Ground lesson #9: Icing, #10: Departures

what are the biggest influences on icing?

Answer explanation

orographically lifted clouds by mountains/terrain, high altitudes and visible moisture. ice accretion is the highest when encountering freezing rain (droplets that freeze upon impact) just below the freezing level, also along thunderstorms, active cold fronts, and unstable warm fronts.

define ice accretion

Answer explanation

the buildup of ice over an object, which varies with the temperature/size of water droplets, the water content of the air, and the shape of airfoils. smaller droplets will freeze upon impact while larger droplets will spread across a larger surface area.

ice affects the four forces of flight by

• decreasing lift

• increasing drag (induced)

• increasing weight

• increasing drag (parasitic)

Answer explanation

ice also causes pitot-static system blockages, and decreases fuel efficiency because more thrust is required to counter the increase in drag.

supercooled droplets

• are in a liquid state above freezing point

• resist freezing because the temperatures are high enough

• will always freeze instantly on the wing

• contribute to clear ice formation

Answer explanation

super cooled droplets are below the freezing point, still in the liquid state. they resist freezing because of kinetic motion (the droplet falling through the air), and contribute to clear ice formation when they impact the airfoil and part of the droplet may freeze, while the other flows back over the wing and freezing as it moves. most dangerous = large supercooled water droplets.

rime ice forms primarily from

• small droplets

• large droplets

Answer explanation

rime ice is primarily from small droplets that freeze upon impact. it appears more milky from the air pockets, easier to remove. tends to stay by the leading edge of the wing. common in stratiform clouds.

clear ice forms primarily from

• small droplets

• large droplets

Answer explanation

clear ice forms primarily from larger supercooled droplets. they're common in cumuliform clouds/freezing rain. as the droplet impacts the surface, it'll partially freeze while moving across the surface of the wing. causes ice accretion to buildup beyond the reach of device boots, and is clear/glossy hard to see. heavy, hard to remove, and the most dangerous type of ice.

clear ice forms between the temps of [_] to negative [__] degrees celsius

Answer explanation

mixed ice is typically between -10 to -15 degrees celsius.

rime ice forms between the temps of negative [__] to negative [__] degrees celsius

how does ice affect airflow over the wing?

Answer explanation

ice causes an early separation of airflow, which leads to the aircraft reaching the critical angle of attack sooner (aka stalling at higher airspeeds). it also worsens the boundary layer by increasing it. the boundary layer is established due to the fact that the surface of the wing isn't smooth, so as air passes over a layer of air molecules will adhere to the surface because of friction. combined with the viscosity of air, the boundary layer restricts airflow and the resulting force produced is drag.

so as the boundary layer grows larger due to ice, drag increases.

describe induced drag.

Answer explanation

as a wing produces lift, it experiences the Venturi effect. air over the wing is compressed, which increases speed and creates an area of lower pressure. air beneath the wing is undisturbed, and an area of higher pressure. this creates a pressure differential, and as high seeks low pressure, it creates wing vortices (eddies, induced drag).