Pilots use the radar to find and assess convective weather in the flight path. This assessment can then be used to plan the avoidance maneuver before.
The onboard weather radar is one of the most important pieces of equipment in an aircraft to ensure safe operation. Pilots use the radar to find and assess convective weather in the flight path. This assessment can then be used to plan the avoidance maneuver before it is encountered by the aircraft.
How does the weather radar work?
The airborne weather radar consists of the following:
- A transmitter
- A receiver
- An antenna
- The cockpit control and display.
The antenna of the weather radar is attitude stabilized using inertial data sent by the aircraft inertial reference units. This means that the antenna position remains static regardless of the motion of the aircraft. This ensures proper radar imagery for the pilots.
The radar works on the echo principle. The radar transmitter produces a signal which is reflected by water droplets in the clouds. The reflected signals are then collected by the receiver and processed to give the weather display to the pilots. The signal consists of a narrow radio beam with a width of about 3 degrees.
The beam width needs to be as narrow as possible, as wider beams can cause the radar to interpret the weather incorrectly. This is because wider beams reduce the effectiveness of the radar at a distance. The pilots need to know the weather well ahead of the aircraft so that they can plan out their deviation maneuvers. When the beam is wide, the radar may interpret two separate convective clouds as one, until the aircraft gets too close to the clouds.
The use of narrower beams is therefore essential. However, narrower beams require a large antenna. This is not very practical as there is a limit to the size of the antenna that can be carried by aircraft. The good news is that there is another way by which the beam width can be reduced – using waves with a shorter wavelength. Consequently, the radar operates at a significantly high frequency of about 9375 MHz. This gives a wavelength of about 0.032 m or 3.2 cm. It is calculated using the wave equation as follows:
Lambda (wavelength) = 300 m / 9375 MHz
= 0.032 m/ 3.2 cm
This wavelength is also approximately equal to the diameter of a large water droplet. So, with the frequency and the wavelength, very accurate weather interpretation is possible.
Weather radar display and controls
The weather is displayed to the pilot in the cockpit navigation display. In older aircraft, the weather radar has its own display.
Most modern weather radars have color displays. The colors are based on the intensity of the rainfall in a weather cell. The color codes are as follows:
- BLACK – Less than 0.7 mm/ hr (very light to no returns)
- GREEN – 0.7 to 4 mm/hr (light returns)
- YELLOW – 4 to 12 mm/hr (medium returns)
- RED – Greater than 12 -15 mm/hr (strong returns)
- MAGENTA – Greater than 50 mm/hr
The pilot can control the weather radar using various input options on the weather radar control panel. One of the most important of these controls is the radar tilt. The radar tilt is the angle between the radar beam and the horizon. As discussed earlier, the radar is unaffected by the aircraft’s pitch, yaw and roll unless the pilot plays with the tilt control.
The tilt must be adjusted by the pilot so that the radar is always pointing to the most convective part of the storm cell. When climbing, the radar tilt is lowered for this reason, and during descent, the tilt is progressively moved up. When cruising at about 35,000 ft, the radar tilt is positioned about -1.50 degrees below the horizon. This allows the radar to look at the bottom areas of the clouds where the most convective weather exists. If the radar’s tilt is too high when flying at high altitudes, it may only cut through the upper parts of the cell, which mainly consists of ice crystals that are hard to detect. This might give the pilots a wrong impression of the weather ahead.
The next available control is the gain control. This is an important tool that can be utilized by pilots when analyzing the weather. When the gain of the radar is increased, the color calibration of the weather radar is adjusted so that the weather appears stronger. The gain can be used to assess a cell well far away from the aircraft. However, when the weather is close by and in heavy rainfall, an increased gain can oversaturate the weather display. So, its use must be limited only to study distant weather.
The weather radar operation is highly enhanced in the hands of a knowledgeable pilot. Understanding the behavior of storm cells and the effective use of radar control requires a fair amount of knowledge.
One of the major fallacies of the weather radar is known as the attenuation effect. This happens when heavy rainfall (highly reflective) blocks the convective weather behind it. This can give the pilots a wrong display of the conditions ahead, as the radar might not be able to detect the hidden weather. This is also known as storm shadows effect.
In 2002, a Garuda Indonesia Boeing 737 was forced to make a water landing after a two-engine flameout. The cause of the flameout was the ingestion of heavy rain and hail by the engines. The following investigation concluded that the pilots entered an area of strong convection unknowingly because of radar attenuation. It was found out that the airline did not formally train their pilots to use the weather radar.
Due to the attenuation effect, it is never recommended to go through a storm cell even if the end of the cell shows no sign of strong convection in the radar display. Some radars have a function called Rain Echo Attenuation Compensation Technique (REACT). The REACT can detect attenuation by measuring the intensity of the signals and highlighting the areas where the interpreted weather is doubtful.
How do pilots use the weather radar to avoid storm cells?
Foremost, the weather is detected by using the radar and its control functions. Then the weather radar display is analyzed to find the greatest area of convection. A red or a magenta target, for instance, is considered an area of the highest risk. Once the analysis phase is finished, the actual avoidance can begin. It is highly recommended to start the avoidance maneuver as quickly as possible. Typically, once the weather is within 80 NM range, the decision of where and which direction to deviate must be made.
As a general rule, the weather must always be avoided laterally by at least 20 NM from the area of the greatest danger. It is also recommended to deviate to the upwind side of the cell as the weather tends to move with the wind. So, if the deviation is made downwind, the weather might catch up with the aircraft, requiring an even larger deviation.
Vertical deviations by trying to ‘climb up’ the weather are highly discouraged. One of the reasons for this is, at high altitudes, jet aircraft are close to their low and high-speed buffet margins and are performance-limited. In such conditions, getting into turbulence might not be such a great idea as there is a chance of loss of control. The other reason is that at high altitudes, clouds are highly unpredictable and powerful. It is important to keep in mind that only a very highly convective cloud can sustain itself at 30,000+ ft. altitudes. The cloud might build up vertically at such a rate that it could very well engulf the aircraft before it could climb out of it.
Source: Simple Flying