Professional Pilot, May 2017

WX BRIEF Wind The simple movement of air remains one of the most challenging weather factors for pilots Wind is an oft invisible danger to aviation and the leading weather factor in accidents Visual cues such as these cap clouds above a mountain ridge provide valuable clues about potentially dangerous wind behavior A s the pilots brought their bizjet in for landing on the final leg of a long transcontinental flight they heard Charlie One Niner windshift to 270 at 25 gusting to 40 Possible windshear cleared to land runway 18L At 400 ft they were still in wind that was right down the centerline for runway 18L so low level shear near the surface was a distinct likelihood not to mention a stiff crosswind landing As the airplane neared the approach end of the runway at about 50 ft there was a noticeable buffeting and the aircraft immediately drifted to the left of the centerline A strong gust hit the aircraft side on and the pilot at the controls instinctively lowered the right wing and pushed rudder Fortunately his more experienced colleague wrested the wings level again while simultaneously pushing the throttles forward for a go around The right wing cleared the runway by less than 3 feet By the time the aircraft made its 2nd approach the shear zone had dissipated and the flight made an uneventful landing 72 PROFESSIONAL PILOT May 2017 Wind is something that every pilot deals with on every flight Most of the time we know exactly how to handle it but at the same time it is also the weather factor most cited as contributing to aviation accidents Unlike a thunderstorm or fog wind is an invisible foe that may turn sour without much if any warning But understanding the likelihood of encountering an adverse wind event will make it much more manageable Wind is nothing more than the force that air molecules exert as they run into us or our aircraft while flowing from one place to another But that force can be enough to knock us over or push an aircraft off course unless we account for it Force is defined as mass multiplied by acceleration so wind tends to pose the greatest threat to aviation where relatively dense air is accelerating For pilots these places would be in close proximity to the ground and in highly dynamic environments such as in the vicinity of thunderstorms or jet streams The movement of air is the result of density differences between those 2 places If a spot is heated its air molecules move apart lowering density Conversely if a spot cools the molecules huddle more closely increasing density The result is that air density varies from place to place depending on the heating received Since the atmosphere is always attempting to maintain equilibrium a mass of molecules will flow from the high density region toward places of lower density Sea breeze A classic example of this flow based on heating differentials is sea breeze circulation A sea breeze is established in the daytime when the land heats up more quickly and to a greater degree than the adjacent water which has a much higher capacity to absorb heat The air over the heated land also warms and the molecules spread apart lowering the density Simultaneously the air above the water remains cool leaving the air density relatively higher than the air over the land The difference establishes a circulation flow At the surface air molecules flow in shore from the water to equalize the lower density over the land Simultaneously the heated air over the land also begins to flow upward Aloft air is drawn downward to fill the area over the ocean vacated by the landward flowing surface air and in turn the air that ascended over the land is moved out aloft over the ocean to complete the circulation At night the land cools faster than the water and so the surface heat differential is reversed setting the circulation in motion the other way This circulation is a common feature of many coastal locations and it often results in daily storm fronts that move onshore during the late morning and migrate offshore after dark Pressure gradient In the atmosphere this differential heating between 2 places can occur at just about any scale from an entire hemisphere to the end of a By Karsten Shein Comm Inst Climate Scientist