.
altitude deviation becomes more than 900 feet

.
a new altitude is selected.

Altitude Alert Profile

NO INDICATIONS
Acquisition - Aural warning tone momentarily sounds. A box shows around the selected altitude display and the box around current altitude becomes bold.
Deviation - Aural warning tone momentarily soundsand current altitude box turns amber and flashes.
Copyright . The Boeing Company. See title page for details.
15.20.10  June 1, 2003

Warning Systems -System Description 737 Flight Crew Operations Manual
Ground Proximity Alerts
The GPWS provides alerts for potentially hazardous flight conditions involving imminent impact with the ground.
The GPWS monitors terrain proximity using an internal world wide terrain data base. Proximate terrain data shows on the navigation display. If there is a potential terrain conflict, alerts are provided based on estimated time to impact. These alerts are “look-ahead terrain alerts.”
The GPWS provides alerts based on radio altitude and combinations of barometric altitude, airspeed, glide slope deviation, and airplane configuration. The alerts are for:
.
excessive descent rate

.
excessive terrain closure rate

.
altitude loss after takeoff or go-around

.
unsafe terrain clearance when not in the landing configuration

.
excessive deviation below an ILS glide slope

These alerts are “radio altitude based alerts.”
Ground proximity alerts are accompanied by voice aural alerts and the PULL UP annunciation on the attitude indicators or, for deviation below glide slope alert, the BELOW G/S light.
Note:  Terrain ahead of the airplane may exceed available climb performance. A ground proximity alert does not guarantee terrain clearance.
Look-ahead terrain alerts and radio altitude based alerts are prioritized based on the level of hazard and the required flight crew reaction time. Look-ahead terrain alerts and radio altitude based alerts are inhibited by an actual windshear warning (airplane in windshear).
Look–Ahead Terrain Alerting
The GPWS terrain data base contains detailed terrain data near major airports, and data in lesser detail for areas between airports. Terrain within 2,000 feet of airplane barometric altitude shows on the navigation display. The terrain data is not designed to be an independent navigation aid.
YA613 - YC413, YK101 - YK104 Note: The GPWS terrain data base, look-ahead terrain alerting, and terrain
display do not account for man made obstructions.
The terrain display is generated from a data base contained in the GPWS computer and correlated to GPS position.
Copyright . The Boeing Company. See title page for details.
Warning Systems -System Description

Terrain and weather radar cannot show together on a display. If one pilot selects terrain and the other pilot selects weather radar, each display updates on alternating sweeps. All other displays (TCAS, LNAV routing, etc.) can show with terrain data.
Look-ahead terrain alerts are based on the airplane’s position, barometric altitude, vertical flight path, and ground speed.
Look Ahead Terrain Alerts
AURAL ALERT  VISUAL ALERT  DESCRIPTION 
TERRAIN  PULL UP on both  20 to 30 seconds from projected impact with 
TERRAIN  attitude indicators  terrain shown solid red on the navigation 
PULL UP  Red TERRAIN message on navigation display (all modes) Solid red terrain on navigation display  display (in expanded MAP, center MAP, expanded VOR, or expanded APP modes only). Moving the ground proximity terrain inhibit switch to TERRAIN INHIBIT inhibits the alert. 
CAUTION  Amber TERRAIN  40 to 60 seconds from projected impact with 
TERRAIN  message on  terrain shown solid amber on the navigation 
navigation display  display (in expanded MAP, center MAP, 
(all modes)  expanded VOR, or expanded APP modes 
Solid amber terrain  only). 
on navigation  Moving the ground proximity terrain inhibit 
displays  switch to TERRAIN INHIBIT inhibits the 
alert. 
TOO LOW,  PULL UP on both  Descent below unsafe radio altitude while too 
TERRAIN  attitude indicators  far from any airport in the terrain database. Moving the ground proximity terrain inhibit switch to TERRAIN INHIBIT inhibits the alert. 

Copyright . The Boeing Company. See title page for details.
15.20.12  June 1, 2003

Warning Systems -System Description 737 Flight Crew Operations Manual
Radio Altitude Based Alerts
AURAL ALERT  VISUAL ALERT  DESCRIPTION 
　
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