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B735, Denver USA, 2008

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Summary
Runway Side Excursion During Attempted Take-off in Strong and Gusty Crosswind Conditions.
Event Details
When December 2008
Actual or Potential
Event Type
Fire Smoke and Fumes, Human Factors, Runway Excursion, Weather
Day/Night Night
Flight Conditions On Ground - Normal Visibility
Flight Details
Aircraft BOEING 737-500
Operator Continental Airlines
Domicile United States
Type of Flight Public Transport (Passenger)
Origin Denver International Airport
Intended Destination Houston Intercontinental
Flight Completed No
Flight Phase Take Off
TOF
Location - Airport
Airport Denver International Airport
FIRE
Tag(s) Post Crash Fire
HF
Tag(s) Manual Handling,
Inappropriate crew response - skills deficiency
RE
Tag(s) Directional Control,
Significant Crosswind Component,
Off side of Runway
WX
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
Outcome
Damage or injury Yes
Aircraft damage Hull loss
Injuries Many occupants
Causal Factor Group(s)
Group(s) Aircraft Operation,
Air Traffic Management
Safety Recommendation(s)
Group(s) Aircraft Operation,
Air Traffic Management
Investigation Type
Type Independent

Description

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.

B735 Denver 2008 Runway Excursion
Ground track taken by the aircraft to its final resting position - taken from the official report

Investigation

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
56.54 m/s
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
13.878 m/s
, which was equivalent to a cross wind component of 27 knots compared to the AFM limit of 33 knots61.116 km/h
16.962 m/s
. 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.

Safety Recommendations

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.

(A-10-105)

  • 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.

(A-10-106)

  • 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.

(A-10-107)

  • 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.

(A-10-108)

  • 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.

(A-10-110)

  • 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.

(A-10-111)

  • 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.

(A-10-112)

  • 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.

(A-10-113)

  • 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.

(A-10-114)

  • 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.

(A-10-115)

  • 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.

(A-10-116)

  • 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.

(A-10-117)

  • 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.

(A-10-118)

The Final Report of the NTSB Investigation was adopted on 13 July 2010

Related Articles

Accidents and serious incidents which include Runway Excursion (Directional Control) as an outcome:

  • DH8D, Aalborg Denmark, 2007 (On 9 September 2007 the crew of an SAS Bombardier DHC8-400 approaching Aalborg were unable to lock the right MLG down and prepared accordingly. During the subsequent landing, the unlocked gear leg collapsed and the right engine propeller blades struck the runway. Two detached completely and penetrated the passenger cabin injuring one passenger. The Investigation found that the gear malfunction had been caused by severe corrosion of a critical connection and noted that no scheduled maintenance task included appropriate inspection. A Safety Recommendation to the EASA to review the design, certification and maintenance of the assembly involved was made.)
  • E145, Hannover Germany, 2005 (On 14 August 2005, a British Airways Regional Embraer 145 overran Runway 27L at Hannover by 160 metes after flying a stable approach in daylight but then making a soft and late touchdown on a water covered runway. Dynamic aquaplaning began and this was followed by reverted rubber aquaplaning towards the end of the paved surface when the emergency brake was applied. The aircraft suffered only minor damage and only one of the 49 occupants was slightly injured.)
  • B738, Nuremburg Germany, 2010 (On 8 January 2010, an Air Berlin Boeing 737-800 attempted to commence a rolling take off at Nuremburg on a runway pre-advised as having only ‘medium’ braking action. Whilst attempting to position the aircraft on the runway centreline, directional control was lost and the aircraft exited the paved surface onto soft ground at low speed before the flight crew were able to stop it. The event was attributed to the inappropriately high taxi speed onto the runway and subsequent attempt to conduct a rolling take off. Relevant Company standard operating procedures were found to be deficient.)
  • AT72, Copenhagen Denmark, 2013 (On 14 January 2013, selection of the power levers to ground idle after an ATR 72-200 touchdown at Copenhagen produced only one of the two expected low pitch indications. As the First Officer called 'one low pitch' in accordance with SOP, the Captain selected both engines into reverse. He was unable to prevent the resultant veer off the runway. After travelling approximately 350 metres on grass alongside the runway as groundspeed reduced, the runway was regained. A propeller control fault which would have prevented low pitch transition on the right engine was recorded but could not subsequently be replicated.)
  • DH8C, Kimberley South Africa, 2010 (On 16 July 2010, a South African Express Airways Bombardier DHC 8-300 hit an animal during a night landing at Kimberley after a passenger flight from Johannesburg. The nose landing gear took a direct hit and collapsed but after a temporary loss of directional control, the runway centreline was regained and the aircraft brought to a stop. The Investigation found wildlife access to the aerodrome was commonplace and the attempts at control inadequate.)

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