FLTA alerting is triggered by either a projected imminent impact with terrain or obstacle or reduced terrain and obstacle clearance. Projected imminent impact with terrain occurs when the aircraft flight path is projected to intersect with terrain up to 3.0nm (yellow caution) or up to 1.5nm (red warning) in front of the aircraft flight path. By way of reference, this means an aircraft traveling at 180 knots of ground speed will have 60 seconds of notice for a caution and 30 seconds for a warning. Reduced terrain or obstacle clearance occurs when the aircraft flight path is not projected to impact the terrain but the projected clearance between the aircraft flight path and nearby terrain or obstacles falls below a designated safe vertical distance. The reduced terrain and obstacle clearance distance varies with phase of flight and aircraft dynamics per TAWS TSO C-151b but generally ranges from 300’ to 1000’.
Either terrain or obstacles can trigger FLTA alerts and they are distinguished via the CAS messaging and aural alerting. The difference between FLTA warnings and cautions is exclusively based on distance-to-go to projected impact points or reduced clearance areas. The projected imminent impact location or the projected reduced terrain and obstacle clearance area is depicted on the maps with a solid yellow (caution) or red (warning) “flashlight” elliptical depiction. Depending on aircraft dynamics, it may be possible for a solid red FLTA “flashlight” projected terrain impact point to appear on top of a larger hatched yellow TA depiction.