Tuesday, March 12, 2013
Weather Avoidance Techniques
Staying Out of Trouble When it isn’t CAVU
To use it effectively, you'll need to understand antenna tilt, attenuation, how the manufacturer maps intensity to color, storm gradient and shape, range settings, gain, how to estimate radar tops and optional features like stabilization, turbulence detection, wind shear detection and sensitivity timing control. Failing to understand these features may result in misinterpreting the radar picture and flying into an area where you really don't want to be!
One key thing to understand is that any radar, whether airborne or ground-based (including NEXRAD) detects only water. It can't show clear air turbulence, and also can't detect ice or dry hail. Tops of thunderstorms at very high altitudes may not be detectable as they consist almost entirely of ice.
As a practical matter, onboard radar is available only on multi-engine aircraft and the largest singles. It's expensive, and radar performance improves with antenna size—the smallest available is generally around 10 inches in diameter. You're not going to fit something like that on a typical piston single. An alternative technology that can be used on any airplane with an electrical system is lightning detection.
Lightning detectors are essentially sophisticated directional radio receivers that listen for the radio noise emitted by lightning strokes. They estimate distance based on the strength of the noise pulse. The result is a display that shows approximately where the lightning is relative to your aircraft. It has two advantages over radar-based systems: It works whenever there's lightning, whether there's water present or not, and it gives you a 360-degree view, showing activity to the sides and even behind you (onboard radar generally is limited to a narrow arc directly ahead).
Just as with data link and onboard radar, onboard lightning detection has limitations. The most significant is that only cells generating lightning are shown. Lightning detectors can't warn you about heavy rain (or other precipitation) that isn't generating lightning. This equipment also has a range limit, and there are other issues that vary from one type of detector to the next—you'll want to study the manual before flying with one in heavy weather.
Practice And Plan
For all types of sensors, practice is essential whether you have radar, data link, lightning detection or some combination of different sensors, you should use them regularly—even on flights where you don't expect to encounter significant weather. You don't want to find yourself trying to figure out the fine points of your equipment while in the soup on a rough day!
Before departure on any flight where significant weather is expected, get a complete preflight briefing so that you know what to expect—and make sure whatever weather avoidance equipment you have is working: Preflight inspection of radomes/antennas and ground tests of each sensor are critical before takeoff.
If you're launching into active weather, turn on whatever sensor(s) you have, but bear in mind that data link weather can be old—if you're looking at cells, check the age of data and think through how they may have moved/developed.
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