(b)
Pressing the GPWS/G/S pushbutton switch on the main instrument panel for mode 5 (glide slope) visual and aural warning. This inhibition is temporary and the mode will be automatically reactivated for a new envelope penetration.
(c)
Pressing the G/S MODE pushbutton switch on the overhead panel for mode 5 (glide slope) visual and aural warning (permanent inhibition).
(d)
Pressing the SYS pushbutton switch on the overhead panel for inhibition of all the modes (visual and aural warnings).
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(e) The GPWC is automatically inhibited if a stall warning is in progress.
(4) Faults which prevent normal GPWC operation are stored in the BITE memory and may be read on the BITE display by operating the STATUS/ HISTORY switch located on the GPWC face below the BITE display.
B. Detailed Operation - Modes 1 through 5 This section describes the five warning modes of the GPWC and describes the operation of each of the modes. The voice messages are illustrated for the basic vocabulary, i.e., no audio select program pins connected.
(1) Mode 1 - Excessive rate of descent
(Ref. Fig. 007)
(a)
The mode 1 provides the pilot with an alert/warning for the high descent rates into terrain. The pilot also receives a timely alert for rapidly building sink rates near the runway when landing. The figure shows the outer and inner boundaries whose penetrations trigger the alert/warnings.
(b)
The outer boundary is a straight line which has an equation of:
H warn (ft.) = - 572 ft. - 0.6035 X Altitude rate.
NOTE : The descent rate is considered negative during the
____ descent. The formulas provide design goals only.
(c)
The penetration of this outer boundary will activate the GPWS legends and produce the voice warning SINK RATE. This will continue until the boundary penetration is corrected.
(d)
Further penetration of the first boundary will reach the inner boundary where the voice warning will change from SINK RATE to WHOOP WHOOP PULL UP. This inner boundary is composed of two straight lines. The equation which describes the lower line is:
H warn (ft) = - 1620 ft. - 1.1133 X Altitude rate.
The equation which describes the upper line is:
H warn (ft) = - 400 ft. - 0.400 X Altitude rate.
(e) Both boundaries are cut off at the top by a radio altitude of 2450 ft. The lower is cut off at 10 ft. A delay equivalent to 0.8 seconds is included in the upper function to minimize the warnings caused by the momentary boundary penetration.
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(2) Mode 2 - Excessive closure rate with terrain (Ref. Fig. 008)
(a)
The mode 2 provides a warning based on the radio altitude between the aircraft and the ground and on how rapidly the radio altitude decreases. The barometric altitude of the aircraft is not important for the initiation of this warning.
(b)
The altitude rate is complemented in the filter to provide lead information. The increase of the altitude descent rate will tend to speed up the warning occurrence. The reduction of the altitude descent rate or the initiation of a climb will tend to delay the warning occurrence or reduce the time the warning is on.
(c)
This mode has two areas of application that are generally referred to as mode 2A and mode 2B. The mode 2A is applied when the landing flaps are not down and the aircraft is not in the glide slope beam. The mode 2B requires that the landing flaps be down or that the aircraft be in the glide slope beam within +/- 2 dots of deviation during an ILS approach. The figure shows the warning boundary for the mode 2A (landing flaps up). The closure rate is the computed change in the radio altitude between the aircraft and the ground. It is considered positive when the altitude decreases. The lower sloped line has an equation of :
H warn (ft.) = - 1579 ft. + 0.7895 X Closure Rate (FPM).
The upper sloped line has an equation of:
H warn (ft) = 522 ft + 0.1968 X closure rate (FPM).
(d)
The normal upper limit of the boundary is horizontal at 1650 ft. radio altitude due to a computed maximum closure rate limit of 5733 FPM. As the airspeed increases from 220 Kts up to 310 Kts, the maximum rate limit is linearly increased to 9800 FPM. This allows the upper boundary to also linearly increase up to 2450 ft. The upper boundary is limited at certain airports to reduce the warning sensitivity and minimize the nuisance warnings.
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