Lesson 4
GROUND PROXIMITY WARNING
SYSTEM
CONTENT
1. GPWS INTRODUCTION
2. GPWS OPERATION MODES
3. ENHANCED GPWS
4. REVIEW
1. GPWS INTRODUCTION
1.1. Function
The purpose of the Ground Proximity Warning System is to warn the flight
crew when the aircraft mistakenly comes too close to the ground.
This is done by an aural alert using a synthetic
voice, and there are also annunciation lights in
front of the pilot. In some aircraft there are also
alert messages on the primary flight displays.
The basic GPWS system operates automatically when
the aircraft height is less than two thousand five hundred
feet.
1.2. Standard GPWS
- GPWS computer
- Control panel
- Warning devices
The basic Ground Proximity Warning System needs the
following main input signals:
- Radio altitude,
- Vertical speed,
- Glide slope deviation
- Gear and flap positions
- GPWS computer analyze information from radar.
Computer can identify hazardous situations (very close
to the ground/terrain and loss of altitude)
- GPWS display gives visual and audio warning signals to
the pilot.
1.3. Enhanced ground proximity warning system.
It shows the terrain around the aircraft on the
Navigation Display
The system also gives a warning when the aircraft gets
too near to the terrain. These functions are independent
of the radio altitude.
2. GPWS OPERATION MODES
2.1. Mode 1 “Excess sink rate”
It alerts the flight crew when the aircraft descents
below two thousand five hundred feet with a high
vertical speed.
In addition to the voice alert the red GPWS lights
illuminates or the “Pull Up” message appears on the
PFD
2.2. Mode 2 “ excessive terrain closure rate”
An alert is triggered when the aircraft height decreases
dramatically, for example when the aircraft approaches
rising terrain, like a mountain
Aural Alert: “TERRAIN, TERRAIN”
If the height continues to decrease the alert changes to a
warning: “ PULL UP “
2.3. Mode 3 « Loss of altitude after take-off »
Mode 3 alerts the flight crew when the aircraft loses
altitude during a take - off. The first step is the aural alert
accompanied by the red lights.
Aural Alert: “DON’T SINK”
If the aircraft continues to sink the aural alert changes to
a warning. “ TOO LOW, TERRAIN”
2.4. Mode 4: « Insufficient terrain clearance »
Mode 4 becomes active when the aircraft approaches the
ground with incorrect gear and flap positions.
For example when the aircraft descends below 500 ft and
the gear is not extended the aural alert sounds and the red
lights come on.
Below 250 ft without the correct flap setting the system
will give the flap alert.
2.5. Mode 5: « Descent below the G/S »
Mode 5 becomes active if the aircraft deviates below the
glideslope during an approach below a radio altitude of
1000 ft.
When a critical deviation is exceeded, an aural alert
sounds and the amber glideslope light illuminates.
2.6. Mode 6:” RA callouts/Bank angle”
The synthetic voice of the ground proximity warning
system is also used in some aircraft for the radio altitude
callouts. An aural alert can also happen if the bank angle
of the aircraft exceeds a critical value.
2.7. Mode 7: “Windshear”
This Mode warns the flight crew about a dangerous
windshear condition during take off or approach.
GPWS detects this situation by comparing the true
airspeed from the air data computer with the ground
speed from the inertial reference unit
3. ENHANCED GPWS
The first function provides a Terrain display on the
navigation display. This is called the Terrain Awareness
function or TA in short.
To create this map the EGPWS computer compares the
actual aircraft position calculated by the FMS or GPS
with a world wide terrain database stored inside the
GPW computer.
The Navigation Display can only show either the terrain
or the weather radar image, therefore it is necessary to
switch to the required image. Usually the pilot can
manually switch between the images on the ND
An automatic ND image switching happens when the
ground proximity warning system or the weather radar
detects a critical situation.
The weather image is selected automatically when the
weather radar system detects a Windshear situation
without a terrain threat from the GPW system.
Detection for Terrain Threat
The EGPWS can detect a terrain threat in two ways.
- The terrain awareness mode gives a caution alert when
the aircraft is due to reach the terrain in the next 60 sec.
On the ND amber blocks show the area with the
potential terrain conflict and an amber terrain message
is also displayed. In addition the aural alert sounds.
The terrain and the message change to red on the ND
when the aircraft position is only 30 sec from the terrain.
In addition the red GPWS light illuminates or pull up is
displayed on the PFD accompanied by a new aural
warning.
- Mode “Terrain clearance Floor – TCF”.
In this mode the computer compares a minimum height
around the runway, called the terrain floor envelope.
When the airplane descends through the floor of the
envelope, the system shows the amber terrain and
message on the ND and an aural alert sounds
If the airplane continues to descend the terrain and
message on the ND become red and the pull up warning
sounds.