Ch 12 - Icing Flashcards
Supercooled Water Droplets (SCWD)
Only occur below 0C
Where there is no nuclei to freeze onto
Size depends on temp; Large 0-20/25C, Small -20/25-40/45C, Colder than -40 = ice crystals
In CU you can find all of the above
0->-10C is where you will find the largest concentration of SCWD
Size of droplets determine the type of icing
Cirrus clouds = nil icing (ice crystals bounce of airframe)
Dangers of SCWD
Decreased thrust Loss of control surfaces Increased weight Decreased lift Increased drag
Hoar Frost
Aligns molecules - Knocks them about to align
Airframe icing but not with SCWD
Happens when parked at night, when there is water vapour in the atmosphere that comes into contact with airframe <0C - sublimation (vapour into ice)
Can also happen in the descent coming down from the cruise into lower levels with increased moisture vapour, cool airframe allows Vapour to sublimate onto airframe
Reduces lift by 10/15% and needs to be de-iced and anti-iced before flight
Rime Ice
Caused by SCWD and is the most common form of airframe icing
Forms at temps; -20/25C - -40/45C with small SCWD
From alto-type clouds (stratus as droplet size is smaller)
Will freeze at 1/80th for every 1C below 0 on impact with airframe
- Bigger droplets freeze far more slowly
- Small SCWD will freeze almost instantly
Traps air in it and gives it a Milky/White appearance and forms on the leading edges. Will be a problem if nothing is done after about 20 minutes
Clear Ice
Also known as glaze ice is caused by contact with Large SCWD in temperatures between 0C and -20/25C from Cumuliform clouds CU/CB
Slow process (large), latent head released as freezing takes place
The SCWD flow over the wing towards control surfaces and freeze as they do it which can leave the control surfaces inoperable
Air escapes before it freezes, hence the transparency
Far more dangerous than rime ice and in less than 20 minutes it will be trouble - difficult to remove
FZDZ
Caused by large SCWD falling as rain and being broken up into drizzle on the way down below 0C level. Usually from NS
Far more dangerous than clear ice as they freeze very quickly and can be very intense,
In only a few minutes it can cause complete failure
Avoid at all costs, climb above 0C level, descend to MSA with terrain and ATC clearance, or turn around and fly away
Mixed Ice
Caused by a combination of large and small water droplets, usually from NS and creates a combination of clear ice and rime ice on the airframe
Cloudy Ice / Packed Snow
If the airframe is wet, ice crystals then stick to the airframe and eventually build up and form a milky white cloudy colour
Also happens from snow flakes
Only happens on a wet airframe
Kinetic Heating
Movement generated heating (Friction)
Makes the severity worse as it slows down the freezing process, given the SCWD chance to reach your control surfaces
Orographic Uplift (Intensification)
Orographic uplift increases the amount of SCWD in the atmosphere
When it rains, it can evaporate, reduce OAT lowering the freezing level
All of this combines to increase the severity of the icing
Aerofoil Shape on Icing
Thicker wing; slower process and is not as severe
More severe with thinner wing
Severity and Reporting of Ice Accretion
Trace Icing - Rate of accumulation larger than the rate of sublimation, not a hazard
Light icing - Maybe a problem, use equipment if you have, not that hazardous
Moderate icing - Divert needed potentially, anti/de-icing needed
Severe - Immediate diversion necessary, anti-icing equipment is inadequate reduction/control of problem, obligated to report this
Engine Icing
Occurs in the carburettor
Dangerous because you wont expect it (clear and sunny conditions)
Most dangerous at engine idle on the descent/landing and sometimes in the cruise
Use carburettor heaters to reduce the chance of engine icing.
Wet conditions between +10C and -10C are the most dangerous so put on icing system
Turbine Engine Icing
Happens in the first stages of the engine intake during the convergence - decreased static pressure, which leads to a decrease in temperature which causes turbine engine icing
Ice on the core then gets sucked through
Ice Crystal Icing
Ice crystal icing can occur outside regions of expected
Ice crystals in the engine can lead to a complete engine failure
Most common in the subtropical regions 30N/S can be higher in summer reaching America and japan
- Alt 20-35,000ft
- Temp +10 to -10C
To avoid, find the gap and avoid whole bad pressure system by a wide margin, if not an option, divert.
