B735, Denver USA, 2008
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|Runway Side Excursion During Attempted Take-off in Strong and Gusty Crosswind Conditions.|
|Actual or Potential
|Fire Smoke and Fumes, Human Factors, Runway Excursion, Weather|
|Flight Conditions||On Ground - Normal Visibility|
|Type of Flight||Public Transport (Passenger)|
|Origin||Denver International Airport|
|Intended Destination||Houston Intercontinental|
|Flight Phase||Take Off|
|Location - Airport|
|Airport||Denver International Airport|
|Tag(s)||Post Crash Fire|
Inappropriate crew response - skills deficiency
Significant Crosswind Component,
Off side of Runway
|Tag(s)||Strong Surface Winds,|
Mountain Waves"Mountain Waves" is not in the list (In Flight Airframe Icing, In Flight Icing - Piston Engine, In Flight Icing - Turbine Engine, CAT encounter, En route In-cloud air turbulence, Hail damage, Volcanic Ash Effects, Fog, In Cloud on Visual Clearance, Precipitation-limited IFV, ...) of allowed values for the "WX" property.,
Low Level Windshear
|Damage or injury||Yes|
|Aircraft damage||Hull loss|
|Causal Factor Group(s)|
Air Traffic Management
Air Traffic Management
On 20 December 2008 a Boeing 737-500 being operated by Continental Airlines on a scheduled passenger flight from Denver to Houston departed the left side of runway 34R during a normal visibility night take-off roll in gusty crosswind conditions and then travelled across moderately rough but essentially level terrain before coming to stop still upright but with the fuselage broken into two pieces. A post crash fire followed. Six of the 115 occupants were seriously injured and a further 41 sustained minor injuries.
An Investigation was carried out by the National Transportation Safety Board (USA) (NTSB). It was established that the aircraft had left the runway at a speed of approximately 110 knots203.72 km/h
and that the total distance then travelled before stopping had been approximately 730 m2,395.013 ft
, crossing both a taxiway and an airport service road and including a short airborne excursion following impact with uneven terrain. The cabin evacuation was found to have been promptly accomplished by the cabin crew with assistance from positioning operator flight crew. There were no announcements from the flight deck in connection with the evacuation. A fire developed on the right side of the aircraft but all occupants had been successfully evacuated before it breached the cabin.
It was established that there was no evidence of any defects in or failures of the engines, structures or systems of the aircraft, including the nosewheel steering system.
Huge amounts of recorded data on wind velocity across the airport at the time of the accident were available to the investigation, including that from 32 Low Level Wind Shear sensors from which information was also able to be displayed in the TWR.
It was confirmed that mountain wave conditions, which were widely known to commonly prevail at Denver, were extant at the time of the accident and had created strong westerly surface winds with very localised and intermittent gusts as high as 45 knots across the runway during the take-off roll. However, performance calculations made during the investigation indicated that the aircraft rudder was capable of producing enough aerodynamic force to offset the weather-vaning tendency created by even the strongest winds encountered during the accident take-off roll.
ATC were found to have cleared the aircraft for take off with a spot wind of 270° / 27 knots50.004 km/h
, which was equivalent to a cross wind component of 27 knots compared to the AFM limit of 33 knots61.116 km/h
. This was in keeping with prevailing ATC procedures which did not require the controller to provide additional information about variations in wind velocity. Use of the accident runway by other aircraft had not been followed by any adverse comment related to the gusting crosswinds which prevailed.
The conclusions of the investigation included the following:
- The Captain's use of tiller and full right control wheel in the 3 seconds before the excursion likely resulted from acute stress stemming from a sudden, unexpected threat, perceived lack of control, and extreme time pressure.
- The unexpectedly strong and gusty crosswinds the airplane encountered as it accelerated during the take-off roll made maintaining directional control during this take-off a more difficult control task than the captain was accustomed to dealing with; however, had the Captain immediately reapplied significant right rudder pedal input as the airplane was continuing its left turning motion, the airplane would not have departed the runway.
- The captain's initiation of a rejected take-off was delayed by about 2 to 4 seconds because he was occupied with the nosewheel steering tiller and right control wheel input, both of which were ineffective and inappropriate for steering the airplane.
- Although the departure wind information the Captain received with the take-off clearance from the Denver International Airport (DEN) air traffic control tower (ATCT) local controller indicated that the winds were out of 270° at 27 knots (which resulted in a stronger-than-expected 26.2-knot crosswind component), the reported winds did not exceed Continental's maximum crosswind guidance of 33 knots, and the captain could reasonably conclude that the winds, as reported by DEN ATCT, did not exceed either his or the airplane's crosswind capabilities.
- If the accident pilots had received the most adverse available wind information (which was displayed as airport wind on the Denver International Airport air traffic control tower local controller's ribbon display terminal and indicated a 35-knot crosswind with 40-knot gusts), the Captain would likely have decided to delay the departure or request a different runway because the resultant crosswind component exceeded Continental's 33-knot crosswind guidelines.
- The Denver International Airport air traffic control tower runway selection policy does not clearly account for crosswind components when selecting a runway configuration.
- Because Continental's simulator training did not replicate the ground-level disturbances and gusting crosswinds that often occur at or near the runway surface, and it is unlikely that the accident captain had previously encountered gusting surface crosswinds like those he encountered the night of the accident, the Captain was not adequately prepared to respond to the changes in heading encountered during this take-off.
- Because there are no standards for the development of enhanced crosswind guidelines for transport-category airplanes, Boeing did not adequately consider the dynamic handling qualities of the Boeing 737 during take-off or landing in strong and gusty crosswinds; it is likely that the enhanced crosswind guidelines developed by other manufacturers are similarly deficient.
- The accident pilots' injuries would have likely been lessened or eliminated if their seats had been designed to meet the crashworthiness requirements of 14 CFR 25.562, to which other airplane seats are designed.
- A flight attendant jump seat that is weakened due to undetected metal fatigue could fail under lower-than-expected crash loads and injure a cabin crewmember who might subsequently be needed to perform critical safety duties, such as evacuating passengers.
- The adhesive-only fastening method used for the latch plate in the aft galley of the accident airplane and similarly equipped airplane galleys was not adequate for securing galley drawers or other items of mass because it can fail over time and/or with exposure to the elements.
The Investigation concluded that the Probable Cause of the event was: “The Captain's cessation of right rudder input, which was needed to maintain directional control of the airplane, about 4 seconds before the excursion, when the airplane encountered a strong and gusty crosswind that exceeded the Captain's training and experience.” It was considered that the following factors had contributed to the occurrence:
- An air traffic control system that did not require or facilitate the dissemination of key, available wind information to the air traffic controllers and pilots
- Inadequate crosswind training in the airline industry due to deficient simulator wind gust modelling.
As a result of the Investigation, a total of 14 Safety Recommendations were made, all to the FAA: These Recommendations were to:
- 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.
- Archive all low-level windshear alert system (LLWAS) data obtained from Denver International Airport and other airports that experience similar wind conditions and make those data available for additional research and the potential future development of an improved LLWAS algorithm for crosswind and gusty wind alerts on air traffic control tower ribbon display terminals.
- Modify Federal Aviation Administration Order 7110.65 to require air traffic controllers at airports with multiple sources of wind information to provide pilots with the maximum wind component, including gusts, that the flight could encounter.
- Review the required documentation for all low-level windshear alert system (Low Level Wind Shear)-equipped air traffic control towers to ensure that a letter to airmen has been published and is easily accessible describing the location and designation of the remote sensors, the capabilities and limitations of the system, and the availability of current LLWAS remote sensor wind information on the request of a pilot, in compliance with Federal Aviation Administration Order 7210.3.
- Require air traffic control towers to locally develop and implement written runway selection programs that pro-actively consider current and developing wind conditions and include clearly defined crosswind components, including wind gusts, when considering operational advantage with respect to runway selection. (A-10-109) Gather data on surface winds at a sample of major U.S. airports (including Denver International Airport) when high wind conditions and significant gusts are present and use these data to develop realistic, gusty crosswind profiles for use in pilot simulator training programs.
- Require 14 Code of Federal Regulations Part 121, 135, and 91K operators to incorporate the realistic, gusty crosswind profiles developed as a result of Safety Recommendation A-10-110 into their pilot simulator training programs.
- Once realistic, gusty crosswind profiles as asked for in Safety Recommendation A-10-110 are developed, develop a standard methodology including pilot-in-the-loop testing, for transport-category airplane manufacturers to establish empirically based, type-specific maximum-gusting-crosswind limitations for transport-category airplanes that account for wind gusts.
- Once a methodology as asked for in Safety Recommendation A-10-112 has been developed, require manufacturers of transport-category airplanes to develop type-specific, maximum-crosswind take-off limitations that account for wind gusts.
- Until the actions described in Safety Recommendation A-10-113 are accomplished, require manufacturers of transport-category airplanes to provide operators with interim crosswind take-off guidelines that account for wind gusts.
- Work with U.S. airline operators to review and analyse operational flight data to identify factors that contribute to encounters with excessive winds and use this information to develop and implement additional strategies for reducing the likelihood of wind-related runway excursions.
- Require cockpit crew seats installed in newly manufactured airplanes that were type certificated before 1988 to meet the crashworthiness standards contained in 14 Code of Federal Regulations 25.562.
- Require operators to perform periodic inspections on the Burns Aerospace model 2501-5 jumpseats for fatigue cracks within the jumpseat structure and replace the jumpseat if fatigue cracks are found.
- Require that operators of transport-category airplanes that use galley latches or latch plates secured solely by adhesives that may degrade over time modify the latches to include mechanical fasteners.
Accidents and serious incidents which include Runway Excursion (Directional Control) as an outcome:
- B735, Denver USA, 2008 (Runway Side Excursion During Attempted Take-off in Strong and Gusty Crosswind Conditions.)
- JS31, Skien Norway, 2001 (On 30 November 2001, a BAe Jetsream 31 operated by European Executive Express ran off the side of runway 19 on landing at Skien Airport, Geiteryggen, Norway. The runway excursion was the consequence of an unstable non-precision approach, with airframe ice accretion, and a very heavy touchdown, which caused severe aircraft damage and loss of control.)
- AT43, Jersey Channel Islands, 2012 (On 16 July 2012, the left main landing gear of a Blue Islands ATR 42-300 collapsed during landing at Jersey. The aircraft stopped quickly on the runway as the left wing and propeller made ground contact. Although the crew saw no imminent danger once the aircraft had stopped, the passengers thought otherwise and perceived the need for an emergency evacuation which the sole cabin crew facilitated. The Investigation found that the fatigue failure of a side brace had initiated the gear collapse and that the origin of this was a casting discontinuity in a billet of aluminium produced to specification.)
- B462, Stord Norway, 2006 (On 10 October 2006, a BAE Systems 146-200 being operated by Danish airline Atlantic Airways on a passenger flight from Sola to Stord overran the end of runway 33 at destination at a slow speed in normal visibility at dawn (but just prior to the accepted definition of daylight) before plunging down a steep slope sustaining severe damage and catching fire immediately it had come to rest. The rapid spread of the fire and difficulties in evacuation resulted in the death of four of the 16 occupants and serious injury to six others. The aircraft was completely destroyed.)
- A320, Gold Coast QLD Australia, 2017 (On 18 December 2017, an A320 crew found that only one thrust reverser deployed when the reversers were selected shortly after touchdown but were able to retain directional control. The Investigation found that the aircraft had been released to service in Adelaide with the affected engine reverser lockout pin in place. This error was found to have occurred in a context of multiple failures to follow required procedures during the line maintenance intervention involved for which no mitigating factors of any significance could be identified. A corrective action after a previous similar event at the same maintenance facility was also found not to have been fully implemented.)