WAFSOPSG/6-IP/13 3/3/ /11 WORLD AREAA FORECAST SYSTEM OPERATIONS GROUP (WAFSOPSG) SIXTH MEETING Dakar, Senegal, 21 to 24 March 2011 Agenda Item 6: Development of the WAFS 6.1: Improved GRIB 2 forecasts for convective clouds, icing and turbulence OPERATIONS CONCEPT ICING FORECAST (Presented by WAFC Provider States) SUMMARY This paper discusses a possible operational service concept and requirement for airframe and jet-engine icing (high ice water content) forecasts. 1. INTRODUCTION 1.1 The WAFC Provider States are presenting Working Paper 14 (WP)14 at this Sixth Meeting of the WAFS Operations Group (WAFSOPSG/6) titled, Guidance for the Use of WAFS Gridded Icing Forecasts for Extended Range Operations by Turbine-Engined Aeroplanes. The purpose of this Information Paper is to discuss a process of how best to define current and future operational requirements in support of icing forecasts. 1.2 Icing forecasts can be used in the terms in supporting flight planning with regard to airframe icing such as in an extended range operations by twin-engined aeroplanes (ETOPS) situation, and tactical situations where there are in-flight encounters with airframe and jet-engine icing. 1.3 Airframe icing: In basic terms, it is when the accumulation of ice disrupts the airflow over the lifting surfaces (e.g., wing) such as to destroy lift. The severity of icing is dependent on the unsuccessful removal of the icing by using the aircraft s icing protection system. Unsafe icing is considered Severe, but could also be Moderate, and even Light under prolonged duration if the icing protection system fails or is inoperable. Severe icing may occur at any ice accumulation rate when the icing rate for ice accumulation ns exceeds the tolerance of the aircraft. Icing certification implies an increased tolerance to icing intensities up throughh moderate. (6 pages) WAFSOPSG.6.IP.013.6.en.docx
WAFSOPSG/6-IP/13-2 - 1.3.1 Jet-engine icing, or turbine-engine core icing, or High Ice Water Content (H-IWC): In basic terms, turbine-engine core icing is caused by high concentrations of ice crystals lifted to higher altitudes by strong thunderstorm activity. The tiny crystals bounce off the cold airframe, but partially melt and adhere to warmer engine surfaces. Turbine-engine core icing is thought to have contributed to more than 100 jet-engine power loss events since 1989, leading to temporary shutdowns because of surge, stall or flameout. 1 2. CURRENT USE OF ICING FORECASTS 2.1 Flight Planning: As noted in the discussion section of WP/14, fuel requirements for ETOPS flights increase when icing is forecast along the route. Icing forecasts are also used by flight dispatchers to determine if a change in aircraft is needed due to requirements contained in the aircraft s Master Minimum Equipment List (MMEL), e.g., an inoperative portion of an aircraft s icing protection system. ETOPS decisions are based on the potential of icing, not just icing severity. MMEL decisions are based on known or forecast icing, with no reference to icing severity 2. 2.2 In Flight - avoidance of flight into severe icing: Even airplanes approved for flight into known icing conditions should not fly into severe icing. Many Airplane Flight Manual Limitation Sections require an immediate exit when these types of conditions are encountered 3. Pilots change an aircraft s heading and/or altitude when ice accumulation or icing encounter become a concern or requires an action due to aircraft or airline operational procedures. Pilots and airline operations centers may adjust the flight route to avoid known areas of severe icing (e.g. SIGMET). 3. CURRENT PROVISION OF ICING INFORMATION. 3.1 Airframe icing information currently is provided in the following products: SIGMET, AIRMET, Significant Weather (SIGWX), AIREP, WAFS Icing Potential (in trial), as well as national icing forecasts from various NMS (e.g., US s Current and Forecast Icing Products). 3.1.1 SIGMETs provide information on observed and forecasts of non-convective severe icing and are issued by meteorological watch offices (MWOs) for their respective flight information region (FIR). A few MWOs issue AIRMETs, which are forecasts for non-convective moderate icing. 3.1.2 SIGWX forecasts contain non-convective moderate and severe icing forecasts. Since these phenomena primarily occur below FL 250 they are rarely seen on the SIGWX high level forecast charts, but are commonly shown on the SIGWX medium level forecast charts for certain regions of the world (e.g., Europe, North Atlantic, Middle East, and Southern Asia). 3.1.3 Icing forecasts for convective clouds (e.g., CBs, thunderstorms) are not explicitly provided. Icing associated with convective clouds is implied on the SIGWX forecasts if the CBs meet SIGWX criteria. A warning label on the SIGWX forecasts states, CB implies TS, GR, MOD or SEV TURB and ICE. Icing SIGMETs are only for non-convective severe icing. SIGMETs for thunderstorms are not to include references to associated icing (Annex 3 Meteorological Service for International Air Navigation, Appendix 6, paragraph 1.1.5 refers). 1 Flights to gather data on jet-engine core icing, Aviation Week and Space Technology, 23 Dec 2010 2 FAA MMELs for B-747, B-767, and B-777, http://fsims.faa.gov/picresults.aspx?mode=publication&doctype=mmel 3 FAA AC-91-74A
- 3 - WAFSOPSG/6-IP/13 3.2 H-IWC forecasts are not provided, other than the standard warning label on SIGWX forecasts that states, CB implies TS, GR, MOD or SEV TURB and ICE. 4. SERVICE NEEDS: 4.1 Safety of flight. 4.1.1 Knowledge of areas that contain or will contain atmospheric conditions to produce unsafe airframe and/or engine icing. 4.1.1.1 Flight planners, dispatchers, airline operation centers, and pilots use this knowledge to avoid flight into known unsafe icing, and may avoid flight over (i.e., above) known areas of unsafe icing if aircraft unable to maintain fly-over altitudes. Pilots use this information to make tactical changes in heading and altitude during unsafe icing encounters. 4.2 Risk Management. 4.2.1 Flight planners, dispatchers, airline operation centers plan for contingencies in the unlikely event of having to enter any icing conditions such that extra fuel is needed to operate anti-icing or de-icing systems or to divert to an alternate destination. This is the principle of requiring more fuel for extended range operations by twin-engined aeroplanes (ETOPS) flights when any level icing is forecast. 5. ALTERNATIVE ANALYSIS OPTIONS: 5.1 Ideally the industry needs an icing index. Icing severity is aircraft dependent. Wings that are thin or have sharp leading edges are more efficient ice collectors. For that reason smaller, thin aerofoils may accrete more ice faster than large thick airfoils. A large transport aircraft will accrete proportionally less ice than a smaller aircraft traversing the same environment. 4 Thus moderate icing to a Boeing 747 would likely be reported as severe by smaller aircraft. This becomes the dilemma for airframe icing severity forecasts. Without an icing intensity index, the traditional intensity terms used in current forecast products can be misleading to service users. 6. GENERAL SERVICE REQUIREMENTS: 6.1 International Air Transport Association (IATA), in WP/20 has requested that the WAFC s consider the development of an icing forecast that takes into account probability of the severity or intensity of icing, and provide the information in a single visualized product with other SIGWX information. As noted above to provide this information can be misleading and may not provide the information to operators for a more informative decision. 6.2 The following are seen as general service requirements for icing forecasts: a) geographic location and altitude of where airframe icing is possible, either from convective or non-convective clouds; 4 A Pilot s Guide to In-Flight Icing, NASA.
WAFSOPSG/6-IP/13-4 - b) geographic location and altitude of where H-IWC is possible; c) rate of accumulation of airframe icing per aircraft type (icing index); d) rate of H-IWC per engine make/model (if possible); and e) the probability of occurrence for each of the above. 6.3 In addition, WP/20 states that the icing forecasts should be provided in a single visualized product in combination with the depiction of the probability of severe turbulence and other SIGWX forecasts. From a visualization perspective, to provide all of this information into a simple single product that can be easily used will be a challenge. 7. DETAILS FOR SERVICE REQUIREMENTS: 7.1 In order to provide the icing information to meet service needs, additional information is also required. The providers need to know the expected resolution and precision required with the icing forecasts. Some issues that need to be addressed and considered are: a) what is the spatial resolution required? What is the minimum size area of a forecast? One 1.25 degree grid box? 3,000 square miles? b) what is the minimum vertical resolution? 1,000 feet, 500 feet? What is the temporal resolution needed? 4 hours, 2 hours, 1 hour, 30 minutes? c) what are the valid time steps needed? 3 hourly forecasts through 36 hours? Hourly forecasts through 6 hours? d) what is the update cycle needed? 6 hour, 3 hours, 1 hour? e) what is the expected accuracy? and f) do the requirements change for en-route vs. terminal areas? 7.2 Finally, what probability values are required for each of the above to equate to an operational decision? This is perhaps the most important question to ask. At what probability value does the user do something differently? In other words, what probability of occurrence results in the users making a change to the previous objective due to the probability of the event occurring? 7.2.1 To further complicate the issue, there may be differences in application of probability forecasts for flight planning and operations depending on the valid time of the forecast. For example, an airline may treat a flight differently for a forecast of 40 per cent airframe icing at FL150 valid for the next hour, compared to the planning decision for the same forecast, but valid in 12 hours.
- 5 - WAFSOPSG/6-IP/13 8. USE OF ICING FORECASTS INCLUDING INDEX FORECASTS 8.1 ETOPS planning and aircraft changes (due to MMEL decisions) are done based on the potential of any ice, not necessarily icing severity, thus their need should be met with WAFS icing potential forecasts, as proposed in WP/14. 8.2 Flight plans for short-haul flights can be adjusted based on icing index forecasts that provide the necessary resolution to provide an optimum flight path that avoids unsafe icing conditions. 8.3 Flight plans for long-haul flights may be adjusted for the first portions of the flight path based on forecasts of icing index, but later portions of the flight path may need to wait for updated forecasts in order to provide the optimal flight route. 8.4 In-flight guidance to avoid unsafe icing conditions should continue with a SIGMET, but also have the option to use fine resolution icing index forecasts or icing Nowcasts, which could be provided to the dispatcher as well as the aircrew. 8.5 But as was mentioned in 7.2, what probability value results in an operational decision for each of these? 9. SCENARIOS 9.1 A trans-oceanic flight at FL350 is planned to over-fly an area of forecast severe icing from FL050-FL150 in the middle of the Pacific. What decisions does the operator make? Would an operator load extra fuel since it is severe icing compared to the fuel load for moderate icing, or for that sake any icing intensity? Would an operator reroute around that low altitude area since it is severe? 9.2 Suppose a flight is planned from Sydney, Australia to Los Angeles, US, and a WAFS forecast has severe icing forecast over the Southern California (e.g., the Los Angeles area) from FL050- FL150 for the arrival time (which means that the WAFS forecast is likely a 24 hour forecast from the model run time). Would the airline cancel the flight due to a forecast of severe icing at the destination? Would they load extra fuel in anticipation of holding until the icing conditions improved? What would the airline do differently as compared to the same scenario with a "70% Icing Potential" forecast (e.g., the output of the WAFS gridded icing product)? What would the airline do differently with a 50 per cent probability of severe icing? 9.3 Answers to the questions in the scenarios may vary from airline to airline according to their business model and safety regulations. The question on the use of probability forecasts likely can t be answered, since guidance has not likely been developed for the use of probability forecasts. 10. PROVISION OF SERVICE: 10.1 Airframe icing primarily is a concern during transition from the aerodrome to cruising altitudes, and when established in a prolonged holding pattern at lower altitudes. H-IWC is primarily a concern at higher cruising altitudes, and near cumulonimbus clouds. As the need for icing forecasts is spread across both terminal and en-route environments, how best should this information be provided? Global numerical weather prediction models are necessary for the provision icing forecasts for long-haul
WAFSOPSG/6-IP/13-6 - en-route flights, but regional scale models, with their finer resolution, may provide for detailed information for short-haul en-route flights (i.e., regional) as well as the terminal and transition areas. 10.1.1 An icing Nowcast product, produced from numerical weather prediction models with fine temporal, vertical and spatial resolution would likely provide greater benefit for tactical avoidance of severe icing than a WAFS scale icing severity forecasts valid from 6 hours to 36 hours. 10.2 WAFS icing potential forecasts, as well as any icing index forecasts, are not likely to be of use for H-IWC in the near term. Global models with coarse resolution are not likely to provide the needed accuracy for H-IWC near cumulonimbus clouds. H-IWC forecasts will require numerical weather prediction models with increased resolution, new algorithms, as well as in situ observations, in order for the forecasts to be useful for operational decisions. These may not be possible, and alternative solutions may be needed, such as onboard remote sensors to detect potential H-IWC conditions near the thunderstorms. 11. CONCLUSION 11.1 Icing severity forecasts are not based on aircraft type. What is severe to one aircraft may not be severe to another aircraft type. Therefore an objective icing index is needed for airframe icing intensities. Unfortunately, the development of such an index is beyond the role of the WAFCs. 11.2 Avoidance of severe icing, for any airframe, is best done tactically, on a short timeframe, with SIGMETs or Nowcasts produced from fine resolution numerical weather prediction models. 11.3 H-IWC forecasts are related to thunderstorms or cumulonimbus clouds. WAFS icing forecasts are intended for airframe icing, and have not been examined for application for jet engines. Given the life-time cycle of a thunderstorm, WAFS gridded forecasts, with their 1.25 spatial resolution, and 3 hourly temporal resolutions, would not provide the necessary H-IWC forecasts needed by pilots and dispatchers. 11.4 Users need to plan for contingencies for unlikely events. For this, forecasts of any icing may be obtained from the WAFC gridded icing forecasts (currently in trial mode). 11.5 This paper raises several issues that need to be considered in the development of any new product that takes into account icing intensity to ensure that the development of the product meets the operational needs so the information provided is not misleading and can withstand the scrutiny of a safety risk assessment. Any request for a new product, whether for the WAFS or any other group, should be accompanied by a mature operational concept which defines the specific operational service requirement, as well as how this new product will be used to meet this service requirement. 12. ACTION BY THE WAFSOPSG 12.1 The WAFSOPSG is invited to note the information contained in this paper. END