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Section 2
TAWS Operation
Nearest Airport Criteria & TAWS Phase of Flight
Changes to the Nearest Airport Criteria Page (Figure 2-9) affects how TAWS performs. The nearest air-
port list is used to derive the TAWS destination airport when there is no loaded active flight plan or when
off the active flight plan or when deviating from the active flight plan (whenever the TAWS algorithms go
into estimation mode). In turn, the nearest airport list is used to determine the TAWS phase of flight. The
TAWS phase of flight dictates the Required Terrain Clearance (RTC) levels at which Forward Looking Ter-
rain Avoidance (FLTA) alerting occurs, as described on page 55. TAWS Premature Descent Alerting (PDA)
alerting is also affected by nearest airport list configuration; airports that are not in the nearest list would
not have PDA alerts issued.
As an example, if the pilot configures the unit to include only airports with runways 5000’ or longer,
only these are added to the nearest airport list. The TAWS phase of flight is affected in that all airport
runways under 5000’ are NOT included in TAWS calculations. During a cross-country flight with this
configuration, only airports runways 5000’ or greater cause the unit to go into Terminal or Approach Mode
from Enroute mode. If the pilot makes an emergency landing at an airport that did not show up in the
nearest airport list, FLTA alerts would be issued throughout the descent.
TAWS Destination Airport
The TAWS destination airport is considered to be the next airport in the flight plan while adhering
to the flight plan path. If no flight plan is entered, or if the aircraft is not adhering to the entered flight
plan path, the TAWS destination airport is estimated using the Nearest Airport Criteria as described in the
section above.
Figure 2-9 Nearest Airport Criteria Page
