c. Other Operational Considerations. Precipitation and Turbulence Detection. The TWR-850 system and the
RTA-85X system have been designed to accurately detect and display rainfall and precipitation-related turbulence.
While it is not uncommon for a weather radar system to detect and display solid or dry objects, weather radar should
never be used for that purpose. The systems can also detect and display areas of turbulence in the 5 to 50-NMI range
when WX+T or TURB mode is selected. The Doppler turbulence detection techniques used in the systems relies on
the presence of at least light precipitation. It is important to note that the systems are not capable of detecting clear
air turbulence. Detected turbulence is shown in magenta and consists of areas of precipitation where horizontal
wind velocity shifts in excess of 16.4 feet per second (5 meters per second or about 11 mph) are detected. (For
reference, the National Weather Service deines wind velocity shifts of 0 to 9 feet per second (0 to 5.79 meters per
second) as light turbulence.) The systems cannot detect dry air turbulence. Areas of detected turbulence will also
be displayed in magenta if WX+T or TURB mode is selected and the range control is positioned to the 5, 10, 25, or
50-nmi range. If WX+T mode is selected and a radar display level 2 is being painted, selecting TURB mode removes
weather radar targets from the display except for areas of turbulence which appear in magenta. On the WXP, this
position of the MODE switch is spring loaded (i.e., the MODE switch must be held in the TURB position). Releasing
the MODE switch on the WXP returns it to the WX + T position, restores it to the WX + T position, and restores the
full weather radar display. TURB mode is useful for closely analyzing areas of turbulence that have been detected
while in the WX+T mode. By removing the green, yellow, red, and magenta precipitation targets from the display,
the areas of turbulence can be more easily identiied. It is important to note that when the GAIN control is in the CAL
position and magenta is being displayed, it cannot be determined from the display alone if the magenta area is a
VIP level 3, 4, 5, or 6 storm. If the information is necessary, it must be acquired during prelight or from Flight Watch
when enroute. If WX or WX+T mode is selected and the display shows areas of magenta, it can be determined if
the magenta areas are due to heavy rainfall or if it is detected turbulence. If TURB mode is selected and magenta
targets remain on the display, then those targets are areas of detected turbulence. If no targets are being displayed
after selecting TURB mode, then the targets that were being painted in the WX+T mode are due to heavy rainfall.
However VIP level 3, 4, 5, or 6 storms can be hazardous, and the pilot is cautioned that light near these areas
can be hazardous and should be avoided. Proper use of the GAIN control allows the experienced pilot to estimate
rainfall rates by reducing the gain. Targets still being displayed in a reduced gain condition would imply that severe
turbulence, hail, and/or very heavy rainfall are likely.
3-23. TRANSPONDER (AN/APX-119).
a. The purpose of the digital transponder set is to provide pertinent platform information based on the type
of interrogation received. The digital transponder provides cooperative Mark XII Identiication Friend or Foe (IFF)
capability using full diversity selection, as well as Mode Select (Mode S) capability. In addition, transponder operation
provides interface capability with the aircraft s Trafic Collision and Avoidance System (TCAS). The transponder
receives pulsed radio frequency interrogation signals in any of six modes, (1, 2, 3/A, 4, S, and 5), decodes the
signals, and transmits a pulse-coded reply. The Mark XII IFF operation includes Selective Identiication Feature
(SIF) Modes 1, 2, 3/A and C, as well as secure cryptographic Mode 4. Mode 5 operational capability is available.
b. The control panel contains the controls and indicators required for operation of the transponder. The controls
are used to enable, disable, or modify the operation of the transponder. Information pertaining to the operation
coniguration or status of the transponder is indicated on an eight character alphanumeric display. Figure 3-13 and
Table 3-4 identify and describe the function of each operating control and indicator on the control panel.
c. Controls and Indicators.