1. Lecture 9: Ground Proximity Warning System (GPWS)

2. 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.
The pilots are alerted through visual and audio warnings by GPWS display inside cockpit.

3. 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 .

4. CFIT= Controlled Flight Into Terrain
Prior to the widespread implementation of GPWS, CFIT was an all too common occurrence.
CFIT accidents involve a normally operating aircraft which contacts the ground due to loss of situational awareness, or other pilot error.
GPWS has greatly reduced the number of these incidents.
Since 1960s, a series of Controlled Flight Into Terrain (CFIT) accidents killed hundreds of people. 4

5. 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)

6. How GPWS Works? RA measures how far aircraft from the ground.
Visual
Audio
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.
(RA)
What is the different between radar altimeter and barometric altimeter?

7. 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.
The primary limitation of standard ground proximity warning system designs is that it looks only directly below the aircraft.
It can detect when the airplane is too low or losing altitude, but if the terrain itself rises steeply, the GPWS won't be able to inform the pilot until the aircraft is already over the rising terrain. Such a warning may come too late, especially if darkness or weather conditions have obscured the ground.
In civilian aircraft, this weakness was addressed by the development of the enhanced ground proximity warning system (EGPWS), an advance made possible by the creation of the global positioning system (GPS). The EGPWS contains an electronic terrain database and maintains constant contact with the GPS to track the aircraft's own location, allowing it to provide pilots with accurate data on terrain beyond the range of its own radar.

8. 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.

9. 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 (don’t sink)
if the aircraft is too low and slow, with landing gear retracted (“too low, gear”)

10. GPWS Modes MODE WARNING Mode 1: Excessive Descent Rate.
when Rate of Descent(ROD) increase rapidly
“sink rate, sink rate”
“whoop, whoop, pull up, pull up”
Mode 2:
Excessive Terrain Closure Rate
when closure rate with terrain is too high.
“terrain, terrain”
Mode 3:
Descent after take-off
warn loss of altitude after take-off.
“don’t sink”
Mode 4:
Unsafe Terrain Clearance
Too low, landing gear/flap configuration.
“too low, terrain”
“too low, gear” too low, flaps”
Mode 5:
Descent below Glide Slope
Too low
“Glideslope”
Mode 6: Situation Awareness
Bank Angle, Altitude
“Minimums”
“Bank Angle”
Mode 7: Wind shear Information
“Wind Shear”

11. MODE 1: Excessive Descent Rate..
Warns of excessive Rate of Descent(ROD)
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.

12. MODE 2: Excessive Terrain Closure Rate
Warns crew when closure rate with terrain is too high.
Designed to warn crew when rising terrain is a threat.
“terrain, terrain”
“whoop, whoop, pull up, pull up”
There may be no change in barometric altitude but the radar altitude is decreasing.
Recovery: Continue climb until clear of terrain.

13. MODE 3: Altitude Loss After Take-off
Warns of loss of altitude after take-off.
If the aircraft sinks 10 percent of its radar altitude “don’t sink” will be heard.

14. MODE 4: Unsafe Terrain Clearance
Terrain clearance while aircraft take-off, climb, cruise, and descent.
If the airspeed is higher the warning will be “too low, terrain”
Alert the configuration of landing gear during approach.
if the aircraft is too low and slow, with landing gear retracted (not open).
“too low, gear”
Alert the configuration of flaps during landing
When the gear is selected down, but the flaps are still retracted (not open)
“too low, flaps”
Warns the crew of glideslope deviations.
When the aircraft receives a valid glideslope and sinks two dots below “glideslope” will be heard.
The warning will continue with increasing intensity if the problem is not corrected.
This warning may be canceled by the pilot.

15. Mode 5, 6, 7 Mode 5: Descent below Glide Slope
Mode 6: Situation Awareness (Bank Angle, Altitude)
Mode 7: Wind shear Information