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New Helicopter Emergency Medical Services Weather Tool

by Larry Buehler
Reprinted with permission from FAA Aviation News

A review of the commercial Helicopter Emergency Medical Services (HEMS) accidents from January 1998 through December 2004 revealed a need for more detailed flight planning. While this study concerned only HEMS accidents, flight planning information sources are relevant to every facet of aviation. One of these sources is the Aviation Digital Data Service (ADDS), which was introduced in 1997 as an experimental digital data program that would contain weather observations and forecasts important to the aviation community and would be available via the Internet.

The ADDS has added a new tool to help HEMS operators make better weather related go/no-go decisions. This system is currently in the experimental phase and, as such, there are restrictions placed on its use for flight planning. While The ADDS HEMS Weather Tool is intended for HEMS operators, it is available to the public and may prove useful for many other segments of the aviation community.

Every one knows that HEMS operate in a very demanding environment. They provide an invaluable service to the public by providing crucial, safe, and efficient transportation of critically ill and injured patients to medical care facilities. While the contribution of HEMS is profound as a component of the nation's medical infrastructure, from an operational standpoint, it is a commercial aviation activity performed by FAA certificated air carrier operators. Therefore, operations must have the highest level of safety. HEMS operators are generally certified under Title 14 Code of Federal Regulations (14 CFR) part 135 as on-demand air carriers. Legally they operate like a normal charter business even though their role is very different.

The HEMS Weather Summit

In order to prevent controlled flight into terrain (CFIT) and loss of control (LOC) accidents, the FAA, the HEMS industry, and the University Center for Atmospheric Research (UCAR) (which is a consortium of universities) conducted a HEMS Weather Summit in early 2006. One of the conclusions of the summit was that there was an absence of usable ceiling and visibility data between reporting and forecasting stations. Often, HEMS operators conduct entire HEMS flights in the area between such stations, with reliance on off-course stations and area forecasts to make critical flight planning decisions. One of the outputs of this summit was a commitment to provide the HEMS operating community access to information, which might support better weather decision-making in VFR operations.

Following the weather summit, FAA asked UCAR's Research Applications Laboratory (RAL) to develop a weather tool as a part of the ADDS experimental Web site. This weather tool would provide access to 'gridded' ceiling and visibility assessment in areas between Aviation Routine Weather Report (METAR) and Terminal Area Forecast (TAF) reporting/forecasting sites. The ceiling and visibility data are 'assessments,' meaning they are determinations of the ceiling and visibility that are likely to exist in the grid block based on terrain influences, technical assessments, and observations provided to software forecasting models that automatically generate the graphical product. The user must understand that this weather product is not a report of an observation or a forecast. It is an assessment of the ceiling and visibility at the time chosen by the user and at the location of the grid block(s).

In addition, this product has not completed a rigorous Flight Standards operational suitability assessment. This assessment establishes the product as either a primary weather product that meets all safety and regulatory requirements (e.g., METAR or TAF) or as a supplementary weather product for increased situational awareness. Until that assessment, the product currently is experimental, but highly informational.

The user should be aware that the weather tool derives ceiling and visibility by interpolating the nearest METAR data. This interpolation process, in effect, 'stretches' limited-area METAR observations across a broader area between stations accounting for terrain effects on ceiling height. The results are the likely conditions between METAR stations. However, a critical issue is that the reliability of the information generally degrades as distance from a METAR site increases. Thus, users should apply practical judgment when considering the 'likely' weather conditions that are remote from a METAR site. To aid in this judgment, the product provides confidence fields that integrate a variety of product quality factors. Accordingly, indications of a 'likely' weather condition that indicates a no-go condition should strongly influence the decisions of certificate holders authorized to use this product.

The ADDS HEMS Weather Tool allows the user to identify gridded weather assessments in 5 km by 5 km blocks. The weather data available includes flight category, ceiling, visibility, radar, convection, icing, temperature, relative humidity, and wind. Overlays on the graphical data include wind barbs, METARs, PIREPs, AIRMETs/ SIGMETs, TAFs, VORs, state and county boundaries, and a base map of terrain and cultural information.

This data became available November 1, 2006, on the ADDS website at The site contains a tutorial as well as 'frequently asked questions.' In addition, you can find a technical report on the performance of this weather tool at under the 'Supplementary Weather Products' menu. Taking the tutorial and reviewing the report will help users understand how to use the tool most effectively.

There are some limitations, however. Operators may not use this weather tool in any way to support IFR operations. The only approved use of this weather tool is in VFR operations, and then only in the context of supporting a 'no-go' decision. Operators may not use the HEMS Weather Tool as the sole source for decisions to 'Go.' They may only use established primary products, such as METARs, TAFs, area forecasts, weather depiction charts, prognosis charts, etc., to make both 'Go' and 'No-Go' decisions. Here are a few examples:

  • If primary products, such as METARs, TAFs, and area forecasts, indicate a proposed flight would encounter weather conditions worse than those required by Operations Specification (also known as Op Spec, which is a set or rules or guidelines that apply to part 135 and 121 operators) for VFR operations, and the HEMS Weather Tool indicates that conditions meet Op Specs minima, an operator cannot use the HEMS Weather Tool to support a 'Go' decision not supported by primary products.
  • If the primary products indicate that an operator could complete a flight in conditions at or above the OpSpec minima, and the HEMS Weather Tool indicated weather lower than required along the route of flight, the HEMS Weather Tool can support a 'no-go' decision. This is particularly important since many primary products (such as area forecasts) do not have the specificity to identify highly localized low weather conditions. The HEMS Weather Tool can resolve assessments at the 5km by 5km grid level.

The following table reflects the relationship of the HEMS Experimental Weather Tool to primary weather products:

Hems Experimental Weather Tool

In cases where the weather tool supports an initial 'No-Go' decision, depending on the extent of the area of assessed low ceilings and/or visibilities, it may also provide information which would support re-routing a flight to avoid indicated hazardous ceiling and visibility conditions. In these cases, the primary products, as well as the ADDS HEMS Weather Tool would then both indicate acceptable conditions along the re-routed flight path, meeting the criteria of condition 2 in Table 1.

In some cases, a certificate holder may choose to disregard the ADDS HEMS Tool on the basis of direct observations, pilot reports, or other data. However, should the certificate holder elect to operate when the weather tool indicates unacceptable conditions, knowledge of the weather tool's assessment may increase the pilot's situational awareness and support more timely in-flight decisions to divert or land short if the pilot observes deteriorating conditions in-flight. Delaying such in-flight decisions substantially increases the potential for inadvertent instrument meteorological conditions (IMC) encounters, with the resulting increased risk of a CFIT or LOC accident.

HEMS part 135 certificate holders are encouraged to consider adopting this HEMS Weather Tool into their approved weather program under Operations Spec A010, Aeronautical Weather Data. To obtain approval to use the ADDS HEMS Weather Tool, the certificate holder must provide access to the ADDS HEMS weather tool tutorial and the technical performance report to pilots and other personnel, before using the product. Also pilots and other personnel who will use the ADDS HEMS Weather Tool must review these documents.

After reviewing this documentation and establishing the method to accomplish the training, the Part 135 certificate holder may request authorization to use the ADDS HEMS Weather Tool to support VFR flight operations under their OPSpecs. This authorization is accomplished by amending existing OpSpec paragraph A010 through coordination with your Principal Operations Inspector.

After using this weather tool briefly, it is my opinion that it is a major step forward in the presentation of weather information. In the future this HEMS Weather Tool could serve as the basis for a larger scale general aviation version that would be very useful in VFR planning and enhancing situational awareness. Like HEMS opera'tors, GA pilots often fly to areas that lack weather-reporting systems. The system also can provide a good overview of possible conditions likely to be encountered along routes with limited reporting points. This visual display allows pilots to rapidly ascertain the likely weather situation at all points along the flight. By presenting the information in graphical form a pilot can clearly see a line where marginal conditions start and how far they might continue. This could help prevent pilots from trying to push their luck as conditions deteriorate. This would be useful not only for HEMS operators, but for any pilot.

As was stated earlier, the HEMS Weather Tool is still experimental and can only be used in a very limited way. But as a pilot you should check it out. While it may not be usable directly today, it can provide an excellent picture of what you can expect to see on your flight. In many cases it could be the only way to get any weather information for the area you're headed to. I think this weather tool could be very useful. Now it's your turn to try it and tell us what you think.

The main source of information on this new ADDS tool is a notice to aviation safety inspectors concerning its use and implementation (Notice N8000.333). Questions, comments, and other feedback concerning this tool may be directed to the Flight Standards Service, Commuter, On Demand, and Training Center Branch, AFS-250, at (202) 267-8166.

Larry Buehler is an aviation safety inspector with the Air Transportation Division's Commuter, On-Demand, and Training Center Branch in Flight Standards Service.

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