(g)
Mode 2B is also selected when the aircraft is performing an ILS approach and the glide slope and localizer deviations are less than +/- 2 dots. However, the warning envelope is slightly different from the flaps down case in that the lower boundary is controlled only as a function of radio altitude, having a constant lower cutoff of 30 ft. above ground level. When the flaps are selected to landing configuration in the ILS beams, the lower boundary is activated, (Mode 2B inhibit).
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(h)
When the envelope boundary conditions for Mode 2B are violated, the GPWS legends come on, and the voice message is repeated until the boundary is exited. If the gear or the flaps are up, then the message is TERRAIN TERRAIN followed by PULL UP if the condition persists. If both gear and flaps are in the landing configuration, the message is TERRAIN.
(i)
Not Applicable
R **ON A/C 106-149, 220-299, 301-399, 401-499,
Post SB 34-1287 For A/C 220-228,301-302,
(2) Mode 2 - Excessive closure rate with terrain
(Ref. Fig. 011)
(a)
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 tends to speed up the warning occurrence. The reduction of the altitude descent rate or the initiation of a climb tends 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. Mode 2A is applied when the landing flaps are not down and the aircraft is not in the glide slope beam. 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 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).
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(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.
(e)
Upon penetration of the boundary, either on the slope or from the top, the PULL UP legends come on and the voice message is TERRAIN TERRAIN. If the boundary penetration lasts beyond these two TERRAIN messages, approximately 1 second, then the warning switches to PULL UP repeated continuously until the boundary is departed. When the Mode 2A envelope is exited, after having been violated for more than 3 seconds, an altitude gain feature is automatically activated. First the altitude at this instant is sampled and stored. The PULL UP legends remain on and the voice message is TERRAIN. After three hundred feet of altitude are gained from the stored value, or 45 seconds have elapsed from the point where the PULL UP envelope was exited, the PULL UP legends and voice stop. If another boundary penetration occurs during this altitude gain time, and it lasts long enough to restart the PULL UP warning plus 3 seconds, then the whole process begins again with a new reference altitude for the 300 ft. after boundary separation. In this manner the aircraft is directed up and over the terrain to a safer altitude.
(f)
Lowering the flaps to the landing position automatically switches the GPWS to Mode 2B. The static boundary for Mode 2B is the same as the Mode 2A boundary except the upper limit has been lowered to 789 ft. at a maximum closure rate of 3000 FPM. The lower part of the Mode 2B boundary is controlled as a function of radio altitude and altitude rate when flaps are in full landing configuration.
(g)
Mode 2B is also selected when the aircraft is performing an ILS approach and the glide slope and localizer deviations are less than +/- 2 dots. However, the warning envelope is slightly different from the flaps down case in that the lower boundary is controlled only as a function of radio altitude, having a constant lower cutoff of 30 ft. above ground level. When the flaps are selected to landing configuration in the ILS beams, the lower boundary is activated, (Mode 2B inhibit).
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