Professional Pilot, May 2019

lightning stroke as a mature cell So pilots need to rely on information from forecasters and briefers to understand where storms are likely to occur and anticipate that even if the information calls for few or widely scattered storms they may pop up anywhere in the forecast area and at any time during the forecast period Things are a little more straightforward with frontal storms which will form and decay just behind the surface cold front or along a prefrontal squall line Storms may also form as part of a larger storm complex such as a hurricane or Mesoscale Convective System MCS It should go without saying that any region of organized storms should be avoided if possible Dangers Obviously few pilots will intentionally penetrate a storm or even fly close to one because they recognize the dangers these storms pose But in the rare instance where a pilot inadvertently flies into a thunderstorm they can expect the worst forms of many of the dangers they face any time they take to the skies Ironically while lightning the main indicator of a thunderstorm is a danger and may temporarily blind a pilot fry avionics or leave a small hole in the aircraft it doesnt bring down aircraft 74 PROFESSIONAL PILOT May 2019 Most towering cumuli including cumulonimbus rise through the freezing level Since cloud and rain droplets can remain in liquid state to temperatures of around 40 C 40 F the subfreezing regions of the storm are where severe to extreme icing can quickly coat the aircraft Supercooled large droplets will transform into solid ice when they strike the subfreezing skin of the aircraft and the ice will likely accrete at a rate far exceeding any ability of the aircraft to shed it Even a descent may not help as the freezing level may be too far below to melt the full quantity of accreted ice before it becomes unmanageable The air currents within the storm pose one of the greatest dangers Up and downdrafts along with the eddies they shear can strike an aircraft from any direction or multiple directions simultaneously The associated turbulence is normally characterized as severe to extreme While the actual force of a 100 kt wind striking an aircraft is unlikely to do physical damage on its own when the aircraft is struck by strong winds from different directions at the same time the torque can be enough to bend and break steel spars More likely however is that a single strong eddy is able to cause a loss of control and the gain or loss of hundreds if not thousands of feet of altitude Hail is another danger inherent to thunderstorms An aircraft within a mature thunderstorm can be struck repeatedly by hailstones cycling through the storm cell Eventually the hail produced within the storm either falls from its base or is ejected from the top front sides or rear Ejected hail has been observed traveling 20 miles or more from the storm that produced it Where the updraft tilts forward into the anvil of a large storm hailstones can be thrown from the anvil even greater distances This is one of the reasons why staying at least 20 miles from any storm is important A safer margin is avoidance by at least 1 mile horizontally for every 1000 ft of storm height and 1000 ft above the storm top for every 10000 ft of storm height It is never a good idea to fly beneath a thunderstorm or even near the base of one Not only can hail strike your aircraft or rain cause you to lose visibility at a crucial moment this is also the realm of strong outflow including downbursts and microbursts Microbursts produce low level windshear that has slammed many aircraft into the ground as they were attempting to land Because they were already low and slow they had little chance of recovery from the loss of control After several high profile commercial crashes many airports installed windshear alert systems and pilots received training in microburst recovery But the best safety is Mature thunderstorm discharges a lightning stroke Thunderstorms are frequently characterized as stratosphere reaching anvil headed giants but they come in all sizes Even the smallest thunderstorm can pose extreme risk to aircraft Downburst from an airmass thunderstorm over LYB Edward Bodden Airfield Little Cayman Cayman Islands Even the weakest tropical airmass storms can produce strong downdrafts and low level windshear Photo by Karsten Shein Photo courtesy WMO