Aviation Weather Hazards

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Aviation Weather Hazards. Weather radar, observing equipment and balloon launching on roof. Mark Sinclair Department of Meteorology Embry-Riddle Aeronautical University Prescott, Arizona. ERAU Academic Complex. Weather center. Talk Overview. Survey of weather related accidents Turbulence - PowerPoint PPT Presentation

Text of Aviation Weather Hazards

  • Aviation Weather HazardsMark SinclairDepartment of MeteorologyEmbry-Riddle Aeronautical UniversityPrescott, ArizonaWeather centerWeather radar, observing equipment and balloon launching on roofERAU Academic Complex

  • Talk OverviewSurvey of weather related accidentsTurbulenceLow-level turbulence and surface windThermal turbulenceMicroburstsMountain wave turbulenceIMC conditions

  • All weather related accidentsThe following data are from the FAAs National Aviation Safety Data Analysis Center (NASDAC), Office of Aviation Safety, Flight Standards Service and are based on NTSB accident data. Data from all accidents, the majority non-fatalhttp://www.asias.faa.gov/aviation_studies/weather_study/studyindex.html

  • Weather related accidents

  • Nearly 87% or 7 out of 8 of these involved general aviation operationsCommuterAgAir carrierGAGeneral Aviation

  • 19,562 total accidents4,159 (21.3%) weather relatedMain cause = wind

  • GA weather-related fatalities a study by D.C. Pearson (NWS)http://www.srh.noaa.gov/topics/attach/html/ssd02-18.htmLooked at NTSB data from 2,312 GA fatal accidents in the US during 1995-2000Weather a factor in 697 or 30% of all GA fatalitiesA similar study by AOPA showed an average of 35% but decliningWeather a bigger factor in FATAL accidents than for non-fatal

  • GA weather related fatalities (cont.)NTSB cited NWS weather support to be a contributing factor in only two (0.3%) of the 697 weather-related fatal accidents. NTSB cited FSS support to be a factor in only five (0.7%) of the accidents.NTSB cited inadequate ATC support only nine times (1.3%)Combined, NWS, FSS and ATC = 2.3%Pilot error accounted for remaining 97.7%Continued flight into IMC the leading cause of GA weather-related fatalities

  • Flight Safety and WeatherClearly, the responsibility for flight safety is YOU, the pilotYou need to brief (up to 41% dont)Clear sky and light wind now does not mean it will be that wayOne hour from now50 miles from here1,000 ft AGL

  • Fatal GA accidents

  • Causes of

  • Aviation Weather HazardsSurface wind is the major listed hazard in in ALL weather related GA accidentsContinued flight into IMC conditions (reduced visibility and/or low ceilings) the leading cause of FATAL GA accidents

  • A. TurbulenceBumpiness in flight Four types Low-level turbulence (LLT)Turbulence near thunderstorms (TNT)Clear-air turbulence above 15,000 ft (CAT)Mountain wave turbulence (MWT)Measured asLight, moderate or severeG-load, air speed fluctuations, vertical gust

  • Turbulence in PIREPsTurbulence FrequencyTurbulence Intensity

  • Turbulence Can be thought of as random eddies within linear flow+Hi! Im an eddy

  • Turbulence Linear wind and eddy components add to gusts and lulls, up and down drafts that are felt as turbulence

  • Low-level Turbulence (LLT)Occurs in the boundary layerSurface layer of the atmosphere in which the effect of surface friction is feltTypically 3,000 ft deep, but varies a lotFriction is largest at surface, so wind increases with height in friction layer Vertical wind shear turbulenceImportant for landing and takeoffsResults in pitch, yaw and roll

  • Low-level Turbulence (LLT)

  • Factors that make low-level turbulence (LLT) strongerUnstable air encourages turbulenceAir is unstable when the surface is heatedAir is most unstable during the afternoon Cumulus clouds or gusty surface winds generally indicate an unstable atmosphereStrong windMore energy for turbulent eddiesRough terrainWhen LLT is stronger than usual, the turbulent layer is deeper than usual

  • Low-level turbulence (LLT)MechanicalCreated by topographic obstacles like mountains, and by buildings and treesIncreases with increasing flow speed and increasing surface heating (afternoon)ThermalOccurs when air is heated from below, as on a summer afternoonIncreases with surface heating

  • Mechanical TurbulenceCreated by topographic obstacles in flowIncreases in both depth and intensity with increasing wind strength and decreasing stability. Worst in afternoonExtends above 3000 ft for gusts more than 50 ktStrongest just downwind of obstaclesOver flat terrain, mechanical turbulence intensity is usually strongest just above surface and decreases with height

  • Mechanical Turbulence (cont.)Over flat terrainMaximum surface wind gusts are typically 40% stronger than the sustained windModerate or greater turbulence for surface wind > 30 ktWhen sustained surface wind exceeds 20 kt, expect air speed fluctuations of 10-20 kts on approachUse power on approach and power on landing during gusty windsSudden lulls may put your airspeed below stall

  • Thermal turbulence Produced by thermals (rising bubbles of warm air) during day in unstable airmassCommon on sunny days with light windStronger above sun-facing slopes in pmTurbulence intensity typically increases with height from surface and is strongest 3-6,000 ft above the surface

  • Thermal turbulence (cont.) Generally light to moderateCommonly reported CONT LGT-MODUsually occurs in light wind situations, but can combine with mechanical turbulence on windy daysOften capped by inversionTop of haze layer (may be Sc cloud)~3,000 ft, but up to 20,000 ft over desert in summerSmoother flight above the inversion

  • Deep summer convective boundary layer causes thermal turbulenceup to 20,000 MSL(more stable air above)thermalthermalHot, dry, unstable airdust devil

  • Towering cumulus over PrescottFall 2000Photo by Joe Aldrich

  • Dry microbursts from high based thunderstormsWhen precipitation falls through unsaturated air, evaporative cooling may produce dry microburstsResult in very hazardous shear conditionsVisual clue: fallstreaks or virga (fall streaks that dont reach the ground)

  • Downburst (Phoenix, AZ)July 2003Photo by Phillip Zygmunt

  • Downburst (Prescott Valley, AZ)1999Photo by Jacob Neider

  • The nocturnal boundary layerClear nights, moderate flowShallow friction layerGreatly reduced turbulenceLack of mixing possibility of strong vertical shearSurface air decoupled from gradient flow in free air above friction layerSurface flow often unrelated to pressure pattern (and flow above friction layer)May have super-gradient flow and turbulence at top of inversion

  • Strong turbulence during day means a deep layer is stirredMixing means 3,000 ft wind better mixed down to surfaceStronger turbulence, reduced vertical wind shearReduced turbulence means only a shallow layer is mixedSuppressed downward mixing means surface wind falls to near zero at nightStronger vertical shear

  • Diurnal variation of surface wind

  • 2. Mountain Wave Turbulence

  • In mountainous terrain ...Watch for strong downdrafts on lee sideClimb above well above highest peaks before crossing mountain or exiting valleyIntensity of turbulence increases with wind speed and steepness of terrainHighest wind speed directly above crest of ridge and on downwind side Maximum turbulence near and downwind of mountain

  • MountainStrongest wind speed and turbulence on downwind side, also warm and dryAir flow over mountainsOrographic cloud and possible IMC conditions on upwind sideUpwindDownwindAirflowDesired flight pathActual flight pathSplat!

  • Mountain wave turbulence (MWT)Produces the most violent turbulence (other than TS)Occurs in two regions to the lee of mountains:Near the ground and Near the tropopauseTurbulence at and below mountain top level is associated with rotorsTurbulence 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 crestFor mountain top winds between 25 and 50 kt, expect mod turb at all levels between the surface and 5,000 ft above the tropFor 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 tropopauseSevere turb in boundary layer. May be violent downslope windsDust may indicate rotor cloud (picture)

  • Mountain wave terminologyFohn cloudwallHydraulic jumprotorBreaking wavesInversionWave clouds (altocumulus lenticularis)

  • Mountain WavesMountain waves become more pronounced as height increases and may extend into the stratosphereSome pilots have reported mountain waves at 60,000 feet. Vertical airflow component of a standing wave may exceed 8,000 feet per minuteVertical shear may cause mountain waves to break, creating stronger turbulenceOften happens below jet streak or near front

  • Breaking Wave RegionVertically-propagating waves with sufficient amplitude may break in the troposphere or lower stratosphere.

  • Rotor cloudRotor cloudcap cloud

  • Lee WavesLee waves propagate horizontally because of strong wind shear or low stability above.These waves are typically at an altitude within a few thousand feet of the mountain ridge crest.

  • Lee waves (cont.)Lee waves are usually smooth, however, turbulence occurs in them near the tropopauseAvoid lenticular cloud with ragged or convective edgesWatch for smooth (but rapid) altitude changesLee wave clouds in NZ

  • Satellite photo of lee waves over ScotlandLee wave photos

  • Flow over/around mountainsStrongest flow near top and on downwind sideFor stable air and/or lighter winds, air will tend to go around rather than over mountainFor less stable air and strong winds, air will go over mountain

  • Mountain Wave AccidentsIn 1966, a mountain wave ripped apart a BOAC Boeing 707 while it flew near Mt. Fuji in Japan. In 1992 a Douglas DC-8 lost an en