Transcript Icing and Turbulence

Aviation Hazards
What we must consider:
forms of icing
Packed snow
Hoar frost
Rain ice
Engine / airframe icing
Icing
Engine Icing can occur when the environment is
above zero and is due to the intake
of air cooling adiabatically
Airframe Icing (our main concern)supercooled droplets freeze on the
aircraft body
Engine Ice
Due to:
1. Decrease of pressure inside
engine manifold
2. Extraction of latent heat due
to vaporisation
ICING TYPES:
• Rime Ice: Rough, milky, opaque ice
formed by the instantaneous freezing of
small supercooled water droplets.
• Clear Ice: A glossy, clear, or translucent
ice formed by the relatively slow
freezing of large supercooled water
droplets.
Airframe Icing
Small supercooled water droplet
• Freezes quickly on contact with airframe
Large supercooled water droplet
• Begins to freeze on initial impact
• Latent heat release as droplet freezes
Rime Ice
White
Opaque
Porous
Brittle
Clear Ice
Clear
Tough
Dense
Adhesive
Icing
Question: Which airframe will
have
the greatest icing?
150 KT
A
150 KT
B
Answer
B
150 KT
A
150 KT
B
Kinetic Heating
Generated by:
Increased pressure
at leading edges
Friction
between aircraft skin and atmosphere
Kinetic Heating
Advantages/disadvantages:
Raise skin temp above ZERO
no ice will form
Warm cold objects to just below ZERO
increase severity
Factors affecting severity
Droplet
size
temperature
concentration
Factors affecting severity
Aerofoil
shape
speed
temperature
Effects on Aircraft
Aerodynamics - shape altered
All up weight - increases
Engine - intakes may become blocked
Undercarriage - retraction problems
Control surfaces - may jam or become stiff
Pitot tubes - blocked
Communications - affected
Vision - impaired
Vibration - due to uneven loading
Category
Description
Trace
Ice becomes perceptible. Rate of accumulation slightly
greater than rate of sublimation. It is not hazardous even
though de icing/anti-icing equipment is not utilized, unless
encountered for an extended period of time (over 1 hour).
Light
The rate of accumulation may create a problem if flight is
prolonged in this environment (over 1 hour). Occasional use
of de icing/anti-icing equipment removes/prevents
accumulation. It does not present a problem if the deicing/anti
icing equipment is used.
Moderate
The rate of accumulation is such that even short encounters
become potentially hazardous and use of deicing/anti-icing or
diversion is necessary.
Severe
The rate of accumulation is such that deicing/anti-icing
equipment fails to reduce or control the hazard. Immediate
diversion is necessary.
Airframe Icing
• Requires supercooled droplets
• Airframe icing is most efficient when the cloud
droplets are between 0 0C and -15 0C
• Below -20 0C only very small supercooled
droplets can exist
• Large supercooled droplets are a greater
hazard as they spread out to a greater extent
Icing probability in stratiform cloud
Icing reports vs temperature
Icing in stratiform cloud
-3 C
0C
4C
Accretion
Depends on:
• Aircraft characteristics
• Cloud type
• Length of time flying through the cloud,
(especially significant for layered cloud)
Forecasting icing
•
•
•
•
Types of cloud
Temperature of cloud
Horizontal extent of cloud
Changes in airmass characteristics
Icing
• Entrainment of dry air at the top of a cloud will
reduce the water content and therefore its icing
ability
• Vertical motion within cloud will increase the
liquid water content significantly
• Strong vertical motion in convective clouds
produce the most severe icing
• Vertical motion due to fronts or orography can
also give severe icing problems
Cloud Types and Icing
Cloud Type
Probability of Icing
CB, CU, NS
High
SC, AC, ACAS
50%
AS
Low
ST
Low
Cloud Types and Icing
Cloud Type
Severity of Icing
CB, CU, NS
May be severe
SC, AC, ACAS
AS
Rarely more than
moderate
Moderate or light
ST
Light
Further points
• Maritime clouds have fewer, larger droplets
• LWC in stratiform cloud can be up to 10X
lower than in convective cloud
• Icing layer usually around 2-3000ft thick
and is seldom >5000ft
• Autopilots can mask the effect of icing until
it is too late!
Packed Snow
Affects:
Leading edges
Air intakes
Packed Snow
Occurs when:
 flying through wet snow
 in or beneath clouds
with temperature near ZERO
Hoar Frost
Soft
White
Light
Crystalline
Hoar Frost
Forms in clear air when:
Airframe temperature below ZERO
Air is cooled below dew point
Hoar Frost can occur
1. Aircraft parked outside on
clear winter nights.
2. Rapid descent into warmer
(clear), moist air, (cold soak).
3. Frosty morning with warmer,
moist air above.
Rain Ice
Clear
Tough
Dense
Adhesive
Result of freezing drizzle
on approach to Gander
Rain Ice
Occurs when:
•Airframe temp. below ZERO
•Rain is falling from above
Rain Ice
Circumstances:
Rapid descent
(after prolonged flight at high level)
Ahead of a warm front
Plan view
0 C isotherm
Warm Air
0C
isotherm
Cold Air
Cross Section
HT
0 C isotherm
Melting
Zone
Super
Cooled
Rain
0 C isotherm
T
Rain Ice
Clear
Tough
Dense
Adhesive
Accumulates
quickly!
Moderate or Severe Icing may be expected in:
Cumuliform clouds
Nimbostratus
Active cold fronts
Stratocumulus formed from Cu
Over hills and mountains
Ahead of some warm fronts
Summary
• Be thoroughly aware of customer practices
• Examine cloud type / structure carefully
• Main concern- helicopters / light aircraft
• Good communication with flyers helps