G.4.8. NON-REFLECTIVE WEATHER
Small cumulus clouds may not content enough water. Weather radars may not detect them due to their weak reflectivity. Nevertheless, this kind of clouds may produce light to moderate turbulence.
Figure G - 5: Turbulence due to cumulus The turbulence is due to the alternation of updrafts that form the clouds and downdrafts between the clouds.
APPENDIX H – LOW LEVEL WIND SHEAR EFFECTS ON AIRCRAFT
PERFORMANCES
This appendix provides a summary of wind shear effects on aircraft performances at low level. For a more detailed analysis of these effects, refer to the ICAO Manual on Low Level Wind Shear and Turbulence, Doc 9817 (see References).
H.1. HORIZONTAL WIND SHEARS
There are two kinds of horizontal wind shears: longitudinal wind shears and crosswind shears. As runways are aligned on dominant winds, the most frequent wind shears encountered at low levels are longitudinal. Nevertheless, crosswind shears have some significant effects on the aircraft flight path.
Wind shears affect the aircraft in a transient way. They affect the airspeed, the altitude, the angle of attack or the drift depending on their direction (longitudinal, lateral, or vertical). The initial effects of wind shear are the ones that mostly affect aircraft performances.
H.1.1. LONGITUDINAL WIND SHEARS
Climb Level Descent
Figure I - 1: Equilibrium of aerodynamic forces The equilibrium of aerodynamic forces illustrated in Figure I - 1 assumes that: -The flight is straight (no turn) and is not accelerating -The thrust is along the flight path. γ is the angle of climb/descent.
Climb Level Descent
Figure I - 2: Resultant flight path vector – Decreasing headwind/Increasing tailwind Figure I - 2 illustrates the initial effect (resultant flight path vector R) due to the transient decrease of airspeed following a decreasing headwind or an increasing tailwind, until the aircraft reaches a new equilibrium.
Figure I - 3: Resultant flight path vector – Increasing headwind/Decreasing tailwind Figure I - 3 illustrates the initial effect (resultant flight path vector R) due to the transient increase of airspeed following an increasing headwind or a decreasing tailwind, until the aircraft reaches a new equilibrium.
Considering the transient effect of wind shears on airspeed, an increasing headwind is equivalent to a decreasing tailwind, and vice-versa.
When the aircraft passes the wind shear, the aircraft naturally returns to equilibrium thanks to its longitudinal stability. However, the flight crew must often take the controls to avoid the aircraft starting phugoid oscillations (airspeed and height oscillations with a period of approximately 40 seconds).
Figure I - 4 illustrates the effects of longitudinal wind shears on the flight path at take-off and landing.
H.1.2. CROSSWIND SHEARS
The initial effect of a crosswind shear affects the drift and the sideslip angles, without any initial effects about airspeed and altitude. When the aircraft encounters a crosswind shear, the aircraft:
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