Denver overrun spurs crosswind scrutiny by NTSB

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US safety officials are urging the FAA to improve how crosswind data is collected and shared with pilots at airports downwind of mountainous terrain.

The National Transportation Safety Board (NTSB) today formalised recommendations stemming from its investigation into the December 2008 Continental Airlines Boeing 737-500 overrun at Denver International airport.

NTSB determined the main cause of the overrun of Runway 34R was the captain not properly controlling the aircraft for the extreme and unexpected crosswind conditions during the takeoff run.

But the board also stressed that when the pilots received takeoff clearance they were advised of a 26kt (48km/h) crosswind when data available to controllers that is not typically supplied to pilots showed the winds were stronger. NTSB says a study conducted by the National Centre for Atmospheric Research (NCAR) showed mountain wave conditions were present at the time of the accident, resulting in strong, localised westerly winds with intermittent gusts as high as 45kt that crossed the aircraft's path during takeoff ground roll.

As a result the NTSB is recommending that FAA conduct research into mountain wave conditions at airports subject to those circumstances including Denver, Colorado Springs, Anchorage, Salt Lake City and Reno.

The board concludes that existing low level windshear alert systems (LLWAS) at airports could supply a better understanding of mountain wave conditions, but the ability of those systems to alert air traffic control to gusts or crosswinds could be improved.

NTSB is urging FAA to archive all LLWAS data obtained from airports experiencing mountain wave conditions and make the information available for use the in the potential development of an improved LLWAS algorithm for crosswind and gusty wind alerts.

The board also states that airport wind displayed on the controller's ribbon display terminal at the time of the accident would have shown 35kt winds from the west with 40kt (74km/h) gusts. Continental's maximum crosswind speed is 33kt (61km/h) during takeoff and Boeing's recommendation is 40kt (70km/h). But controllers did not display that information and were not required to do so, says NTSB, which adds no established criteria exists for controllers to supply alternate wind information to pilots.

Given those circumstances NTSB recommends that FAA should require air traffic controllers at airports with multiple sources of wind information to provide pilots with the maximum wind component a pilot could encounter.

Another conclusion reached by NTSB is that the Denver air traffic control tower's runway selection policy doesn't account for crosswind components when selecting a runway configuration. The board believes FAA should require towers to develop written runway selection programmes that factor in current and developing wind conditions when considering operational advantage in runway selection.

NTSB has sent a total of 14 recommendations to FAA resulting from its investigation into the Continental overrun, including a recommendation that aircraft manufacturers develop type-specific, maximum crosswind takeoff limitations. Other suggestions pertain to crosswind training for pilots and cockpit seat upgrades. NTSB's letter to FAA is available here.