(b)
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.
(c)
The equation for the sloped portion of the warning curve is:
H warn (ft.) = - 1083 ft. + 8.333 X Computed Airspeed (Kts)
(5) Mode 5 - Descent below glide slope (Ref. Fig. 011)
(a)
The mode 5 provides two levels of warning when the aircraft flight path descends below the glide slope beam on front course ILS approaches. The upper limit of 1000 ft. nominal allows the capture of the beam before enabling this mode. Other upper limits are used at certain airports to increase the warning envelope. The deviation boundaries are shown in dots below the beam where one dot equals 0.0875 DDM. The first warning occurs whenever the aircraft is more than 1.3 dots below the beam. It is called a soft glide slope warning because the volume level of the GLIDE SLOPE warning is approximately one half (- 6 db) that of the other warnings. A second warning boundary occurs below 300 ft. radio altitude and below 2 dots deviation. It is called loud glide slope because the volume level is increased to that of the other warnings.
(b)
Both boundaries allow additional deviation below 150 ft. of radio altitude to allow for normal beam variations near the threshold.
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(c)
The repetition rate of the GLIDE SLOPE messages also varies as a function of radio altitude and glide slope deviation. It is slow at 1000 ft. and 1.3 dots and speeds up as altitude is lowered or deviation is increased. Actual time between glide slope messages is controlled by the equation:
Radio Alt (ft.)
Time = ----------------------X 0.0067
G/S Deviation (Dots)
(d)
The GLIDE SLOPE warning can be manually cancelled by the crew. This can be done any time below 1000 ft. nominal radio altitude. Cancel can be reset by ascending above 1000 ft. nominal, or descending below 30 ft. Cancel can also be reset by reselecting the ILS frequency in certain installations.
(e)
The change from constant deviation to a slope below 150 ft. radio altitude is shown along with the constant separation 0.7 dots between the two boundaries. A delay of approximately 0.8 seconds is inserted between the warning output and the enabling logic during a warning condition. The delay shall also turn off the warning output when two successive samples show a loss of the enabling logic. The repetition rate of the glide slope message is controlled by the radio altitude and the glide slope deviation as shown in the equation above.
(f)
The gear-down logic requirement for mode 5 is deleted at certain airports.
(g)
The glide-slope cancel latch will be set by a momentary cancel command that lasts for at least two successive processor cycles. Reset of the latch is accomplished by:
(h)
The reset condition must last for at least two successive processor cycles. A reset condition will inhibit the set command.
(i)
The state of the cancel latch must be retained during loss of computer power.
1_ Climb above 1000 ft. radio altitude.
2_ Descent below 30 ft. radio altitude.
3_ Change of valid ILS select data.
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C. Envelope Modulation
(1) Background During the past 20 years, experience with GPWS have shown that normal approaches to certain airports can be incompatible with the normal warning envelopes and signal filtering. A number of enhancements to the envelopes and filters have been developed during this time in an attempt to accommodate these few airports, without compromising the overall GPWS effectiveness for all the other "normal" airport approaches. However, there remain a limited number of cases which are still not satisfactory despite these efforts. All of the noticeable cases have been due to nuisance warnings for approaches and departures at particular airports. The majority of nuisance warnings involves mode 2 closure rate due to terrain under the approach path or rising terrain just before the runway threshold. Others involve mode 4 terrain clearance warnings during initial approach. A few mode 1 warnings are the results of steeper than normal approaches over terrain which slopes down to the runway at some airports. A different type of case is the inadequate warning protection during ILS approaches when the upper limit of mode 5 is 1000 ft. radio altitude. There are airports located at a significantly higher altitude than the surrounding terrain. In some instances this difference is over 1000 ft., thus requiring the aircraft to be below the runway elevation before a mode 5 warning is possible during most of the approach. Until recently there has been no reasonable way to accomodate these few special cases without compromising the GPWC at all other airports. What is needed is the ability to recognize when the aircraft is approaching one of these airports and then adjust the warning criteria to suit the particular approach. The availability of accurate, low drift, latitude and longitude information from the latest generation inertial navigation equipment now makes individual airport recognition possible. After recognizing the approach to or departure from one of these airports, it is also important to verify the aircraft is at a reasonable altitude before desensitizing any warning criteria. If the aircraft is already low, further warning reduction is not desirable.
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