Revision Deck Flashcards
Define Air frame Icing
Icing that accumulates on the outside of the aircraft both during flight and on the ground.
What are the types of airframe icing (6)
- Rime ice
- Clear (glaze) ice
- Mixed ice
- Hoar frost
(5. Freezing drizzle - Freezing Rain)
What are the two forms of aerosols
- Condensation nuclei (very common)
- Freezing nuclei (only a small number of these - required for a SCWD to freeze)
What are super cooled water droplets
SCWD are water droplets that have reached zero degrees but do not have a suitable freezing nuclei, so they remain liquid below zero.
Characteristics of Rime ice. Any consequences?
SCWDs
TEMPS
APPEARANCE
DANGERS
- Forms at higher altitudes
- Colder temperatures (-20 to -40 degrees)
- Bright white appearance and brittle (due to air being trapped during rapid freezing process)
- Not heavy
- 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
Characteristics of Clear (glaze) ice. Any consequences?
SCWDs
TEMPS (3)
APPEARANCE
DANGERS
- Most severe form of icing.
- Found in clouds with high liquid water content and warmer temps (zero to -20 degrees). most severe between -5 to -8 degrees. (most common -15 to -25)
- SCWDs are large and numerous.
- Clear, sheet - like appearance. Hard to see, heavy.
- 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
Formation of clear (glaze) ice
- Because the SCWDs are large and numerous, freezing process is not instantaneous.
- Portion of droplet will freeze on contact, latent heat released slows the freezing process, allowing droplet to spread back across the wing before freezing.
- droplets join, air bubbles are expelled, strong adherence to SFC of wing.
Characteristics of Mixed ice
SCWDs (what clouds?)
TEMPS
APPEARANCE
DANGERS
- Combination of a range of SCWDs sizes. (suggests both cumuliform and stratiform clouds)
- Rime ice visible on leading edges.
- Clear/glaze ice not visible on the rest of the wing.
- Occurs between -10 to -25 degrees (most likely between -10 to -15).
- General rule: treat all mixed ice as a case of clear ice.
Characteristics of Hoar Frost.
Can it occur during flight?
- Forms when moist air/water vapour comes in contact with a sub-zero a/c surface. (DEPOSITION).
- Can cover entire air frame and dangerous to take off without getting rid of it (disrupts lift = stall).
- Generally occurs on the ground but 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.
Cloud types associated with Rime ice
- If the FZL is very low: Stratocumulus and stratus
- Altostratus and Altocumulus
Cloud types associated with clear ice
- Cb, Tcu, Nimbostratus, altocumulus lenticularis clouds (updrafting portion in special conditions).
General height ranges relative to FZL (rime/clear/mixed)
- Clear: 1500 - 6000ft above FZL (commonly) or updrafting section of lenticularis_
- Mixed: 5000 - 12500 above FZL (commonly)
- Rime: 7500 - 15000ft above FZL
Hazards of air frame icing in flight (aeros based) (3)
- Changes in the 4 forces acting on the a/c in flight (D and W increase, L and T decrease)
- Tail Plane stalling (generally before the main plane)
- Main plane stalling
Hazards of air frame icing (Structure based damage) (5)
- Damage to surfaces from chunks of ice breaking off forward surfaces.
- Damage to engines from ice ingestion.
- Uneven ice distribution resulting in severe vibration and structural failiure.
- Poor radio comms due to ice build up on antennas.
- Poor visibility due ice on windshield.
Hazards of air frame icing (Performance based) (6)
- Intake icing (reduces intake of air into engine, less power generated).
- Pitot tubes/static vents icing over.
- Propeller icing (can alter shape of blades = less thrust.)
- Landing gear doors frozen shut
- Control surfaces freezing solid
- Uncommanded full deflection of control surfaces
Conditions required for Cb development
- An adequate amount of water vapour at low levels
- Conditional instability through depth in the atmosphere (= release of latent heat)
- Trigger mechanism to initiate lifting
Types of trigger mechanisms for TS development
- Convection (parcels of air gaining buoyancy through contact with a warm SFC)
- Orography (ascent via contact with mountains etc)
- Frontal lifting (widespread ascent)
Classifications of Thunderstorms
- Airmass (parcel of air with similar characteristics)
- Frontal (boundary b/t two different types of airmasses, triggered by 3 main mechanisms)
The lifecycle of a TS
- Cumulus stage
- Mature stage
- Anvil/dissipating stage
Overall time of life cycle is 1.5 hrs.
Describe the cumulus stage in the life cycle of a TS
Any dangers at this stage? Time taken?
- Trigger initiates lifting, a TCU begins to form. (lots of latent heat released)
- There are only updrafting winds in a TCU, which carries SCWDs upwards.
- Some SCWDs freeze, creating snow and ice. Eventually enough of these form at the top for it to fall, melt and create the first rain at the SFC.
- Nil dangers except possible severe icing at this stage (dude large qty of SCWDs lifting, turb only light - moderate (no downdrafts to worry about). 30 mins.
Describe the mature stage in the life cycle of a TS
Any dangers? Time Taken?
- First heavy rain at SFC
- An anvil will begin to form at the top of the cloud as the SCWDs begin to spread out horizontally beneath the tropopause.
- Any of the EIGHT hazards now exist, tornadoes are unlikely however from stationary Cbs. 30 mins.
Describe the Anvil/Dissipating stage of a TS
Any dangers? Time taken?
- The top of the anvil will start to become fibrous (due to formation of cirrus)
- Updrafts cease and hazards start to dissipate. 30 mins.
List the hazards created by Cb (8)
- Severe turbulence (due up/down drafts - centre to base should be avoided)
- Severe icing (in clusters of Cb, area above FZL 0 to -12 degrees should be avoided)
- Electrical phenomena (Lightning/static charges)
- Hail (ice crystals cycled in up/down drafts)
- Poor Visibility (due heavy precip)
- Tornadoes (in severe TS, over water they are called waterspouts, both are rare in NZ)
- Microbursts
- First gust/gust front. (sudden strong downdraft of cold, dense air = Low level wind shear, characterised by roll clouds)
Define Microbursts
Characterised by?
Intensity and lifespan?
2 Types
- Localised severe wind pattern driven by extremely strong downdrafts from dense, cooled air. Can exceed 100kts in the vertical.
- Microburst winds intensify 5 mins after touch down, general lifespan is 15 mins.
- Can be Dry (occur with high based TS, rain evaporates before reaching SFC. More dangerous)
- Can be Wet (When precip accompanies microburst to SFC. More common in NZ due moist lower atmosphere)
Development of Hailstones
- Only occur within Cb clouds with up/down drafting air.
- Number of times the ice crystal cycles up and down the Cb before it gets too heavy and falls out defines size and number of layers of:
Rime ice (upper Cb)
Clear ice (lower Cb) - Hail can fall out of: Anvil/sides/bottom of Cb.
How to avoid TS
- Avoid in planning phase! (use radar in flight)
- have an alternate route
- Don’t fly under orographic TS (downdrafts)
- Cross frontal TS at right angles
What influences the flow of wind (5)
- Wind strength
- Angle of wind flow near the ranges
- Shape of the mountain range
- Stability of the air
- Vertical profile of wind speed and direction
Requirements favourable for lee wave development (5)
- Min 20kts at ridge top level
- Increasing wind speed with height (creates overturning motion)
- Wind perpendicular to ridge line (or within 30 degrees to perpendicular)
- Wind direction not varying with height
- Stable layer at about ridge top level.
Define the formation of Rotor streaming
Occurs when strong winds blow across a ridge line (Easterlies), with a marked decrease in speed above ridge level (Westerlies). Lee waves will not form. However, a single rotor will format ridge top, lee of the range and can migrate further lee of the range.
Conditions required for rotor streaming
- Winds near perpendicular to ridge line
- Strong winds at ridge top height, but decreasing above.
Features of rotor streaming
- Severe turb in rotor zone on the lee side and in line with or just below of the ridge line.
- Relatively smooth airflow above ridge top level.
Define the Fohn wind
Where does it occur often in NZ?
A warm, dry, very gusty wind blowing down the lee side of a major mountain range.
The Strong NW that blows across the canterbury plains to CHCH is a Fohn wind.
How is a Fohn wind formed
How is this related to the lapse rate.
- Air rises on windward side of a range
- Cloud forms, releasing latent heat (from condensation)
- Rainfall will occur on the windward side, reducing water vapour in the air.
- wind on lee side is therefore hot, dry and strong
Windward side: DALR (-3 deg per 1000ft) and SALR (-1.5 deg per 1000ft)
Top of range down lee side: - DALR (-3 deg per 1000ft)
Features of a Fohn wind
- Fohn gap (band of clear skies in lee of the ranges due descending air)
- High cloud bases on lee side
- Turbulent conditions
- Lee wave activity
- Warm temperatures to the east of the country
Dangers of flying in Lee Wave conditions
- Strong downdrafts
- Rising ground
- Low ground speed
- Risk of rotors with sever turb.
Features of lee waves (7)
- Bands of lifting and sinking air parallel to range
- AC Lenticularis and rotors present (visible w sufficient moisture)
- Severe turb in rotor zones
- High level clear air turb if jet stream is present
- Smooth flying in wave system above friction layer (downdrafts can exceed climb performance of a/c)
- Downdrafts can touchdown to SFC = localised strong winds
- Severe icing in updrafting portion of wave clouds under special circumstances.
Describe Lee Wave formation
- Air displaced upwards over the Range hits a stable layer at ridge - height, this provides restoring force for the descent of the air on the leeward side.
- A pendulum effect is formed downstream
What are the components of air flow?
- Translation
- Rotation (clockwise or anticlockwise)
- Divergence (and) Convergence
- Deformation
Define translation
The movement of a parcel of air from A to B (in any direction) without rotating/altering its shape or volume.
Define Divergence
Occurs when a body of air expands horizontally. More precisely, a fixed volume of air is divergent when horizontal outflow EXCEEDS the horizontal inflow.
Define convergence
Occurs when a body of air contracts horizontally. More precisely, a fixed volume of air is convergent when horizontal inflow EXCEEDS the horizontal outflow.
Define confluence and diffluence and describe how they work relative to conversion/diversion
Confluence: Wind streamlines coming together
Diffluence: Wind streamlines moving apart.
-Both can influence conversion/diversion generated, however, Confluence/Diffluence can exist without conversion/diversion.
Generic rules for Divergence and Convergence (3)
- The net inflow must equal the net outflow.
- Divergence and convergence only work within the horizontal.
- The horizontal deficit/excess is made up for by vertical inflow (DIV) or outflow (CON)
Describe how conversion works in a parcel of air
- Inflow of 20kts in horizontal
- Outflow of 10kts in the horizontal
- Therefore a loss of 10kts vertically.
- Loss of 10kts in the vertical induces low pressure system at the SFC. Tropopause will sink.
Describe how diversion works in a parcel of air
- Inflow of 10kts in horizontal
- Outflow of 20kts in horizontal
- Therefore a gain of 10kts vertically
- Gain of 10kts in the vertical induces high pressure system at the SFC. Tropopause will rise
Why are conversion and diversion important?
What causes them?
Because they lead to vertical motions in the atmosphere, which in turn results in cloud and precip formation, as well as cloud suppression.
Changes in absolute vorticity
How does pressure in a Low/high system increase or decrease due to DIV and CONV?
- Increase in CONV at the SFC = increase in ASCENT of air, therefore results in an increase in the low pressure system at the SFC (and vice versa)
- Increase in DIV at the SFC = Increase in DESCENT of air, therefore results in an increase of high pressure system at the SFC (as more air gets stacked on top of the point - subsidence)
- At the top of the troposphere, CONV leads to descent, and DIV leads to ascent.
Define curvature vorticity
Occurs in the bends of both anticyclones and cyclones and rotates in the direction of the wind around the pressure system.
Define Shear vorticity
Occurs in straight isobars around both Low and High pressure systems, and occurs due to the difference in faster and slower moving air. (like a barrier jet).
Define relative vorticity
The sum of the curvature and shear vorticities gives the relative vorticity. Which is relative to the SFC of the earth and can be cyclonic or anticyclonic in nature.
Define the Earths vorticity
Arises from the shear produced by the earths rotation around its axis, and increases in strength with latitude (towards the poles).
Define Absolute vorticity
(3) points about absolute vorticity
Is the sum of relative vorticity plus the earths vorticity.
1. Earths vorticity is much greater than Relative vorticity.
2. Earths vorticity is always cyclonic (in Southern Hemisphere)
3. Therefore Absolute vorticity is ALWAYS cyclonic in nature.
What does change in Absolute vorticity cause?
- Conversion/diversion are affected
- Ascent or subsidence of the columns of air above these systems are affected (High/Low pressure systems)
- Therefore the development or decay of cloud/precip is affected.