OPERATIONS Optimum use of weather radar

Optimum use of weather radar In recent years, there have been a number of flights where passengers or crew suffered injuries due to severe turbulence. In some other instances, the aircraft structure was substantially damaged following a hailstorm encounter. Clearly adverse weather can pose a threat to the safe and comfortable completion of a flight, thus it needs to be detected and avoided in a timely manner.

DAVID CHRISTIAN LAURENT MARCONNET NORDEN VIDAL Flight Operations Director Flight Surveillance Systems Support & Training Operations & Manager Standards Safety Training Policy Enhancement Safety First #22 | July 2016 023

The airborne weather radar system is an essential tool for pilots to assess the intensity of convective weather ahead of the aircraft. In this respect, it enables the strategic and tactical planning of a safe flight trajectory. Weather radar technology has evolved significantly in the last few years and a range of enhanced products is now available. If properly used, they permit pilots workload to be significantly reduced while substantially reducing encounters with adverse weather. This article offers an overview of the existing weather radar technologies, and provides information and tips on how to tune the system and correctly interpret the available displays.

WEATHER RADAR PRINCIPLE AND OPERATION

The weather radar system installed the crew to carefully optimize its Prevention on-board aircraft provides the pilot with use. This relies primarily on a good the necessary information to avoid - not meteorological knowledge of weather through anticipation penetrate - adverse weather. phenomena, along with a good is essential. To obtain the maximum benefit from understanding of the available radar the weather radar system requires functions. Flying in adverse weather: lessons learned

The aviation industry experience shows aircraft damage (fig.1). that although aircraft are equipped with These events have led us to wonder airborne weather radars, incursions why such encounters happen, and into very active convective cells still clearly show that prevention through occur, resulting in injuries or substantial anticipation is essential. (fig.1) Radome and windshield after hail encounter

Weather When it comes to understanding is the active monitoring of the overall radar is of help, why aircraft fitted with technologically meteorological situation by the crew, but the crew overall advanced weather radars can end in addition to the optimum use of the up flying in such unfavorable weather weather radar and correct understanding assessment patterns, we have to consider that getting of the information displayed. We must of the weather the best out of technology onboard is not forget that weather radar is of help, just a part of the answer. A key element but the crew overall assessment of the situation plays of adverse weather avoidance strategies weather situation plays the central role. the central role. OPERATIONS Optimum use of weather radar

HOW IS A CUMULONIMBUS, OFTEN CALLED “THUNDERSTORM”, STRUCTURED?

Three common threats to aircraft are turbulence (which Understanding how a cumulonimbus cloud is structured is caused when two masses of air collide at different and evolves is key in dodging the associated weather speeds), hail and windshear. All three of these are disturbances. by-products of thunderstorms. High concentration of Ice Crystals

FL 350 -40°C

23 000 ft -15°C

16 000 ft Freezing Level

WIND SHEAR TURBULENCE WIND SHEAR TURBULENCE DUST Super-cooled liquid droplet Ice crystals

Turbulence:

Turbulence associated with a cumulonimbus is not it is necessary to apply recommendations for weather limited to inside the cloud. Therefore, when flying in avoidance as summarized in this article. an area where cumulonimbus clouds have developed,

Hail:

The presence of hail within a Risk of encountering hail relative to cumulonimbus cloud position cumulonimbus varies with altitude and wind:

- Below FL 100, hail is equally likely to be encountered under the storm, in the cloud or around it (up to 2 NM).

- Between FL 100 and FL 200, approximately 60 percent of hail is encountered in the cumulonimbus and 40 percent is encountered outside the cloud, under the anvil.

- Above FL 200, hail is most likely to be encountered inside the cloud.

When hail is encountered outside driven upward within the cloud by is less risk of hail in humid air than the cloud, usually the threat of strong drafts. It then freezes and is in dry air. In fact, moisture in the air hail is greater downwind of the transformed into hail before being behaves as a heat conductor, and cumulonimbus because moisture is blown downwind. Paradoxically, there helps to melt the hail. Safety First #22 | July 2016 025

Weather radar principle

A knowledge of the radar principle tune this system and interpret the On the Airbus is paramount in order to accurately weather radar display correctly. fleet, all weather Reflectivity radars have full capability to allow Weather detection is based on the The weather radar echo returns vary reflectivity of water droplets(fig.2) . The in intensity as a function of the drop- wet turbulence weather echo appears on the Naviga- let size, composition and quantity. For detection. tion Display (ND) with a color scale that example, a water particle is five times goes from red (high reflectivity) to green more reflective than an ice particle of (low reflectivity). the same size.

High Re ectivity (fig.2) Wet Hail Weather radar principle

Good Re ectivity

Liquid Rain water

Wet Snow

Dry Hail

Dry Snow Low Re ectivity Fog Drizzle Low Re ectivity

Some weather radars are fitted of light rainfall, depicted in green in with a turbulence display mode. normal mode, is shown in magenta This function (the TURB function) is when there is high turbulence activity. based on the Doppler effect and is The TURB function is on most weather sensitive to precipitation movement. radars only active within a range of 40 Like the weather radar, the TURB NM (Doppler measurement capability) function needs a minimum amount of and should only be used in wet precipitation to be effective. An area turbulence. DID YOU

Weather radar operation KNOW Each type of weather radar has its own The flight crew uses four features to has an essential influence on the particularities. To get all the information on operate the radar: optimum tilt setting. the characteristics, limitations and opera- - Antenna tilt: this is the angle between - Gain control: this adjusts the tional recommendations of each weather the centre of the beam and the sensitivity of the receiver. radar model, the user guide of the radar horizon (fig.3). - Radar modes: weather (WX) or manufacturer needs to be studied. - Range control of the ND: this weather + turbulence (WX + T).

(fig.3) Definition of the TILT OPERATIONS Optimum use of weather radar

Weather radar limitations

Weather radar detection capability Reflectivity One of the weather radar limitations is winds produce large scale uplifts of that it indicates only the presence of dry air. The resulting weather cells have is not directly liquid water. The consequence is that much less reflectivity than mid-latitude proportional to a thunderstorm does not have the convective cells. However, turbulence same reflectivity over its altitude range in or above such clouds may have a the level of risk because the quantity of liquid water in higher intensity than indicated by the that may be the atmosphere decreases with the image on the weather radar display. encountered. altitude (fig.4). Yet, the convective On the other hand, air close to the cloud and associated threats may sea can be very humid. In this case, extend significantly above the upper thermal convection will produce clouds detection limit of the weather radar that are full of water: these clouds will (called ‘radar top’). This means that have a high reflectivity, but may not reflectivity is not directly proportional to necessarily be a high threat. the level of risk that may be encountered: a convective cloud may be dangerous, Consequently, limitations of weather even if the radar echo is weak. radars must be well understood and complemented by basic meteorological This is particularly true for equatorial knowledge of the crew and, where overland regions where converging possible, visual observation.

Turbulence Area Turbulent area, not detected Re ectivity by the weather radar Visible Top

Radar Top

0°C

(fig.4) Reflective image of a cumulonimbus

The weather radar detects: The weather radar does not detect:

- Rainfall - Ice crystals, dry hail * and snow - Wet hail and wet turbulence - Clear air turbulence - Windshear - Sandstorms (solid particles are almost transparent to the radar beam) - Lightning *

* The latest generations of weather radars offer hail and lightning prediction functions (see the following sections). Safety First #22 | July 2016 027

The beam attenuation phenomenon Another limitation of the weather radar and intensity of that weather may not be A black hole is called ‘shadowing’ or ‘attenuation’. accurately displayed to the pilot. What The weather radar display depends appears to be a thin or inexistent band behind a red area on signal returns: the more intense the of precipitation (fig.5) could in fact be on a weather radar precipitation, the less distance the radar the leading edge of a much larger area can see through. Therefore when the of precipitation. Secondly, any weather display should always radar echo is unable to make the two behind such strong shadowing cells be considered way trip through heavy precipitation, a will not be detected. This can result in as a zone that is ‘shadowing’ effect occurs. unexpected weather unfolding only after The result is twofold. First, the size, shape the cell has been circumnavigated. potentially very active and shadows weather further down the scanned path.

(fig.5) Attenuation caused by moderate to extreme precipitation

WEATHER RADAR TECHNOLOGY: THE DIFFERENT TYPES OF WEATHER RADAR

In cooperation with its suppliers, These continuous improvements have Airbus has continuously and pro- allowed the crew to be provided with actively supported weather radar optimized observation features and technology evolution over the years. weather threat assessment functions. NOTE Radars with manual controls Early generation of full manual Full manual control radars controlled radars without auto tilt are: Rockwell Collins WXR- Early generations of radars are not the expected weather on path and 701X family up to weather radar equipped with an automatic tilt the ND range selection. Then the pilot transceiver Part Number 622-5132- function; therefore the antenna tilt needs to analyze and understand the 624 and Honeywell RDR-4B family needs to be manually adjusted up individual radar slices of weather up to weather radar transceiver Part and down as the flight progresses displayed in order to get an overall Number 066-50008-0407. according to the aircraft’s altitude, picture. OPERATIONS Optimum use of weather radar

Autotilt radar (Honeywell RDR-4B family PN 066-50008-0409)

Honeywell introduced the first weather the EGPWS terrain database and radar featuring an automatic tilt automatically adjusts the antenna tilt computation named “Autotilt”. based on the aircraft position, altitude, When in Autotilt mode, the radar uses and the selected ND range (fig.6).

(fig.6) Honeywell Autotilt

Fully automatic radars Automatic The next generation of radars included - Offer independent pilot control and automatic functions, which: display selection. radars optimize - Scan airspace ahead of the aircraft weather detection with multiple beams These new radars optimize weather and decrease - Feature a three dimensional (3D) detection and decrease significantly buffer to store weather data the pilots’ workload necessary to significantly the - Automatically compute and adjust understand the complete picture of pilots’ workload. the antenna tilt the weather ahead.

A320 & A330 families: Multiscan radar (Rockwell Collins WXR-2100 family)

The WXR-2100 Multiscan weather antenna tilt settings. The image that is radar is part of this new generation displayed on the ND is the result of the of weather radars that offers an stored and combined information from automatic computation of tilt and gain each beam. control at all ranges, all altitudes and The radar automatically adjusts the gain all times (fig.7). and tilt based on various parameters This weather radar is designed to work in (aircraft altitude, geographical area, Multiscan automatic mode. Pilots select season, time of the day) to obtain only the desired range for the display the best weather display in each and the radar alternatively scans at two geographic region.

(fig.7) Rockwell Collins Multiscan Safety First #22 | July 2016 029

UpperUpper Beam Beam LowerLower Beam Beam

Upper Upper and lower Beam beams data merge Upper Upper and lower Beam beams data+ merge Ground Lower Clutter+ Suppression Beam Ground(GCS) Lower Clutter Suppression Beam (GCS)

A320 & A330 & A350 & A380 families: Honeywell RDR-4000

The Honeywell RDR-4000 model is part A380) to show the en route weather of the new generation of weather radars picture, as well as automatically scan including a 3D volumetric buffer. from the ground up to 60 000 feet to provide information targeted at various It can probe hundreds of miles ahead altitudes. The required display data (up to 320 NM on A320 & A330 families are then accessed from the 3D buffer and up to 640 NM on A350 and (fig.8 and 9).

(fig.8) Honeywell RDR-4000 control panel on A320 & A330 families OPERATIONS Optimum use of weather radar

(fig.9) Honeywell RDR-4000 control panel on A350 & A380

When it is activated in the automatic ‘OFF PATH’) depending on the flight mode, the radar RDR-4000 takes into profile. Weather conditions along the account a vertical trajectory envelope plane’s trajectory are displayed in solid (nominally +/- 4000 ft) along the vertical colors, while more distant vertical flight path of the aircraft based on the echoes are shown in striped pattern to flight path angle. It then defines if the help pilots determine whether weather weather echo is inside this envelope avoidance maneuver or rerouting is (relevant ‘ON PATH’) or not (secondary necessary (fig.10).

(fig.10) Honeywell RDR-4000 display Relevant ‘ON PATH’ weather displayed in solid colors

Secondary ‘OFF PATH’ weather displayed in striped pattern

The RDR-4000 can also be used in - the ND, for views along the vertical manual mode (elevation mode) as a flight path (in AUTO mode) or along tool for analyzing weather at user- the selected altitude (in ELEVN NOTE selected altitudes and thus, assess mode) or along the selected tilt angle the vertical expansion and structure (in TILT mode) These enhanced weather radars of convective clouds. are provided at Entry Into Service This system is available on the A380 - as well as on the Vertical Display for new programmes (A380, A350) and also on the A350 with an additional (VD) for views along the lateral flight and as a retrofit option for A320 and ‘weather ahead’ alerting function. On path (in AUTO mode) or along the A330 families. these aircraft, the weather displayed is selected azimuth (in AZIM mode) a computed image on: (fig.11). Safety First #22 | July 2016 031

(fig.11) Weather information displayed on the Vertical Display

Hail and lightning prediction: the new functions introduced by ‘step 2’ automatic radars

In continuity of RDR-4000 and AHEAD’) to alert the crew when the Multiscan WXR-2100, a new step of ND is not in weather mode development recently introduced: - Hazard functions offering: - Hail and lightning prediction • Lightning and hail prediction - Improved weather information. • Rain Echo Attenuation Compen- sation Technique (REACT): this Honeywell RDR-4000 (V2) function indicates areas where includes new features to improve the intensity of the radar echo has weather hazard assessment by been attenuated by intervening automatically providing the following weather. additional information (fig.12): • Extended turbulence detection - Weather alerting (‘WEATHER (up to 60 NM instead of 40).

(fig.12) Honeywell RDR-4000 V2 display

REACT

Lightning risk area Hail risk area 60 NM Turbulence OPERATIONS Optimum use of weather radar

Rockwell Collins Multiscan the corresponding threat based on WXR-2100 (V2) includes an automatic reflectivity characteristics(fig.13) . weather threat assessment (“Track This new radar also provides hazard While Scan” function). In continuity of functions, namely: the Multiscan, the aim of this version - Lightning and hail prediction is to provide not only a depiction of - Predictive OverFlight (this function the reflectivity of surrounding weather alerts the crew to growing cells cells, but also a threat assessment for that are potentially on the aircraft each cell detected. trajectory) Weather cells are first tracked and - Improved turbulence detection then, additional vertical scans are able to display an additional level of performed automatically to assess moderate turbulence.

(fig.13) Rockwell Collins Multiscan V2 threat Predictive OverFlight detection and analysis (convective phenomenon)

Two levels of turbulence (severe: plain magenta, moderate: magenta speckles)

Associated threats (area with hail and lightning probability)

NOTE

Honeywell RDR-4000 V2 and Rockwell Collins Multiscan WXR-2100 V2 weather radars were certified in July 2015 for A320 and A330/A340 families and are available as retrofit options. Safety First #22 | July 2016 033

Coming next… the future evolution of weather information

Airbus in cooperation with weather on aircraft. The next generation of radars suppliers, maintain their efforts weather radars is expected to benefit in designing and producing new from this research work and enable weather surveillance functions. Today, the detection of ice crystals to avoid at a research level, a strong focus is convective weather linked to ice placed on three main dimensions with crystal icing. the aim to improve pilots’ awareness of the weather ahead. 2. Weather display fusion to offer a single display of weather data 1. High Altitude Ice Crystal (HAIC) covering all weather threats. detection to avoid flying in ice The feasibility of collecting all “weather crystals areas. on board” information together with There are multiple threats attributed weather information collected by to ice crystals; for example, engine the radar (reflectivity, turbulence and vibrations, engine power loss, engine hazards) and merging them in a single damage or icing of air data probes. display is currently being studied. In fact, the formation of ice crystals at high altitude and their effect on 3. 3D weather analysis: automatic aircraft performance is recognized re-routing as an industry wide issue. Airbus in Work is also being carried out to allow the particular is leading the HAIC research automatic computation of an optimized project with several partners. This deviation route based on: the actual project aims to characterize and weather (weather on board and radar identify the environmental conditions data), the ongoing traffic and the stored of ice crystals, to improve aircraft flight plan. Such a function is expected operations through the development to facilitate pilot’s decision making and of appropriate detection and re-routing planning if needed. Additionally awareness technologies to be fitted it improves comfort. OPERATIONS Optimum use of weather radar

PUTTING THEORY INTO PRACTICE: HOW TO MAKE AN OPTIMUM USE OF THE AIRBORNE WEATHER RADAR

The weather radar is a tool for detecting, charts and online simulation) and during analyzing and avoiding adverse weather flight (update on weather information) and turbulence. As with any other - Adapted use of the weather radar, tool, adequate skills and the crew’s with the crew regularly assessing involvement are needed in order to use the range, gain and tilt, and making it efficiently. In fact, the management use of weather threat assessment of adverse weather still relies primarily functions when available in order to on the crew to actively monitor the display an optimum weather radar meteorological situation throughout the picture on the ND. flight, and make a full use of the available - Regular manual vertical and horizontal technology thanks to: scanning by the crew to increase - Awareness of weather radar capabilities situation awareness. and limitations, according to the - Correct understanding of the radar specificities outlined in the FCOM and image displayed. the manufacturer’s user guide. - Adequate strategic (mid-term) and - Preflight briefing (knowledge of the route tactical (short term) decision making climatology and weather forecast – for trajectory planning. How to optimally tune the weather radar and manage flights in convective weather? Flight planning: the importance of weather briefing and weather reports

Weather avoidance already starts in on both the weather briefing and their the briefing room before commencing knowledge of local climatology. Changing the flight, with a thorough assessment the flight route could be an option as well of en-route weather and decisions on as taking additional fuel for enhanced possible mitigation means. strategic and tactical options in flight. Before boarding, a weather briefing Once airborne, the weather radar should Once airborne, should reveal areas of predicted be used and tuned regularly in combination the weather radar significant weather activity. Equally, this with all available information, e.g. pre- should be used briefing should include the assessment of flight briefing, pilot’s knowledge and typical weather patterns in the area. For experience of the area’s typicality, reported and tuned regularly example in the tropics, cumulonimbus turbulence, updated weather reports… If in combination with intensity and development is greater at possible, the weather information should certain times of the day. The crew have be updated in flight regularly. Information all available weather the opportunity at this stage to plan a sought by ATC of turbulence encounters information. route to avoid active weather based are an additional means. INFORMATION

Safe operation in convective weather requires good theoretical knowledge of meteorol- ogy, particularly on the formation, development and characteristics of convective clouds in different regions of the world. This knowledge is usually provided in pilot licencing and operational training and is not covered by aircraft documentation (FCOM and FCTM). Safety First #22 | July 2016 035

Weather radar antenna tilt Effective management of the antenna The flight crew needs to periodically The automatic tilt along with an appropriate ND range scan: selection, are key tools to obtaining an - Vertically, using the antenna tilt function mode should be informative weather radar display on - Horizontally, using the range change. used as the default the ND. If available, the automatic mode mode, for detection The ND might not display cells at should be used as the default and initial evaluation aircraft flight level, only cells that are mode (unless mentioned differently of displayed weather. cut by the radar beam are shown in the FCOM), for detection and (fig.14). For this reason, the antenna initial evaluation of displayed Then, manual tilt needs to be adjusted up and down weather. Then, if adverse weather control should be regularly to scan weather ahead, is suspected (e.g. according to and it needs to be adjusted to the information gathered during the pre- used periodically ND range selection (except with the flight briefing), manual control should to analyze the most recent radar models where this be used regularly and actively to weather. adjustment is made automatically). analyze the weather ahead.

(fig.14) Display along radar beam

BEST PRACTICE

Even when the tilt is adjusted automatically, pilots are advised to reverse to the manual mode “MAN” regularly in order to scan the immediate weather ahead. This action allows the crew to assess the vertical structure and expansion of convective clouds.

Factors that can affect the relevancy of - The shape of thunderstorms the ND display and that should trigger - A pilot report from another aircraft in a tilt adjustment are: the vicinity. - A heading change In the case of a change in heading - An altitude change, or even a regular or altitude, leaving the antenna tilt on flight profile change (e.g. from climb auto may induce a risk of overlooking to cruise) weather or underestimating the OPERATIONS Optimum use of weather radar

severity of the weather. For example, convective cell because the tilt is at take-off or in climb, the tilt should be set incorrectly (too high in this case) set up if adverse weather is expected while in the auto-tilt mode. When the above the aircraft. Figure 15 is an antenna is tilted down, the ND shows example of radar overshooting a a much stronger activity. (fig.15) Weather radar display at different tilt settings

Overscanning

Correct storm display Presence of To analyze a convective cell, the flight then be used to scan the area verti- crew should use the tilt knob to obtain cally. Presence of yellow or green areas yellow or green areas a correct display and point the weather at high altitudes, above a red cell, may at high altitudes, radar beam to the most reflective part indicate a very turbulent area. of the cell. At high altitude, a thunder- above a red cell, storm may contain ice particles that In most cases in flight, the adequate may indicate a very have low reflectivity. If the tilt setting is antenna tilt setting shows some ground turbulent area. not adapted, the ND may display only returns at the top edge of the ND, which the upper (less reflective) part of the may be difficult to differentiate from convective cloud (overscanning). As genuine weather echoes. A change in a result, the flight crew may underes- antenna tilt rapidly changes the shape timate or not detect a thunderstorm. In and color of ground returns and eventually order to get accurate weather detec- causes them to disappear. This is not the tion, the weather radar antenna should case for weather echoes. Some weather also be pointed toward lower levels (i.e. radars are fitted with a Ground Clutter below freezing level), where water can Suppress (GCS) function. When turned still be found. If a red area is found at ON, it suppresses the ground return from a lower level, the antenna tilt should the display.

Display range management

To maintain a comprehensive situation therefore, the following ranges should be awareness, the flight crew needs to selected on the NDs: monitor both the short-distance and - Pilot Monitoring (PM) adjusts ranges to DID YOU long-distance weather. To this end, the plan the long-term weather avoidance KNOW crew should select different ranges on strategy (in cruise, typically 160 NM the Pilot Monitoring (PM) and Pilot Fly- and below). The FCTM provides useful guidance ing (PF) ND. - Pilot Flying (PF) adjusts ranges to moni- to correctly tune the weather radar in To avoid threatening convective weather, tor the severity of adverse weather, and accordance with the flight phase. the flight crew should make deviation decide on avoidance tactics (in cruise, decisions while still at least 40 NM away; typically 80 NM and below as required). Safety First #22 | July 2016 037

Course changes to avoid adverse seem safe when using a low range ND weather should be determined using display may reveal a blocked passage both displays. This prevents the “blind when observed at a higher range alley” effect: a course change that may (fig.16).

The crew should select different ranges on the Pilot Monitoring (PM) and Pilot Flying (PF) ND.

(fig.16) Blind alley effect

Gain adjustment

The sensitivity of the receiver may significant positive ISA deviations in a vary from one type of radar system to very humid atmosphere (typically the another. In the CAL (AUTO) position, Indian monsoon). In these cases, slowly the gain is in the optimum position to reducing the gain allows the detection detect standard convective clouds. of threatening areas: most red areas Manual settings are also available and slowly turn yellow, the yellow areas can be used to analyze weather. turn green and the green areas slowly disappear. The remaining red areas – At low altitudes, reducing the gain i.e. the red areas that are the last to turn might be justified for proper weather yellow, - are the most active parts of analysis. Due to increased humidity the cell and must be avoided (fig.17). at lower levels, convective cells are usually more reflective and the weather At high altitudes, water particles are radar display may have a tendency to frozen and clouds are less reflective. show a lot of red areas. This can also In this case, gain should be increased be the case at higher altitude with for threat evaluation purposes.

Gain decreased

(fig.17) Effect of gain reduction OPERATIONS Optimum use of weather radar

Turbulence and weather threats detection

Turbulence can be difficult to predict, the radar standard) (fig.18). Remem- but signs such as frequent and strong ber that the TURB function needs lightning and/or the specific shape of humidity; therefore clear air turbulence The TURB clouds (see the next section) can alert will not be displayed. function needs the crew to the likely presence of severe turbulence. If necessary and when In addition, the flight crew may be humidity; therefore available (according to the standard alerted by visual cues provided by the clear air turbulence of weather radar onboard), the TURB latest generations of weather radars function can additionally be used to that offer weather threat assessment will not be confirm the presence of wet turbulence functions, such as hail or lightning displayed. up to 40 NM (or 60 NM depending on predictions.

(fig.18) Turbulence detection (in magenta) Safety First #22 | July 2016 039

HOW TO CORRECTLY TUNE THE WEATHER RADAR AT A GLANCE

Study the weather radar’s specificities and limitations through Before flight the FCOM, FCTM and weather radar user guide.

Gather information about the Before forecasted weather and update and regularly during flight: weather during flight briefing, route climatology knowledge, reported turbulence…

Set the antenna tilt to auto as the default mode for detection and initial evaluation of weather, During flight and periodically use the manual modes to scan and analyze the weather situation.

In cruise the combination of the following ranges provides good weather awareness and allows to During flight avoid the “blind alley effect”: - 160 NM on the PM ND - 80 NM on the PF ND.

Use gain in AUTO/CAL mode by default, then regularly reduce During flight the gain for weather severity assessment.

Be attentive to the visual and oral cues provided by the weather During flight threat and hazard assessment functions (as installed). OPERATIONS Lithium batteries: safe to fly?

Weather radar data understanding: how to decide on an effective avoidance strategy?

Before any avoidance maneuver is to conduct an in-depth analysis of the initiated, the analysis the flight crew convective weather situation on-path makes of the weather radar display is and off-path and eventually, initiate essential. Doing so, the crew is able action if needed.

Correctly understanding the weather display is paramount

After the weather radar has been tuned conditions are too dangerous to fly in. correctly, the data displayed should be supplemented with the available weather Some ND displays contain specific charts, reports and the meteorological cues that should alert the flight crew. knowledge of the pilot. Altogether these Clouds shapes, in addition to colors, data enable the flight crew to get a should be observed carefully in order complete weather picture and establish to detect adverse weather conditions. an “area of threat”. This “area of threat” Closely spaced areas of different colors corresponds to the zone where the usually indicate highly turbulent zones flight crew estimates that the weather (fig.19).

(fig.19) Indication of a threat: closely spaced areas of different colors Some shapes are good indicators of changing shapes, whatever the form severe hail that also indicate strong they take, also indicate high weather vertical drafts (fig.20). Finally, fast activity.

Finger Hook (fig.20) Shapes indicative of adverse weather

U-Shape Scalloped Edges Safety First #22 | July 2016 041

Avoidance strategy Consider a The flight crew needs to remain vigilant been taken, flight crews need to bear in and active in using and tuning the mind the following advisory precautions minimum distance weather radar in order to be able to and limits before actually deciding the tra- of 40 NM from initiate an avoidance maneuver as early jectory of the avoidance maneuver. as possible. Indeed, weather radar a threatening information becomes more intense as If possible, it is preferable to perform convective cloud to the aircraft gets nearer the convective lateral avoidance instead of vertical initiate the avoidance weather zone, thus making avoidance avoidance. Indeed, vertical avoidance is decisions more difficult. For this reason, not always possible (particularly at high maneuver. crews should consider a minimum altitude) due to the reduction of buffet distance of 40 NM from the convective and performance margins. In addition, cloud to initiate the avoidance maneuver. some convective clouds may have a significant build-up speed, that extends Once the decision to deviate course has far above the radar visible top.

Lateral avoidance

• When possible, it is advisable to try (fig.21). An additional margin may be to avoid a storm by flying on the applied in case the convective clouds upwind side of a cumulonimbus. are very dynamic or have a significant Usually, there is less turbulence and build-up speed. hail upwind of a convective cloud. • If the aircraft trajectory goes • The “area of threat” identified by the between several convective clouds, flight crew (e.g. a cumulonimbus cloud) if possible maintain a margin of at should be cleared by a minimum of least 40 NM with the identified “area 20 NM laterally whenever possible of threat”.

Vertical avoidance

• Do not attempt to fly under a all indications (visual judgement, convective cloud, even when you weather radar, weather report, pilot’s can see through to the other side, report, etc) before they take the final due to possible severe turbulence, decision. windshear, microbursts and hail. If an aircraft must fly below a convective • If overflying a convective cloud cannot cloud (e.g. during approach), then the be avoided, apply a vertical margin of flight crew should take into account 5 000 feet (fig.21). OPERATIONS Optimum use of weather radar

5,000 feet 20 NM If possible, it is preferable to perform lateral avoidance instead of vertical avoidance.

(fig.21) Lateral and vertical circumvention margins

Practical example: typical scenario of severe weather avoidance strategy

Figure 22 shows a typical weather Route C: radar display indicating multiple areas of this route looks like a possible escape severe weather. What route would look route because it goes around most of like the preferable option? the storms by a wide safety margin. However, while doing so, the flight Route A: crew would need to keep a look at the this is the most direct route to destination cell to the left of this route, and see but it navigates right through the most whether it develops rapidly or not. In severe and active zone; therefore it is the addition, this route leads away from the path that carries the biggest risks and initial flight plan and therefore it could should not be an option. have operational implications such as fuel consumption or delays. Route B: this route could be tempting since it Route D: requires little deviation to the mainstream this route would be the preferable route and it looks like the most active option in terms of risks mitigation. red areas are avoided. Nevertheless, this trajectory leads downwind of the When faced with a situation where convective area, thus increasing the weather ahead reveals an extensive risk of encountering severe weather. storm system, several options are Additionally, the convective cells beneath always possible. Before the flight crew might be developing fast and upwards, makes a decision, it is prudent to ana- thus closing off the gap in between the red lyze weather carefully by scanning the zones. Before this option is considered, vertical expansion of the various cells, the flight crew would need to tilt the radar and if possible, consider deviation to antenna down to analyze weather and an alternate route. see what is below the apparent gap. Safety First #22 | July 2016 043

Route C

Route A Route B

Route D

(fig.22) Available options to avoiding weather

Regardless of how you locate a severe weather area – visual, by radar, or from a report – a key parameter to successful route planning and avoidance strategy is time. The weather radar, and enhanced models more particularly, can help you to analyze and understand distant weather accurately and evaluate weather scenarios from a distance. This system is a key tool to planning ahead to avoid last- minute decisions, and making a decision on circumnavigating a nasty convective cell with a comfortable safety margin. In addition to technology, you need to stay active in maintaining situation awareness throughout the flight. Regularly complement the radar images displayed by a manual vertical scan of surrounding cells, as well as gain and tilt adjustments as required. Last but not least, adhere to your knowledge of meteorology basics, local climatology and weather briefing to adopt the best course of actions, and navigate safely, effectively and comfortably to destination.