June 30, 2015

Dramatic images of damaged airliners following encounters with severe hail this summer drive home the need for business aircraft flight crews to be aware of the conditions in which they might encounter this damaging weather phenomenon.

“Overall, avoiding hail usually goes hand in hand with avoiding thunderstorms,” said John Kosak, weather project manager at NBAA Air Traffic Services. “One of the criteria for a severe thunderstorm is hail greater than one inch in size, while hail of three-quarters of an inch or greater in size is one of the criteria for issuance of a convective (significant meteorological information) SIGMET.”

Flight crews have encountered hail in completely clear air, as far as 20 miles downwind from storm activity and at altitudes up to FL450. “All you need is a nasty little updraft,” Kosak said. “Moisture accretes as it rises and falls within the storm, sometimes over the course of several minutes, with hailstones ultimately ‘spit out’ to fall in any direction the winds carry them.”

Parked aircraft sometimes require extensive repairs to hail-dented external surfaces, but inflight encounters with hail can cause even greater damage. In the past month alone, two airliners have made emergency landings – one in Denver, the second in Beijing – with mangled radomes, cracked outer windscreens and other damage from hail.

To avoid such events, Kosak encouraged business aviation pilots to utilize advanced forecasting tools such as the Collaborative Convective Forecast Product, Extended Convective Forecast Product and Collaborative Aviation Weather Statement. View these resources, along with the NOAA/NWS Storm Prediction Page.

Kosak also stressed that pilots must be wary of relying on in-cockpit Nexrad weather displays to stay clear of convective activity. “Latency issues mean the ‘real-time’ information displayed may actually be as much as 15 to 20 minutes old,” he said. “That’s scary for a flight crew trying to skirt the edge of the storm.”

Lastly, the Aeronautical Information Manual section on thunderstorms likely offers the best advice for exiting a hailstorm encounter: “Don’t turn back… A straight course through the storm most likely will get the aircraft out of the hazards most quickly. In addition, turning maneuvers increase stress on the aircraft.”