Aircraft Systems - Weather Radar Basics

Aircraft Systems - Weather Radar

• General Information: This manual provides basic knowledge of onboard weather radar systems. Refer to the manufacturer's user guide for detailed information on specific radar models.
• Weather Detection: Weather detection relies on the reflectivity of water droplets. Radar displays weather intensity with a color scale, ranging from red (high reflectivity) to green (low reflectivity). Reflectivity is dependent on droplet size, composition, and quantity with water particles having greater reflectivity than ice particles of the same size.
• Weather Radar Principle: The radar detects various weather phenomena. High reflectivity indicates features like hail or rain; low reflectivity indicates fog or drizzle. Different precipitation types (wet hail, wet snow, dry hail, dry snow, drizzle) are indicated by various intensities.
• Radar Limitations: Weather radar does not detect clouds or fog, or clear air turbulence; these phenomena have small or no droplets.
• Storm Cell Detection: The purpose of weather radar is to detect and avoid storm cells (e.g., cumulonimbus). Storm cells vary in reflectivity depending on altitude, decreasing with height. Radar has an upper detection limit called the radar top.
• Radar Top: The radar top isn't the visible top of the storm; the storm cell and associated turbulence extend significantly above the radar top.
• Manual Tilt Management: The tilt angle refers to the angle between the antenna beam and the horizon. The tilt knob is used to direct the beam to the most reflective part of a storm cell to avoid an overscan of the storm cell.
• Gain Setting: Using calibrated gain (CAL or AUTO) is the default setting for weather detection. A standard display of colors is ensured using this setting enabling the flight crew to manually adjust if needed.
• Range Management: Monitoring both long and short-distance weather is crucial for efficient flight planning. Accuracy is lower at long distances due to beam widening and signal attenuation, but higher for short distances.

Weather Detection

• Controls and Functions: Flight crews use TILT, GAIN, and RANGE controls to operate the weather radar.
• Manual Tilt Management: The tilt angle is adjusted to focus the radar beam on the most reflective portion of a storm, preventing overscanning.

Specific Weather Shapes

• Interpretation: The flight crew should focus on shapes, rather than color in order to detect potentially adverse weather conditions.
• Areas of similar color: Areas of the same or similar color represent areas of high turbulence.
• Shapes: Different shapes (such as closely spaced, finger, hook, U-shape, or scalloped edges) can indicate adverse weather conditions.

Attenuation Effect

• Heavy Precipitation: In areas of heavy precipitation, the weather radar signal is often reflected by the front of the precipitation, with the region behind showing less reflectivity or as black areas (storm shadows)
• Radar Attenuation: This occurs, and the radar signals lose strength, allowing the flight crew to identify very active areas of precipitation. Some radar systems have visual indicators that highlight areas with significant attenuation.

Manual Gain for Weather Analysis

• Manual Gain Adjustment: Manual gain adjusts the color calibration to show weather as stronger or weaker. This can be used to assess overall weather conditions.
• Heavy Rain Scenarios: When operating in heavy rain, the weather radar can become saturated. Reducing the gain helps to identify areas of heaviest rainfall.

Radar Interference

• High Power Sources: High-power external radio frequency sources nearby can cause interference. Radar returns may appear as a wedge on the ND or as a bright or unusual area, depending on the source distance.
• Spoke/Alien Radar: Interference may be known as “spoking" or “alien radar" on the radar display. This doesn't harm the radar system.

Operations in Convective Weather

• General Advice: The flight crew should use operational recommendations along with meteorological principles and knowledge to navigate through convective weather.

Ice Crystals

• Detection: Ice crystals are difficult to identify on weather radar screens as they have a low reflectivity. Ice crystals are most common around convective weather systems.
• Indications: Ice crystals are frequently linked to or associated with heavier, more intense precipitation, and can be identified by visual phenomena like "rain" appearing in too low of temperatures, a "shhh" noise, or ozone smells.
• Precautions: Flight crews should avoid areas of ice crystals and may need to use additional flight maneuvers/altitude adjustments to navigate safely

Operational Recommendations for Ice Crystals

• Weather Radar Use: The radar system needs to be carefully analyzed to understand the shape and intensity of the weather system, to determine the risks involved and possible avoidance actions
• Avoidance: The best approach is generally to deviate upwind of the expected or potential threat areas
• Flight Maneuvers: Avoid flying in areas that show potential hazards for ice crystals and comply with recommended margins and operational actions

Emergency Procedures

• ECAM Alerts: If ice encountered despite avoidance, ECAM alerts (Engine Control and Monitoring System) and other emergency procedures in the QRH (Quick Reference Handbook) might be triggered and need to be followed.