Skip Ribbon Commands
Skip to main content
Safety Recommendation Details

Safety Recommendation A-10-105
Details
Synopsis: On December 20, 2008, about 1818 mountain standard time, Continental Airlines flight 1404, a Boeing 737-500, N18611, departed the left side of runway 34R during takeoff from Denver International Airport (DEN), Denver, Colorado. A postcrash fire ensued. The captain and 5 of the 110 passengers were seriously injured; the first officer, 2 cabin crewmembers, and 38 passengers received minor injuries; and 1 cabin crewmember and 67 passengers (3 of whom were lap-held children) were uninjured. The airplane was substantially damaged. The scheduled, domestic passenger flight, operated under the provisions of 14 Code of Federal Regulations (CFR) Part 121, was departing DEN and was destined for George Bush Intercontinental Airport, Houston, Texas. At the time of the accident, visual meteorological conditions prevailed, with strong and gusty winds out of the west. The flight operated on an instrument flight rules flight plan.
Recommendation: TO THE FEDERAL AVIATION ADMINISTRATION: Conduct research into and document the effects of mountain wave and downslope conditions at airports, such as Denver International Airport, that are located downwind of mountainous terrain (including, for example, airports in or near Colorado Springs, Colorado; Anchorage, Alaska; Salt Lake City, Utah; and Reno, Nevada), identify potential mountain-wave-related hazards to ground operations at those airports, and disseminate the results to pilots and airport air traffic control personnel to allow for more informed runway selection decisions.
Original recommendation transmittal letter: PDF
Overall Status: Open - Acceptable Response
Mode: Aviation
Location: Denver, CO, United States
Is Reiterated: No
Is Hazmat: No
Is NPRM: No
Accident #: DCA09MA021
Accident Reports: Runway Side Excursion During Attempted Takeoff in Strong and Gusty Crosswind Conditions Continental Airlines Flight 1404, Boeing 737-500, NN18611
Report #: AAR-10-04
Accident Date: 12/20/2008
Issue Date: 7/29/2010
Date Closed:
Addressee(s) and Addressee Status: FAA (Open - Acceptable Response)
Keyword(s): Air Traffic Control

Safety Recommendation History
From: NTSB
To: FAA
Date: 6/12/2015
Response: We note that your research determined that the Low-Level Wind Shear Alert System (LLWSA) would provide better detection of airport wind conditions if a very high resolution numerical weather model were used, but that the associated costs to implement this improvement would be too expensive. Although we appreciate your efforts to improve LLWSA technology, we do not believe that such improvements are needed to satisfy this recommendation, the intent of which is to provide pilots and ATC personnel with guidance on identifying potential mountain-wave-related hazards to ground operations. Accordingly, we encourage you to apply the results of your research to developing such guidance, and then to disseminate the information to pilots and ATC personnel to enable them to make more informed decisions when selecting a runway at airports downwind of mountainous terrain. Pending your completion of these actions, Safety Recommendation A 10-105 is classified OPEN—ACCEPTABLE RESPONSE.

From: FAA
To: NTSB
Date: 3/27/2015
Response: -From Michael P. Huerta, Administrator: The Federal Aviation Administration (FAA) has completed its research on the use of high-resolution models in forecasting events. We performed an assessment of the feasibility of utilizing a high-resolution nested forecast model to provide enhanced turbulence and gust hazard information for airports located downwind of mountainous terrain. This assessment involved computer simulations of the Continental Airlines flight 1404 accident using an experimental numerical weather prediction model similar to the Low-Level Wind Shear Alert System (LL WSA) observations at the time of the event. It was found that this model, with a horizontal resolution of 3 kilometers, was able to capture the mesoscale environment (the strong westerly flow over the mountain and the large scale mountain wave), but not the shorter, trapped lee waves and strong, gusty surface winds. As mentioned in our December I 0, 2012 letter, the FAA sponsored the development of Graphical Turbulence Guidance (GTG), which provides incremental improvements to the capabilities of a turbulence analysis and forecast. We found that the current operational model utilized as an input to the GTG has a resolution of 13 kilometers. At this resolution, the event was not detected. The FAA has completed ongoing research to address the Board's recommendations. We conducted an analysis of the feasibility and associated costs to operationally implement a very high-resolution numerical weather model to improve upon the current LL WSA detection of airport wind conditions. This analysis determined that such a model was scientifically feasible but very expensive; an annual cost of approximately $60 million, plus $400 million in start-up costs. These figures do not include downstream costs associated with model dissemination and integration within operational air traffic management systems. In response to this result, we examined the 30-year NTSB accident history for airports known to be particularly vulnerable to mountain wave activity; Anchorage, Colorado Springs, Reno, and Denver. We determined there were 22 accidents at those airports associated with mountain wave induced cross-wind conditions. Assuming a repeat of those same accidents over the next 30 years, a notional benefit of eliminating those accidents was calculated to be about $86 million. Comparing that figure to the 30 year cost of running the model-based mountain wave detection and forecast capability, which is, $2.2 billion, it becomes clear that very high-resolution numerical modeling is not a cost-effective approach to mitigating mountain wave impact at vulnerable airports. Further, the LL WSA technology available in 2008 remains capable of detecting and forewarning of highly variable wind conditions in the runway environment associated with mountain waves. I believe the FAA has effectively addressed this recommendation and consider our actions complete.

From: NTSB
To: FAA
Date: 2/11/2013
Response: In our December 16, 2011, reply to the FAA’s previous letter, we identified three areas of concern regarding Safety Recommendation A-10-105. In its current letter, the FAA addressed each concern, describing several ongoing research and assessment programs that address the effects of mountain wave and downslope conditions at airports located downwind of mountainous terrain, with the goal of developing the ability to identify potential mountain wave related hazards to ground operations at such airports. In our earlier letter, we indicated our concern that the systems the FAA was developing do not archive data and cannot be used to reconstruct accident conditions. The FAA replied that it recognizes this concern; however, the agency believes it to be outside the scope of Safety Recommendation A-10-105. We agree that the archiving of such data for use in accident investigations is beyond the scope of Safety Recommendation A-10-105. The program of research and assessments described in the FAA’s letter responds to Safety Recommendation A-10-105; accordingly pending completion of that program, the recommendation is classified OPEN—ACCEPTABLE RESPONSE.

From: FAA
To: NTSB
Date: 12/10/2012
Response: -From Michael P. Huerta, Acting Administrator: In the Board's December 16, 2011, letter, it identified three areas of concern. 1. The Federal Aviation Administration (FAA) has not conducted any additional research concerning mountain wave turbulence at airports downwind of mountainous terrain. The FAA has ongoing research into the effects of mountain wave and downslope conditions at airports to identify potential mountain wave-related hazards to ground. During Fiscal Year (FY) 2012, the FAA commenced an assessment of the feasibility of utilizing a high-resolution, nested forecast model to provide enhanced turbulence and gust hazard information for airports located downwind of mountainous terrain. This assessment will involve computer simulations of the Continental Airlines flight 1404 accident using an operational numerical weather prediction model compared to the Low-Level Wind Shear Alert System (LL WAS) observations at the time of the event. A final report that will provide results of the comparisons and any model sensitivity studies is planned for completion by the middle of FY 2013. Additionally, the FAA will conduct the first demonstration of the Flexible Terminal Sensor Network (FTSN) concept. FTSN integrates the capabilities currently provided by the Automated Surface Observing System, Automated Weather Observing System, Automated Weather Sensors System, Digital Altimeter Setting Indicator, Wind Transmitter, Runway Visual Range, and LL WAS, while expanding the availability of legacy and raw sensor data to any user who requires it. FTSN will thoroughly integrate sensor data that is currently employed by today's stove-pipe systems onto modem, cost-effective system architecture. The demonstration is expected to confirm that the integration of legacy services results in higher resolution sampling that better captures surface weather hazards over terminal airspace. In addition, as part of the Aviation Weather Research Program, the FAA continues to sponsor research aimed at providing incremental improvements to the capabilities of a turbulence analysis and forecast product known as the Graphical Turbulence Guidance (GTG), which is operational on the Web-based Aviation Digital Data Service. The current version of GTG forecasts clear air turbulence above 10,000 feet while the next version will include forecasts at all flight levels to 45,000 feet, including clear air and mountain wave turbulence, and is anticipated to be operational later in FY 2013. There is ongoing research to extend GTG coverage down to the surface and to analyze the effects of mountain wave and downslope conditions at airports to identify potential mountain wave-related hazards to ground operations. 2. The GTG systems described do not archive data and cannot be used to reconstruct accident conditions. The FAA recognizes this concern; however, we find this to be outside the scope of the initial recommendation. In the Board's letter dated July 29, 2010, it referenced archiving data in regards to Safety Recommendation A-I 0-1 06. We responded to that recommendation on October 14, 2010, which the Board classified closed-acceptable. 3. The resolution ofGTG-4 "will not cover specific airports." Although the GTG-4 product does not have the resolution to provide specific turbulence forecasts for individual airports, the FAA is funding an assessment of the feasibility of utilizing high-resolution, nested forecast models to provide enhanced turbulence and gust information for airport terminal areas downwind of mountainous terrains. If the use of high-resolution models is successful in forecasting turbulence events, the FAA plans to develop and analyze service alternatives and associated costs and benefits to determine whether it is cost beneficial to invest in the development and implementation of this capability. I will keep the Board informed of the FAA's progress and provide an updated response to this recommendation by August 30, 2013.

From: NTSB
To: FAA
Date: 12/16/2011
Response: The FAA indicated that (1) it is continuing the development of the graphic turbulence guidance (GTG) system and the NEXRAD Turbulence Detection Algorithm and (2) it plans to begin operating GTG-3, which will incorporate mountain-wave turbulence forecasts, by fiscal year (FY) 2013. By FY 2015, the FAA intends to release GTG-4, which will provide coverage down to the surface. In its October 14, 2010, letter to the NTSB, the FAA stated that the resolution of GTG-4 “will not cover specific airports.” Please provide us a list of any airports that will not be covered and the reasons for this lack of coverage. The FAA is also developing a nowcast version of the GTG system (GTG-N), which it plans to release in FY 2015. NEXRAD Turbulence Detection Algorithm (NTDA) is an additional input to GTG-N, which is used to estimate in-cloud eddy dissipation rates; this technology shows turbulence at near-surface altitudes on a three-dimensional grid. The FAA will distribute the information in the GTG-N and NTDA systems to pilots and air traffic controllers via the NextGen 4-D weather cube. The FAA’s actions to provide general turbulence modeling, as described above, do not address the specifics of Safety Recommendation A-10-105. The NTSB has several concerns with the FAA’s response, including the following: 1. The FAA has not conducted any additional research concerning mountain wave turbulence at airports downwind of mountainous terrain. 2. The GTG systems described above do not archive data and cannot be used to reconstruct accident conditions. 3. The resolution of GTG-4 “will not cover specific airports.” Therefore, pending the FAA’s researching and documenting the specific effects of mountain wave and downslope conditions at airports located downwind of mountainous terrain, Safety Recommendation A-10-105 is classified OPEN--UNACCEPTABLE RESPONSE.

From: FAA
To: NTSB
Date: 10/14/2010
Response: CC#201000393: - From J. Randolph Babbitt, Administrator: The Federal Aviation Administration's Weather Research Program will sponsor a turbulence analysis and forecast product known as the graphic turbulence guidance (GTG) to address turbulence caused by mountain waves. GTG provides values of turbulence intensity as measured by the state of the atmosphere metric, eddy dissipation rate (EDR), which is the International Civil Aviation Organization (ICAO) standard. Currently GTG-2, which forecasts clear air turbulence above 10,000 feet is operational on the Web-based Aviation Digital Data Service (ADDS). GTG-3 will add mountain-wave turbulence forecasts and is anticipated to be operational in Fiscal Year (FY) 2013. In FY 2015, the GTG-4 will include airspace at all altitudes down to the surface. However, the resolution will not cover specific airports. A nowcast version of GTG (GTG-N) is also under development and is anticipated to be operational in FY 2015 covering all altitudes. Input to the GTG includes frequent updates from aircraft equipped to automatically downlink EDR data. This information is also used to issue advisories to pilots called significant meteorological information, or SIGMETs (for severe turbulence) and airmen's meteorological information, or AIRMETs (for moderate turbulence) during mountain wave events. The GTG forecast information is generated by the National Weather Service and disseminated on NOAAport. Graphics representing GTG output also appear on ADDS at http://aviationweather.gov/adds/turbulence/. In addition to GTG development, the FAA has also sponsored the development of the NEXRAD Turbulence Detection Algorithm (NTDA). The NTDA uses NEXRAD data to compute estimates of in-cloud EDR which are mosaiced onto a 3-D grid to depict turbulence down to near-surface altitudes. The NTDA data will also be utilized as an input to GTG-N. The plans for the dissemination of the information above will be in a gridded format to all users including pilots and airport air traffic control personnel via the NextGen 4-D weather data cube. The FAA is planning to assess the feasibility of high resolution nested forecast models to provide enhanced turbulence hazard information for selected airport terminals.