R **ON A/C 106-149, 220-299, 301-399, 401-499,
 (Ref. Fig. 014)
 **ON A/C 001-049, 051-099, 101-105, 151-199, 201-219,
 (Ref. Fig. 014A)

R **ON A/C ALL
 The equation for the sloped portion of the warning curve is:
 H warn (ft) = -1083ft + 8.333 x Computed Airspeed (Kts)
 Mode 4C is active diring takeoff phase with either gear or flaps not in landing configuration. It provides a warning based on minimum radio altitude clearance during takeoff. This warning is similar to the Mode 4A warning that is active during the cruise and approach phases of flight, only in this case, the minimum terrain clearance is a function of the radio altitude of the aircraft. The slopped portion of the envelope boundary is described by the equation: MIN TERRAIN CLEARANCE (ft) = 0.75 (RADIO ALTITUDE (ft)
 Mode 4C is based on a minimum terrain clearance, or floor, that increases with radio altitude during takeoff. A value equal to 75 per cent of the current rad io altitude is accumulated in a long term filter that is only allowed to increase in value. If the radio altitude later decreases, the filter stores its maximum attained value. Further decrease of radio altitude below the stored filter value with gear or flaps up results in the warning TOO LOW TERRAIN (or equivalent). A simplified example illustrates this operation. First assume the radio altitude increase rapidly from 0ft to 400ft. The filter begins charging to 75 per cent of 400ft or 300ft. In 20 seconds, the filter has charged up to approximately 220ft. Now if the radio altitude decreases so that 75 per cent of this value results in something less than 220ft (i.e., approximately 295ft
R 1EFF : ALL 1 34-48-00Page 92 1 1 Config-1 Aug 01/05 1 1 1CES 1

 or less), the filter remains at a value of 220ft . Further
 reductions in radio altitude below 220ft result in the TOO LOW
 TERRAIN warning. This warning is provided to prevent inadvertent
 controlled flight into the ground during takeoff climb into
 terrain that produces insufficient closure rate for a Mode 2
 warning. After takeoff, and mode change over from Takeoff to
 Cruise or Approach, then Mode 4A and 4B provide this protection. 
R  The basic audio de-clutter feature applies a ratched function to 
R  the Mode 4C voice warning which is equivalent to the ratcheting 
R  voice message described above. Once the message is given, the 
R  envelope is biased down by 20 per cent and further warnings are 
R  held off until this additional 20 per cent radio altitude is 
R  lost. The lamp is not affected and remains on until the terrain 
R  clearance problem is rectified.
 (b) The Mode 4A/4B maximum altitude is lowered from 1000 to 800 ft.
 of radio altitude when an overflight is detected. An overflight 
R  can be detected by sensing the sudden change in radio altitude 
R  (-2200ft/s). The 800 ft limit is maintained for 60 seconds after
 an overflight to ensure that a nuisance warning is not issued.
 (c) When the landing gear or flaps are lowered, the upper boundary
 decreases to 245 ft. To maintain the same airspeed expansion
 function up to 1000 ft. nominal at 250 Kts, the lower level
 corner is at 159 Kts.
 Penetration below 159 Kts results in TOO LOW GEAR messages with
 gear up or TOO LOW FLAPS message with gear down and flaps not in
 landing configuration, while above 159 Kts the message is TOO LOW
 TERRAIN.
 (d) The equation for the sloped portion of the warning curve is: 
R  H warn (ft.) = - 1083 ft. + 8.333 X Computed Airspeed (Kts)
 (e) The basic audio de-clutter feature applies a ratched function to
 the Mode 4 voice warning which is similar to the Mode 3
 ratcheting voice message. Once the message is given, the envelope
 is biased down by 20 per cent and further warnings are held off
 until this additional 20 per cent radio altitude is lost. The
 lamp is not affected and remains on until the radio altitude
 becomes greater than indicated on the curves.
 A simplified example illustrates this operation. First assume the
 radio altitude increases rapidly from 0 ft. to 400 ft. The filter
 begins charging to 75 per cent of 400 ft., or 300 ft. In 20
 seconds, the filter has charged up to approximately 220 ft. 

R 1EFF : ALL 1 34-48-00Page 93 1 1 Config-1 Aug 01/05 1 1 1CES 1

 Enhanced GPWS - Unsafe Terrain Clearance - Mode
 Figure 014 (SHEET 1)

R 1EFF : 106-149, 220-299, 301-399, 401-499, 1 34-48-00Page 94 1 1 Config-1 Aug 01/05 1 1 1CES 1
　
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