AA WXR-700X Operating Manual Weather Radar Description 

General 

The Collins WXR-700X Weather Radar consists of: 

* One receiver / transmitter (RT) unit 

* One flat plate antenna 

* One dual (split) mode control panel 

The radar system displays weather and ground targets on the Captain's and F / O's Horizontal Situation Indicator (HSI). These radar targets can display only when the HSI is operating in the Expanded VOR, Expanded ILS, or MAP modes. 

Antenna 

The antenna scans 90' to either side of the airplane's heading and can tilt from 15' above to 15' below the horizon. The antenna is driven by electric stepper motors that provide precise control of azimuth and elevation. This type of antenna drive causes the antenna to stop suddenly at the end of each scan. This sudden stop may create a small bump noise which is of no concern.   Antenna stabilization corrects for pitch and roll of the airplane. Normally, the left Inertial Reference Unit (IRU) provides stabilization. The center IRU with the ALTN IRS Switch provides antenna stabilization, if required. 

Split Control Feature 

The dual-mode control panel (Figure 1) has split control which allows for different gain and tilt settings to be individually selected for left and right HSls. If different tilt settings are selected, the antenna scans left to right at the tilt angle selected by the left mode, adjusts to the tilt angle selected by the right mode, and scans right to left at that angle. This abrupt change in tilt angle at the end of each sweep may create a second bump which, again, may not be noticeable and is of no concern. Display of tilt angle is by the scale around the tilt control knobs on the control panel.  This split control feature also results in a slower update rate when the weather display is active on both HSls. The left HSI is updated only during the.four second left to right scan while the right HSI display is updated only during the four second right-to-left scan. This process results in the weather display presentation being refreshed during every other scan. However, if weather is displayed on only one HSI, that display updates normally with every sweep of the antenna. 

Figure 1

Power is applied to the radar with the HSI Control Panel WXR Switch. When either WXR Switch is pressed, (Figure 2) the system immediately begins operating in the mode(s) selected by the Radar Control Panel. If radar is powered-up while the IRUs are in the early portion of alignment, the HSI display indicates WXR FAIL-ANT

Figure 2

The radar returns are presented in four colors (Figure 3) representing different.levels of precipitation or reflectivity: 

* GREEN Light – precipitation 

* YELLOW – Low density precipitation 

* RED – High density precipitation 

* MAGENTA – Very strong returns 

Figure 3

* WX for normal weather detection 

* WX / T for doppler turbulence detection 

* MAP for ground mapping 

Reliable weather returns are only received from wet precipitation. Returns from snow or dry hail are only three percent as strong as wet hail. While the radar system, with frequent tilt and gain adjustments by the flight crew, can determine the wet top of a storm cell, the system cannot determine the actual top of the storm itself. 

Figure 4

Flight hazards due to weather conditions result, primarily, from turbulence and hail. Radar can detect wet hail, but turbulent air by itself does not provide a radar echo (Examples are clear air turbulence and airplane vortexes). Areas having high rainfall rates are ordinarily associated with turbulence; it is from this rainfall that radar echoes are reflected and the accompanying turbulence associated with the rainfall is implied. In some instances, the radar echoes may be severely attenuated while passing through large areas of moderate rainfall rate or small areas of high rainfall rate. This condition may mask some targets completely or cause strong targets at a farther range to appear much less intense than they actually are.  Severe attenuation can generally be identified by a complete blackout of data behind a weather cell. This blackout is much like that seen when ground-mapping a mountain peak. When a weakened return occurs, caution is required.    A yellow PAC Alert Bar identifies these attenuated returns. This PAC Alert, at the outermost range mark, indicates the azimuth direction of the attenuating storm cell. The PAC Alert is active for all storm cells within 80 nm. 

A typical storm cell can show several levels of activity (Figure 5): 

* GREEN indicates areas of light rainfall rate 

* YELLOW indicates areas of moderate rainfall rate 

* RED and / or MAGENTA indicates areas of heavy rainfall. 

* A remaining level of intensity is the BLACK SCREEN around the perimeter of the storm cell. It indicates that no detectable rainfall is present in those areas, and flight hazards due to weather in those areas should be minimal.

Figure 5

CAUTION 

Because the radar determines turbulent areas by measuring precipitation velocity, it can only function in the presence of precipitation. Consequently, the system is not capable of detecting clear air turbulence. 

Some clouds, often of the cumulus and stratus types, do not contain sufficient moisture to reflect a detectable echo; however, these clouds are not usually a hazard to flight. A non-hazardous phenomenon that is occasionally encountered is ducting of radar signals (Figure 6). This occurs with certain temperatures and humidity conditions and causes targets to be detected at distances farther than normal. A duct, or broad tunnel that guides radar signals in a curving path, can be formed when temperature increases and humidity decreases with altitude. These gradients occur in inversion conditions and might not remain stable for a long period of time.  Radar signals must be located in or close to the duct to be trapped in the signals. Signals at an angle one or two degrees or more to the duct are not trapped. However, elevating the antenna a few degrees gets the beam out of the duct. With this technique, distant thunderstorms can be differentiated between ducted ground echoes that may be mistaken for cloud targets. 

Figure 6 

The weather radar can detect and display storm-related turbulence as well as precipitation rate. The radar system determines areas of hazardous turbulence by measuring the velocity of precipitation within the target. The turbulence processor compares changes in the measured precipitation velocity to a predetermined shear threshold. The system displays turbulent areas in MAGENTA where this threshold is exceeded (Figure 7).   The system will only display turbulence when the selected range is 40 nm or less.

Figure 7

Updrafts and downdrafts in thunderstorms carry water through the cloud.  The more severe the drafts, the greater the amount of weather resulting from this area of moisture.  Assumptions can be made about the turbulence involved (Figure 8): 

* When the displayed target intensity is moderate (red), due to large amounts of water, the turbulence is considered severe. 

* The steeper gradient (rate of change in the rainfall rate) of a target, the stronger the turbulence. 

* Areas that show a red or magenta display should be avoided by a wide margin. These areas are associated with hail or turbulence.  Careful tilt management is necessary to scan these targets for areas of maximum intensity. 

Along squall lines (Figure 8), individual cells are in different stages of development. Areas between closely spaced, intense echoes may contain developing clouds not having enough moisture to produce an echo.  The lightest level (green) may or may not be displayed which would indicate light rainfall rates or no rainfall; yet, these areas could have strong updrafts or downdrafts.  In penetrating a squall line (Figure 8), the airplane must fly as far from building cells as possible. Avoid red and magenta areas.  Targets showing wide areas of green are generally precipitation without severe turbulence. The tilt control should always be used to scan the detected targets and determine the areas of heaviest precipitation or severe turbulence. 

Developing cells may show only light rainfall (green) but still exhibit areas of turbulence.  Turbulence activity can be identified by irregularities in the echo signature. Irregularities that can be identified are hooks, fingers, or a scalloped edge (Figure 8) between the black and green areas of the display.  These irregularities may be present in cells which do not display any yellow, red, or magenta areas of rainfall and should be avoided by the same distances as a high intensity rainfall area. 

Thunderstorm development is rapid. A course that appears clear may contain cells a short time later. When the pilots view the shorter ranges, they should periodically select one of the longer ranges to observe distant conditions.  This permits early planning of necessary avoidance maneuvers. 

Figure 8

The extreme case of severe turbulence is a tornado.  Cumulonimbus mammas clouds producing tornadoes have, in a few instances, been related to a characteristic target display.  The display is not usually ditferent from that of a regular thunderstorm.  Radar displays of clouds from which tornadoes were confirmed have, on occasion, shown the formation of a hook pattern in connection with the tornado. 

A narrow, finger-like portion extends from the cloud display and, in a short time, curls into a hook and closes on itself.   Other echoes associated with tornadoes are V- shaped notches and doughnut shapes.   These shapes do not always indicate tornadoes, nor are tornado echoes limited to these characteristic patterns.  Of the confirmed radar observations of tornadoes from target thunderstorms, most displays have not shown shapes different from those of a normal thunderstorm display.

Hail results from updrafts carrying water high enough to freeze.  Consequently, the greater the height of the thunderstorm echo, the greater the probability that it contains hail.  The height can be estimated by the amount of antenna uptilt required to view the upper part of the target echo.  Altitude can be roughly estimated by multiplying range x tilt angle x 100.   For example, an up-tilt for one degree and a target at 100 nm would be 10,000 feet above the airplane.  In the upper regions of a cloud where ice particles are "dry" (no liquid coating on the particle), echoes are less intense.  Liquid water reflects about five times more radar energy than solid ice particles of the same mass. 

Since hailstones are considerably larger than water drops, and are usually coated with a thin layer of water, the echo intensity from "wet" hail is greater than from rainfall.  Thunderstorm targets having an intensity greater than that associated with maximum rainfall usually contain hail.   lt is not always possible to determine from the display whether the echo is from hail or from rain. Instances have been reported of hail targets producing fingerlike protrusions up to five miles long and blunt protuberances up to three miles from the edge of thunderstorm echoes.  In parts of the world where hail occurs often, extensions from thunderstorms, shown in red, generally indicate a high probability of hail.   This same type display is also associated with new convective cells that may not yet contain hail. 

As with tornadoes, no uniquely distinctive displays are in all cases associated with hail.  Echoes from hail can appear quickly and along any edge of a storm cell.  These echoes can also change in shape and intensity in a very short time. For this reason, close and careful monitoring of the dispiay is required. 

Turbulence can be assumed to be present above any storm with severe turbulence at the storm center. 

Terrain mapping should be done with the MAP mode selected, one of the shorter ranges selected, and the antenna tilted down. The gain may require adjustment to increase ground return in the display. 

The interpretation of terrain maps displayed on the indicator is largely a matter of experience and understanding of the factors involved when the radar beam strikes a ground target. The extent to which ground targets are displayed depends upon the selected range, antenna tilt, and airplane altitude and attitude. The strength of the return signal depends upon the angle at which the radar beam strikes the ground target (incidence angle) (Figure 9) and the reflectivity properties of the ground target. The attributes of various ground targets are discussed below. The gain control should be adjusted so coastlines become apparent and cities are well defined. Maximum gains should be avoided as their use causes the display to paint between targets, thus obscuring some landmarks.

Figure 9

Cities usually provide a good return signal, though their intensity depends on selected range gain setting, airplane attitude and altitudes, and antenna tilt if one of the longer ranges is selected.  (Figure 10)

Figure 10

Bodies of water such as lakes, rivers, and oceans usually do not provide a strong return and appear as dark areas on the display. This is because the radar beam is reflected away from the antenna and very little, or none, of the signal is returned.  However, if any portion of or all of the water is rough or choppy, the angle of incidence changes and a stronger return is provided. 

Great Plains Quadrant Effect 

In the great plains regions of the United States, property lines, utility poles, barns, houses, and fences are all laid out in a strict north-south, east-west orientation. This orientation causes the radar beam to strike more ground targets at a highly reflective angle in the directions of north, south, east, and west than at any angle. Thus, the return signal will be stronger in those directions than most any other, and the indicator will display a strong return signal line in those directions. 

Mountains, like cities, provide a strong return signal to the antenna, but also mask the areas behind them from the radar beam.   Mountains, however, provide some unique exceptions to this.  The radar beam can be reflected back and forth in the mountain passes or canyon walls to the point that no return signal is received or other features of the mountains are masked out.  So, while the display wiII show a dark spot in the mountains which would indicate a pass, no such pass exists.  The appearance of mountains on the indicator display will also change with the selected range, airplane altitude and attitude, and antenna tilt.    Proper interpretation of weather and ground targets by the pilot make the weather radar system a valuable tool in the flight deck, performing many functions from advance warning of thunderstorms to a backup navigation system providing a position fix.   The capabilities and limitations of the radar system must be remembered

The transmitted radar beam and the returned signals from precipitation targets provide the weather information displayed on the indicator. To interpret the indicator display, it is necessary to understand how well the radar sees precipitation targets. 

The radar beam covers an area ahead of the airplane as shown in the diagram (Figure 11) below. The beam width of the antenna is determined by the width of the beam at the half-power points (antenna has a beam width of 3.5'). The diameter of the beam cross-section becomes very large as the range increases. Therefore, the resolution of the radar is much less at longer distances than at shorter distances. 

Figure 11

Another point should be considered: not all storm targets are equal in their ability to return a signal.  The chart below (Figure 12) shows the reflective levels one might expect from various weather targets.  Each target also absorbs or refracts a portion of the transmitted signal which makes targets behind areas of heavy precipitation much harder to detect.

Figure 12

The most important thing to remember is that the targets displayed on the weather radar indicator are large enough and / or intense enough to provide a processable return signal. Return signals from targets beyond a large storm cell are attenuated, and the displayed target does not accurately rep- resent the real storm cell. Two small intense targets may appear to be one, if the beam width is not smaller than the gap between the two targets.  The Figure 13 illustrates these points:

Figure 13

1. Radar Display Switch (Figure 15)

Pressed to activate radar and display radar returns on respective pilot's HSI in all modes except PLAN and Full VOR / ILS.  If both pilots' switches are out, radar is deactivated. 

2. GCS Switch (Figure 14)

When Ground Clutter Switch is pressed along with WX, ground returns are suppressed to aid in weather interpretation. 

CAUTION

This feature should only be used to identify ground targets. Continuous operation of the ground clutter suppression mode is not recommended because some precipitation returns may also be reduced in intensity. 

3. MAP Switch (Figure 14)

Pressed to display ground mapping.

4. TEST Switch (Figure 14) 

Pressed – Initiates test of the Radar System. The test pattern should contain three concentric ARCs consisting of green, yellow, and red. 

– Displays three magenta wedges (left, center, and right) indicating the Radar System is equipped for turbulence detection 

– Displays test pattern on both HSls in Expanded or MAP modes.

5. WX Switch (Figure 14)

Pressed – Displays weather returns. Depending on TILT, some ground returns may also display.

6. WX / T Switch (757 / 767 -300) (Figure 14) 

Pressed – Displays weather and turbulence if range selected is 40 nm or less. 

– Displays turbulence (magenta) when inside 40 nm.

7. Gain / UCAL Light (Figure 14)

llluminates to indicate gain control is in an uncalibrated position or in TEST.

8. TFR Switch (Figure 14)

Pressed to select opposite display unit to display the same modes, tilt and gain.

CAUTION 

The Weather radar system must not be operated with both the left TFR push-button and the right. TFR push-button selected at the same time. The mode, GAIN, GCS, and TIL T of the system can no longer be determined by viewing the control panel. The weather radar will default to the test mode until one of the TFR buttons is deselected. Note: To deselect a TFR button, one of the other functions must be selected. 

9. Tilt Control (Figure 14)

Varies plane-of-rotation from horizontal by tilting the antenna. The antenna scans 90' to either side of the airplane's heading and can be tilted while in level flight from 15 above to 15' below the horizon. 

Antenna stabilization corrects for pitch and roll of the airplane. Normally stabilization input is from the Left IRS. Antenna stabilization can also be supplied by Center IRS if selected using the ALTN IRS Switch.

Figure 14

Figure 15

System gain is manually controlled in the MAP mode. It can also be manually controlled in the weather detection modes.

Rotated Counter Clockwise – Takes gain out of CALibrated mode and decreases the gain from MAX uncalibrated to MIN uncalibrated.

Rotated Full Clockwise (CAL) – Presets the gain to detect precipitation at the rates shown in the chart below. 

Figure 16

When operating in the MAP Mode, the display is referenced to the aircraft track. The nose of the aircraft will be offset on the display to an amount corresponding to the drift angle as indicated by the white Heading Pointer. This is best demonstrated by selecting the TEST Mode while airborne with a significant crosswind component and observing the offset on the display. (See figure 17)

This offset can make a target on the display appear to be in a different position when the pilot observes the same target while looking outside. Notice that the target in the second illustration (Figure 18) intuitively appears to be to the left of the nose of the aircraft, however, by referencing the Heading Pointer you will see that the target is actually directly in front of the aircraft. Pilots should keep this offset in mind when applying information from the radar display to the picture they will see when looking outside. 

Figure 17                                               Figure 18

WXR FAIL – Indicates a failure in the weather radar system. 

WEAK – Indicates a detected loss of receiver calibration. 

ATT – Annunciates detected attitude input failure. 

STAB – Annunciates system stabilization failure. 

WX – Indicates precipitation detection mode. 

MAP – Indicates ground target detection mode. 

TEST – Indicates self-test mode of operation. 

TURB – Indicates turbulence detection mode. 

WX+T – Indicates turbulence and precipitation detection mode. 

VAR/(mode) – Indicates gain is manually controlled and the mode of operation. 

PAC ALERT BAR – A yellow arc painted at outermost range mark, identifying azimuth direction of intervening, severely attenuating precipitation within 80 nm of aircraft.

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