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MET 2 > Aircraft Icing > Flashcards

Aircraft Icing Flashcards

1
Q

Define Air frame Icing

A

Icing that accumulates on the outside of the aircraft both during flight and on the ground.

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2
Q

What are the two forms of aerosols

A
  1. Condensation nuclei (very common)

2. Freezing nuclei (only a small number of these)

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3
Q

Explain what freezing nuclei are required for

A

When water droplets cool to zero degrees, a freezing nuclei is required for the droplet to change from liquid to solid state.

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4
Q

What are super cooled water droplets

A

SCWD are water droplets that have reached zero degrees but do not have a suitable freezing nuclei, so they remain liquid below zero.

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5
Q

General content of SCWDs compared to air temp.

A

Generally, as it gets colder, the more SCWDs will freeze.
zero to -10: mostly SCWDs
-10 to -20: mixture of SCWDs and Ice crystals
-20 to -40: mostly ice crystals

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6
Q

What is the special circumstance for the freezing of SCWDs?

A

if SCWD are disturbed, e.g. by collision with a/c, they will immediately begin to freeze. (a/c make good freezing nuclei!)

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7
Q

When does severe airframe icing generally occur?

A

Often when warm, moist, tropical/sub-tropical air is advected onto NZ, and then lifted either by frontal or orographic systems. These are called ‘conveyor belts’

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8
Q

What are the types of airframe icing (6)

A
  1. Rime ice
  2. Clear (glaze) ice
  3. Mixed ice
  4. Freezing Rain
  5. Freezing drizzle
  6. Hoar frost.
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9
Q

Characteristics of Rime ice. Any consequences?

A
  1. Forms at higher altitudes
  2. Colder temperatures (-20 to -40 degrees)
  3. Bright white appearance and brittle (due to air being trapped during rapid freezing process)
  4. Not heavy
  5. If left to build up over a long period of time it can begin to affect control/lift of a/c. Easy to get rid of via manoeuvering.
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10
Q

Formation of rime ice

A
  1. Higher ALT means super small, less numerous SCWDs
  2. Super small SCWDs freeze instantly on a/c. Only a tiny amount of latent heat is lost, not affecting the freezing process.
  3. Rime only forms on leading edges exposed to the airflow. e.g. nose cone/wing leading edges.
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11
Q

Characteristics of Clear (glaze) ice. Any consequences?

A
  1. Most severe form of icing.
  2. Found in clouds with high liquid water content and warmer temps (zero to -20 degrees). most intense between -5 to -8 degrees.
  3. SCWDs are large and numerous.
  4. Clear, sheet - like appearance. Hard to see, heavy.
  5. If left too long, ridges and horns can develop on top and below the wing (at 45 degree angles). Profound effect on lift and can cause uncommanded deflection and accidents. Extremely hard to get rid of
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12
Q

Where in NZ is severe clear/glaze icing most common?

A

IN the updrafting portion of wave clouds east of the main divide, particularly in the Kaikoura region. Occurs between -12 and -25 degrees.

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13
Q

Formation of clear (glaze) ice

A
  1. Because the SCWDs are large and numerous, freezing process is not instantaneous.
  2. Portion of droplet will freeze on contact, latent heat released slows the freezing process, allowing droplet to spread back across the wing before freezing.
  3. droplets join, air bubbles are expelled, strong adherence to SFC of wing.
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14
Q

Characteristics of Mixed ice

A
  1. Combination of a range of SCWDs sizes. (suggests both cumuliform and stratiform clouds)
  2. Rime ice visible on leading edges.
  3. Clear/glaze ice not visible on the rest of the wing.
  4. Occurs between -10 to -25 degrees (most likely between -10 to -15).
  5. General rule: treat all mixed ice as a case of clear ice.
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15
Q

Characteristics of Hoar Frost.

Can it occur during flight?

A
  1. Forms when moist air/water vapour comes in contact with a sub-zero a/c surface. (DEPOSITION).
  2. Can cover entire air frame and dangerous to take off without getting rid of it (disrupts lift = stall).
  3. Can occur during flight in clear air above the FZL, whe the a/c is cooled to sub-zero temps and then flies into high humidity.
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16
Q

What climate does Freezing rain require?

A

A mountainous, continental climate, e.g. Canada
Rare in NZ, if it were to occur here it would be light and short - lived.
(A mountain range and warm front is required)

17
Q

Formation of freezing rain

A
  1. Snow forms mid - high level troposphere. (-10 to -25 degrees)
  2. Snow falls into a warmer layer and melts (but stays at zero degrees). Super large SCWDs
  3. Before the snow can continue to warm up, it comes into contact with another sub - zero layer and cools further, BUT DOES NOT REFREEZE.
    (this is because the original freezing nuclei that was suitable at the OG level of -10 to -25 is now no longer suitable at the new level).
  4. Tens of times worse than severe clear ice. Lots of spread and slow freezing
18
Q

Cloud types associated with Rime ice

A
  • If the FZL is very low: Stratocumulus and stratus

- Altostratus and Altocumulus

19
Q

Cloud types associated with clear ice

A
  • Cb, Tcu, Nimbostratus, altocumulus lenticularis clouds (updrafting portion in special conditions).
20
Q

Cloud types associated with mixed ice

A

-Nimbo stratus, Altostratus, Altocumulus. Embedded Cb or Tcu.

21
Q

General height ranges relative to FZL (rime/clear/mixed)

A
  1. Rime: 7500 - 15000ft above FZL
  2. Clear: 1500 - 6000ft above FZL (commonly)
  3. Mixed: 5000 - 12500 above FZL (commonly)
22
Q

Enhancing factors for rime ice

A

Warm fronts or warm sectors

23
Q

Enhancing factors for Clear ice

A
  1. Source - air is moist or from sub-tropics

2. Forced lifting (orographic/Frontal/lee waves). Creates larger and more numerous SCWDs

24
Q

Enhancing factors for Mixed ice

A
  1. Active cold fronts
  2. Moist source air
  3. Forced lifting (same as clear ice)
25
Q

Four factors that influence the rate of ice accumulation

A
  1. Size of SCWDs (larger = more ice)
  2. The number of SCWDs (more =more ice)
  3. The speed of the aircraft (as speed increases, collision with SCWDs also increases = more ice). EXCEPTION, above 450kts, frictional heating and adiabatic warming (from RAM air) against leading edges will combat ability of ice to form.
  4. The design of the aircraft (particularly the wings). Ice will form more readily on areas it gets ‘trapped’ (e.g. seam of de-icing boot)
26
Q

Hazards of air frame icing in flight (aeros based) (3)

A
  1. Changes in the 4 forces acting on the a/c in flight (D and W increase, L and T decrease)
  2. Tail Plane stalling (generally before the main plane)
  3. Main plane stalling
27
Q

Hazards of air frame icing (Structure based damage) (5)

A
  1. Damage to surfaces from chunks of ice breaking off forward surfaces.
  2. Damage to engines from ice ingestion.
  3. Uneven ice distribution resulting in severe vibration and structural failiure.
  4. Poor radio comms due to ice build up on antennas.
  5. Poor visibility due ice on windshield.
28
Q

Hazards of air frame icing (Performance based (6)

A
  1. Intake icing (reduces intake of air into engine, less power generated).
  2. Pitot tubes/static vents icing over.
  3. Propeller icing (can alter shape of blades = less thrust.)
  4. Landing gear doors frozen shut
  5. Control surfaces freezing solid
  6. Uncommanded full deflection of control surfaces
29
Q

Methods to mitigate air frame icing (7)

A
  1. avoidance in planning - forecast of freezing level is accurate.
  2. Note temperature in flight, use weather radar
  3. Avoid Cb and Tcu
  4. Avoid flight parallel to mountain ranges in updrafting portion of lenticularis.
  5. If unable to descend, climb above cloud or as high as possible (as icing rate is lower at higher ALTs)
  6. Fly faster than 450kts
  7. Use de-icing/anti-icing equipment where possible.
30
Q

Moderate icing

A

Change of heading/altitude is desirable, ice accretion increases but not at a rate serious enough to affect safety of flight.
If continued for an extended period of time, airspeed may be lost.

31
Q

Severe icing

A

Change of heading/altitude is considered essential, ice accretion continues to build up on the a/c and begins to seriously affect performance and maneuverability of the a/c.

MET 2 (12 decks)

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