Aoa 737ngx Groundwork Gpws Handout

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6

Ground Proximity Warning System

The material covered in this document is based off information obtained from the original manufacturers’ Pilot and Maintenance manuals. It is to be used for simulation purposes only.

Copyright © 2011 by Angle of Attack Productions, LLC All rights reserved Page 6-1

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Table of Contents GPWS/EGPWS Overview G’nd Proximity Warning Computer GPWS Operating Modes Overview GPWS Operating Modes Mode 1 – Excessive Descent Rate Mode 2 – Excessive Terrain Closure Rate Mode 3 – Excessive Alt’ Loss after Takeoff or GA Mode 4 – Unsafe Terrain Clearance Mode 5 – Excessive Deviation below ILS Glideslope Mode 6 – Advisory Callouts Mode 7 – Reactive Windshear

EGPWS Functions Overview Envelope Modulation Terrain Clearance Floor Runway Field Clearance Floor Look Ahead Terrain Alerting ND Terrain Display Controls and Indications

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Table of Illustrations 4 5 9 10 10 11 14 15 19 21 23 26 27 28 30 31 32 37

Figure 6-1. GPWC Diagram Figure 6-2. Mode 1 Alert Envelopes Figure 6-3. Mode 1 Diagram Figure 6-4. Mode 2A Alert Envelopes Figure 6-5. Mode 2A Diagram Figure 6-6. Mode 2B Alert Envelope Figure 6-7. Mode 2B Diagram Figure 6-8. Mode 3 Alert Envelope Figure 6-9. Mode 3 Alert Envelope Figure 6-10. Mode 4A Alert Envelopes Figure 6-11. Mode 4A Diagram Figure 6-12. Mode 4B Alert Envelopes Figure 6-13. Mode 4B Diagram Figure 6-14. Mode 4C Alert Envelope Figure 6-15. Mode 4C Diagram Figure 6-16. Mode 5 Alert Envelopes Figure 6-17. Mode 5 Diagram Figure 6-18. Mode 6 Callouts Diagram Figure 6-19. Mode 7 Windshear Alert Diagram Figure 6-20. TCF Alert Envelopes Figure 6-21. Runway Field Clearnce Floor Diagram Figure 6-22. Look Ahead Terrain Alerting Diagram

Ground Proximity Warning System

8 10 10 12 12 13 13 14 14 16 16 17 17 18 18 20 20 22 24 29 30 31

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Table of Illustrations (Cont.) Figure 6-23. Low Altitude Terrain Colours Figure 6-24. High Altitude Terrain Colours Figure 6-25. GPWS Control Panel

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Ground Proximity Warning System

Notes

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GPWS/EGPWS Overview GPWS stands for Ground Proximity Warning System. EGPWS stands for Enhanced Ground Proximity Warning System. Both systems are designed to help prevent accidents caused by CFIT – Controlled Flight Into Terrain. This is defined by a pilot flying unknowingly into water or obstacles. Following a string of deadly CFIT accidents in the 1960s and a subsequent enquiry into what could be done about it, the original GPWS system was developed and subsequently made mandatory for part 121 aircraft by the US FAA in 1974.

database to overcome the limitations of the earlier GPWS systems. This allows the system to look ahead of the aircraft and anticipate collision risks well in advance of projected impact. EGPWS has been instrumental in increasing air safety in the past decade. The database also allows for terrain data to be displayed on the Navigation Display, greatly increasing situational awareness. The 737NG is equipped with Enhanced GPWS.

The original GPWS systems had a critical weakness however. They used the radio altimeter as their core reference, which only monitors terrain directly below the aircraft. Very steep rising terrain could therefore trigger warnings far too late for the pilots to react in time. A more advanced system – Enhanced GPWS (EGPWS) – was developed during the 1990s by Honeywell. Enhanced GPWS was introduced in 1996 and uses GPS derived positioning information married to a digital terrain Page 6-4

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G’nd Proximity Warning Computer The heart of GPWS on the 737NG is the Ground Proximity Warning Computer, or GPWC.

Notes

The GPWC is located on the E1-1 shelf in the Electronic Equipment compartment (top left on the forwardmost shelf as you look forward). (Figure 6-1) The GPWC receives inputs from many systems and components on the aircraft, including the following systems. Air Data Inertial Reference System (ADIRS): ●● Air data ○○ Computed airspeed ○○ True airspeed ○○ Uncorrected altitude ○○ Barometric altitude ○○ Barometric altitude rate ●● Inertial data ○○ Inertial altitude ○○ Inertial vertical speed (IVS) ○○ Pitch attitude ○○ Roll attitude ○○ Body longitudinal acceleration Page 6-5

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GPWC (Cont.) ●● Inertial data (Cont.) ○○ Body normal acceleration ○○ Inertial vertical acceleration ○○ Pitch rate ○○ Ground speed ○○ True track ○○ True heading ○○ Inertial reference mode ●● If FMC input is invalid, ADIRS also supplies: ○○ Latitude ○○ Longitude ○○ Magnetic track

Multi-Mode Receivers (MMRs): ●● Glideslope deviation ●● Localizer deviation

Flight Management Computer System (FMCS): ●● Latitude ●● Longitude ●● Magnetic track ●● If FMC input is invalid, the ADIRS supplies this data

Mode Control Panel (MCP): ●● Selected approach course.

Stall Management Yaw Damper computers (SYMDs): ●● Indicated AOA ●● Corrected AOA ●● Stick shaker AOA ●● Flap position ●● Minimum operating speed. (VMIN)

Radio Altimeters ●● Radio Altitude Page 6-6

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Display Electronic Units (DEUs): ●● Selected display range, ●● TERR selection, ●● DATA selection, ●● Radio minimums, ●● Baro minimums.

Weather Radar (WXR): ●● Aural prioritization logic – the GPWC prioritizes the GPWS, TCAS and PWS systems to prevent conflicts.

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GPWC (Cont.) Ground Proximity Warning Module (GPWM): ●● Crew-selected FLAP/GEAR/TERRAIN inhibits, initiation of system test. ●● Located on the First Officer’s forward panel.

The GPWC uses these inputs to generate alerts and terrain data on the Navigation Display. The GPWC also outputs data to several aircraft systems, including: ●● System status and caution and warning data for display on the Display Electronic Units (DEUs). ●● Terrain display data for the DEUs for display on the Display Units via the Terrain/Weather Relays. ●● Flight Data Acquisition Unit passes system status and caution and warning data to the Flight Data Recorder (FDR).

The model of GPWC installed on the 737NG is also available on other aircraft types, so a number of ‘Program Pins’ are used to define the operational environment of the system. During installation of the unit, Program Pins are positioned to specify several parameters, including aircraft make and model, electrical interfaces and feature selections. Airlines may adjust GPWS features such as the range of Mode 6 approach callouts to suit their operation and SOPs. Many more customizations are possible. These selections are made on the ground by maintenance via the Program Pins on the GPWC. They are not pilot alterable.

Additionally to performing calculations for the generation of GPWS alerts, the GPWC incorporates terrain and airport databases used by the GPWS Enhanced Functions. GPWS is a very versatile system available on many types of aircraft. Page 6-7

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L ADIRU R ADIRU

GPWS

EXTERNAL FAULT

ENHANCED

COMPUTER OK COMPUTER FAIL PUSH TO EJECT

RA 1 RA 2

DEU 1

PRESS TO SELF TEST

HEADPHONES

OK PROG

CARD CHNG

MMR 1

XFER COMP

MMR 2

XFER FAIL

TERR/WXR RELAYS DEU 2

FMC DFCS MCP

SMYD 1 FDAU

SMYD 2

WXR

GPWC

Figure 6-1. GPWC Diagram Page 6-8

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GPWS Operating Modes Overview The Ground Proximity Warning System on the 737NG monitors and provides alerts for seven different sets of conditions. There are also several enhanced functions, introduced in the 1990s with EGPWS.

in atmospheric pressure to indicate altitude. Barometric altitude is therefore a distance above a pre-determined datum, usually sea level. This altitude is indicated on the Primary Flight Display altitude tape.

These are the seven ‘classic’ GPWS operating modes:

Radio altitude is determined by transmitting radio waves downwards beneath the aircraft and timing how long it takes them to be reflected back again. Because the speed of the waves is a known quantity, the distance can then be calculated to determine radio altitude.

●● Mode 1 – Excessive Descent Rate, page 10 ●● Mode 2 – Excessive Terrain Closure Rate, page 11 ●● Mode 3 – Excessive Alt’ Loss after Takeoff or GA, page 14 ●● Mode 4 – Unsafe Terrain Clearance, page 15 ●● Mode 5 – Excessive Deviation below ILS, page 19 ●● Mode 6 – Advisory Callouts, page 21 ●● Mode 7 – Reactive Windshear, page 23

Modes 2, 4 and 6 have various sub-modes. These are discussed in detail later. To understand GPWS properly, you need to know the difference between radio and barometic altitude. Both terms will be used when outlining the various GPWS modes.

Radio altitude is therefore the altitude above the terrain directly beneath the aircraft. It is important to understand these differences, as the GPWC uses both barometric and radio altitude as inputs. The GPWC also uses inertial altitude as an input for Modes 2 and 3. Inertial altitude is an ADIRU derived altitude used by several aircraft systems, including the Flight Control Computers (FCCs), Flight Management Computers (FMCs) and the GPWC.

Atmospheric pressure drops with an increase in altitude. The aircraft’s barometric altimeters use those changes Page 6-9

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GPWS Operating Modes Mode 1 – Excessive Descent Rate

3000

Mode 1 provides alerts for excessive descent rates with respect to altitude above ground level. The ‘descent rate’ input for Mode 1 is inertial vertical speed (IVS), an ADIRU derived quantity not prone to the lag of barometric vertical speed.

5007 FPM

7125 FPM

2450 2000 RADIO ALTITUDE (FEET)

SINK RATE

ALERT AREA WARNING AREA

1000

WHOOP WHOOP PULL UP

1710 FPM

10 1000 1500 2000

3000

4000

5000

6000

7000

DESCENT RATE (FEET PER MINUTE)

If inertial vertical speed is not available, the GPWC uses an internally calculated altitude rate. If both the IVS and the internally calculated rates are invalid, the barometric altitude rate from the ADIRS is used. When barometric altitude rate is used, the base of the Mode 1 envelopes changes from 10 feet to 30 feet. The rate of descent at which an alert is triggered is dependent on the current radio altitude. The greater the radio altitude, the greater the rate of descent needed to trigger a Mode 1 alert. (Figure 6-2)

Figure 6-2. Mode 1 Alert Envelopes

"SINKRATE SINKRATE" "PULL UP"

"SINKRATE"

Mode 1 has two alert envelopes – a caution envelope and a warning envelope. Envelope dimensions are defined by radio altitude and descent rate. (Figure 6-3)

"PULLUP"

Figure 6-3. Mode 1 Diagram

Should an excessive rate of descent develop for the Page 6-10

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GPWS Operating Modes (Cont.) aircraft’s current radio altitude, a SINK RATE aural caution message will be triggered. This occurs in the caution envelope.

Mode 2 is based on the rate of change of radio altitude. A positive closure rate means that the aircraft is rapidly approaching the ground, as sensed by the radio altimeter.

If the rate of descent continues or worsens, a continuous PULL UP aural warning message will be triggered once the aircraft enters the warning envelope.

Mode 2 exists in two forms: 2A and 2B.

When any Mode 1 alert is triggered, PULL UP annunciates red on the Primary Flight Displays as a visual alert. The PULL UP visual alert is also relayed through the Headup Guidance System (HGS) if installed. Mode 2 – Excessive Terrain Closure Rate

Mode 2 provides alerts for rapidly rising terrain with respect to the aircraft. This detection is based on the current radio altitude and the terrain closure rate. ‘Terrain closure rate’ in this case is a rate of change of radio altitude. This differs from ‘descent rate’ monitored by Mode 1. Mode 1 is based on the rate of change of barometric altitude.

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These two sub-modes are selected as a function of flap position. ●● Mode 2A when Flaps < 30 ●● Mode 2B when Flaps ≥ 30

Mode 2A provides alerts for excessive terrain closure rate during climbout, cruise, and initial approach with flaps 25 or less and glideslope deviation more than 2 dots. Mode 2A has two alert envelopes – a caution envelope and a warning envelope. Envelope dimensions are defined by radio altitude and terrain closure rate. The greater the radio altitude, the greater the terrain closure rate needed to trigger a Mode 2A alert. (Figure 6-4) The size of the Mode 2A alert envelopes is dependent on airspeed. At higher airspeeds the time to impact will be reduced, so the envelopes expand to provide increased alert times. For airspeed less than 220 knots, the caution

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GPWS Operating Modes (Cont.) envelope tops out at 1650ft radio altitude. For airspeed greater than 220 knots (220-310kts), the envelopes expand to a maximum of 2450ft. If the aircraft penetrates the Mode 2A caution envelope, a TERRAIN, TERRAIN aural caution message is generated. If the condition is not corrected and the aircraft penetrates the Mode 2A warning envelope, a PULL UP aural warning message sounds, and is repeated continuously until the aircraft exits the warning envelope. (Figure 6-5)

2450 FEET

2500 2000 RADIO ALTITUDE (FEET) 1500

TOP LIMIT GOES TO CAUTION AREA 2450 FT FOR AIRSPEED BETWEEN 220 AND 310 KTS 1650 FEET TERRAIN TERRAIN WARNING AREA

1000 WHOOP WHOOP PULL UP

500

Once the aircraft has exited the Mode 2A envelopes, PULL UP continues to show on the PFD until barometric altitude has increased by 300ft or after 45 seconds has elapsed. A TERRAIN aural caution message will sound during this period if radio altitude continues to decrease.

30 FEET 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 RADIO ALTITUDE CLOSURE RATE (FPM)

Figure 6-4. Mode 2A Alert Envelopes

Mode 2B provides alerts for excessive terrain closure rate when the flaps are in the landing configuration. (30 or 40). It is also active for the first 60 seconds after takeoff. The Mode 2B envelope is desensitized to permit normal landing approach maneuvers close to terrain without triggering unwanted alerts. It is inevitable during approach and landing that the aircraft will approach terrain. Page 6-12

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Figure 6-5. Mode 2A Diagram www.flyaoamedia.com

GPWS Operating Modes (Cont.) In some cases, the runway threshold may be in close proximity to rapidly rising terrain, resulting in radio altitude spikes on approach that could potentially trigger nuisance alerts.

1000 RADIO ALTITUDE (FEET) 500

TERRAIN TERRAIN OR WHOOP WHOOP, PULL UP

2000

3000

4000

5000

6000

RADIO ALTITUDE CLOSURE RATE (FPM)

Figure 6-6. Mode 2B Alert Envelope

“TERRAIN TERRAIN”

The upper limit of the envelope is fixed at 789ft radio altitude, however the lower limit varies between 600ft and 30ft as a function of inertial vertical speed and flap configuration.

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LOWER LIMIT DEPENDS ON IVS CLOSURE RATE AND FLAP CONFIG.

1000

Mode 2B has a single envelope. Envelope dimensions are defined by radio altitude and terrain closure rate. The greater the radio altitude, the greater the terrain closure rate needed to trigger a Mode 2B alert.

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600 FEET

30 FEET

The Mode 2A alerting envelope could trigger nuisance alerts during this phase, so Mode 2B is automatically activated upon selection of flaps 30 or 40, or once established on an ILS approach with localizer and glideslope deviation less than 2 dots. This reduces the potential for nuisance alerts during an approach. (Figure 6-6) Mode 2B is also active for the first 60 seconds after takeoff to prevent nuisance alerts during this period.

799 FEET

Ground Proximity Warning System

“PULL UP”

Figure 6-7. Mode 2B Diagram www.flyaoamedia.com

GPWS Operating Modes (Cont.) If the aircraft penetrates the Mode 2B envelope with either the gear or flaps not in the landing configuration a TERRAIN, TERRAIN aural caution message is generated. If this condition lasts for more than 1.6 seconds, a PULL UP aural warning message will sound.

RADIO ALTITUDE (FEET) 1500

1500 FEET BASELINE

1000

If the aircraft enters the envelope with the landing gear down, there will be no aural PULL UP messages, but a TERRAIN message will be repeated until the envelope is exited.

DONT SINK

30 50

100

150

200

250

300

LOSS OF INERTIAL ALTITUDE (FEET)

When any Mode 2 alert is triggered, PULL UP annunciates red on the Primary Flight Displays as a visual alert. The PULL UP visual alert is also relayed through the Head-up Guidance System (HGS) if installed.

Figure 6-8. Mode 3 Alert Envelope

Mode 3 – Excessive Alt’ Loss after Takeoff or GA

Mode 3 provides alerts for excessive altitude loss after takeoff or go around below 245ft. There is a single Mode 3 envelope. Envelope dimensions are defined by radio altitude and altitude loss. The altitude loss permitted is a function of radio altitude. The greater the radio altitude, the greater the altitude loss needed to trigger a Mode 3 alert. Page 6-14

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Ground Proximity Warning System

Figure 6-9. Mode 3 Alert Envelope www.flyaoamedia.com

GPWS Operating Modes (Cont.) Notes If the aircraft penetrates the Mode 3 envelope, a DON’T SINK aural caution message is generated. The DON’T SINK aural message is enunciated twice for each 20% degradation in altitude. The message will cease once a positive rate of climb is established. (Figure 6-8) When a Mode 3 alert is triggered, PULL UP annunciates red on the Primary Flight Displays as a visual alert. The PULL UP visual alert is also relayed through the Head-up Guidance System (HGS) if installed. Mode 4 – Unsafe Terrain Clearance

Mode 4 provides alerts for insufficient terrain clearance with respect to phase of flight, aircraft configuration and speed. Modes 1 and 2 are based around rates of descent and terrain closure, while mode 4 prevents inadvertent flight into terrain where the rate of descent or terrain closure is too gradual to trigger a Mode 1 or 2 alert. Mode 4 exists in three forms: 4A, 4B and 4C. These three modes are selected as a function of gear and flap position and phase of flight. Page 6-15

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GPWS Operating Modes (Cont.) Mode 4A is active during cruise and approach with the gear up and flaps not in the landing configuration (30 or 40). It provides alerting for inadvertent flight into terrain where the aircraft is not descending excessively and terrain closure is not excessive. (Figure 6-11) Mode 4A has two alerting envelopes. Envelope dimensions are defined by radio altitude and computed airspeed. Below 500ft radio altitude and less than 190 knots airspeed, the Mode 4A aural alert is TOO LOW GEAR.

1000 RADIO ALTITUDE (FEET) 500 TOO LOW - GEAR 30 100

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300 COMPUTED AIRSPEED (KNOTS)

“TOO LOW TERRAIN”

For either Mode 4A aural alert, further alert messages are generated for each 20% degradation in altitude. Page 6-16

200

Figure 6-10. Mode 4A Alert Envelopes

Below 190 knots airspeed, the envelope upper limit is 500ft radio altitude. Above 190 knots airspeed, the envelope is shaped by increasing terrain clearance, and generates a different aural message. As airspeed increases, the envelope expands to incorporate greater terrain clearances. The envelope expands to 1000ft at 250 knots. This airspeed compensation ensures a more timely alert, as the time to impact reduces at higher airspeeds. Above 190 knots airspeed the Mode 4A aural alert is TOO LOW TERRAIN.

TOO LOW - TERRAIN

Ground Proximity Warning System

“TOO LOW GEAR”

Figure 6-11. Mode 4A Diagram

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GPWS Operating Modes (Cont.) Mode 4B is active during cruise and approach with the gear down and flaps not in the landing configuration (30 or 40). Like Mode 4A, Mode 4B provides alerting for inadvertent flight into terrain where the aircraft is not descending excessively and terrain closure is not excessive. (Figure 6-12)

1000 RADIO ALTITUDE (FEET) 500 245

TOO LOW - FLAP

30

100

Mode 4B has two alerting envelopes. Envelope dimensions are defined by radio altitude and computed airspeed. Below 245ft radio altitude and less than 159 knots airspeed, the Mode 4B aural alert is TOO LOW FLAPS. Below 159 knots airspeed, the envelope upper limit is 245ft radio altitude. Above 159 knots airspeed, the envelope is shaped by increasing terrain clearance (radio altitude), and generates a different aural message.

“TOO LOW TERRAIN”

For either Mode 4B aural alert, further alert messages are generated for each 20% degradation in altitude.

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300 COMPUTED AIRSPEED (KNOTS)

200

Figure 6-12. Mode 4B Alert Envelopes

As airspeed increases, the envelope expands to incorporate greater terrain clearances. The envelope expands to 1000ft at 250 knots. Above 159 knots airspeed the Mode 4B aural alert is TOO LOW TERRAIN.

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TOO LOW - TERRAIN

Ground Proximity Warning System

“TOO LOW FLAPS”

159

FLAPS

Figure 6-13. Mode 4B Diagram

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GPWS Operating Modes (Cont.) Mode 4C is active during the takeoff phase or during a low altitude go around with the gear or flaps not in the landing configuration. The Mode 4C envelope is constructed dynamically as the aircraft climbs, using radio altitude and computed airspeed to define the top limit of the envelope. During the takeoff roll the minimum terrain clearance is zero feet. As the aircraft climbs, the minimum clearance is increased to 75% of the aircraft’s Radio Altitude, based on an average taken over the previous 15 seconds. This value is not allowed to decrease, and forms the top of the Mode 4C alert envelope.

1000 TOP LIMITS OF ALTITUDE THE FILTER CAN STORE THE FILTER ACTIVATES AT APPROX 150 FEET

500

MODE 4C LIMIT TOO LOW TERRAIN

30 190 250 COMPUTED AIRSPEED (KNOTS)

Figure 6-14. Mode 4C Alert Envelope

The envelope is limited to 500ft radio altitude for airspeeds less than 190 knots. Above 190 knots, the envelope expands to a limit of 1000ft at 250 knots. If the aircraft’s radio altitude enters the envelope, a TOO LOW TERRAIN alert is generated. The alerts cease once the envelope is exited. (Figure 6-14) When any Mode 4 alert is triggered, PULL UP annunciates on HGS (if installed) and shows in red on the Primary Flight Displays as a visual alert. Page 6-18

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Ground Proximity Warning System

Figure 6-15. Mode 4C Diagram www.flyaoamedia.com

GPWS Operating Modes (Cont.) Mode 5 – Excessive Deviation below ILS Glideslope

Notes

Mode 5 provides alerts for excessive deviation below the ILS glideslope. It uses radio altitude and glideslope deviation as its references. There are two levels of Mode 5 alerting – soft alerts and hard alerts. A soft alert occurs when the aircraft is below 1000ft radio altitude with an ILS deviation 1.3 dots or greater below glideslope. In this case, a reduced volume GLIDESLOPE audio message is triggered. A hard alert occurs when the aircraft is below 300ft radio altitude with an ILS deviation 2 dots or greater below glideslope. In this case, a full volume GLIDESLOPE audio message is triggered. (Figure 6-16) As deviation below glideslope increases, additional GLIDESLOPE alerts are generated at a progressively faster rate. Mode 5 alerts are enabled only when: ●● Aircraft is below 1000ft radio altitude ●● Localizer signal is captured (deviation within 2 dots) Page 6-19

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GPWS Operating Modes (Cont.) ●● Glideslope signal is valid ●● Landing gear is down ●● A front course approach is selected.

1000 GLIDE SLOPE LOW VOLUME AREA: 6DB BELOW NORMAL INTERVAL BETWEEN MESSAGES: 5.15 SEC

MESSAGE INTERVALS RADIO ALTITUDE (FEET)

The Ground Proximity Warning Computer identifies a front course approach by comparing the aircraft’s magnetic track to the ILS approach course. If the difference is less than 90 degrees, it assumes that a front course approach is being conducted, and Mode 5 alerts will be available.

300

1.55 SEC

1.0 SEC

0.59 SEC GLIDE SLOPE NORMAL VOLUME AREA

150 30

If the difference is greater than 90 degrees, it assumes that a back course approach is being flown, and Mode 5 alerts will be inhibited to prevent nuisance alerts. The upper boundary of the Mode 5 envelope is affected by aircraft descent rate. For descent rates greater than 500 feet per minute, the upper boundary is set to the normal 1000ft radio altitude. A vertical speed greater than 500 feet per minute is to be expected on a normal ILS approach, so this is the normal condition.

1.3 DOTS 2.0 DOTS 2.7 DOTS 3.4 DOTS DEVIATION BELOW GLIDESLOPE SIGNAL

Figure 6-16. Mode 5 Alert Envelopes

SOFT ALERT HARD ALERT

Figure 6-17. Mode 5 Diagram

For descent rates less than 500 feet per minute, the upper boundary is reduced to a minimum of 500ft radio altitude. Additionally, both hard and soft modes are progressively desensitized below 150 feet radio altitude to allow for Page 6-20

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GPWS Operating Modes (Cont.) normal ILS glideslope variations near to the ground. This reduces the possibility of nuisance alerts. A Mode 5 alert causes the below glideslope switches to illuminate on the left and right forward panels. Pushing either switch inhibits Mode 5 glideslope alerts when below 1000ft radio altitude. Mode 6 – Advisory Callouts

Mode 6 provides advisory callouts based on barometric altitude, radio altitude and bank angle. This modes callouts include the following: ●● Radio and barometric altitude callouts ●● Minimums callout ●● Approaching minimums callout ●● Bank angle callouts

The set of callouts used is selectable by program pins on the Ground Proximity Warning Computer. This allows airlines to customize Mode 6 callouts to their operational environment. (Figure 6-18) Mode 6 provides altitude callouts during approach based on radio and barometric altitudes. Page 6-21

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The callouts at 1000ft and 500ft are based on barometric altitude above the landing field elevation. The rest are based on radio altitude. A ‘smart’ 500ft callout is also available, which sounds only during a non-precision approach. When the ‘smart’ 500ft callout is selected, 500 is only called during a non-precision approach – it is removed from precision approaches. The Ground Proximity Warning Computer determines a nonprecision approach when glideslope deviation is greater than two dots, or if a back course approach is being flown. Mode 6 also provides callouts for Decision Height or Minimum Descent Altitude based on the altitude set by the captain’s Minimums Selector on the EFIS control panel. These callouts may be based on radio or barometric altitude depending on the position of the Minimums Reference Selector. There are four callouts available, up to two of which will sound during an approach: PLUS HUNDRED sounds 100 feet above the DH or MDA. APPROACHING MINUMUMS sounds 80 feet above the DH or MDA. MINIMUMS or

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2500 FEET

TWENTY FIVE HUNDRED...OR...RADIO ALTIMETER

2000 FEET

ONE THOUSAND

DECISION HEIGHT PLUS 100 FEET DECISION HEIGHT PLUS 80 FEET

PLUS HUNDRED APPROACHING MINIMUMS

DECISION HEIGHT

MINIMUMS, MINIMUMS...OR..MINIMUMS

500 FEET

FIVE HUNDRED

400 FEET

FOUR HUNDRED

300 FEET

THREE HUNDRED TWO HUNDRED

200 FEET 100 FEET

ONE HUNDRED

50 FEET

FIFTY

40 FEET

FORTY

30 FEET

THIRTY

30 FEET

TWENTY

10 FEET

TEN

NOTE: All possible altitude callouts shown

Figure 6-18. Mode 6 Callouts Diagram Page 6-22

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GPWS Operating Modes (Cont.) MINIMUMS MINIMUMS or DECISION HEIGHT sounds when passing through the DH or MDA. The MINIMUMS callout has priority over other altitude callouts. For example, if the DH is set to 200ft and both TWO HUNDRED and MINIMUMS are valid callouts, then only MINIMUMS will be called out at 200ft. Mode 6 provides bank angle callouts to advise of an excessive roll angle. Between 30 and 130 feet radio altitude, a BANK ANGLE, BANK ANGLE callout is generated when the aircraft bank angle exceeds 10 degrees. Above 130 feet radio altitude, the callout occurs at 35 degrees, 40 degrees and 45 degrees of bank. The callout sounds once as the aircraft passes through each threshold. (In other words), a callout will sound as the aircraft passes through 35 degrees, but will not be given again until the bank angle passes through 40 degrees. A further callout will then be given when 45 degrees is exceeded. When any one of the thresholds is exceeded, the bank angle must reduce below 30 degrees for the system to be reset before additional bank angle callouts will be Page 6-23

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provided. If the roll rate is so great that the next threshold is reached before the previous threshold’s callout is complete, then no callout is given for the bypassed limit. The vertical extent of the bank angle alert envelope is limitless. Mode 7 – Reactive Windshear

Mode 7 is designed to provide alerts if the aircraft enters windshear. The term ‘windshear’ refers to a sudden and drastic change in wind direction or speed over a relatively short distance along the aircraft’s flight path. Most wind usually travels more or less horizontally, but under certain conditions can be provoked to travel in a vertical direction. The most dangerous phenomenon associated with windshear is microburst, as it poses a great threat to aircraft on approach or shortly after departure. A microburst is a localized, very concentrated column of sinking cool air,

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GPWS Operating Modes (Cont.) usually emitting from the base of a thunderstorm. As the air strikes the ground, it spreads horizontally. An aircraft flying through a microburst will first see an increase in airspeed as it enters the zone of increased headwinds. It will also experience an increased sink rate as the sinking air pushes it down. CAUTION WINDSHEAR

As the aircraft flies through the core of the microburst, tailwinds start to increase, with a corresponding decrease in airspeed. The combination of reduced airspeed and the downdraft from the microburst may force the aircraft into terrain if prompt correction is not applied. (Figure 6-19) Several airliners have been lost to this phenomenon. Mode 7 is designed to provide alerts if the aircraft enters windshear.

WINDSHEAR! WINDSHEAR! WINDSHEAR!

Figure 6-19. Mode 7 Windshear Alert Diagram

Two alerting envelopes provide either a Windshear Caution or Windshear Warning alert. Mode 7 windshear alerting is based on rapid rates of change of headwind, tailwind, updraft and downdraft. Note that this is an entirely different system to Predictive Windshear. Mode 7 is purely reactive, and provides Page 6-24

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GPWS Operating Modes (Cont.) alerts based on currently occurring changes, rather than conditions detected ahead of the aircraft. An increasing headwind and/or increasing updraft may result in the aircraft entering the windshear caution envelope. This would trigger a Windshear Caution alert, which generates a CAUTION, WINDSHEAR aural message. This would be indicative of the aircraft entering a microburst. A decreasing headwind, which could also equate to an increasing tailwind, coupled with an increasing downdraft is a far more dangerous situation. This region is covered by the warning envelope, penetration of which will trigger a Windshear Warning, and the aural alert WINDSHEAR, WINDSHEAR, WINDSHEAR. When a Mode 7 windshear alert is triggered, WINDSHEAR annunciates red on the PFDs and conveyed on HGS (if installed), both as a visual alerts.

Other inputs to the warning envelope include: ●● Available climb performance ●● Flight path angle ●● Airspeed significantly different to normal approach speed ●● Unusual fluctuations in Static Air Temperature

Mode 7 windshear alerting is active under the following flight conditions: ●● During takeoff; from rotation until 1500ft AGL ●● During approach from 1500ft down to 10ft AGL ●● During a missed approach until 1500ft AGL

Remember that GPWS Mode 7 is an entirely separate system to Predictive Windshear. PWS uses the weather radar to look ahead of the aircraft at wind trends that signify the existence of windshear. Mode 7 is simply reactive to current windshear conditions.

The visual alert remains present for as long as the aircraft is exposed to conditions that place it in the warning envelope.

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EGPWS Functions Overview Additionally to the seven classic GPWS modes already discussed, EGPWS adds several enhanced functions.

Notes

These enhanced functions are made possible by an internal database in the GPWC, that consists of four general subsets: ●● A worldwide terrain database ●● A frequently updated obstacles database ●● A worldwide airport database containing information on runways 3500 feet or longer ●● An Envelope Modulation database referenced by the Envelope Modulation function covered later in the lesson

The aircraft’s present position is overlaid on these databases to determine proximity to terrain, obstacles and areas that require envelope modulation. This allows terrain alerts to be generated accordingly. The GPWC determines aircraft position using inputs primarily from the GPS 1 system. If GPS 1 position is not valid, GPS 2 position is used. When neither GPS 1 nor GPS 2 positions are valid, ADIRU position may be used for short periods.

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Envelope Modulation Early GPWS equipment was plagued by nuisance alerts, which could cause pilots to distrust the equipment even when actual hazardous conditions existed. Certain airports around the world have terrain features and approach procedures that have resulted in nuisance or missed alerts in the past. With the introduction of the EGPWS terrain and airport database, it is possible to identify these areas and adjust the normal alerting envelopes to compensate. This is Envelope Modulation. Envelope Modulation uses the database to both desensitize and expand alerting envelopes where required to provide alerting protection consistent with normal approaches.

parameters to verify that all the data required for Envelope Modulation is valid and within tolerances. Monitored inputs include: ●● Latitude ●● Longitude ●● Radio altitude ●● Localizer deviation ●● Magnetic track ●● Selected runway heading ●● Barometric corrected altitude

Once the snapshot requirements have been verified, one or more of the GPWS modes can be modified upon entry to the Envelope Modulation Area.

Data for airports that require envelope modulation are stored in the Envelope Modulation database. Each effected airport has a Snapshot Area and an Envelope Modulation area, both defined by latitude and longitude.

The Envelope Modulation function adjusts the Mode 1, 2 4 and 5 envelopes to prevent nuisance or missed alerts. Mode 1 – adjusted to allow greater descent rates. Mode 2 – adjusted to allow greater terrain closure rates. Mode 4 – adjusted to allow less minimum terrain clearance. Mode 5 – adjusted to allow glideslope warnings at higher radio altitudes, including with the landing gear up.

As the aircraft enters the Snapshot Area, the Ground Proximity Warning Computer looks at several aircraft

Envelope Modulation is automatic and requires no flight crew action.

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Terrain Clearance Floor The Terrain Clearance Floor function alerts the pilots of a descent below a defined Terrain Clearance Floor. The standard GPWS Modes 2 and 4 are desensitized when the aircraft is in landing configuration, and thus fail to trigger alerts when landings are attempted where there is no airport. Since EGPWS features a terrain and airport database which contains the exact positions of all allowable airport runways (3500ft or longer), it is possible to define an additional alert envelope at all areas where there are no runways. This is the Terrain Clearance Floor.

The Terrain Clearance Floor function has been progressively improved since the introduction of EGPWS in the mid-1990s. Construction of the Terrain Clearance Floor envelope in the immediate vicinity of the runway varies depending on the EGPWS version installed on the aircraft. When the aircraft descends into the Terrain Clearance Floor envelope, a TOO LOW TERRAIN voice alert is generated. The alert is repeated with further 20% decreases in radio altitude. Additionally to the aural alert, TERRAIN is annunciated on the Navigation Display in amber.

When the aircraft is 15nm or more from a runway, the floor is 700ft AGL. As the aircraft approaches an airport in the database, the Terrain Clearance Floor steps down closer to terrain. Between 12nm and 15nm from the runway the envelope steps down gradually to 400ft AGL. The floor remains at 400ft AGL from 12nm to 4nm, then steps down further. (Figure 6-20) The lower limit of the Terrain Clearance Floor within 4nm of the the runway is dependent on an envelope bias factor which varies as a function of aircraft position accuracy. Page 6-28

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245’ Improved TCF Envelope

700’ 400’ 4 NM 12 NM 15 NM

Figure 6-20. TCF Alert Envelopes

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Runway Field Clearance Floor The Runway Field Clearance Floor function augments the Terrain Clearance Floor by providing an additional alert envelope for runways that are significantly higher than the surrounding terrain.

300 ft RFCF Alert Envelope

The envelope is contained within a circle within 5.5nm of the runway threshold, and extends to a ceiling 300ft above field elevation. (Figure 6-21) Penetrating the Runway Field Clearance Floor envelope results in a TOO LOW TERRAIN voice alert. The alert is repeated with further 20% decreases in radio altitude. Additionally to the aural alert, TERRAIN is annunciated on the ND in amber.

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TCF Envelope

5.5 NM

Figure 6-21. Runway Field Clearnce Floor Diagram

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Look Ahead Terrain Alerting Look Ahead Terrain Alerting is accomplished based on aircraft position, flight path angle, track, and speed relative to the terrain database image ahead of the aircraft. Two alert ribbons are constructed ahead of the aircraft to provide advanced warning of conflict with terrain and obstacles. These ribbons project down, forward, then up from the aircraft. They have a width starting at ¼ nm, and extend out at 3 degrees laterally. The width extends if the aircraft is turning.

A terrain conflict penetrating the warning ribbon generates a TERRAIN, TERRAIN, PULL UP aural message. This alert is typically given 20 to 30 seconds from projected impact. Additionally to the aural alert, TERRAIN is annunciated on the ND in red. (Figure 6-22) Equivalent alerts are generated in the event that an obstacle in the EGPWS database should penetrate the caution or warning envelopes. The aural messages are CAUTION OBSTACLE, CAUTION OBSTACLE and OBSTACLE, OBSTACLE, PULL UP.

The look-down and look-up angles are a function of aircraft flight path angle. The look-down distance is a function of aircraft altitude with respect to the nearest or destination runway. The look-ahead distance is a function of aircraft speed and distance to the nearest runway. These measures prevent nuisance alerts when taking off or landing.

WARNING

A terrain conflict penetrating the caution ribbon generates a CAUTION TERRAIN, CAUTION TERRAIN aural message. This alert is typically given 40 to 60 seconds from projected impact. Additionally to the aural alert, TERRAIN is annunciated on the ND in amber.

CAUTION

Figure 6-22. Look Ahead Terrain Alerting Diagram Page 6-31

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ND Terrain Display Terrain data from the EGPWS terrain and obstacle database can be displayed on the Navigation Display. The Terrain Display is selected by pushing the TERR select switch on the EFIS control panel. The terrain display is available in the following Navigation Display modes: ●● Expanded MAP ●● Center MAP ●● Expanded VOR ●● Expanded APP

The Terrain Display is a graphical plan view of surrounding terrain designed to enhance vertical and horizontal situational awareness. The display design was subject to human factors studies which recommended a minimum of complexity to ensure easy interpretation.

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The older type of EGPWS terrain display did not display terrain greater than 2000ft below aircraft altitude. The terrain display would thus typically be blank during the en-route portion of a flight. Later versions of EGPWS incorporate the ‘Peaks Display’ type of terrain display as a customer option selectable via a Program Pin.

It is not possible to have both the Terrain Display and weather radar returns selected at the same time. Selecting the Terrain Display ON deselects the weather radar automatically.

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Terrain displayed on the ND is colour coded. Colour and density vary based on the relative altitude of the aircraft to the terrain.

The Peaks Display feature adds further terrain information even for terrain greater than 2000ft below aircraft altitude. This allows the depiction of a mountain range below the aircraft in the cruise for example, useful for situational awareness during an emergency descent. The Peaks Display feature also provides a digital readout of the highest and lowest terrain elevation currently displayed. Terrain is displayed on the ND in varying densities of green, amber and red. Each specific colour and density

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ND Terrain Display (Cont.) represents terrain below, at and above the aircraft’s altitude based on the position of the aircraft with respect to the terrain in the database. Since the introduction of the Peaks Display feature, there are now two sets of ‘rules’ that dictate the logic used by the system to draw terrain: ●● Aircraft at a low relative altitude to highest terrain on the Terrain Display ●● Aircraft at a high relative altitude to highest terrain on the Terrain Display

We will first discuss the coloured terrain display indications active when the aircraft is at a low relative altitude to terrain. These indications are active when the aircraft is 500ft (250ft with gear down) or less above the highest terrain in view on the display. (Figure 6-23) Low density green indicates terrain from 2000ft to 1000ft below aircraft altitude. High density green indicates terrain from 1000ft to 500ft below aircraft altitude. With the landing gear down, the envelope expands to indicate terrain from 1000ft to 250ft below aircraft altitude. Page 6-33

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Low density amber indicates terrain from 500ft below to 1000ft above aircraft altitude. With the landing gear down, the envelope shrinks to indicate terrain from 250ft below to 1000ft above aircraft altitude. High density amber indicates terrain from 1000ft to 2000ft above aircraft altitude. High density red indicates terrain greater than 2000ft above aircraft altitude. Solid amber indicates terrain that has triggered a look-ahead terrain caution condition. Solid red indicates terrain that has triggered a lookahead terrain warning condition. Magenta indicates that no terrain data is available for that area. Cyan indicates terrain identified as water. This is a program pin-selectable customer option, available only with the Peaks feature.

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High Density Red +2000' High Density Yellow Low Density Yellow High Density Green Low Density Green Black

+1000' -500’ -1000' -2000'

Figure 6-23. Low Altitude Terrain Colours

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ND Terrain Display (Cont.) Terrain greater than 2000ft below aircraft altitude is not indicated – these areas are black. (Also,) terrain within 400ft of the nearest airport runway elevation is not indicated. When the aircraft is at a high relative altitude to terrain the Terrain Peaks feature adds additional colour gradations to further define terrain below the aircraft. These indications are active when the aircraft is greater than 500ft (250ft with gear down) above the highest terrain in view on the display. (Figure 6-24) The shades of green displayed are based on the distribution of terrain elevations within the display range. This is a mathematical calculation based on terrain elevation and is independent of aircraft altitude. Each shade of green relates to a separate band of terrain elevations.

Low density green indicates the lower terrain band.



High density green indicates the middle terrain band.

Terrain below the lowest elevation band is not displayed. This is typically terrain less than 50% of aircraft altitude. This is variable! The Peaks Display feature also provides a digital readout of the highest and lowest terrain elevation currently displayed. The elevation values are expressed in hundreds of feet above sea level, the colour of these values matches the corresponding terrain bands on the display. The Terrain Display automatically pops up when a lookahead terrain alert occurs. The Terrain Display updates with a display sweep similar to the weather radar display. TERR is displayed in cyan on the left side of the display whenever the Terrain Display is enabled.

Solid green indicates the highest terrain band on the display - peaks. Page 6-35

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High Density High Density Low Density

High Density Low Density

Figure 6-24. High Altitude Terrain Colours

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Controls and Indications Thus far the following controls and indications have been discussed: ●● TERR select switch on each EFIS control panel ●● Below glideslope switches ●● PULL UP and WINDSHEAR indications on the PFD and HGS ●● EGPWS Terrain Display and TERRAIN annunciation

The remainder of the GPWS controls and indications are located on the right forward panel. The three capped INHIBIT switches allow flight crew to manually inhibit GPWS alerts. All three switches are capped to the NORM position. (Figure 6-25)

GROUND PROXIMITY GPWS

INOP

FLAP INHIBIT

GEAR INHIBIT

TERR INHIBIT

NORM

NORM

NORM

SYS TEST

The FLAP INHIBIT switch simulates a flaps landing position in the GPWC, inhibiting the Mode 4B TOO LOW FLAPS alert.

Figure 6-25. GPWS Control Panel

The GEAR INHIBIT switch simulates the landing gear in the extended position, inhibiting the Mode 4A TOO LOW GEAR alert. The TERR INHIBIT switch inhibits the look ahead CAUTION TERRAIN, CAUTION TERRAIN and TERRAIN, TERRAIN, Page 6-37

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Controls and Indications (Cont.) PULL UP alerts. It also inhibits the Terrain Display. The INOP light illuminates when the GPWC suffers a malfunction or power loss. The INOP light illuminates under four sets of conditions:

Notes

●● GPWC has suffered a malfunction or power loss ●● GPWC cannot calculate windshear conditions ●● Failure of a critical input to the GPWC ●● A GPWS self-test is active

The SYS TEST switch initiates a self-test of the system. Pushing the SYS TEST switch quickly gives a short confidence test, and pushing for 5 seconds gives a full vocabulary test. Several indications on the flight deck cycle on and off as the test completes: ●● Below glideslope switches ●● GPWS INOP light ●● PULL UP alert on the PFD and HGS ●● WINDSHEAR alert on the PFD and HGS ●● WINDSHEAR annunciation on ND ●● TERR FAIL annunciation on the ND ●● TERR TEST annunciation on the ND ●● EGPWS terrain display test pattern on the ND

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