article: stall > reactive windshear > PWS > GPWS/TAWS > TCAS. RAAS (fitted) is the EGPWC's ground side-job, with three station-calls. Per FCOM DSC-34-20-40-30:

The RAAS uses the EGPWS to trigger aural alerts in the three following cases:

"Approaching three four left" (approaching a runway), "On runway three four left" (aligned on a runway), and the last line of defence. Per FCOM DSC-34-20-40-30:

When the aircraft speed exceeds 40 kt, while the aircraft is not on a runway

(triggers "On taxiway! On taxiway!" — a guard against a taxiway take-off).

9. Handling and discipline — hear it and pull; talk about inhibition only inside 15 NM

A TAWS warning (the PULL UP family) is a memory item, unconditional full climb: AP OFF + full backstick and hold + TOGA + check the speed brakes retracted + wings level. Per FCOM PRO-ABN-SURV:

Pull up to full backstick and maintain in that position.

Aircraft obtains the best climb performance when the wings are as level as possible. The flight crew can adjust bank while climbing, provided that turning is the safest action.

DO NOT CHANGE CONFIGURATION (SLATS/FLAPS, GEAR) UNTIL CLEAR OF OBSTACLE.

The confidence to go to full backstick comes from fly-by-wire: in normal law, pulling to the stop takes all available performance and the angle-of-attack protection guards against the stall for you (the essential difference from "pull to the stick-shaker" on conventional aircraft). If a PULL UP is followed by «AVOID TERRAIN/AVOID OBSTACLE», the flight crew must adjust bank as necessary. A TAWS caution splits by light: night or IMC — the same full climb as a warning; daylight VMC with terrain/obstacle in sight throughout —

Adjust pitch, bank and thrust to silence the alert.

The single-caution thresholds are worth memorising: SINK RATE / GLIDESLOPE — below 1000 ft AAL IMC (500 ft VMC) consider a go-around, above it adjust flight path; TOO LOW GEAR/FLAPS — go around (PERFORM, not CONSIDER); DON'T SINK — adjust flight path to rebuild the climb. Finally, three LIM disciplines. Per FCOM LIM-SURV:

Aircraft navigation must not be based on the use of the terrain display.

The predictive GPWS functions should be inhibited (TERR pushbutton to OFF, on the GPWS panel) when the aircraft position is less than 15 NM from the airfield :

The decision to inhibit the predictive GPWS functions must not be based on flight crew judgement only.

Turning off the prophet is an operator-level decision (only at airports named by the manual/notice), not "the captain finds this alert annoying." Outside 15 NM, everything stays on.

Key numbers

Self-test

[!note]- Q1. Level flight toward a cliff, V/S = 0 — which basic mode calls, and what does it measure? Mode 2 — it measures the rate of change of the RA reading, not the aircraft's V/S.

[!note]- Q2. Mode 5's source receiver? On the ground with it failed, the panel light and ECAM? Why "silent knock-on"? ILS 1's G/S deviation. On the ground, ILS 1 failure inhibits only mode 5, no FAULT light, no NAV GPWS FAULT — so the loss of glideslope protection is silent.

[!note]- Q3. TAD's 60/30 s envelope at GS 480 kt vs 250 kt, and in a turn? It reaches farther the faster you go (time-based); the envelope bends to follow the flight path in a turn.

[!note]- Q4. A 10 hPa wrong QNH lands you at the wrong altitude — which function stops you, and what does it use as "true altitude"? Geometric altitude, which is independent of the barometric setting; TAD compares terrain against it.

[!note]- Q5. TERR STBY appears — what is lost, what remains? The most common trigger scenario? Lost: the enhanced TCF/TAD (and the ND terrain image). Remains: the five basic modes. Most common: insufficient position accuracy under GPS interference.

[!note]- Q6. Why is GPWS in the ADR 1 FAULT INOP list? And IR 1? The EGPWC's inertial/air data connect only to ADIRU 1 (no channel 2/3 backup), so ADR 1 or IR 1 loss takes the GPWS.

Key takeaways

References

• FCOM DSC-34-20-40-10 to -40 — GPWS purpose, five basic modes, TAD/TCF/RFCF/RAAS, position degradation, panel, ILS 1 silent knock-on, TERR ON ND, Peaks, memos.
• FCOM PRO-ABN-SURV — TAWS caution/warning memory items.
• FCOM LIM-SURV — terrain display not for navigation, 15 NM inhibition, not on crew judgement alone.
• AMM 34-48-00 — geometric altitude mechanism, position chain, FOM self-inhibit, ADIRU 1 dedicated line, envelope modulation, audio priority, RAAS three calls.

Independent study material, not an Airbus publication and not endorsed by the manufacturer. Always defer to the current operator FCOM, FCTM, and QRH for operational use.