Page 14 - April 2016

HAIL?NT O! he way to not get hailed onsuccessfully diverted into Chattanooga, using airspeed/ altitude relays from ATC.”An experience like that tends to make you wonder if perhaps being shot out of a cannon at the circus wouldn’t be a safer occupation.How to avoid hail encounters is the question. An obvious answer is to stay out of thunderstorms. That’s only a partial answer, however. Hail encounters have occurred 10, 15 even 20 miles entirely outside thunderstorms. In addition, hail damage has occurred in areas where there were only rain showers 15 minutes earlier. Sudden, explosive hail storms like that will only increase with global climate change, we are told.What is the answer, then, to avoiding a hail strike incident? You must begin before flight by discovering where hail is likely to be encountered during the day. For the USA that’s simple; NWS has made it easy for us to begin that investigation. It’s where you should begin first thing each morning, whether you’ll be flying that day or not.Call up spc.noaa.gov. You’ll be rewarded with a USA map. Select “Convective Outlook” on the right side and you’ll get another USA map with colors indicating where thunderstorms are likely to occur within the next few hours. At top left, you’ll find three tabs. Select “HAIL” and you’ll get a revised USAAvoiding Hail Hazardsby Archie Trammellmap showing where hail is likely tooccur this day.What you get is not a simple forecast of hail; it’s a “Probabilistic Forecast” of where hail is likely to be encountered. “Probabilistic” and “Forecast” are vastly different things. A forecast is based on current and expected atmospheric conditions at a location; a “probabilistic forecast” is based on what several meteorologists at SPC, after long experience, have observed typically happens in a certain location when a particular set of atmospheric conditions exist there. Big difference.Note that the spc Outlook doesn’t indicate hail will occur in the location, only that it typically does. Check it out day to day and your confidence in its accuracy will grow. That doesn’t mean, however, hail is not possible elsewhere, given certain atmospheric conditions. What are they? Just five principal ones of interest to pilots; storm height above the freezing level, wind direction and velocity at higher levels, speed of storm movement, radar reflectivity and storm shapes.Many NEXRAD uplink vendors provide storm height, movement and speed of movement. Otherwise, go to www.aviation weather.gov/windtemp and you’ll be covered with tons of WX information. For a quickie, Flight Service should also be able to give you storm heights and speed of movement. There are numerous websites for aviationis to avoid places where it’shailing. Talk about a ruined day! It happens a dozen or so times each year. Most of the time it’s small hail, so no damage is done except chipped paint and an apology to your laundress. But, occasionally, hail is a big problem. For example, the following is from an NTSB report in reference to a hail encounter in a DC-9 during climb-out from Atlanta:“They were examining the line of thunderstorms on their radar and requested a turn to 330 for weather, which was approved. On that heading they could see a 10 nm-wide gap between two cells. They could also see there was no weather on the other side of the line. Suddenly, as they entered the gap, an egg-sized hailstone smashed into the center windshield, shattering the outer pane. Turbulence began and lasted about 10 seconds. Large hail continued for about four seconds. It took out all outer panes of the windshield and the entire radome departed, going through the right engine. The pitot and static systems went with the radome, leaving the crew with no airspeed and unreliable altitude information. Of course, there was extensive damage to all leading edges and the right engine, which did continue to put out power.They exited the weather on the north side of the line of thunderstorms and the crew12 • TWIN & TURBINEAPRIL 2016