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

WhiteWhiteOpaqueOpaquePorousPorousBrittleBrittle

Clear Ice

ClearClearToughToughDenseDenseAdhesiveAdhesive

Icing

Question:Question: Which airframe will Which airframe will havehave

the greatest icing?the greatest icing?

A

150 KT150 KT

B

150 KT150 KT

A

150 KT150 KT

B

150 KT150 KT

Answer B Answer B

Kinetic Heating

Generated by:

Increased pressure at leading edges

Frictionbetween aircraft skin and atmosphere

Kinetic Heating

Advantages/disadvantages:

Raise skin temp above ZERO

no ice will formWarm cold objects to just below ZERO

increase severity

Factors affecting severity

Dropletsizetemperatureconcentration

Factors affecting severity

Aerofoilshapespeedtemperature

Effects on Aircraft

AerodynamicsAerodynamics - shape altered

All up weight All up weight - increases

EngineEngine - intakes may become blocked

UndercarriageUndercarriage - retraction problems

Control surfaces Control surfaces - may jam or become stiff

Pitot tubes Pitot tubes - blocked

CommunicationsCommunications - affected

VisionVision - impaired

VibrationVibration - due to uneven loading

Category Description

Trace Ice becomes perceptible. Rate of accumulation slightlygreater than rate of sublimation. It is not hazardous eventhough de icing/anti-icing equipment is not utilized, unlessencountered for an extended period of time (over 1 hour).

Light The rate of accumulation may create a problem if flight isprolonged in this environment (over 1 hour). Occasional useof de icing/anti-icing equipment removes/preventsaccumulation. It does not present a problem if the deicing/antiicing equipment is used.

Moderate The rate of accumulation is such that even short encountersbecome potentially hazardous and use of deicing/anti-icing ordiversion is necessary.

Severe The rate of accumulation is such that deicing/anti-icingequipment fails to reduce or control the hazard. Immediatediversion 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

0 C

4 C

AccretionDepends 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 Rarely more thanmoderate

AS 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

Leading Leading edgesedges

Air intakesAir intakes

Affects:Affects:

flying through wet snowflying through wet snow in or beneath cloudsin or beneath clouds

with temperature near with temperature near ZEROZERO

Packed Snow

Occurs when:Occurs when:

Hoar Frost

SoftSoftWhiteWhiteLightLight

CrystallineCrystalline

Hoar Frost

Forms in clear air when:Forms in clear air when:

Airframe temperature below Airframe temperature below ZEROZERO

Air is cooled below dew pointAir is cooled below dew point

Hoar Frost can occur

11.. Aircraft parked outside on Aircraft parked outside on clear winter nights.clear winter nights.

2.2. Rapid descent into warmer Rapid descent into warmer (clear), moist air, (cold soak).(clear), moist air, (cold soak).

3.3. Frosty morning with warmer, Frosty morning with warmer, moist air above.moist air above.

Rain Ice

ClearClearToughToughDenseDenseAdhesiveAdhesive

Result of freezing drizzle Result of freezing drizzle on approach to Ganderon approach to Gander

Rain Ice

Occurs whenOccurs when::

•Airframe temp. below ZEROAirframe temp. below ZERO

•Rain is falling from aboveRain is falling from above

Rain Ice

Circumstances:Circumstances:

Rapid descentRapid descent(after prolonged flight at high level) (after prolonged flight at high level)

Ahead of a warm Ahead of a warm frontfront

Plan viewPlan view

Cross SectionCross Section

Warm AirWarm Air

Cold AirCold Air

0 C isotherm0 C isotherm

0 C 0 C isothermisotherm

0 C isotherm0 C isotherm

0 C 0 C isothermisotherm

TT

HTHT

Melting Melting ZoneZone

Super Super CooledCooledRainRain

Rain Ice

ClearClearToughToughDenseDenseAdhesiveAdhesive

AccumulatesAccumulatesquickly!quickly!

Moderate or Severe Icing may be expected in:Moderate or Severe Icing may be expected in:

Cumuliform clouds Cumuliform clouds

NimbostratusNimbostratus

Active cold frontsActive cold fronts

Stratocumulus formed from CuStratocumulus formed from Cu

Over hills and mountains Over hills and mountains

Ahead of some warm frontsAhead 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