Technical data

Product Specification
HIF-2121/R5 CAGE CODE: 97896 SCALE: NONE SIZE: A DWG NO: 965-1176-601 REV: D SHEET 43
6 System Functions
Revision History
Effectivity
Date - Modified By Description of the Updates App. Cfg.
29-NOV-99 Peter Bateman Initial Release and entry into PVCS -001 -001
24-MAY-00 Susie Wright SCR 4942: General document cleanup. -003 -003
06-JUN-00 – Susie Wright Document only change – Deleted proprietary note from footer. -003 -003
6.0 Mode Control
EGPWC uses mode control to enable specific features in the EGPWC modes. The current flight phase of the aircraft is
identified, such as takeoff or approach, and is used to select the modes of the EGPWC. Various flight phases are described in
the following sub-sections.
6.0.1 Air/Ground Mode
The system must be able to determine if the aircraft is airborne to control warning modes, maintenance functions, and fault
isolation logic. The airborne value is stored and immune to power interruptions to prevent inadvertent change of state during
power loss.
When airspeed is greater than 60 knots and radio altitude is greater than 30 feet for more than 1 second, then the system will
go ‘In Air’.
The aircraft is considered not airborne (i.e. on ground) when the airspeed drops below 40 knots and the radio altitude
becomes less than 30 feet.
6.0.2 GPWS Takeoff/Approach Mode
Takeoff/approach GPWS mode status is used to control portions of Modes 3, 4, 5 and 6. Mode 3 and Mode 4C are only
active during the takeoff phase of flight, while Modes 4A and 4B are only active during the cruise and approach phases of
flight. Mode 5 is active during the approach mode with gear down and can be active in the takeoff mode with both gear and
flaps in landing configuration. Mode 6 utilizes the takeoff to approach mode switching to re-enable callouts.
Approach mode to takeoff mode switching is accomplished when the aircraft passes below the 150 foot Mode 4B floor
without a warning (i.e., gear down and flaps in full landing configuration). At this time, the Mode 3/Mode 4C warning logic
is activated. The state of this switching function is maintained in nonvolatile memory to avoid inadvertent selection of an
improper mode during power loss.
The basic configuration requires that the following algorithms be satisfied before the switch from takeoff to approach mode
can occur. For this algorithm, the Mode 4C minimum terrain clearance filter described in section 6.2.4 is used to control the
switching from takeoff to approach. After takeoff, the switch to approach mode will normally be enabled when the filter
value exceeds 500 feet. This will occur at or above 667 feet radio altitude, depending on the time allowed to charge the filter.
In the event that airspeed has increased to greater than 178 knots, the mode switching will be further delayed until the floor
reaches the expanded Mode 4A warning boundary. A maximum expansion of 750 feet occurs at airspeeds greater than 200
knots and will result in mode switching being enabled at or above 1000 feet radio altitude.
This is the basic configuration of the MKVI/MKVIII and is an improvement over the original switching algorithm from the
MKII.
The MKII uses a fixed radio altitude level to switch modes, with its algorithm it is possible to switch modes prematurely and
get a nuisance Mode 4 warning. Two examples will illustrate the problem using a 700-foot radio altitude for mode switching.
In the first MKII example, passing over a river valley, or other large terrain depression during takeoff, may temporarily
indicate above 700 feet radio altitude and allow switching to Mode 4. If radio altitude then decreases below 500 feet, a Mode
4 warning will occur.
In the second MKII example, emphasizing a speed increase over climb gradient during takeoff can allow the airspeed to
exceed 214 knots before reaching 700 feet radio altitude. Switching to Mode 4 at 700 feet then produces a “Too Low
Terrain” warning.
By using both the MKVI/MKVIII algorithm, the previous potential problems are eliminated and the noise abatement
procedures can be handled without creating nuisance warnings due to premature takeoff to approach mode switching.