Thursday, March 1, 2007
Severe Weather Flying
Dennis Newton’s book reviewed
More Severe Weather
Other by-products of thunderstorms receive Newton’s treatment:
Low-level wind shear. “The downburst,” as Newton describes, is defined as a “localized, intense downdraft with vertical currents exceeding a downward speed of 720 feet per minute at 300 feet above the surface.” Caught in a downburst unaware, the pilot of even the most capable aircraft might not react in time to recover before impact—borne out by severe air carrier accidents that helped scientists discover the phenomenon, and the smaller yet even more intense microburst. When a downburst hits the ground it veers outward in a circular “outburst”, creating the classic Low-Level Wind Shear pattern and the Low-Level Wind Shear Alerting System (LLWAS). Newton discusses research into LLWAS and the FAA’s three programs for improving the system at major airports. More importantly, he provides a table of Microburst Wind Shear Probability Guidelines, correlating observed weather phenomena (for example, virga or temperature/dew point spreads) and the probability of dangerous wind shear. And as always, Newton gives specific avoidance techniques, and how to escape if those attempts at avoidance fail.
Lightning. The most obvious indicator of a thunderstorm, albeit not the most dangerous, lightning nonetheless poses a threat. “There have been many, many lightning strikes on airplanes that have done no more damage than a small hole somewhere in the skin,” Newton writes. “Once in a while, though, something…happens.” In one case, illustrated in the book, “about the last six feet of wing came wide open at the first line of rivets aft of the leading edge. Once in a while also [a lightning-struck airplane] goes down.” Newton tells us “lightning protection provisions in modern aircraft are excellent” and research “has certainly proved that a properly protected and maintained airplane can take an essentially unlimited number of strikes. Nonetheless, the power of lightning should be treated with great respect.”
Newton details the formative process of lightning and relates that to research conducted in lightning-test aircraft. “More has probably been learned about lightning since about 1980 than in the whole of human history” as a result of testing for aviation applications. He pinpoints the altitudes and areas around storms where lightning most commonly formed, relating it particularly to the freezing level, and describes the types of damage most commonly resulting from lightning strikes. This leads to a list of four recommendations for avoiding adverse effects of lightning when flying in proximity to storms, and a chapter on in-flight thunderstorm detection equipment (radar and lightning detectors).
Page 4 of 7