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* Global positioning system (GPS)
* Air data inertial reference system (ADIRS)
* Radio altitude (RA) system.
TCF uses this data:
* Latitude
* Longitude
* Radio altitude.
TCF uses GPS data for latitude and longitude. It uses IRS data if
GPS data is not valid.
Terrain Clearance Floor Logic
The GPWC has a runway database in memory. The runway
database is in the terrain database. The runway database
contains the location of all hard surface runways in the world
that are longer than 3,500 feet. TCF makes a terrain clearance
envelope around the runway. The altitude of the envelope
increases as the distance from the airport increases. GPWC
compares airplane latitude, longitude, and radio altitude with
TCF envelope data. If the airplane descends through the floor of
the envelope, the GPWC makes an alert.
TCF makes an alert even if the airplane is in landing
configuration.
The TCF function is active through the entire flight. When the
aircraft is 15nm or more from a runway the floor is 700 feet AGL
until the aircraft approaches another runway that meets the
requirements. Then the floor decreases as shown on the
graphic.
Terrain Clearance Floor Alert
If the GPWC finds that the airplane is below the TCF, it makes
this caution alert:
* Aural message TOO LOW TERRAIN. This message repeats
for each 20 percent loss of altitude.
* Message, TERRAIN on the ND in amber.
GPWS - TERRAIN CLEARANCE FLOOR
EFFECTIVITY
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If descent continues, these alerts happen:
* Aural message PULL UP
* Message, TERRAIN on the ND in red
* Message, PULL UP on the PFD.
The GPWC inhibits TCF alerts for any of these conditions:
* Airplane is on the ground
* Less than 20 seconds after takeoff
* Less than 30 feet radio altitude.
GPWS - TERRAIN CLEARANCE FLOOR
EFFECTIVITY
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GPWS - TERRAIN CLEARANCE FLOOR
GPWS - TERRAIN CLEARANCE FLOOR
EFFECTIVITY
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Description
Runway field clearance floor (RFCF) alerts the flight crew when
the airplane descends too low on approach. RFCF provides
improved alerts for runways at higher elevations when
compared to the terrain below the approach path of the
airplane.
Runway Field Clearance Floor Inputs
The ground proximity warning system (GPWS) receives
airplane data from these systems:
* Global positioning system (GPS)
* Air data inertial reference system (ADIRS)
* Radio altitude (RA) system.
TCF uses this data:
* Latitude
* Longitude
* Radio altitude.
RFCF uses GPS data for latitude and longitude. It uses IRS data
if GPS data is not valid.
Runway Field Clearance Floor Logic
Field clearance (height above the runway) is calculated by
subtracting the elevation of the selected destination runway
from the current altitude. Geometric altitude is used to find
current altitude.
Geometric altitude is a calculated airplane altitude with these
inputs:
* Pressure altitude
* GPS altitude
* Radio altitude
* Terrain elevation data
* Runway elevation data.
Geometric altitude decreases errors in corrected barometric
altitudes which may have been caused by temperature
extremes, non-standard altitude conditions or altimeter misssets.
The RFCF creates a circular band centered over the selected
runway. Unlike the TCF envelope, the RFCF envelope extends
only 5 nautical miles (NM) past the end of the runway and rises
to a maximum height of 300 feet above the runway. The RFCF
function also calculates a runway position error (KRF) based
upon geometric altitude and runway data quality.
Runway Field Clearance Floor Alert
If the GPWC finds that the airplane is below the RFCF, it makes
this caution alert:
　
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