Ch 12 Turbulence Introduction A characteristic of most

Ch 12 - Turbulence

Introduction • A characteristic of most naturally occurring fluids is that • • they contain some degree of turbulence This means that you can usually find some part of the fluid where the velocities are fluctuating in a chaotic manner. The atmosphere is one of those fluids The velocity fluctuations found within the atmosphere are often weak and barely noticeable in flight. Occasionally, however, atmospheric turbulence is so strong that passengers and crew are injured and the aircraft is damaged or destroyed

Introduction • The purpose of this chapter is to provide information that • • will help you avoid or at least minimize the effects of turbulence on your flight. When you complete this chapter, you will understand the basic types of turbulence and their causes, and you will know the large-scale conditions under which turbulence occurs. Also, you will have learned some rules of thumb that will help you anticipate and deal with the turbulence problem.

Ch 12 - Turbulence • Section A – Turbulence Defined – Aircraft and Pilot Response – Turbulence Measures • Section B – Turbulence Causes & Types – Low-Level Turbulence (LLT) • Mechanical Turbulence • Thermal Turbulence • Turbulence in Fronts • Wake Turbulence

Ch 12 - Turbulence – Turbulence in and Near Thunderstorms (TNT) • Turbulence Within Thunderstorms • Turbulence Below Thunderstorms • Turbulence Around Thunderstorms – Clear Air Turbulence (CAT) – Mountain Wave Turbulence (MWT) • Lee Wave Region • Lower Turbulent Zone

Ch 12 - Turbulence • Section A: Turbulence Defined – Aviation turbulence – Based on descriptions from pilots, crew, and passengers, aviation turbulence is best defined simply as “bumpiness in flight. ” • This definition is based on the response of the aircraft rather than the state of the atmosphere.

Ch 12 - Turbulence • Aircraft and Pilot Response – Turbulent gusts – Atmospheric motions produced by turbulent eddies are often referred to as turbulent gusts – Maneuvering – If the pilot (or autopilot) overreacts, control inputs may actually add to the intensity of bumpiness.

Ch 12 - Turbulence • Turbulence Measures – Turbulence reporting criteria – Turbulence intensity varies from light, moderate, severe to extreme and is related to aircraft and crew reaction and to movement of unsecured objects about the cabin. – G-load – Also known as gust load, this force arises because of the influence of gravity.

Ch 12 - Turbulence • Section B: Turbulence Causes and Types – Low-Level Turbulence – Defined as that turbulence which occurs primarily within the atmospheric boundary layer. • The boundary layer is the lowest few thousand feet of the atmosphere; – that is, where surface heating and friction influences are significant. – Mechanical Turbulence – Over flat ground, significant LLT occurs when surface winds are strong.

Ch 12 - Turbulence • ***The type of approach and landing recommended during gusty wind conditions is a power-on approach and a power-on landing

Ch 12 - Turbulence – Turbulent wake – Typically, a trail of turbulent eddies is produced downwind of an obstacle with a sheared layer between the ground-based turbulent region and smooth flow aloft. – Funneling effect – Similar to the increase in the speed of the current of a river where it narrows, strong local winds with substantial LLT and wind shear are created when a broad air stream is forced to flow through a narrow mountain pass. • Strong winds due to this funneling effect may extend well downstream of the pass.

Ch 12 - Turbulence – Thermal Turbulence • Thermal turbulence – Thermal turbulence is LLT produced by dry convection in the boundary layer. – It is typically a daytime phenomenon that occurs over land under fair weather conditions.

Ch 12 - Turbulence • ***The characteristics of an unstable cold air mass moving over a warm surface are cumuliform clouds, turbulence, and good visibility. – A stable air mass is most likely to have smooth air.

Ch 12 - Turbulence • Capping stable layer – This layer is caused by a very slowly sinking motion aloft; – typically associated with a macro scale high pressure region.

Ch 12 - Turbulence • Turbulence in Fronts – Wake turbulence – The term wake turbulence is applied to the vortices that form behind an aircraft that is generating lift.

Ch 12 - Turbulence • ***The greatest vortex strength occurs when the generating aircraft is heavy, clean and slow. – Wake turbulence is near maximum behind a jet transport just after takeoff because of the high angle of attack and high gross weight. • ***The wind condition that prolongs the hazards of wake turbulence on a landing runway for the longest period of time is a light quartering tailwind

Ch 12 - Turbulence • Turbulence in and near Thunderstorms – Turbulence in and near Thunderstorms (TNT) – Turbulence which occurs within developing convective clouds and thunderstorms, in the vicinity of the thunderstorm tops and wakes, in downbursts, and in gust fronts.

Ch 12 - Turbulence • ***When landing behind a large aircraft, the pilot should avoid wake turbulence by staying above the large aircraft’s final approach path and landing beyond the large aircraft’s touchdown point. – When departing behind a heavy aircraft, the pilot should avoid wake turbulence by maneuvering the aircraft above and upwind from the heavy aircraft.

Ch 12 - Turbulence • Turbulence within Thunderstorms – Overshooting tops – Although updrafts weaken above the equilibrium level, in intense thunderstorms, they may penetrate several thousand feet into the stratosphere before they are overcome by the stability. • The strongest updrafts can often be identified by cumuliform bulges that extend above the otherwise smooth anvil top of the thunderstorm. – These are called overshooting tops and they are evidence of very strong thunderstorms and turbulence.

Ch 12 - Turbulence • Turbulence below thunderstorms – The downdrafts, downbursts and micro bursts define the primary turbulent areas below the thunderstorm. • These phenomena produce intense turbulence as well as wind shear. – Strong winds in the outflow from the downdraft generate mechanical turbulence, which is especially strong along the edge of any microburst and/or gust front.

Ch 12 - Turbulence • Turbulence around thunderstorms – Overhang – A turbulent wake occurs under the anvil cloud downwind of the thunderstorm. • This is one of the most hazardous regions outside of the thunderstorm and above its base. • Sometimes identified as the region under the overhang (anvil), it is an area well known to experienced pilots and is a location of severe turbulence and possibly hail.

Ch 12 - Turbulence • Clear Air Turbulence – Turbulence which occurs in the free atmosphere away from any visible convective activity. – Billow clouds – In the clouds that show evidence of shearing-gravity wave activity, the “herring bone” pattern of billow clouds is a common feature in high cloud layers subjected to vertical shear.

Ch 12 - Turbulence • ***When a pilot enters an area where significant CAT has been reported, an appropriate action when the first ripple is encountered is to adjust airspeed to that recommended for rough air

Ch 12 - Turbulence – Shearing gravity waves – Short atmospheric gravity wave disturbances that develop on the edges of stable layers in the presence of vertical shears. – Jet stream front – In the vicinity of the jet stream, there are two specific regions where CAT occurs most frequently. • One is in the sloping stable layer below the jet core. – This is a high-level frontal zone, also called a jet stream front.

Ch 12 - Turbulence • ***A sharply curving jet stream is associated with greater turbulence than a straight jet stream

Ch 12 - Turbulence • Mountain Wave Turbulence (MWT) – Turbulence produced in connection with mountain lee waves. • It is responsible for some of the most violent turbulence that is encountered away from thunderstorms.

Ch 12 - Turbulence – Lee Wave Region • Lee Wave Region – Lee waves are more often smooth than turbulent, but if turbulence does occur in the lee wave region, it is most likely to occur within 5, 000 feet of the tropopause.

Ch 12 - Turbulence • Lower Turbulent Zone – ***One of the most dangerous features of mountain waves is the turbulent area in and below rotor clouds

Summary • Aviation turbulence is caused by a number of different • atmospheric phenomena. In this chapter we have considered the four most common types: – Turbulence generated in the boundary layer (LLT) – Turbulence caused by strong convection (TNT) – Turbulence in the vicinity of the jet stream (CAT) – Turbulence caused by mountain waves (MWT)

Summary • You now know why and where this turbulence develops. • You have some useful conceptual models which help you • • connect the various types of turbulence to the larger scale circulations in which they are embedded. Finally, you have learned some rules of thumb to aid you in turbulence avoidance. In Part IV, we will cover some of the aviation weather products that are available from the NWS and the FAA that help you identify turbulence areas