(c)
To provide additional nuisance warning protection at the runway threshold during steeper approaches, the lower limit of mode 2B varies as a function of barometric rate as shown in Fig. 5.
(d)
The GPWC cannot provide a warning if an airplane is flying directly towards a vertical cliff with no initial terrain slope at the bottom. Mode 2 warnings are inhibited below 50 feet of radio altitude.
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737-300/400/500MAINTENANCE MANUAL
(3)
Takeoff Altitude Loss Warning (Mode 3)
(a)
A warning consists of visual PULL UP and aural DON'T SINK is provided when the airplane is in takeoff mode before acquiring 700-foot terrain clearance with landing gear or flaps up and a barometric altitude loss is detected as shown in Fig. 6. The altitude loss required to activate the warning varies with the height of the airplane above the ground at the time inadvertent descent occurs. At a climbout altitude of 100 feet any loss of altitude will activate the warning. At 700 feet, an altitude loss of 70 feet will activate the warning.
(b)
A magnetic latch provides logic between mode 3 and 4. During takeoff, the latch is set to arm the takeoff mode and disarming the terrain clearance warning. Upon acquiring 700 feet of altitude, the latch resets, arming the terrain clearance warning.
(4)
Unsafe Terrain Clearance Warning (Mode 4)
(a)
With landing gear not down, the computer will provide a warning if the airplane is below 500 feet and speed below 0.35 mach, as shown in Fig. 7, (Mode 4A). The warning consists of the red PULL UP light coming on and an aural of TOO LOW-GEAR is repeated every 0.75 seconds. The warning may be inhibited by setting the GEAR INHIBIT switch to the INHIBIT position. For speed above 0.35 mach, the aural warning changes to TOO LOW-TERRAIN, repeated every 0.75 seconds and the warning threshold increases linearly to a maximum of 1000 feet above the terrain.
(b)
With landing gear down, the minimum warning threshold decreases to 245 feet and a TOO LOW-FLAPS warning comes on if the flaps are not in landing configuration and the airspeed is less than
0.29 mach as shown in Fig. 7 (Mode 4B). The warning may be inhibited by setting the FLAP INHIBIT switch on the ground proximity warning module to INHIBIT position. This will simulate a flaps down condition if the pilot prefers to land with less than normal landing flaps. For speeds above 0.29 mach, the aural warning is "TOO LOW-TERRAIN" repeated every
0.75 seconds. The warning altitude threshold increase linearly as airspeed increases to a maximum of 1000 feet above terrain. A latch is used to select mode 4B so that power interruption or missed approach below 500 feet prevent reactivation of mode 4A until at least 700 feet of terrain clearance is reached.
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737-300/400/500MAINTENANCE MANUAL
(5)
Glide Slope Deviation Warning (Mode 5)
(a)
The glide slope warning is armed when an ILS frequency is selected. The warning envelope consists of two regions as shown in Fig. 8. The BELOW G/S indicators accompanied by a reduced volume of aural warning GLIDE SLOPE come on, if the deviation of the airplane below the glide slope exceeds 1.3 dots between 1000 and 150 feet of radio altitude and the gear is down. The aural warning increases to full volume, hard warning, if the deviations of the airplane below the glide slope exceeds 2 dots when the airplane is between 300 and 150 feet of radio altitude.
(b)
In order to allow the pilot to purposely descend below the glide slope without triggering a warning, the glide slope warning mode may be inhibited by pressing the BELOW G/S indicator when the radio altitude is below 1000 feet. To again enable Mode 5, the airplane is required to ascend to a terrain clearance of greater than 1000 feet.
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