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Part Three: Section 3
TAWS Alerts
Departure Phase
- Defined as when all five of the
following conditions are met:
• It has been determined that the aircraft was
previously “on-ground.”
• The aircraft has a vertical velocity greater than
zero.
• The aircraft has a ground speed greater than
35 knots.
• The aircraft has an altitude greater than 150 feet
AGL and less than 1500 feet AGL.
• The aircraft is within 15 NM of the departure
airport.
NOTE:
There are additional factors beyond what is
outlined above that may alter the determined TAWS
flight phase.
Nearest Airport Criteria & TAWS Phase of Flight
Changes to the Nearest Airport Criteria Page affects
how TAWS performs. The nearest airport 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 estima-
tion 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 Terrain Avoidance
(FLTA) alerting occurs. TAWS Premature Descent
Alerting (PDA) alerts are also affected by nearest
airport list configuration. Airports that are not in the
nearest list would not have PDA alerts issued.
Nearest Airport Criteria Page
As an example, if the pilot configures the unit
to include only airports with runways 5000 feet 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 feet are NOT included in TAWS
calculations. During a cross-country flight with this
configuration, only airports runways 5000 feet or
greater cause the unit to go into Terminal or Approach
Mode from En Route 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.
Summary of Contents for GNC 420W
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