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Denver Crash Was In Conditions That Appear To Exceed Crosswind Limit

Sat, Jul 25, 2009 — David Evans

Articles, Featured

The air traffic controller at Denver International Airport called out, “Roll the equipment, I have a crash.” It may be that the cause of the crash is the vulnerability of an airplane with winglet to cross winds. This needs to be determined by the National Transportation Safety Board (NTSB) as it continues its investigation into the 20 December 2008 crash on takeoff of Continental Airlines flight 1404, a Boeing B737-500.

The airplane skidded off the left side of runway 34R at about 90 mph, ploughed through grass, bumped violently down an incline, and came to rest in a drainage basin with the fuselage cracked and fuel spilling out of a ruptured tank, which caught fire and basically consumed the airplane.

Aerial view of ground scars and wreckage.

Aerial view of ground scars and wreckage.

Fortunately, all 115 passengers and crew survived, although 37 passengers and crew were injured. The crew injuries were severe. A flight attendant in the first class section suffered a broken ankle and had to be assisted off the airplane. Captain David Butler and First Officer Chad Levang in the cockpit were stunned and in pain. They were incapable of doing anything for one or two minutes. They did not order an evacuation or perform the evacuation checklist. The darkened cockpit had lost all electrical power. Richard Greene, a Continental B737 captain, was deadheading in the first class cabin of the accident airplane. He recalled going to the cockpit, the door of which was opened by Levang, and seeing Captain Butler on his hands and knees.

Interior of accident airplane, looking forward. Note fire damage on overhead bins.

Interior of accident airplane, looking forward. Note fire damage on overhead bins.

The flight attendants initiated the evacuation. There was some panic, pushing and shoving around the left overwing exit, but all got out of the aircraft in about a minute, the flight attendants estimated. Richard Lowe, a Continental first officer deadheading on the accident flight, said the windows were melting and popping, and passengers were screaming “we’re gonna burn” and “it’s gonna explode.” Lowe recalled passengers were climbing over seats and that some 30 people were trying to get out of the left overwing exit at the same time. He attempted to slow them down and maintain order. One of the most incredible sights was a male passenger who ran over a woman with small children in her arms. Lowe said because of the fire on the right side, it was the most extreme human behavior he had ever seen and frantic did not adequately describe the evacuation.

Right hand side of the wreckage. The definition of "substantially damaged" seems a mite optimistic.

Right hand side of the wreckage. The definition of "substantially damaged" seems a mite optimistic.

These and other details are contained in the extensive docket released a few days ago by the NTSB. The investigation is far from complete, but the susceptibility of a B737-500 with winglets to crosswinds will doubtless be a point of inquiry.

First, a description of the situation in the cockpit. The crew was aware of the Automated Traffic Information System (ATIS) report that winds were at 270 degrees and 11 knots on runway 34R. In other words, wing was blowing at some 70º across runway 34R.

Then, as they were lining up for takeoff, the tower contacted the flight crew and reported that winds were at 27 knots and cleared them for takeoff. The wind report surprised the crew, because it was higher than that reported by ATIS. The wind was gusting up to 32 knots. From the cockpit voice recorder:

Local Time

& Speaker                    Content

 

18:16:47

Captain Butler             Looks like you got some wind out there.

18:16:48

FO Levang                    Yeah.

18:16:57

Capt. Butler                Oh yeah, look at those clouds moving.

18:17:26

Tower                         Continental 1404, wind 270 at 27, right turn heading 220

                                      Runway 34 right cleared for takeoff.

18:7:34

FO Levang                   Heading 220, cleared for takeoff runway 34 right, Continental

                                       1404.

18:17:38

Capt. Butler                Left cross wind twenty ah seven knots.

18:17:44

FO Levang                   Huh.

18:17:49                      [Sound of increasing engine noise.]

18:18:00

Capt. Butler                Check power.

18:18:04

FO Levang                   Power’s set 90.9%.

18:18:13

Not clear who             Jesus

18:18:15                      [Sound of snap.]

18:18:15

FO Levang                   Oh # # # [Expletives deleted.]

18:18:17                      [Sound of increasing background noise.]

18:18:21

Capt. Butler                Reject

18:18:22

FO Levang                  * ject

18:18:27                     End of recording.

From the Operations/Human Performance Group Chairman’s report:

“The captain was the flying pilot and began a reduced-power takeoff … He noticed a difference in the thrust being generated by the two engines, but the two engines matched as he increased N1 to 90%. After verifying this, he … called out ‘check power.’ The first officer responded that thrust was set at 90.9%. The captain applied a left control wheel correction, applied forward pressure to the yoke, and used variable light rudder to keep the airplane aligned with the runway centerline. He recalled that it felt at first like a ‘normal crosswind takeoff.

“The captain recalled that as the airplane was getting up to speed it suddenly yawed to the left as if hit by a ‘massive gust of wind,’ or as if the tires had hit a patch of ice and lost traction. He recalled using full right rudder but seeing the airplane continue to veer left. The first officer recalled that as airspeed was increasing from 87 to 90 knots he looked up and saw the airplane drifting left of the runway centerline. He thought the captain was correcting back to the right, but the airplane suddenly yawed 30 to 45 degrees to the left. At appeared to the first officer as if there was ‘zero directional control.’ He recalled feeling the rudder pedals with his feet and he believed the captain was applying full right rudder.

“The captain recalled facing the edge lights on the left side of the runway. He believed the airplane was going to exit the left side of the runway and, as a last resort, he reached down with his left hand and grabbed the tiller [for nose wheel steering] for a second or two. He attempted to steer the airplane back onto the runway using the tiller, but this did not work so he put his left hand back on the yoke.

“The captain recalled using right control wheel to keep the wins level as the airplane departed the left side of the runway. He said that he did this because he thought the ground next to the runway sloped down and he feared that the aft end of the fuselage would slide down that incline and cause the airplane to ‘tumble on its side.’ After the airplane had completely exited the runway, the captain said ‘reject’ and tried to deploy the thrust reversers … he was unable to deploy the reversers because the ride was very rough.”

As the Structures Group reported, the airplane was “substantially damaged.” The word “totaled” comes to mind, given the engine torn off, the cracked fuselage, the twisted and ruptured wing, and so forth.

The airplane, a B737-500, was a “Classic” model of that series. It came from the factory without the winglets that characterized the “Next Generation” B737s. In November 2008 the airplane was modified at AAR Aircraft Services in Miami, FL, to install blended winglets. The winglets were upright appendages affixed to the wingtips and they reduced the drag. The winglet installations increased the wingspan from about 97.5 feet to about 102.5 feet.

B737-500 with blended winglets.

B737-500 with blended winglets.

Boeing had established maximum crosswind limits for the original B737-500 of 35 knots. However, in a supplemental type certificate (STC), a maximum demonstrated crosswind component of 22 knots was published for winglet-equipped B737-500s.

In other words, the airplane was legal to take off using the original 35 knot crosswind limits, given that crosswinds were gusting up to 32 knots at the time of the accident. But with the wingtip modification, the airplane was out of limits, with a 22 knot crosswind limit.

Captain Clifford Pittman, the B737 fleet manager for Continental, told NTSB investigators the takeoff crosswind limit in the flight manual was 33 knots. It was based on Boeing’s original 40 knot limitation, but that was downgraded at Continental to 33 knots, the lowest crosswind of all aircraft. When asked to clarify the 22 knot crosswind with winglets, he said that was in the general section and there were no limits. He said it was a demonstrated limit and did not apply to a dry runway.

When asked to rate how difficult a takeoff with a 27 knot crosswind component would be, Pittman said it was not difficult at all. When asked to rate it on a scale of 1-10, he said it was a 4.

If the winds were31 knots, he would rate takeoff difficulty as a 4.4 and declared it was not that big of a difference. When asked about winds blowing at 31 knots and gusting to 37 knots, Pittman said this situation would certainly create an awareness, but in terms of rating on a scale of 1-10, he said it would be the “same story,” about a 4.4.

Captain Gabriel Vaisman, a B737 line check airman for Continental, was asked if there was any performance difference on a B737-500 airplane with winglets or without. He said on landing with winglets the airplane floated a little bit more, but he could not recall a difference on takeoff.

Five pilots were tested on taking off in the simulator with varying crosswinds, ranging from 0 knots crosswind to 35 knot and gusting to 40 knots. Takeoffs were subjectively rated on a scale of 1-7:

1 = “very easy”

2 = “moderately easy”

3 = “slightly easy”

4 = “neither difficult nor easy”

5 = “slightly difficult”

6 = “moderately difficult”

7 = “very difficult”

The takeoff in 35 knot crosswinds was judged by all five pilots as a “5.” One pilot in those tests said:

“Same as my previous experience, pretty easy. Don’t feel lateral G like in the real airplane. Can feel and see the wing pick up easier in the real airplane. One-third to one-half rudder travel was the maximum required. Did not have much aileron in until rotation. Just do what you have to do to keep the wings level. Overall, didn’t seem that difficult. Did not need aileron during ground roll.”

These trials may be highly artificial. As the NTSB noted, “Simulator motion was turned off so that all group members could simultaneously observe inside the simulator cab.”

And one thing is not clear from the voluminous documentation the NTSB released: was that simulator one with a winglet-equipped B737-500?

The 21-knot crosswind limit for a winglet equipped B737-500 hangs out there, demanding further inquiry as to why Boeing established it, and why Continental officials were rather blasé about its importance.


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