The Ground Proximity Warning System is used to alert the flight crew when one of the following thresholds are exceeded below around 2000 feet radio altitude. If the aircraft is flying towards a mountain the aircraft can be at a 1000 feet barometric altitude but if that mountain is higher than a 1000 feet u will still have a problem, the radio altimeter measures the altitude regardless off air data. It sends a radio signal to the earth and it measures the time delay between sending and receiving the signal to determine how far above the ground it is. In the original GPWS system there were 7 mode's

• Mode 1 - Excessive Decent Rate.
• Mode 2 - Excessive Terrain Closure Rate.
• Mode 3 - Altitude Loss After Take - Off or Go Around.
• Mode 4 - Unsafe Terrain Clearance During High Speed Flight or While Not in the Landing Configuration.
• Mode 5 - Below Glideslope Deviation Alert.
• Mode 6 - Callouts/Bank angle.
• Mode 7 - Predictive windshear alert.

Mode 1 - Excessive Decent Rate
Has two boundaries and is independent of airplane configuration it's purpose is to protect the aircraft from a too fast descent.

GPWS Mode 1

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Mode 2 - Excessive Terrain Closure Rate
Monitors mach number, radio altitude and radio rate of change, barometric altitude and airplane configuration. Has two boundaries, the first causes an aural alert of TERRAIN repeated twice, followed by the repeated aural warning of WOOP WOOP PULL UP.
If both boundaries are penetrated while in the landing configuration, only the repeating TERRAIN aural alert will occur.

GPWS Mode 2

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Mode - 3 Altitude Loss After Take off Or Go Around
Provides an alert if a decent is made during initial climb or go around. The aural alert is a voice message "DON'T SINK", indicating the flight crew that they are sinking when they should be climbing.

GPWS Mode 3

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Mode - 4 Two modes 4A and 4B

4A - Unsafe Terrain Clearance With Landing Gear Not Down
This will give a warning to the flight crew when the aircraft is in landing and the gear is not down, the aural alert: TOO LOW GEAR will sound if the speed is close to landing speed and when the speed is higher then landing speed the aural alert TOO LOW TERRAIN will sound

GPWS Mode 4A and 4B

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MACH -IAS Conversion

MACH	SL	5000'	8000'	10000'
0.34	232	211	200	193
0.45	298	272	258	249

4B - Unsafe Terrain Clearance With The Flaps Not In Landing Position
This mode is similar to 4a but this is for the flaps not in the landing configuration. If the aircraft is flying at landing speed the TOO LOW FLAPS alert is sounded and when the aircraft is flying too fast the TOO LOW TERRAIN is sounded.

see picture at 4A

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Mode 5 - Below Glideslope Deviation Alert
This mode alerts you of a descent of more than 1.3 dots below an ILS glideslope. The envelope has two areas of alerting, soft and loud.
In both areas is a repeated aural warning of GLIDESLOPE and illumination of the pilots BELOW G/S lights. The voice message amplitude is increased when entering the loud area In both areas, the aural warning repetition rate is increased as glideslope deviation increases and radio altitude decreases.

Mode 1 through 4 aural alerts and warnings have priority over mode 5 aural alerts, though both PULL UP and BELOW G/S lights could be illuminated at the same time.

GPWS Mode 5

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Mode 6 - Callouts/Bank angle
This mode gives the callouts for the gpws system. in this mode are all the pin-programmed airline-option's programmed as to wich callouts should be given. Also in this mode the bank angle protection is incorperated. Bank angle warnings are given when the aircraft banks too much for it's situation, ie just after take off the bank angle limit is less then when in cruise.

GPWS Mode 6

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Mode 7 - Predictive windshear
This mode warns the flight crew of a possible microburst. The system uses an envellope to look for typical situations that occur when the aircraft enters a microburst.

Microbursts are very dangerous and have caused aircraft crashes worldwide. The newer generation of weather radar nowadays has an option to actually detect wet microbursts.
When an aircraft is in landing and thus close to ground and it flies towards a microburst it will first encounter a strong headwind resulting in high lift causing the autothrottle to reduce thrust in order to keep the aircraft to approach the airfield. When the aircraft then exits the microburst it will encounter a strong tailwind and therefore lose a lot of lift. Because of this loss of lift plus the engines throttle reduced to minimum this can cause the aircraft to fall onto the runway.

aircraft crash due to windshear

With the gpws mode 7 the flight crew can keep the throttles forward and accepting the extra lift to get out of the microburst and stay airborne.

GPWS Mode 7

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But what if an aircraft were to towards a steep mountain like this:

The gpws system sees no problem because the radio altitude is high enough and by the time it sees the ground closing up it is too late to pull up. This danger of controlled flight into terrain (CFIT) led to the Enhanced gpws system or EGPWS wich has a map of the world in its database in wich all mountains, elevations, airports and even obstacle's are incorperated.