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Types of Turbulence. Mechanical Convective Downburst Clear Air Turbulence Aircraft-Produced. Turbulence. Mechanical Boundary Layer Turbulence. Determined by terrain, stability of the atmosphere, and wind speed. Mechanical Turbulence.
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Types of Turbulence Mechanical Convective Downburst Clear Air Turbulence Aircraft-Produced
Mechanical Boundary Layer Turbulence • Determined by terrain, stability of the atmosphere, and wind speed.
Mechanical Turbulence • Mechanical turbulence is caused by mountains, hill, buildings, or other large obstructions. When winds above 15 knots are bent around the objects, this creates the turbulence. • Usually this isn’t a major problem for most aviation, but when they winds become above 25 knots and pass over a large enough obstacle they can create turbulent areas as high as 5,000-10,000 feet.
Mountain Wave Turbulence Strong winds are forced over and around large terrain features Turbulence often is severe Characteristic cloud patterns
Scales of Turbulence and Eddies Whirling eddies of wind are generated at various scales and orientations. Here are horizontal whirls leeward of an obstructing mountain and vertical rotors, or roll eddies, that extend for many kilometers.
Billow Cloud • Billow cloud caused by wind shear (where there was sufficient moisture to show eddy). Courtesy of Dr Moser, ERAU Dept of Aviation
Convective Turbulence • Daytime: surface is hot, air above surface is cooler • In lowest 1 km, when atmospheric lapse rate is greater than the dry adiabatic rate, a rising parcel is warmer than it’s surroundings, and will keep rising • These warm volumes of air carry thermal energy upwards: convective heat transport
Convection Effects • Some surfaces absorb and radiate energy at a much higher rate than others. • This creates a light form of Convective turbulence. • Common when two areas of land heat at different rates (such as a large parking lot next to a field) • When an aircraft flies into this it may experience vertical shifts as it moves over the two areas boundaries. • This is also known as Thermal Turbulence.
Convective Turbulence • A more severe type of convective turbulence is caused when the convective currents form cumulus clouds. • Inside the clouds the air is unstable • Frequently aircraft come out at much higher level or lower levels then they entered. • The downdrafts and updrafts can be strong enough to cause structural damage because the aircraft is moved vertically at such a fast rate.
Thunderstorm Downbursts • Strong downward rush of winds from a mature thunderstorm. • Causes serious takeoff and landing hazards: • When aircraft is most vulnerable—close to the ground and operating near stall speed
Downburst Winds Virga
Microburst – Takeoff and Landing • -3 factors vital to aircraft survival: • Altitude at which shear is experienced • The pilot’s experience • Type of Aircraft
Clear Air Turbulence • Fast winds over slow winds in neutral or unstable environment will cause turbulent eddies within clear atmosphere (no cloud signs). • Often found in the vicinity of the jet stream core • At least, an inconvenience • At worst, cause significant problems • Strength depends on vertical wind shear and vertical temperature profile (stability). • ADDS – Turbulence
CAT—An Aviation Hazard • In December of 1992, a DC-8 cargo plane westbound out of Denver encountered severe clear air turbulence. Despite losing an engine (far right side) and 6 meters of wing, the crew managed to land the plane safely.
Wake Turbulence • All aircraft produce wake turbulence. • Wake vortices are formed any time an airfoil is producing lift. The pressure differential across the wing triggers the rollup of the airflow aft of the wing resulting in swirling air masses trailing downstream of the wingtips. • Viewed from behind the generating aircraft, the left vortex rotates clockwise and the right vortex rotates counterclockwise. • The strength of the vortex depends primarily on aircraft weight and configuration. • The strongest vortices are produced by heavy aircraft, flying slowly, in a clean configuration. • While there have been rare instances where wake turbulence caused structural damage, the greatest hazard is induced roll and yaw. • This is especially dangerous during takeoff and landing when there is little altitude for recovery. • Helicopters also make wake turbulence • Tends to be much stronger than fixed wing aircraft of the relatively same size.
Turbulence Categories • Light turbulence - briefly causes slight, erratic changes in altitude and/or attitude. • Light chop - slight, rapid and somewhat rhythmic bumpiness without noticeable changes in altitude or attitude. • Moderate turbulence - similar to light turbulence, but greater intensity. Changes in altitude/attitude occur. Aircraft remains in control at all times. Variation in indicated air speed. • Moderate chop - similar to light chop, but greater intensity. Rapid jolts without obvious changes in altitude or attitude. • Severe turbulence - large, abrupt changes in altitude/attitude. Large variation in indicated airspeed. Aircraft may be temporarily out of control. • Extreme turbulence - aircraft is violently tossed about and is impossible to control. May cause structural damage.
