Technical data

Product Specification
HIF-2121/R5 CAGE CODE: 97896 SCALE: NONE SIZE: A DWG NO: 965-1176-601 REV: D SHEET 60
Barometric altitude rate is used in the same manner as was described for Mode 1. The Mode 3 alert envelopes are inhibited
close to the ground.
The descent required for an alert varies as a function of flight profile and time. Once a descent begins during the takeoff phase
of flight, as determined by the polarity of the altitude rate signal and takeoff/approach mode logic, the computer will store the
existing value of altitude. Subsequent samples of altitude, altitude rate, and radio altitude are examined for alert conditions.
The original stored value of altitude indicating where the descent began is retained until the aircraft ascends above the stored
altitude value, or the radio altitude is greater than 925 feet AGL, (815 feet AGL with flap override active and 1500 feet for
turbofan). When the polarity of the altitude rate signal indicates ascent rather than descent, the alert is cut off to indicate
recovery is being initiated. A subsequent return to descent prior to regaining the altitude lost enables the alert. The altitude
loss required to resume the message and lamps is based on the initially stored altitude value. In this manner, the possibility of
stair stepping down without Mode 3 alert indication is eliminated.
Mode 3 is biased above 700 feet radio altitude at the rate of 5 additional feet of altitude loss per second. Selection of the
GPWS flap override function increases the allowable altitude. This allows optional pattern work to be performed without
unwanted warnings.
Mode 3 annunciation will typically give two messages, and then will bias the voice alert conditions an amount equal to 20%
of radio altitude. If the aircraft does not lose this additional altitude, no further voice messages will be given. If, however,
this altitude is also lost, then two additional messages will be given and another 20% bias of radio altitude added into the alert
calculation. This process of ratcheting the voice alert continues until the original altitude is recovered. The caution/warning
lamps are not affected and always remains on when the envelope is violated.