AC 00-57 Hazardous Mountain Winds

PART I. REVIEW OF METEOROLOGICAL CONCEPTS1.0 INTRODUCTIONFlight in the vicinity of mountainous terraincan be inspiring and immensely enjoyablefor both pilots and passengers. However,this aspect of aviation also can presentpilots with some of the most challengingand potentially dangerous situationsencountered in air operations. Aircraftperformance degradation because of highdensity altitudes, navigation problemsassociated with en route terrainobstructions, and rapidly changing weatherpatterns can cause difficulties for pilots ofsmaller aircraft operating at lower altitudes.In addition, the crews of high performanceturbine equipment must deal with highaltitude turbulence as well as reductions inaircraft performance caused by densityaltitude conditions. All pilots who fly nearmountainous terrain must deal with thepotential for mountain-induced severe windevents, particularly during takeoff andlanding. Although the effects of densityaltitude and high terrain are of greatimportance to all pilots who are operatingin mountainous areas, our discussion here islimited to the hazardous effects ofmountainous weather systems on aircraftoperations.·The atmosphere is a fluid in motion. Just asthe swiftly flowing water in a streamdevelops waves and eddies as it passes overand around obstructions, so does theatmosphere contain disturbances thatdevelop as it interacts with mountainousterrain. These atmospheric eddies canrange in size from a few centimeters to tensor hundreds of kilometers, and can presentthe pilot with relatively smooth air, or withturbulence of potentially destructiveintensity, and the likelihood of loss ofcontrol. The mountain-induced flow fieldswe will discuss in this AC are frequentlyaccompanied by visual indicators (such aslenticular and rotor clouds or blowing dust).However, this is not always the case, andextremely severe wind events can occurwith little or no visual warning of theirpresence.The purpose of this AC is to assist pilots,and others involved in aviation operations,in diagnosing the potential for severe windevents in the vicinity of mountainous areasand to provide information on pre-flightplanning techniques and in-flight evaluationstrategies for avoiding destructiveturbulence and loss of aircraft control. ThisAC can be used in several ways. For thosereaders who wish to obtain a more detailedunderstanding of the phenomena, the AC

Several points should be noted before weproceed. The first is that we understand agood deal about the mechanisms involvedin the production of mountain-relatedmeteorological disturbances at the largerend of the wavelength spectrum, such as leewaves. 'However, the role of pulsations inthe wind over and around mountain peaksin producing extremely strong, small-scaleeddies, and the range of strengths of thosedisturbances are not well understood.DlmFigure 1-1. States withgeneral aviation accident ratesover 3.0 per 100,000operations, Fiscal Year 1992.2Accident rat is *" lhan 3.0Aeadent,.t.lt g,.aler P'tM 3.0discusses meteorological theory relating tothe development of each type of severewind event. It then provides descriptivesummaries (in boxes) of the major pointsdeveloped for each weather hazard. Thosewho desire only the latter information canomit the background theory. Finally, anatlas of visual indicators has been includedto allow the reader to visually identify thecloud formations in question.Second, it should be remembered that allinformation contained in this AC isadvisory in nature and based upon ourcurrent level of knowledge. Individualpilot actions, as set forth under theFederal Aviation Regulations, are strictlythe decision of the pilot in commandbased upon his or her best evaluation ofthe existing conditions and theperformance characteristics of theaircraft.It is hoped that this document represents thefirst edition of what will become asuccession of training resources foraircrews and other aviation professionals,with revisions based on the results ofplanned research. For now, it cannot bestressed too strongly that much is yet to belearned about the atmosphere as it interactswith high terrain.

2.0 ACCIDENT STATISTICSNumerous aircraft accidents have occurredover mountainous areas involving generalaviation, military, and commercial aircraft.Figure 1-1, taken from U.S. GeneralAccounting Office report GAO/RCED-94-15(1993), summarizes accident statistics ongeneral aviation operations in mountainousareas of the United States. Researchersfound that the accident rate was nearly40 percent higher in 11 western mountainstates than in the other 37 continental states,and 155 percent higher for airports withtowers located in mountainous areas, whencompared with similar airports innonmountainous areas. During the periodfrom 1983 to 1992, 60 percent of theaccidents at 5 selected nontowered, mountainairports were associated with weather-relatedfactors, while 45 percent of accidents wereassociated with weather at 5 nontowered,nonmountain airports. One explanation forthe higher risk associated with operations inmountainous areas was determined to beweather. The implication is that thecombination of weather and mountainousterrain is particularly hazardous.Air carrier and military aircraft also havebeen victims of mountain-induced highwinds and associated turbulence. Table 2-1depicts a partial list of accidents/incidentsthat have occurred during the period fromTable 2-1. Turbulence-related accidents andincidents occurring in thl!! vicinity ofmountains.Accident31 Mar 93Anchorage, AKB-747 turbulence. Loss of engine.Accident22 Dec 92West of Denver, COLoss of wing section and tailassembly (two-engine cargoplane). Lee waves present.Accident09 Dee 92West of Denver, CODC-8 cargo plane. Loss of engineand wing tip. Lee waves present.Unknown Cause;Accident03 Mar 91Colorado Springs, COB-737 crash.Accident12 Apr 90Vacroy Island, NorwayDC-6 crash.Severe Turbulence24 Mar 88Cimarron, NMB-767 1.7 G. Mountain wave.Severe Turbulence22 Jan 85Over GreenlandB-747 2.7G.Severe Turbulence24 Jan 84West of Boulder, COSabreliner, - 0.4G, -0.4G.Severe Turbulence16 Jul 82Norton, WYDC-10, 1.6G, -0.6G.Severe Turbulence03 Nov 75Calgary, CanadaDC-10, 1.6G.Accident02 Dec 68Pedro Bay, AKFairchild F27B. Wind rotorsuspected.Accident06Aug 66Falls City, NBBAC 111. Wind rotor suspected.Accident05 Mar 66Ncar Mt. Fuji, JapanB-707. Wind rotor suspected.AccidentOl Mar 64Near Lake Tahoe, NVConstellation. Strong lee wave.AccidentlOJan 64East of Sangre de Cristo.Range, COB-52. Wind rotor suspected.3

January 1964 to March 1993. It is evidentfrom these data that accidents or incidentsassociated with severe turbulence inmountainous areas are not limited to onelocality or operating altitude, a particulartime of year, or a specific type of aircraft.In many cases, other aircraft operating inthe vicinity of the accident encounteredonly weak turbulence, suggesting thatsevere wind events can be highly localized,extremely violent, and short-lived. As hasbeen shown to be the case for accidentscaused by rnicrobursts, mishaps associatedwith the most severe orographic (of orrelating to mountains) wind events mayrepresent a case of being at the wrong placeat the wrong time. As with the microburstphenomenon, pilots need effective tools fordetecting the presence of orographic strongwinds and turbulence. They also needstrategies for avoiding encounters withthese potentially deadly phenomena andobtaining maximum aircraft performance indealing with an in-flight confrontation.The most severe orographic wind eventsusually occur when the large-scale (or,synoptic) winds are strongest, from late fallto early spring.4During the remainder of the year, when thesynoptic winds are normally much weaker,hazardous winds in the vicinity ofmountains are more likely to be associatedwith thunderstorms and their outflow fields.3.0 THE EFFECTS OF OROGRAPHICWINDS AND TURBULENCE ONAVIATION OPERATIONSOrographic winds and turbulence affect alltypes of aircraft operations. As will bedescribed below, regardless of the type ofaircraft, operations near mountainous areascan be hazardous.3.1 HIGH-ALTITUDE OPERATIONSTurbine-powered aircraft operating at cruisealtitudes above flight level (FL) 180 in thevicinity of mountainous terrain mayencounter moderate or greater turbulenceassociated with orographic winds. Thistype of turbulence may be characterized byrelatively rapid onset and can lead tostructural damage or airframe failure. Forexample, during the winter of 1992 nearDenver, Colorado, mountain-waveturbulence caused the separation of anengine from a DC-8 and loss of theoutboard portion of one wing.

Structural damage is not the only dangerassociated with high-altitude turbulenceencounters. It is possible to operate someturbine-powered aircraft at such weightsand altitudes so that their cruise airspeed isonly a few knots below the onset of Machbuffet and a like speed above stalJ buffet.In this situation (the so-called coffincorner), turbulent airspeed excursions ofmoderate or greater intensity (15 knots (kt)or more) can quickly lead to high-speedupset, Mach tuck, and loss of control. Onemethod for avoiding an upset, if theturbulent area cannot be avoided, is to flythe aircraft at a lower cruise altitude and/orloading to a lower weight.3.2 TAKEOFFANDLANDINGTakeoff and landing concerns includeexperiencing turbulent air with inadequatestall margins, loss of directional control onor near the runway, rolling moments thatsurpass aircraft roll authority, anddowndraft velocities that exceed the climbcapability of the aircraft, particularly forairplanes with high wing- andpower-loading. It is important to realizethat localized gusts in excess of 50 kt, withdowndrafts greater than 1500 feet (ft) perminute, are ·not unusual. Instances ofstructural damage have occurred in suchconditions; for example, on 31 March 1993,a B-747 experienced engine separationshortly after takeoff from Anchorage,Alaska.Vortices spawned by the interaction ofstrong winds and high terrain can lead tosevere turbulence and aircraft rollingmoments that may exceed the pilot's abilityto maintain aircraft control. Although moreresearch is needed, there is evidence thatmoving vortices in the lee of mountains canmarkedly increase the likelihood of loss ofcontrol (NTSB, 1992).3.3 Low-LEVEL MOUNTAIN FLYINGAircraft that engage in low-level flightoperations over mountainous terrain in thepresence of strong winds (20 kt or greater atridge level) can expect to encountermoderate or greater turbulence, strongup- and downdrafts, and very strong rotorand shear zones. This is particularly truefor general aviation aircraft. One suchaircraft was involved in an accident on22 December 1992, when a twin-enginecargo airplane crashed west of Denver,Colorado, in the presence of mountainwaves.The mountain flying literature cites 20 kt asthe criterion for classifying a wind as"strong." As used in the current document,this criterion refers to the large-scale (or5

prevailing wind in the area as opposed to alocal wind gust) wind speed at the crest ofthe ridge or level of the mountain peaks,upwind of the aircraft's position. Such anambient wind flow perpendicular to a ridgewill lead to substantially stronger surfacewinds, with the likelihood of turbulence.Similar wind enhancements can beanticipated near the slopes of an isolatedpeak. Forecast and actual wind speeds atridge level can be determined from the FD(forecast winds and temperatures aloft) andUA (PIREPS) products, respectively. Incontrast, downdrafts over forested areasmay be strong enough to force aircraftdown into the trees, even when the aircraftis flown at the best rate-of-climb speed.This effect on the aircraft is exacerbated byloss of aircraft performance because of thehigh-density altitude.4.0 SOURCES OFMOUNTAIN-INDUCED WINDHAZARDS FOR AVIATION4.1 A REVIEW OF KEY METEOROLOGICALCONCEPTSAs previously noted, the atmosphere is afluid and its motions generally obey ratherwell-understood mathematical relationshipsdescribing fluid motion. Many atmosphericdisturbances occur as periodic events; thatis, they are waves, with a measurable6wavelength, period, phase speed, andamplitude. The wave disturbances thatdevelop in the atmosphere are a result of theinteractions among a number of forces.These forces normally include pressuregFadients, the Coriolis force, gravity, andfriction.Large-scale atmospheric waves (on theorder of 1,000 nautical miles (nm)) exhibitprimarily horizontal motion. The verticalmotion in these waves is several orders ofmagnitude less than the horizontal motion.Examples of this type of wave are thesynoptic- and planetary-scale waves foundon constant pressure analyses (Figure 4-1).Other atmospheric waves, however, aresmaller in horizontal scale.

----- 7Figure 4-1. Example ofa large-scale atmospheric wave pattern as seen on a National Weather Service constant pressure chart(500mb). The solid lines are approximately parallel to the wind flow at this level. Rawinsonde observations are plotted. This examplehappens to be a few hours before a DC-8 experienced engine separation west ofDenver, Colorado (see Table 2-1).7

In these smaller horizontal scale waves, theratio of the vertical motion to the horizontalmotion is much greater than is the case forthe large-scale waves. The most importantwaves exhibiting this property are gravitywaves, so called because the restoring forceis gravity, and shear-induced orKelvin-Helmholtz (K-H) waves. A familiarexample of a gravity wave is a wave on theocean's surface. Atmospheric gravitywaves also are very common, but aregenerally invisible unless clouds are present.Mountain ranges can generate very strong,large amplitude gravity waves that canproduce serious hazards to mountain flying.For that reason, we will consider theirproperties in some detail. Innonmountainous areas, shear-inducedwaves are a primary source of turbulence ataltitude. In the vicinity of mountainousterrain, however, shear-induced waves canoften be found superposed on larger-scalegravity waves, thus constituting animportant source of turbulence.4.2 A REVIEW OF STATIC STABILITY ANDSTABLE/UNSTABLE ATMOSPHERICSTRATIFICATIONSAtmospheric stability describes the verticaldistribution of air density over a givenlocation and at a given time. If relativelyheavy air overlies Jess dense air, the8tendency will be for overturning and mixingto occur until a new, more stableatmospheric "mixture" (with less dense airabove) results. In general, the more rapidlythe atmosphere cools with height, the moreunstable it is (and the less resistant tovertical motions). Conversely, an area ofthe atmosphere that warms with increasingaltitude (an inversion) is quite stable andresistant to vertical motion.The stability of the atmosphere is related tothe vertical displacement of "parcels" of air.Vertically moving parcels of unsaturated airare cooled by expansion (if rising) andwarmed by compression (if descending) at afixed rate (the dry adiabatic lapse rate,3 degrees Celsius/1,000 ft). A review ofstability concepts is shown in Figure 4-2.In order for gravity waves to develop, theatmosphere must possess at least somedegree of static stability. This is because inan unstable atmosphere, an air parcel thatexperiences a vertical displacement (suchunstable air being forced upward when itinteracts with a mountain) will continue torise, rather than be forced back down to itsoriginal level. A stable atmosphere tends tosuppress vertical motions becauseatmospheric stability controls the motionsresulting from vertical deflection of theatmosphere by terrain.

4-2b4-2aTemperature4-2c(Figures 4-2a-c. Determination ofatmospheric stability: (a) unstable case;(b) neutral case; (c) stable file!Level BIc','.I'''Level A---- .-. --"' 'A1rParcelTemperature· Temperature9

Figure 4-2a shows an area of theatmosphere in which the temperaturedecreases rapidly with height (at a rategreater than the dry adiabatic lapse rate). Inthis case, the expansional cooling of a risingparcel moving between level (a) andlevel (b) takes place at a slower rate thanthat of the surrounding atmosphere. As aresult, the parcel will be warmer, thereforeless dense, than its surroundings at anylevel above its starting point, and it willcontinue to rise with no

weak instability (photograph ,NCAR). Figure 6-28. Cap cloud over Mt. Shasta, California, with . low-lying weak convection (photograph , 1972, R. Reinking). Figure 6-29. Banner and cap clouds occurring in the Grand Tetons, Wyoming (photograph , B. Martner). Figures 6·30a-c. Cap cloud, or cloud associated with a bora (photograph , K .