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

2. Ground Proximity Warning System (GPWS)
Since 1960s, a series of Controlled Flight Into Terrain (CFIT) accidents killed hundreds of people.
Thus, a device called a ground proximity warning system (GPWS) had been used to overcome CFIT problem.

3. Controlled Flight Into Terrain
CFIT describes about aircraft collision, under pilot control, inadvertently flies into terrain, an obstacle, or water.
The pilots are generally unaware of the danger until it is too late.
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. 3

4. Korean Airlines Boeing 747: CFIT while attempting to land in heavy rain: 228 of 254 killed (Aug. 6, 1997)
There was heavy rain at Guam so visibility was significantly reduced and the crew was attempting an instrument landing.
Air traffic control in Guam advised the crew that the glideslope Instrument Landing System (ILS) in runway 6L was out of service.
Air traffic control cleared Flight 801 to land in runway 6L at around 1:40 am.
The crew noticed that the plane was descending very steeply, and noted several times that the airport "is not in sight".
At 1:42 am, the aircraft crashed into Nimitz Hill, about 3 nautical miles (5 km) short of the runway, at an altitude of 660 feet (201 m).

5. Ground Proximity Warning System (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.
GPWS has greatly reduced the number of CFIT incidents.

6. HOW GPWS WORKS
The heart of the GPWS is a computer processor which interprets data from a number of sources such as radar, gear and flap indicating system & roll attitude input sensor.
The computer analyzes a number of parameters including aircraft configuration, speed, proximity to terrain, and descent rate in order to issue appropriate warnings.
The pilots are alerted through aural & visual warnings by GPWS display inside cockpit.

7. Aural & Visual Warning
Aural & 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”)

8. AURAL WARNINGS Some typical GPWS warnings are: “DESCENT,DESCENT”
“CLIMB, CLIMB”
“SINK RATE”
“TERRAIN, TERRAIN”
“DON’T SINK”
“TOO LOW, TERRAIN”
“TOO LOW GEAR”
“PULL UP”
These aural warnings will be accompanied by illumination of a GPWS display.

9. MODE 1 Warns of excessive descent rate..
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.

10. MODE 2 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.

11. MODE 3 Warns of loss of altitude after take-off, or go-around.
If the aircraft sinks 10 percent of its radar altitude “don’t sink” will be heard.

12. MODE 4
MODE 4a: if the aircraft is too low and slow, with landing gear retracted.
“too low, gear”
If the airspeed is higher the warning will be “too low, terrain”
MODE 4b: When the gear is selected down, but the flaps are still retracted “too low, flaps”
This warning can be canceled by the pilot in the event of a flapless landing.

13. MODE 5 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.

14. RECOVERY PROCEDURE
Recovery procedure will vary depending on the type of aircraft, but the general response is:
Roll level and simultaneously set maximum power.
Slowly pitch up and maintain Vx.
Retract gear and flaps.
Continue climb until clear of terrain.

15. Enhanced Ground Proximity Warning System
A limitation of GPWS is the fact that with radar altimeter information only the aircraft is only capable of seeing terrain directly below.
In the case of rapidly rising terrain it may not react quick enough to issue a warning in time.

16. Enhanced Ground Proximity Warning System
EGPWS corrects this problem by integrating an aircraft navigational source into the system.
GPS position in conjunction with a terrain database is used to help predict terrain conflict.
Any catalogued man made obstructions are included in the database.
This type of system can be used to display terrain profile to the pilot to improve situational awareness.