Transcript Aviation Weather Hazards

Aviation Weather Hazards
LT Clayton Martin
NAS Patuxent River
Air Operations
Talk Overview
• Survey of weather related accidents
• Turbulence
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–
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Low-level turbulence and surface wind
Thermal turbulence
Microbursts
Mountain wave turbulence
• IMC conditions
Flight Safety and
Weather
• Clearly, the responsibility for flight safety is YOU, the pilot
• You need to brief (up to 41% don’t)
• Clear sky and light wind now does not mean it will be that way
– One hour from now
– 50 miles from here
– 1,000 ft AGL
Fatal GA accidents
Causes of
Aviation Weather
Hazards
• Surface wind is the major listed hazard in in ALL
weather related GA accidents
• Continued flight into IMC conditions (reduced
visibility and/or low ceilings) the leading cause of
FATAL GA accidents
Turbulence
• “Bumpiness” in flight
• Four types
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Low-level turbulence (LLT)
Turbulence near thunderstorms (TNT)
Clear-air turbulence above 15,000 ft (CAT)
Mountain wave turbulence (MWT)
• Measured as
– Light, moderate or severe
– G-load, air speed fluctuations, vertical gust
Turbulence
• Can be thought of as random eddies
within linear flow
+
Turbulence
• Linear wind and eddy components add to
gusts and lulls, up and down drafts that
are felt as turbulence
20 kt gust
15 kt wind
+
updraft
5 kt
eddy
10 kt lull
downdraft
Low-level Turbulence
(LLT)
• Occurs in the boundary layer
– Surface layer of the atmosphere in which the effect of surface
friction is felt
– Typically 3,000 ft deep, but varies a lot
– Friction is largest at surface, so wind increases with height in
friction layer
– Vertical wind shear  turbulence
• Important for landing and takeoffs
• Results in pitch, yaw and roll
Factors that make low-level
turbulence (LLT) stronger
• Unstable air – encourages turbulence
– Air is unstable when the surface is heated
– Air is most unstable during the afternoon
– Cumulus clouds or gusty surface winds generally indicate an unstable
atmosphere
• Strong wind
– More energy for turbulent eddies
• Rough terrain
• When LLT is stronger than usual, the turbulent layer is deeper
than usual
Low-level turbulence
(LLT)
• Mechanical
– Created by topographic obstacles like mountains, and by buildings and
trees
– Increases with increasing flow speed and increasing surface heating
(afternoon)
• Thermal
– Occurs when air is heated from below, as on a summer afternoon
– Increases with surface heating
Mechanical Turbulence
• Created by topographic obstacles in flow
• Increases in both depth and intensity with increasing wind
strength and decreasing stability. Worst in afternoon
– Extends above 3000 ft for gusts more than 50 kt
• Strongest just downwind of obstacles
• Over flat terrain, mechanical turbulence intensity is usually
strongest just above surface and decreases with height
Mechanical Turbulence
• Over flat terrain
– Maximum surface wind gusts are typically 40% stronger than the
sustained wind
– Moderate or greater turbulence for surface wind > 30 kt
– When sustained surface wind exceeds 20 kt, expect air speed
fluctuations of 10-20 kts on approach
– Use power on approach and power on landing during gusty winds
– Sudden lulls may put your airspeed below stall
Thermal turbulence
• Produced by thermals (rising bubbles of warm air) during day
in unstable airmass
• Common on sunny days with light wind
• Stronger above sun-facing slopes in pm
• Turbulence intensity typically increases with height from
surface and is strongest 3-6,000 ft above the surface
Thermal turbulence
• Generally light to moderate
– Commonly reported CONT LGT-MOD
• Usually occurs in light wind situations, but can combine with
mechanical turbulence on windy days
• Often capped by inversion
– Top of haze layer (may be Sc cloud)
– ~3,000 ft, but up to 20,000 ft over desert in summer
– Smoother flight above the inversion
Summer Thermal
Turbulence
(deep convective boundary = more stable air above)
up to 20,000’ MSL
thermal
thermal
dust devil
Hot, dry, unstable air
Dry microbursts from high
based thunderstorms
• When precipitation falls through unsaturated air,
evaporative cooling may produce dry microbursts
• Result in very hazardous shear conditions
• Visual clue: fallstreaks or virga (fall streaks that don’t
reach the ground)
Flight
path of
plane
45 kt
downburst
45 kt
headwind
45 kt
tailwind
Diurnal variation of surface
wind
Wind at 3,000 ft AGL
Wind speed (kt)
30
20
Surface wind is
stronger and
more turbulent
during afternoon
Surface
wind
10
0
Midnight
6am
noon
6pm
Midnight
Mountain Wave Turbulence
In mountainous terrain ...
• Watch for strong downdrafts on lee side
– Climb above well above highest peaks before crossing mountain or
exiting valley
• Intensity of turbulence increases with wind speed and
steepness of terrain
• Highest wind speed directly above crest of ridge and on
downwind side
• Maximum turbulence near and downwind of mountain
Mountain wave turbulence
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1.
2.
Produces the most violent turbulence (other than TS)
Occurs in two regions to the lee of mountains:
Near the ground and
Near the tropopause
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Turbulence at and below mountain top level is associated with
rotors
Turbulence near tropopause associated with breaking waves in
the high shear regions just above and below trop
MWT (cont)
• Severity increases with increasing wind speed at mountain
crest
– For mountain top winds between 25 and 50 kt, expect mod turb at all
levels between the surface and 5,000 ft above the trop
– For mountain top winds > 50 kt, expect severe turb 50-150 miles
downstream of mountain at and below rotor level, and within 5,000 ft
of the tropopause
– Severe turb in boundary layer. May be violent downslope winds
– Dust may indicate rotor cloud (picture)
Mountain Waves
• Mountain waves become more pronounced as height
increases and may extend into the stratosphere
– Some pilots have reported mountain waves at 60,000 feet.
– Vertical airflow component of a standing wave may exceed 8,000
feet per minute
• Vertical shear may cause mountain waves to break,
creating stronger turbulence
– Often happens below jet streak or near front
Flow over/around
mountains
• Strongest flow near top and on downwind side
• For stable air and/or lighter winds, air will tend to go around
rather than over mountain
• For less stable air and strong winds, air will go over mountain
Turbulence PIREPs
Turbulence Information
• http://adds.aviationweather.gov/
– Hit the turbulence button
• http://www.dispatcher.org/brief/adfbrief.html
– Lots of aviation links to real time weather info
– Look down to turbulence section
• These are tools to help pilots better visualize aviation weather
hazards.
• Not intended as a substitute for a weather briefing from a Flight
Service Station
Instrument Meteorological
Conditions
VFR Minimums
and/or
Category
VFR
(Visual flight rules)
MVFR
(Marginal VFR)
IFR
(Instrument flight
rules)
LIFR
(Low IFR)
Ceiling
vis
(feet AGL) (miles)
None or > > 5
3,000
1,000 to 3 to 5
3,000
500 to 1 to 3
1000
< 500
<1
IFR/MVFR/VFR
• VFR- Visible Flight Rules – Pilot must be able to see
the ground at all times.
• MVFR – Marginal VFR conditions. Still legally VFR but
pilots should be aware of conditions that may exceed
their capabilities
• IFR – Instrument Flight Rules – Pilot has special
training and equipment to fly in clouds.
• LIFR – Low IFR.
Visibility IFR/MVFR/VFR
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VFR – Visibility greater than 5 miles.
MVFR – Visibility 3-5 miles.
IFR – Visibility 1-3 miles.
LIFR – Visibility less than 1 mile.
Red IFR
Magenta LIFR
Blue MVFR
Cloud Ceiling
IFR/MVFR/VFR
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VFR - Ceiling greater than 3,000 ft.
MVFR – Ceiling 1,000 to 3,000 ft.
IFR – Ceiling less than 1,000 ft.
LIFR – Ceiling less than 500 ft.
• IFR may be cause by either (or both) ceiling and visibility
restrictions.
Meteorological Causes of IFR
Conditions
• Fog (radiation fog, advection fog)
• Precipitation (snow, heavy rain)
• Low Clouds (lifting, cooling)
• High surface Relative Humidity (RH) common factor in all
causes of IFR
Fog
• Fog = low cloud with base < 50 ft AGL
• Generally reported when vis <5 miles and there is no
precipitation reducing visibility
• Formed by condensation of water vapor on condensation nuclei
• Longer-lived when layer of cloud above
• Need
– A cooling mechanism
– Moisture
• Either lower T (cool) or raise DP (add moisture)
Mist
• Mist (BR) is reported as "A visible aggregate of minute water
droplets or ice crystals suspended in the atmosphere that
reduces visibility to less than 7 statute miles but greater than
or equal to 5/8 statute mile."
Fog
• Can be considered as a low stratus cloud in contact
with the ground. When the fog lifts, it usually
becomes true stratus. This photo shows fog over the
Pemigewasset River basin with clear skies elsewhere.
•
Foggy Weather
Fog types
• Radiation fog
– Air near ground cools by radiation to saturation
– Also called ground fog
– Needs clear night, light breeze < 5 kts and high surface relative humidity
at nightfall
• Advection fog
– Occurs when warm moist air moves over colder bodies of water (sea
fog), or over cold land
– Needs winds up to about 15 kt
– Occurs mostly near coasts, day or night
• California coast (+ other upwelling regions)
• Near Gulf coast in winter in southerly flow
Fog types (cont.)
• Upslope fog
– Occurs on windward side of mountains
– Moist air moves upslope and cools
• Precipitation fog
– Occurs with surface inversion during rain
– Occurs over land areas in winter
– Raindrops fall to cold ground and saturate the air there first
• Three thermodynamic types
– Warm fog (temp > 0°C)
– Supercooled fog (-30°C < temp < 0°C)
– Ice fog (temp < -30°C)
Types of Fog - Upslope
Fog
• Air is lifted by moving up to higher ground.
Types of Fog - Precipitation
Fog
• Rain falling into layer of cold air
• Evaporation below cloud base raises the dew-point and
lowers the temperature
• Typically occurs in winter when there is a surface inversion
• The precipitation itself can also lower visibility to below
IFR criteria in heavy snow or rain conditions
Questions to Ask Before Flight
1. How close is the temperature to the dew point? Do I
expect the temperature-dew point spread to
diminish, creating saturation, or to increase?
2. What time of day is it? Will it get colder and form
fog, or will it get warmer and move further from
saturation?
3. What is the geography? Is this a valley where there
will be significant cold air drainage?
Will there be upslope winds that might cool and
condense?
4. What is the larger scale weather picture? Will it be
windy, suppressing radiation fog formation? Is
warm, moist air moving over a cold surface?
Climatology of IMC
• In west, highest frequency of IFR conditions occur in
– Pacific northwest - lots of cyclones & fronts
• > 40% in winter
– California coast - coastal upwelling & fog
– LA basin - smog
– Elswhere in west < 10% IFR conditions
• Higher frequency in east, particularly in midwest and south
– In IL, IN, OH, PA, > 50% frequency in winter
– Also > 40% along Gulf coast in winter
Climatology of IMC,
winter
10-40
40-50
< 10
10-40
40-50
40-50
10-40
> 50
< 10
10-40
10-40
10-40
40-50
< 10
40-50
10-40
WX Information Sources
• AWC Standard Brief – Satellite with AFC
– AWC - Standard Brief
• ADDS (Aviation Digital Data Service – run by AWC)
Metar regional plots are color coded for IFR
conditions
– ADDS – METARs
• ADDS Interactive Java tool using sky cover
– ADDS - METARs Java Tool
• NCAR-RAP Surface Observations (similar to ADDS
site)
– RAP Real-Time Weather
IFR Forecast Products
• Terminal Area Forecast (TAF) – Text product issued by
WFOs for selected airports. Hourly resolution of
prevailing and temporary surface conditions for up to
24 hours into the future.
• TAF provide visibility and cloud ceilings, which can be
related to IFR conditions
• TAF has standard format so can be decoded and
displayed as graphics or plain text.
Sources of TAF Forecasts
• ADDS – TAFs – Available as plotted maps for a single
time for a given region for prevailing or tempo
conditions. Also available in text form in raw or
translated formats for a given single station (need to
know 4 letter ID).
• ADDS - TAFs Java Tool – Mouse over map for raw TAF
data at any station.
• Aviation Weather Center (AWC) - TAF Graphics –
Mouse over times and data types showing US
prevailing or tempo conditions (3 hour resolution) in
graphical form for IFR conditions.
Area Forecasts
• Text product generated by AWC. Covers state or part of
state VFR conditions for 12 hours into future with 6 hour
outlook.
• Coded format not decoded into graphics.
• Available at http://aviationweather.gov/products/fa/ NWS
plans to develop graphical Area Forecast product in future.
AIRMET
• AIRMET regularly issued for IFR or Mountain Obscuration
conditions covering at least 50% of an area.
• 6 hour forecast with 6 hour outlook
• Text product with graphical products generated from
decoding of “from” lines.
• Available at ADDS - AIRMETs
Online Wx info and Forecasts
– to reiterate:
• These are tools to help pilots better visualize aviation weather
hazards.
• Not intended as a substitute for a weather briefing from Flight
Weather Briefer
Summary
• Issues to do with low-level wind are the main weather hazard
facing GA
– Probably includes cross winds, low-level turbulence, mountain effects
and shear
• Continued flight into IMC conditions the main cause of GA
fatalities
• Get a weather brief from your FSS
• Get a weather brief from your FSS
• Get a weather brief from your FSS