The Thunderstorm Briefing
Thursday, March 1, 2007
Severe Weather Flying
Dennis Newton’s book reviewed
|Severe weather. Who would ever think about flying in it, or around it? Yet a book about severe weather flying has been highly popular and successful for over 20 years, and is now in its third edition.|
Newton provides guidance for obtaining a preflight weather briefing to better understand the severe weather threat. His recommendations for a thunderstorm threat include detailed explanations of the use of the:
- Stability chart, “seldom used or asked for in weather briefings” but which “will alert you at once to a potentially explosive situation.”
- Convective Outlook, “a description of the areas, if any, in which severe thunderstorms are considered possible, as well as a general thunderstorm forecast.”
- Surface chart, “particularly…moisture and lifting” information that he details.
- Low-level Winds Aloft, to “see if there is a general pattern of wind from large bodies of water….”
- Radar Summary, to detect “areas of [already extant] organized activity” and cloud top heights. The chart also contains severe weather watch boxes. Newton cautions pilots to “look at the time the chart was prepared” and to “remember that 2 hours is a long, long time when you are talking about things that can grow at a rate of over 6000 feet per minute.”
- Hourly sequence reports (METARs), to check for dew points, cloud layers, haze and remarks about nearby towering cumulus clouds that indicate possible storm development.
- Convective SIGMETs, which report existing storms and their forecast movement.
- Severe weather watches, as a backup to watch boxes on the radar summary.
Lastly, Newton suggests pilots “look at what the forecast said the weather would be right now,” using forecasts from your earlier weather checks as a guide, “and see how the forecast is panning out so far.”
Airframe icing, according to Newton, is “probably the weather hazard second only to thunderstorms in the opinion of those pilots who spend a lot of time at relatively low altitudes.” Interestingly, “the average experience of pilots involved in icing accidents was nearly 3000 hours, and over 70 percent of them were instrument rated”—confirming ice is a hazard even to “high-time pilots and sophisticated aircraft.”
Newton covers the important aspects of flying under the threat of airframe ice:
- How ice forms on an airplane, and where.
- Icing clouds: where they exist and why.
- The present (“and still sorry”) state of icing forecasts.
- How to recognize and avoid icing clouds in flight.
- The effects of ice accumulation on an airframe.
- The legalities of flight in icing conditions.
Ice formation. “The accretion of ice on the components of an airframe is caused by…factors includ[ing] cloud liquid-water content, temperature, cloud-droplet size, and the size and speed of the ice-collecting object.” The shape of accumulated ice depends on airplane speed and water-droplet size. The type (rime, clear or mixed) is also dependent on all factors including aircraft velocity through the air. The location of ice on an airframe is determined by the size of the airframe component (“small parts will collect ice first”; “it is possible to ice the…empennage with little or no icing” on the wings) and the size of water droplets (freezing drizzle or freezing rain, the result of large water droplets, can cause ice to form aft of protected leading edges of ice-protected airframes).
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