L 13 Weather v2 - Lecture notes 13 PDF

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Greg Fontaine...

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Lesson 13, Long Range and International Weather Application of weather data to Part 121 long range and international flight situations requires a familiarity with various weather products and formats. This lesson provides opportunity to regain that familiarity, and to explore the ICAO format for METARs and TAFs. Lesson objectives, targeted skills, and key terms are listed below. Objectives: Review critical weather situations appropriate to long range planning and flight operations. Analyze international METARs and TAFs to determine their impact on a Part 121 long range international flight in a two engine jet aircraft. Targeted Skills: Accurate interpretation of select weather charts and ICAO METARs/TAFs. Key Terms: jet stream, turbulence, thunderstorm, hail, lightning, weather radar, shadow, hook, finger, gradient, high level significant weather prognostic, meters, MPS, millibars, hectopascals, QNE, QNH, QFE, VOLMET.

CFR 121.599 Weather Familiarity (a) Domestic and flag operations. No aircraft dispatcher may release a flight unless he is thoroughly familiar with reported and forecast weather conditions on the route to be flown. (b) Supplemental operations. No pilot in command may begin a flight unless he is thoroughly familiar with reported and forecast weather conditions on the route to be flown.

CFR 121.613 Weather Required for Dispatch Except as provided in §121.615, no person may dispatch or release an aircraft for operations under IFR or over-the-top, unless appropriate weather reports or forecasts, or any combination thereof, indicate that the weather conditions will be at or above the authorized minimums at the estimated time of arrival at the airport or airports to which dispatched or released.

Weather Charts The following weather chart samples may be used in class to discuss various scenarios, and to emphasize the need for general chart familiarity. Students are encouraged to review previous course work as necessary to be refreshed on chart application prior to class.

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Surface Analysis

Winds Aloft

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High Level Significant Weather Prognostic

Radar Summary

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Convective Weather Managing your flight near thunderstorms involves locating the cells and following a plan to avoid them. Deliberately flying through a full-fledged thunderstorm is not an option. The pilot depends upon a combination of preflight planning and enroute updates to locate the weather. Once underway, the flight crew also relies on air traffic control to provide timely warnings of thunderstorms. This is particularly true when conditions are rapidly changing. An ARTCC is responsible for these warnings. Typically, a meteorologist at the ARTCC will issue a Center Weather Advisory (CWA) as a first step to alert pilots of developing thunderstorms in the center’s airspace. A CWA can be disseminated faster than a Convective SIGMET (WST), and since time may be critical for flight safety, it is often the first warning of new squall lines, embedded thunderstorms or any thunderstorms classified as “severe.” A severe thunderstorm may produce tornadoes, or contain hail of three-quarters of an inch or more, or has winds of 50 knots or more. ARTCC controllers usually broadcast advisories or new SIGMETs just once on center frequencies. Where HIWAS is available, a center may just alert pilots to the fact that a CWA or a new SIGMET has been issued. In this case, controllers would advise crews to “monitor HIWAS” for the full text. (Enroute charts show which VORs have HIWAS.) Terminal ATC facilities receive CWAs and SIGMETs from their ARTCC.

Avoiding Thunderstorm Cells All thunderstorms are potentially hazardous, and a cell which tops 30,000 feet deserves special attention. The hazards of a thunderstorm are turbulence, hail or a lightning strike. Severe turbulence can be present in any cell, but it is also possible a considerable distance away in clear air. Hail can be produced in a large storm and can be thrown out 20 miles or more downwind. Most lightning strikes occur near the freezing level, so they are not a frequent hazard in cruise. Use all available information on the location and severity of cells to make the best possible avoidance plan. Avoidance can be based on lateral deviation around cells, or by going over them. The choice depends upon cloud height, strength of the winds aloft and the apparent severity of cells. Most often, going around strong cells is more satisfactory than attempting to go over them. Lateral deviations should be made upwind of a cell, especially if winds aloft are strong. Deviating on the downwind side increases the chance of running into turbulence or hail. Hail is likely to be found under the anvil of a large storm. Cirrus layers downwind from a thunderstorm top may contain hail, even though the area shows little or no radar return. There is no industry consensus on the margin of clearance needed to go around a thunderstorm. Some airlines provide their crews with general guidelines and expect that good judgment will prevail. Other companies, not content with guidelines, provide definite rules. In the end, the pilot bears the final responsibility for safety, and he or she must make an appropriate decision for each situation.

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Following are two examples of general guidelines from an airline flight operations manual:  

“The stronger the storm, or the stronger the winds aloft, the further the lateral distance needed to avoid the storm.” “Turbulence can be expected downwind of the storm, one nautical mile for every knot of wind speed at flight altitude.”

In contrast to those general guidelines, consider these more specific rules from another company’s operations manual:   

“Above the freezing level, avoid all echoes by a minimum of 10 nautical miles.” “Above 23,000 feet, avoid all echoes by 20 nautical miles.” “Never fly near an echo with a top above 30,000 feet.”

Avoiding cells by attempting to over fly them is the least preferred method of avoidance. There are two main difficulties. First, you must accurately estimate the cloud tops, and secondly, turbulence and hail can be encountered in the clear air well above the tops. It is difficult to judge when a safe margin exists to over fly thunderstorms. There are a number of characteristics you can evaluate, however, in considering an over flight of cells. The possibility of encountering significant turbulence while flying over thunderstorms is increased when any of these factors is present:    

Strong winds at flight level A rapidly growing cell Tops over 30,000 feet Presence of cirrus or cap clouds, indicating shear

The use of radar to judge the height of a cell is inexact. The top of a thunderstorm may be mostly vapor and ice crystals, which will not show up in a radar return. Usually, an educated guess based upon visual observation of a storm will be more accurate than an estimate based on radar. When possible, go around strong cells rather than directly over them.

Clear Air Turbulence A jet pilot must understand high altitude weather. The FAA reports that turbulence is the leading cause of nonfatal accidents involving passengers and flight attendants. Injuries caused by turbulence cost U.S. airlines an estimated $100 million each year, and most occur in the cruise portion of flight. In this section we will look at phenomena which produce turbulence, and consider pilot actions to meet the problem. There are two general categories of turbulence that you might experience at cruise flight levels. The first is convective turbulence, the kind of turbulence you will encounter in cumulous clouds and thunderstorms. The second category is clear air turbulence, which can be caused by a number of factors.

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CAT is the turbulence experienced above 18,000 feet in clear air or in stratiform clouds. It can occur anywhere in the upper atmosphere where layers of air, moving at different velocities, rub against each other. Waves or undulations are created in unpredictable, turbulent patterns. Typically, some degree of CAT can be expected:  Near jet streams.  Downwind of thunderstorm tops.  In or over mountain waves.

Jet Streams and CAT Jet streams are ribbons of high velocity wind moving from west to east. They exist at the boundary between air masses of different temperature. In the northern hemisphere there are three jet streams, but the most northerly of the three, the Polar Front jet stream, is the strongest and the most significant to aviation. The Polar Front jet separates the polar air mass from more tropical air to the south, and it tends to change its latitude with the season of the year. In the summertime, for example, this jet stream might cross the continent near the Canadian border, while in the winter it could lie across the southern United States. The core of the jet stream is about as high as the tropopause, and the height of the tropopause varies according to latitude. In Polar Regions, the height of the tropopause is about 20,000 feet, while near the equator it is about 60,000 feet. A nominal height of the tropopause over the United States is 36,000 feet. While the image below gives the impression a jet stream is shaped like a tube, a more realistic visual would be a ribbon. The reason is that the jet stream is usually depicted as at least 1000 miles long, at least 100 miles wide, and little more than 1 mile deep. Winds as high as 240 knots have been recorded in a jet stream, but a velocity between 80-120 knots is much more common. The faster the wind, and the larger the temperature gradient between air masses, the more likely that moderate or severe CAT exists in some portion of the jet stream.

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Strongest CAT

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It is possible to cruise eastbound in the core of a jet stream and enjoy a smooth, steady flight, helped along by a substantial tailwind component. While CAT can develop anywhere near the jet stream, it is usually located on the polar edge of the core, which is the area of greatest wind and temperature gradient. While cruising in or near a jet stream, if unacceptable turbulence begins, the best evasive action is to change altitude. A relatively short climb or descent will often clear a turbulence layer. Deviating laterally is not a practical alternative, because jet stream turbulence can spread over a wide area. Any region in which a jet stream makes a sharp turn in direction is also a high risk area for turbulence. We can say, then, that a jet stream has good and bad features. On the positive side, it is a ready means to boost the groundspeed on an eastbound flight. On the negative side, a jet stream can generate strong turbulence with little warning.

Mountain Wave Turbulence A mountain wave begins when strong winds blowing perpendicular to a mountain range are lifted by the rising terrain. A wave-like motion of the winds can develop with certain temperature conditions. Turbulence and dangerous downdrafts can be produced for up to 100 miles on the downwind side of mountains. A mountain wave may contain visual cues to signal its presence if there is enough moisture in the atmosphere. Warning signs for a mountain wave include rotor clouds in the rolling air below the wave, and lenticular (lens-shaped) clouds over the tops of the wave action. Mountain waves are forecast when there are strong winds perpendicular to a mountain range and certain thermal conditions. If the winds are part of a jet stream, a vigorous wave can be expected. Pilots should be aware of the following:    

Mountain wave turbulence can extend well above cruise flight levels. A detour around known activity might be the best course of action. Altimeter errors of up to 2,500 feet can occur in mountain waves. The best altitude to fly over a mountain wave is the highest practical flight level which affords adequate mach buffet protection.

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Summing Up Clear Air Turbulence CAT causes at least half of all turbulence incidents in commercial flying. It is more insidious than convective turbulence because CAT often does not provide visual cues. Still, forecasting tools are constantly improving to provide the pilot with more accurate preflight information. Help in detecting CAT while airborne is now available. NASA and industry have developed a laser radar system called Light Detection and Ranging (LiDAR). LiDAR can be loosely compared to radar in that it emits energy and measures the reflected waves. However, the new technology uses infrared light waves from a laser, not a radar beam. The LiDAR transmits a laser pulse, and some of the light is reflected back from particles of dust and vapor along the flight path. The reflected light undergoes a slight shift in frequency, called a Doppler shift, due to the aircraft’s motion. By analyzing the Doppler shift, the changes in wind velocity along the laser beam path can be calculated. LiDAR airborne equipment to detect CAT may be a reality for airliners in the not too distant future.

DC-8 with Missing Outboard Engine from CAT in 1992

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Aircraft Weather Radar Making the best use of radar for weather avoidance depends on following your flight manual procedures. Antenna tilt is critical for getting an accurate picture of approaching weather. If not using an “auto tilt” mode, be sure that tilt is set for the selected range and your altitude. For close-in targets in cruise, you will need to “point down” with the tilt control to get meaningful returns. Radar attenuation occurs when some of the radar beam’s energy is absorbed by a heavy precipitation cell. The absorption of energy reduces the radar return and can cause a heavy precipitation area to be hidden in the shadow of the leading edge of the weather. A good rule of thumb is to not fly into a radar shadow, since it can be the area of heaviest precipitation and turbulence. Patterns emerge on the weather radar display that should be carefully considered. If behind a heavy radar return there appears to be clear weather, a shadow may be occurring, obscuring the real situation. If the normally roundish shape of a rain cell return becomes jagged, with fingers, this could be the indication of an unpredictably severe storm. If those fingers show signs of curving into a hook shape, you are likely seeing the circulation associated with a tornado.

ATC’s Role in Severe Weather Routing The Traffic Management System of ATC plays a leading role in providing safe routing when a region is impacted by thunderstorms. The Air Traffic Control System Command Center (ATCSCC), which monitors and manages traffic throughout the National Airspace System, coordinates procedures to assure an orderly flow of traffic around severe weather. When airways or other parts of the airspace system are blocked by thunderstorms, the area is shut down for operations and alternate routing is called into use. ATCSCC, working with the centers affected by weather, plans and directs procedures which will make the best use of available airspace. For areas in which severe weather frequently occurs, ATC may call into use a prearranged Severe Weather Avoidance Plan (SWAP). A SWAP has the advantage of advance planning, and it may be published and available to both controllers and pilots. If a SWAP is not available, severe weather management teams work out alternate routes for traffic as each situation demands. When routes are shut down due to weather, the ATCSCC directs the centers to broadcast a Special Weather Advisory. A “special” will inform pilots what area or route has been shut down and what alternate routing to expect. This advance information of course helps the flight crews, but individual flights do not actually deviate until receiving a revised clearance. Altering the normal flow of traffic, particularly near crowded urban areas, can be a challenging task for ATC. Routing changes can produce a need for greater in-trail spacing, wide vectors for spacing, or even ground holds at originating airports. In these cases ATC will attempt to reopen normal routes as soon as practical.

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While ATC has become adept at managing severe weather traffic flow, the pilot must always monitor and confirm any thunderstorm situation himself. The pilot has the final decision-making responsibility to keep his airplane out of a thunderstorm. For operations with a company dispatcher (primarily Part 121), the dispatcher can provide valuable assistance on routing around weather. He can check radar displays over a wide area and consult directly with a weather specialist at an ARTCC.

METARs and TAFs This section will emphasize the differences between domestic and international METARs and TAFs, which are actually quite minimal. Fundamentally, international reports use meters for visibility and RVR, millibars instead of inches of mercury for altimeter settings, and occasionally MPS for wind velocity. The trick is to recognize whether or not the report is international. The easiest way is to reference the visibility code. If it is listed in statute miles, the report is not international. How well can you interpret the following weather data? Review and decode the examples below. If you have difficulty, it might be time for some additional review on the following pages. 1) METAR EGLL 2300 21015KT 8000 RASH SCT012 BKN020 10/08 Q1011

2) SPECI LFPO 0625 VRB01MPS 0200 RVR 0400 FG VV003

3) METAR OERK 1700 33025/G36KT 4800 SA SCT100 27/08 Q1030 TEMPO 2400

4) ARP DAL71 3156N06811W 1727 M040 250/40KT

5) TAF LFBD 1903/1912 VRB05KT CAVOK TEMPO 0307 0300 FG VV008

6) TAF KRDR 2217/2317 13009KT 3200 –SN BR BKN007 FM0800 10007KT 1600 –SN BR VV004

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The following decoding information is relevant to both METARs and TAFs: TAF KPIT 091730Z 091818 15005KT 5SM HZ FEW020 WS010/31022KT FM1930 30015G25KT 3SM SHRA OVC015 TEMPO 2022 1/2SM +TSRA OVC008CB FM0100 27008KT 5SM SHRA BKN020 0VC040 PROB40 0407 1SM -RA BR FM1015 18005KT 6SM -SHRA OVC020 BECMG 1315 P6SM NSW SKC METAR KPIT 091955Z COR 22015G25KT 3/4SM R28L/2600FT TSRA OVC010CB 18/16 A2992 RMK SLP045 T01820159 Forecast Explanation Report Message type: TAF-routine or TAF AMD-amended TAF forecast, METAR-hourly, SPECI-special or TESTMMETAR non-commissioned ASOS report KPIT ICAO location indicator KPIT Issuance time: ALL times in UTC "Z", 2-digit date, 4-digit 091730Z 091955Z time Valid period: 2-digit date, 2-digit beginning, 2-digit ending 091818 times In U.S. METAR: CORrected ob; or AUTOmated ob for automated report with no human intervention; omitted COR when observer logs on Wind: 3 digit true-north direction, nearest 10 degrees (or VaRiaBle); next 2-3 digits for speed and unit, KT (KMH or MPS); as needed, Gust and maximum speed; 00000KT 22015G25KT 15005KT for calm; for METAR, if direction varies 60 degrees or more, Variability appended, e.g. 180V260 Prevailing visibility: in U.S., Statute Miles & fractions; above 6 miles in TAF Plus6SM. (Or, 4-digit minimum 5SM 3/4SM visibility in meters and as required, lowest value with direction) Runway Visual Range: R; 2-digit runway designator Left, Center, or Right as needed; "/"; Minus or Plus in U.S, 4digit value, FeeT in U.S. (usually meters elsewhere); 4- R28L/26...