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providing an alert of impending controlled flight into terrain (CFIT). GPWS provides warnings of
potentially unsafe maneuvering flight conditions such as excessive bank angles, excessive sink rates,
gear up landings, floor altitude violations, and altitude loss during recovery. The system is operational
as long as MC1, radar altimeter, and air data systems are ON and functional. The GPWS algorithm
operates in the background of the OFP with no cockpit indications until an actual CFIT warning is
required. The system provides distinctive aural and visual warning cues only, to alert and direct
recovery from an impending CFIT condition. The pilot maintains full control of the aircraft for
recovery.
2.16.4.1 GPWS Sensors/Modes. GPWS is a look-down system with no forward-looking capability.
GPWS uses the radar altimeter as the primary source of terrain clearance information and the FCC air
data function, GPS, and INS as backup altitude sources when radar altitude is invalid. Radar altitude
is considered invalid by GPWS above 4,950 feet AGL or at a pitch or angle of bank greater than 50°.
With valid radar altitude data, GPWS calculates terrain slope from inputs from the INS and the radar
altimeter. Over descending terrain, GPWS assumes the terrain descends indefinitely (until the system
senses a change in terrain slope). This mechanization allows for maximum protection while minimizing
nuisance warnings.
For the first 5 seconds after radar altitude becomes invalid (as indicated by a flashing ²B² in the
HUD or RALT Xd out on the UFCD), GPWS provides no CFIT protection. After 5 seconds, the system
enters ²COAST² mode for a period of up to 2 minutes. While in COAST mode, GPWS calculates an
estimate of the aircraft current height above terrain. COAST mode can only be enabled while the
aircraft is not transonic and was over flat terrain (defined as slope less than 2°). CFIT warnings can still
be generated while in COAST mode. If the aircraft was transonic or was not over flat terrain when
radar altitude data was lost, GPWS transitions into the BYPASS mode. In the BYPASS mode, no
CFIT warnings are generated. Full protection is resumed from both modes when valid radar altitude
data is restored.
2.16.4.2 Altitude Required For Recovery Calculations. GPWS calculations for altitude required for
recovery include the loss of altitude due to persistency timers, pilot reaction time, time to roll wings
level, target g-onset rate, and steady state dive recovery time. GPWS pilot reaction time varies
depending on flight conditions but is a minimum of 0.5 second in the GPWS LAT envelope (±30° AOB,
0 to 30° dive, 450 to 560 KCAS). Pilot reaction time is reduced in the GPWS LAT envelope, where pilot
situational awareness is typically good, in order to reduce false warnings. Time to roll wings level is
based on a ½ to ¾ lateral stick displacement roll at 1g. Target g-onset rate is 80% of the available
g-onset rate up to (1) 5g/sec (less than 400 KCAS or greater than 30° AOB) or (2) 6g/sec (greater than
400 KCAS and less than 30° AOB). Steady state dive recovery time is based on a target sustained-g of
80% of g-available up to (1) 5g (less than 400 KCAS or greater than 30° AOB) or (2) 6g (greater than
400 KCAS and less than 30° AOB). Regardless of which category applies, these g-onset
rates and sustained-g levels require an aggressive pilot response.
2.16.4.3 CFIT Protection Provided.
Above 150 feet AGL -
Above 150 feet AGL, GPWS continuously calculates the altitude required to recover. A CFIT
warning is issued if the altitude required to recover plus a variable safety buffer and an added terrain
clearance altitude is greater than the current altitude above terrain. The terrain clearance altitude
varies between 30, 50, and 90 feet, depending on flight conditions.
A1-F18EA-NFM-000
I-2-121 ORIGINAL
Below 150 feet AGL -
Below 150 feet AGL, GPWS transitions to provide warnings of CFIT conditions related to takeoff
and landing. These warnings are based on (1) the time since a WoffW transition (takeoff or T&G) or
a waveoff and then (2) a combination of landing gear position, airspeed, altitude, and sink rate. GPWS
defines a waveoff as 1000 fpm rate of climb for more than 5 seconds while below both 500 feet AGL and
200 KCAS. If the following sets of conditions are valid for greater than 0.3 seconds when the aircraft
altitude is less than 150 feet, a CFIT warning is provided. The CFIT warning is cancelled when the
condition no longer exists for 0.3 seconds.
1. Less than 60 seconds after WoffW or a waveoff:
a. Floor Altitude - less than 90 feet AGL and greater than 250 KCAS.
b. Takeoff Sink Rate - less than 150 feet AGL, less than 250 KCAS, greater than 300 fpm sink.
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本文链接地址：
NATOPS Flight Manual 飞行手册 1(94)
