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METEOROLOGY > AIRCRAFT ICING > Flashcards

AIRCRAFT ICING Flashcards

1
Q

What is required for a water droplet to change from liquid to solid state?

A

A freezing nuclei
temps below 0deg C

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2
Q

Why do many water droplets not freeze just above the FZL?
What are they called?

A

The air is relatively clean with only a small number of freezing nuclei.
Super cooled water droplets (SCWD)

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3
Q

Describe the different levels of icing

A

SFC to 0deg: No icing
0 to -10deg: Mostly glaze ice
-10 to -20: Mixed ice (glaze and rime)
-20 to -40: mostly rime ice
Above -40: No icing

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4
Q

When does severe airframe icing most often occur in NZ?

A

When warm, moist tropical or sub-tropical air is advected onto NZ, then lifted (orographically or frontal systems).
“conveyor belts”

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5
Q

How does Rime ice form?

A

-Cold temps of -20 to -40, SCWD are smaller & less numerous.
-SCWD freeze instantly on contact with the airframe, trapping air & giving it a bright white appearance.
-Brittle & easy to dislodge.
-Due to being small, they release little latent heat, not enough to slow the freezing process.
-Ice only forms on surfaces exposed to airflow.

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6
Q

How does Glaze ice form?

A
  • Found in clouds with high liquid water content
  • 0 to -20deg
  • Most severe -15 to -25
  • Can be extreme in updrafting portion of wave cloud
  • SCWD are large and numerous
  • Freeze slowly on contact due to excess latent heat produced. Droplet rolls slowly back over control SFCs
  • No air bubbles produced, results in strong adherence.
    Excessive build up results in 45deg horns/ridges
  • Heavy, hard to dislodge, very dangerous
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7
Q

How does Mixed Ice form?

A

-occurs at temp between -10 to -25deg C
- Most dangerous between -10 to -15
- Combination of rime & glaze
- Large range of SCWD sizes (usually in Cu (large) and St (small) type clouds). E.g. Cb embedded in Sc, As & Ns.
- Dangerous as looks like it could just be rime.

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8
Q

How does Hoar frost form?

A

Occurs when moist air comes into contact with sub-zero aircraft SFC. (deposition)

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9
Q

How does freezing rain form?

A
  • Rare in NZ
  • Requires continental climate & mountainous areas.
  • Snow falls through warm layer > melts > water drops stay @ 0deg C > fall to sub zero layer but DON’T REFREEZE (due unstable nuclei).
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10
Q

Why is the freezing nuclei in freezing rain unstable?

A

The aerosols that the snow crystals formed on performed well as a freezing nuclei in that temp range, however they were not suitable as freezing nuclei at temps only a few deg below freezing point

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11
Q

Rime Ice, associated:
- Cloud types
- Temp ranges
- Droplet size
- Height range relative to FZL)
- Enhancing factors

A
  • Sc, St, As and Ac
  • light from 0 to -40 generally, -15 to -30 commonly
  • 7500 to 15000ft above FZL
  • Warm front or warm sectors
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12
Q

Clear Ice, associated:
- Cloud types
- Temp ranges
- Droplet size
- Height range relative to FZL)
- Enhancing factors

A
  • Cb, TCu, Ns, updrafting section of lentics
  • 0 to -20 generally, -3 to -12 commonly, -5 to -8 max frq & intensity. -12 to -25 for sev in lentics
  • large & numerous
  • FZL to 10000ft generally, 1500 to 6000ft commonly, 2500 to 4000ft max frq & intensity. 6000 to 12,500ft for lentics.
  • if source air is moist & from sub-tropics. Forced lifting (e.g. frontal, orographic, convection or lee waves - creates larger & more numerous SCWD).
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13
Q

Mixed Ice, associated:
- Cloud types
- Temp ranges
- Droplet size
- Height range relative to FZL)
- Enhancing factors

A
  • Ns, As & Ac. Embedded Cb or TCu
  • generally -10 to -25 with -10 to -15 being most likely & most dangerous.
  • Combination of large & small SCWD
  • 5000ft to 12500ft generally & 5000-7500ft most SEV
  • Active cod fronts, moist source air, forced lifting
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14
Q

What are the four factors that influence rate of ice accretion?

A
  1. size of cloud droplets: larger = more ice
  2. Number of droplets: more = more ice
  3. Speed of aircraft: speed increase = increase in collision with SCWDs. However ram effect means compression & frictional heating occurs. Both these factors combat ability of ice to form. Jets flying >450kts do not have much/any anti-icing protection
  4. The design of the aircraft (esp wings). Ice tends to form & freeze faster if SCWD can become trapped in a/c structures.
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15
Q

How does icing affect the four forces acting on an aircraft in flight?

A
  • Ice on wings: less/more turbulent airflow = reduced lift
  • Ice on engine: Less oxygen into engine = reduced thrust.
  • Ice on aircraft: increase in weight & drag.
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16
Q

Describe the effect of tailplane stalling prior to main plane due to icing

A
  • Ice builds up proportionally on surfaces regardless of the size differences.
  • Therefore smaller surfaces disproportionately build up more ice in comparison
  • Build up of ice on the tailplane LE therefore is disproportionately more than the LE of the main frame. Tailplane stalls first
17
Q

Describe the effect of main plane stalling due icing

A

With ice on the main plane, stalling may occur, particularly if AoA is increased. this is due to the icing having changed the shape and lift characteristics of the wing

18
Q

What are 11 other impacts that icing can have on an aircraft?

A
  1. Damage to trailing SFCs due to ice shedding from mainplane.
  2. Ice ingestion into engines causing damage
  3. Intake icing: reduces oxy intake & therefore thrust produced
  4. Prop icing: Ice can dramatically alter shape of prop & therefore generate reduced thrust.
  5. Gear doors frozen shut. (usually sev glaze ice case or after T/O from field with slushy ice - e.g. antarctica)
  6. Control surfaces freezing solid
  7. Uncommanded full deflection of CTRL SFCs: (ridges & horns cause air to “bounce” over, hitting ailerons from above downstream)
  8. Uneven ice distribution leading to sev vibration & structural failure (due uneven ice shedding)
  9. Poor radio reception due ice on comms aerials
  10. Poor vis due ice on windsheild
  11. Pitot tube/static ports icing over
19
Q

Define “light” icing

A

No change of course or alt is necessary & no loss of speed caused.

19
Q

Define “moderate” icing

A

Change of HDG & Alt is desirable. Ice acretion will continue to increase, not serious enough to affect safety of flight unless it continues for an extended period of time. Airspeed may reduce

20
Q

Define “Severe” icing

A

Change of HDG & ALT is considered essential. Ice build up will seriously affect performance & manoeuvrability of the aircraft.

21
Q

6x methods to avoid/mitigate icing

A
  1. Consider height of FZL in forecast - avoid in planning stage.
  2. In flight: note OAT, look for active cells, wx radar
  3. Avoid flight parallel to mountain ranges in updrafting portion of Lentics (in temps -25deg or warmer)
  4. If unable descent, fly above cloud top, or as high as possible (if temp is -40deg or colder, should be nil icing).
  5. Fly faster than 450kts (ram air effect = LE heating)
  6. De/Anti-icing kit
22
Q
A

METEOROLOGY (26 decks)

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