[Ref Safety first #22 July 2016 – 1/23]
Weather (WX) Radar Principle
The WX radar sent out a beam from its nose. The beam is converging from the transmitter at the nose.
Weather detection of weather radar is based on the reflectivity of water droplets.
The weather radar display depends on signal returns. The detection result – “echo” – appears on the Navigation Display (ND) with a color scale
In other words, “echo” can indicates only the presence of liquid water!
However, radar echo returns vary in intensity as a function – “echo” intensity affected by 3 factors:
- droplet size
- composition – Ice/ Water/ Hail
Now, the modern radar suppliers have already developed with product with a turbulence display mode. However, and again, radar can only detect water droplets, those turbulence function is just the product of algorithm.
Also, each type of weather radar has its ownnparticularities. Pilots should get all the information on the characteristics, limitations and operational recommendations of each weather radar model – the best way is to study the user guide.
Weather Radar Limitations
2 major limitations:
- detection capability (due to converging toward the transmitter)
- beam attenuation phenomenon (due to penetration difficulty)
Detection capability refers to the fact that in some case, the radar beam cannot cover certain area. For example, in some thunderstorm, the quantity of liquid water in the atmosphere decreases with the altitude (fig.4).
Do you remember?! WX radar’s reflectivity is based on water droplets.
In an other cases, air close to the sea can be very humid. In this case,
thermal convection will produce clouds that are full of water: these clouds will have a high reflectivity, but may not necessarily be a high threat.
Reflectivity is not directly proportional to the level of risk
Beam attenuation phenomenon refers to “shadowing” effect – precipitation is severe enough to block the WX radar’s beam – the more intense the precipitation, the less distance the radar’s beam can pass through and the bounce back energy creates a false signal.
Types/Development Of WX Radar
- Full manual control radars
- Autotilt radar
- Fully automatic radars (Multiscan)
Early generations of radars (Full manual control radars), the antenna tilt needs to be manually adjusted.
Later, Autotilt radar came to market. Honeywell introduced the first (1st) weather radar featuring an automatic tilt computation named “Autotilt”. The radar system combines the usage of the EGPWS terrain database to achieve the auto-tilt function.
Now, Fully automatic radars is ever advancing – Multiscan radar offers an automatic computation of tilt and gain control at all ranges, all altitudes and all times – Pilots select only the desired range for the display.
Multiscan radar sends out multiple beams and the radar alternatively scans at two antenna tilt settings. The image that is displayed on the ND is the result of the stored and combined information from each beam. (機師睇既係兩個角度既合成照片)
Operations Of WX Radar
PUTTING THEORY INTO PRACTICE: Weather radar is of help, but the crew overall assessment of the weather situation plays the central role.
A key element of adverse weather avoidance strategies is : You and Me (Crew)!
The weather radar is a tool. As with any other tool, adequate skills and the crew’s involvement are needed in order to use it efficiently. In fact, the management of adverse weather still relies primarily on the crew to actively monitor the meteorological situation throughout the flight, and make a full use of the available technology. The crew overall assessment of the weather situation plays the central role.
- Awareness of weather radar capabilities and limitations, according to the specificities outlined in the FCOM and the manufacturer’s user guide.
- Preflight briefing
- Regular manual vertical and horizontal scanning
- Adequate strategic (mid-term) and tactical (short term) decision making for trajectory planning
Skills About WX Radar OPS
- Weather radar antenna tilt
- Display range management
- Gain adjustment
Weather Radar Antenna Tilt
The ND might not display cells at aircraft flight level, only cells that are cut by the radar beam are shown. For this reason, the antenna tilt needs to be adjusted up and down regularly to scan weather ahead.
– Vertically, using the antenna tilt function
– Horizontally, using the range change
Factors affect relevancy of the ND display:
In the case of a change in heading or altitude, leaving the antenna tilt on auto may induce a risk of overlooking – only cells that are cut by the radar beam are shown!
Rule Of Thumb About Antenna Tilt
Presence of yellow or green areas at high altitudes, above a red cell, may indicate a very turbulent area.
Display Range Management
The flight crew needs to monitor both the short-distance and long-distance weather.
Pilot Monitoring (PM):
adjusts ranges to plan the long-term weather avoidance strategy
(in cruise, typically 160 NM and below)
Pilot Flying (PF):
adjusts ranges to monitor the severity of adverse weather, and decide on avoidance tactics
(in cruise, typically 80 NM and below as required)
To avoid threatening convective weather, the flight crew should make deviation decisions while still at least 40 NM away;
Benefit Of Monitor Difference Range On ND Display?
To prevents the “blind alley” effect: a course change that may seem safe when using a low range ND display may reveal a blocked passage when observed at a higher range
At low altitudes: reducing the gain
Due to increased humidity at lower levels
At high altitudes: gain should be increased
Due to water particles are frozen and clouds are less reflective
Weather detection of weather radar is based on the reflectivity of water droplets;
Only cells that are cut by the radar beam are shown on ND display
Turbulence And Weather Threats Detection
The TURB function needs humidity; therefore clear air turbulence (CAT) will not be displayed.