Ground Proximity Warning System

1. Introduction & Definitions

2. GPWS Modes — Summary Table

3. Mode 1 — Excessive Barometric Descent Rate

• First boundary: Alert — "SINK RATE, SINK RATE" (repeated every 1.5 seconds)
• Second boundary: Warning — "WHOOP WHOOP PULL UP" until rate of descent corrected
• Visual: PULL UP light

4. Mode 2 — Excessive Terrain Closure Rate

• First boundary: Alert — "TERRAIN, TERRAIN" + repeated warning "WHOOP WHOOP PULL UP"
• In landing configuration: only repeating "TERRAIN" alert (no PULL UP)
• Terrain message repeated every 1.5 seconds
• Mach 0.35–0.45 with gear up: highest RA at which Mode 2 warning occurs increases to 2450 ft
• Higher envelope inhibited with FLAP OVRD switch

5. Mode 3 — Altitude Loss after Take-off or Go-around

• Alert: "DON'T SINK" (repeated every 1.5 seconds)
• Visual: PULL UP light
• Effective between 50–700 ft RA
• Activates when accumulated barometric loss ≈ 10% of existing barometric altitude
• Does NOT arm during descent until below 200 ft RA
• Arms when aircraft descends below 200 ft in landing configuration

6. Mode 4A — Unsafe Terrain Clearance (Gear Not Down)

• Below Mach 0.35: alert "TOO LOW GEAR"
• Above Mach 0.35: alert "TOO LOW TERRAIN" (upper boundary increases to 1000 ft RA)
• Repeated every 1.5 seconds
• Visual: PULL UP

7. Mode 4B — Unsafe Terrain Clearance (Flaps Not Landing)

• Below Mach 0.28: alert "TOO LOW FLAPS"
• Above Mach 0.28: alert "TOO LOW TERRAIN" (upper boundary increases to 1000 ft RA)
• "TOO LOW GEAR" takes priority over "TOO LOW FLAPS"
• Inhibited with FLAP OVRD switch

8. Mode 5 — Below Glide Slope Deviation Alert

• Soft area: Reduced volume "GLIDE SLOPE" (alert)
• Hard area (~2 dots below G/S): Full volume "GLIDE SLOPE GLIDE SLOPE"
• Voice repetition rate increases as deviation increases and RA decreases
• Armed when: valid G/S signal received + gear down + RA ≤ 1000 ft

9. Mode 6A — Below Selected Minimum Radio Altitude

• Alert if descent below the minimum decision altitude cursor in the radio altimeter
• Operates between 50–1000 ft RA
• Aural only: "MINIMUMS, MINIMUMS" — sounded once
• Re-arms when RA becomes greater than cursor-selected altitude

10. Mode 6B — Altitude Call-outs & Bank Angle Alert

• Call-outs at selected altitudes (e.g., 200 ft and 100 ft to decision height) — customer option
• "BANK ANGLE" alert for excessive roll angles (airline-defined limits, not structural limits)
• Bank angle limit reduces with proximity to ground (reduced wingtip clearance to prevent wing tip/engine damage during T/O and landing)

11. Mode 7 — Windshear Alerting

Multiple parameters monitored: ground speed, airspeed, barometric height, rate of descent, radio altitude. Warning generated when initial conditions of entering windshear are detected. Benefit: allows earlier initiation of the windshear go-around procedure, increasing probability of avoiding accident.

12. Action on Receiving GPWS Warnings

Discontinuing the climb is permitted ONLY when:

• Cause positively identified and warning has ceased
• Aircraft operated by day in clear visual conditions
• Immediately obvious the aircraft is not in danger

13. Enhanced GPWS (EGPWS) / TAWS

Classic GPWS limitation: No "look ahead" — Mode 2 warning on high terrain requires the aircraft to already be over rising ground. A sheer cliff would give no Mode 2 warning until the aircraft is over it.

EGPWS Terrain Threat Display

• Shows terrain ahead of flight path in colour-coded display (green → yellow → red)
• Alert timing: 40–60 seconds before predicted impact for caution (yellow); 20–30 seconds for warning (red + solid)
• Messages: "Caution Terrain" / "Terrain Ahead" / "Obstacle Ahead" (if obstacle data) → "Whoop Whoop Pull Up"
• Threatening terrain: yellow (caution) → solid red (warning)

The terrain display can be superimposed on the weather radar display, the ND, or on a dedicated PPI.

14. Terrain Clearance Floor, BITE, Inhibition

Terrain Clearance Floor

Alerts crew to possible premature descent for non-precision approaches regardless of configuration. Uses aircraft position relative to the runway in the EGPWS database (separate from FMS, not subject to regular updates). Future integration with RAAS (Runway Awareness and Advisory System) planned.

Integrity Testing (BITE)

• BITE allows full testing of all functions and visual/aural warnings before flight
• Pre-flight BITE is inhibited in flight
• System continuously monitored in flight; failures automatically indicated
• Short confidence check possible in flight but not a full BITE check
• Test initiated by pressing the test switch

Inhibition of EGPWS Modes

Permitted inhibition cases:

• Mode 5 (G/S) — when G/S signal present but aircraft deliberately not using it (e.g., localizer-only approach, different runway)
• When gear or flap position inputs are known to be non-standard