Ground Proximity Warning System (GPWS)

What is GPWS?

A Ground Proximity Warning System (GPWS) is a type of equipment carried by aircraft to warn pilots if they are at a dangerously low altitude and in danger of crashing.

Purpose of GPWS

The main purpose of these systems is to prevent what is called a Controlled Flight Into Terrain (CFIT)

CFIT is an accident in which an aircraft crashes into the ground, the water, or an obstacle such as a mountain or building .

CFIT and GPWS

CFIT can be the result of factors such as navigation errors, pilot fatigue, or reduced visibility owing to weather conditions.

CFIT incidents have been reduced since GPWS entered widespread use in the 1970s.

The use of a GPWS in large aircraft is required by law in many countries.

In the late 1990s improvements were made and the system was renamed "Enhanced Ground Proximity Warning System (EGPWS)

How GPWS Works?

RA measures how far aircraft from the ground.

GPWS computer analyzed the information from radar.

Computer can identify hazardous situations (very close to ground/terrain, rapid ROD & loss of altitude)

GPWS display gives visual and audio warning signals to the pilot.

How GPWS Works?

A GPWS monitors the aircraft's altitude with a Radar Altimeter, which transmits radio waves downward from the plane to determine how far away the ground is.

Most radar altimeters carried by commercial aircraft are short-range devices with ranges of less than a mile (about 1.6 km).

The information from the radar is monitored and analyzed by a computer that can identify hazardous situations and trends in the data, such as

a dangerously rapid rate of descent,

dangerously close ground during, or

unexpected loss of altitude.

If hazardous conditions are detected, the GPWS gives visual and audio warning signals to the pilot.

Radar Altimeter Vs Barometric Altimeter

What is the different between radar altimeter and barometric altimeter?

A Radar Altimeter (RA) measures the distance between the plane and the ground directly below it.

Barometric altimeter provides the distance above sea level.

Audio & Visual Warning

Audio & Visual Warnings are provided under any of the following conditions:

Excessive rate of descend (sink rate)

when closure rate with terrain is too high (terrain-terrain)

loss of altitude after take-off (dont sink)

if the aircraft is too low and slow, with landing gear retracted (too low, gear)

GPWS Modes

Types of Alerts:

I. Advisory or situational awareness information (Take Note)

- Only Aural Alert is sounded

II. Cautionary alert (Understand the Need for Action)

- Both Aural Alert and Yellow Visual Alert

III. Warning alerts (Act Immediately)

- Loud Aural Alert and Red Visual alert

MODE 1

Warns of excessive descent rate.

If the aircraft is less than 2500 ft, as determined by the radar altimeter, and is losing altitude too quickly.

A GPWS warning light will illuminate and sink rate, sink rate will be heard.

If the situation is not corrected whoop, whoop, pull up, pull up will be heard.

MODE 2 This mode consists of two sub modes:

Mode 2A: The flaps are NOT in the landing configuration

Mode 2B: The flaps are in the landing configuration.

This mode possesses two boundaries like mode 1 (initial penetration area and inner warning area)

Penetrating the 1st

boundary results in an aural alert of TERRAIN,TERRAIN and followed by a repetitive

aural alert of WHOOP WHOOP PULL-UP.

MODE 3

This mode activates if an excessive height loss occurs during initial take-off climb and during a go-around procedure.

If the plane enters the envelops, a repetitive aural alert of DONT SINK sounds until the flight condition is corrected.

This mode is only active during take-off or when either the flaps or undercarriage retracts during a missed approach.

The alert is generated when the accumulated loss in height as detected in the pressure altimeter equals approximately 10% of the existing radio altitude.

Mode 3 does not arm until below 200 ft radio altitude.

MODE 4A

Mode 4A activates whenever the terrain clearance hits 500 ft radio altitude regardless of rate of descend unless the landing gear is fully extended. If the airspeed is too high , the TOO LOW TERRAIN alarm will sound. If the plane is flying too low at low speeds with landing gear retracted , the TOO LOW GEAR alarm will sound. The visual alert would be PULL UP.

MODE 4B

Mode 4B activates when the aircraft encounters unsafe terrain clearance. If the aircraft is flying at below 245 ft and at airspeeds less than 159 knots , an aural alert of TOO LOW FLAPS sounds.

MODE 5

Warns the crew of glide slope deviations.

When the aircraft receives a valid glide slope and sinks two dots below glide slope will be heard.

The warning will continue with increasing intensity if the problem is not corrected.

This warning may be canceled by the pilot.

MODE 6

Mode 6 provides alerts and callouts for descent below predefined altitudes on the radio altimeter . The aural alert is Minimums , Minimums

MODE 7

Mode 7 Provides warning of windshear conditions during take-off.

Windshear is a sudden change in wind direction and/or windspeed over a relatively short distance in the atmosphere and can have a detrimental effect on the performance of an aircraft.

Ground Proximity Warning System (GPWS) In terrains and Geometric Altitudes

(GPWS)

A ground proximity warning system (GPWS) is a system designed to alert pilots if their aircraft is in immediate danger of flying into the ground or an obstacle. The United States Federal Aviation Administration defines GPWS as a type of terrain awareness warning system (TAWS).

Terrain Awareness and Warning System

A terrain awareness and warning system (TAWS) aims to prevent "Controlled Flight Into Terrain" (CFIT) accidents. The actual systems in current use are known as ground proximity warning system and enhanced GPWS. The U.S. Federal Aviation Administration(FAA) developed the TAWS term to encompass all current and future systems which meet the relevant FAA standards.

History of Terrain Awareness and Warning System (TAWS)

Beginning in the early 1970s,

a number of studies looked

at the occurrence of CFIT

accidents, where a properly

functioning airplane under

the control of a fully

qualified and certificated

crew is flown into terrain (or

water or obstacles) with no

apparent awareness on the

part of the crew.

Findings from these studies

indicated that many such

accidents could have been

avoided if a warning device

called a ground proximity

warning system (GPWS) had

been used. As a result of

these studies and

recommendations from the

U.S. National Transportation

Safety Board (NTSB), in 1974

the FAA required all Part 121

certificate holders (that is,

those operating large

turbine-powered airplanes)

and some Part 135 certificate

holders (that is, those

operating large turbojet

airplanes) to install TSO-

approved GPWS equipment.

In 1978, the FAA extended

the GPWS requirement to

Part 135 certificate holders

operating smaller airplanes:

turbojet-powered airplanes

with ten or more passenger

seats. These operators were

required to install TSO-

approved GPWS equipment

or alternative ground

proximity advisory systems

that provide routine altitude

callouts whether or not there

is any imminent danger.[ This

requirement was considered

necessary because of the

complexity, size, speed, and

flight performance

characteristics of these

airplanes. The GPWS

equipment was considered

essential in helping the pilots

of these airplanes to regain

altitude quickly and avoid

what could have been a CFIT

accident.

Installation of GPWS or

alternative FAA-approved

advisory systems was not

required on turbo-propeller

powered (turboprop)

airplanes operated under

Part 135 because, at that

time, the general consensus

was that the performance

characteristics of turboprop

airplanes made them less

susceptible to CFIT accidents.

For example, it was thought

that turboprop airplanes had

a greater ability to respond

quickly in situations where

altitude control was

inadvertently neglected, as

compared to turbojet

airplanes. However, later

studies, including

investigations by the NTSB,

analyzed CFIT accidents

involving turboprop airplanes

and found that many of

these accidents could have

been avoided if GPWS

equipment had been used.[4]

What are the causes of CFIT?

Bad weather

navigation equipment problems

pilot error

Solutions

First generation GPWS

EGPWS (Enhanced- GPWS)

How does it work?

A TAWS works by using digital elevation data and airplane instrumental values to predict if a likely future position of the aircraft intersects with the ground. The flight crew is thus provided with "earlier aural and visual warning of impending terrain, forward looking capability, and continued operation in the landing configuration.

EGPWS

EGPWS (Enhanced Ground Proximity Warning System) is a Terrain Awareness and Alerting system providing terrain alerting and display functions with additional features.

The EGPWS uses aircraft inputs including geographic position, altitude, airspeed and glidescope deviation. These are used with internal terrain, obstacles, and airport databases to predict a potential conflict between the aircraft flight path and terrain or an obstacle. A terrain or obstacle conflicts in the EGPWS providing a visual and audio caution or warning alert

What is the difference between EGPWS and GPWS?

The difference between an early GPWS system and a EGPWS system is that we add the forward looking terrain awareness (FLTA) function, which uses a database to alert the pilot to hazardous terrain or obstructions that are ahead of the aircraft. The FLTA function looks ahead of the aircraft along and below its lateral and vertical flight path and provides suitable alerts if a potential CFIT threat exists.

What are the limitations of using GPWS?

Traditional GPWS does have a blind spot. Since it can only gather data from directly below the aircraft, it must predict future terrain features. If there is a dramatic change in terrain, such as a steep slope, GPWS will not detect the aircraft closure rate until it is too late for evasive action.

There were still some CFIT accidents which GPWS was unable to help prevent, due to the blind spot of those early GPWS systems. More advanced systems were developed.

In the late 1990s improvements were made and the system was renamed "Enhanced

Ground Proximity Warning System"(EGPWS). The system was now combined with a worldwide digital terrain database and relies on Global Positioning System.

EGPWS software improvements were focused on solving two common problems; no warning at all, and late or improper response.

Thus an enhanced system is required, taking inputs not only from the radar altimeter but also from inertial navigation system (INS), Global Positioning System (GPS), and Flight Control System (FCS), and using these to accurately predict the flight path of the aircraft up to 4 to 5 miles (6.4 to 8.0 km) ahead.

No warning

The primary cause of CFIT occurrences with no GPWS warning is landing short. When the landing gear is down and landing flaps are deployed, the GPWS expects the airplane to land and therefore, issues no warning. EGPWS introduces the Terrain Clearance Floor (TCF) function, which provides GPWS protection even in the landing configuration.

Late warning or improper response

The occurrence of a GPWS alert typically happens at a time of high workload and nearly always surprises the flight crew. Almost certainly, the aircraft is not where the pilot thinks it should be, and the response to a GPWS warning can be late in these circumstances. Warning time can also be short if the aircraft is flying into steep terrain since the downward looking radio altimeter is the primary sensor used for the warning calculation. The EGPWS improves terrain awareness and warning times by introducing the Terrain Display and the Terrain Data Base Look Ahead protection

The EGPWS incorporates several enhanced features:

Terrain Alerting and Display (TAD) provides a graphic display of the surrounding terrain on the Weather Radar Indicator, EFIS, or a dedicated display. Based on the aircrafts position and the internal database, the terrain topography (within the display range selected) that is above or within 2000 feet below the aircraft altitude is presented on the system display. This feature is an option, enabled by program pins during installation.

Peaks is a TAD supplemental feature providing additional terrain display features for enhanced situational awareness, independent of the aircrafts altitude. This includes digital elevations for the highest and lowest displayed terrain, additional elevation (color) bands, and a unique representation of 0 MSL elevation (sea level and its corresponding shoreline). This feature is an option, enabled by program pins during installation.

Obstacles is a feature utilizing an obstacle database for obstacle conflict alerting and display. EGPWS caution and warning visual and audio alerts are provided when a conflict is detected. Additionally, when TAD is enabled, Obstacles are graphically displayed similar to terrain. This feature is an option, enabled by program pins during installation.

A process feature called Envelope Modulation utilizes the internal database to tailor EGPWS alerts at certain geographic locations to reduce nuisance alerts and provide added protection.

A Terrain Clearance Floor feature adds an additional element of protection by alerting the pilot of possible premature descent. This is intended for non-precision approaches and is based on the current aircraft position relative to the nearest runway. This feature is enabled with the TAD feature.

In -210-210 and later versions, a Runway Field Clearance Floor (RFCF) feature is included. This is similar to the TCF feature except that RFCF is based on the current aircraft position and height above the destination runway based on Geometric Altitude. This provides improved protection at locations where the destination runway is significantly higher than the surrounding terrain.

An Aural Declutter feature reduces the repetition of warning messages. This feature is optional, and may be disabled by system program pins during installation.

Geometric Altitude, based on GPS altitude, is a computed pseudo-barometric altitude designed to reduce or eliminate altitude errors resulting from temperature extremes, non-standard pressure altitude conditions, and altimeter miss-sets. This ensures an optimal EGPWS alerting and display capability.

Runway Alerting & Advisory System (RAAS) The EGPWS also provides position awareness advisories relative to runways during ground operations and approach to land. This new feature is known as "Runway Awareness and Advisory System" - RAAS (only available in -218-218 or later versions).

Members:

Valerie Zenn Magbanua

Kryzia Faith Calaguas

John Paolo Ocampo

Valentin Maucat