Boeing 727-224 accident at Denver, Colorado, 7 Aug 1975

Accident narrative

On August 7, 1975, at about 1611 mountain daylight time, Continental Air Lines Flight 426 crashed after takeoff from Stapleton International Airport in Denver, Colorado. The Boeing 727-224 climbed to about 100 feet above runway 35L before descending and striking the ground near the departure end of the runway. All 134 persons aboard survived the crash, though 15 were seriously injured. The aircraft was substantially damaged.

### The flight

Continental 426 was a scheduled passenger flight from Portland, Oregon, to Houston, Texas, with intermediate stops at Denver, Wichita, and Tulsa. The aircraft departed the Denver passenger terminal with 127 passengers and 7 crewmembers aboard.

The flightcrew was properly certificated. The captain had 11,465 flight-hours, including 483 as pilot-in-command in the Boeing 727, and the first officer had 6,555 flight-hours, with 998 in the Boeing 727.

### The sequence of events

Before taxiing, the flightcrew received an automatic terminal information service broadcast reporting a temperature of 84°F and winds from 070° at 15 knots. At 1606:37, the local controller cleared the flight to taxi to runway 35L, reporting winds from 300° at 14 knots.

Two flights preceded Continental 426 on the takeoff from runway 35L. About 1605, Braniff International Flight 67 took off and reported "pretty good up and downdrafts out here from two, three hundred feet." About 1607, Frontier Airlines Flight 509 took off and reported "a pretty good shear line there about halfway down 35" at an altitude of about 200 feet. Continental 426, holding in the takeoff position, transmitted that it had copied the Frontier report.

At 1610:11, the local controller cleared Continental 426 for takeoff. The controller informed the flight that winds were 230° at 12 knots and that there had been reports of "pretty stout up and downdrafts and that shear out there at 200 to 300 feet." The flight acknowledged the clearance and the information.

The flightcrew used maximum takeoff thrust. At an indicated airspeed of 132 knots, the captain called for rotation, and the first officer rotated the aircraft to a pitch attitude of between 13° and 15°. The first officer saw a positive rate of climb and called for the landing gear to be raised at 1611:05. The captain stated that the aircraft entered heavy rain about the time of rotation, turned on the windshield wipers, and raised the gear.

According to the flightcrew, the aircraft climbed normally to 150 to 200 feet above the runway and accelerated to an indicated airspeed of about V2 + 5 knots. The airspeed fluctuated and then decreased to V2 - 5 knots, and the first officer relaxed back-pressure on the control column. The captain felt the aircraft sink, saw the airspeed at V2 - 20 knots, and took control. He advanced the power levers to maximum thrust and lowered the nose to a pitch attitude of about 10°. The aircraft continued to descend, and the captain attempted to increase the pitch attitude. Just before the aircraft struck the ground, the stall warning system activated.

The aircraft first struck the ground on the right shoulder of runway 35L, just south of the departure end. It slid about 1,995 feet and came to rest on an airport road at 1611:18.

When the aircraft came to rest, the second officer heard a loud explosive sound and screaming from the passenger cabin. Believing the aircraft was on fire, the flightcrew escaped through the cockpit sliding windows without completing the published aircraft shutdown or passenger evacuation procedures. The captain subsequently returned to the cockpit through the window and attempted unsuccessfully to shut off the engines, which continued to run at high power settings because their control cables had been rendered inoperative by fuselage structural damage.

Inside the cabin, the two forward flight attendants were knocked unconscious when their heads struck an unpadded bulkhead. They were then trapped in their seats by a forward coat closet that broke loose and blocked the main entry door. The two aft flight attendants slid from beneath their seatbelts during the crash but freed themselves to assist passengers. Because the engines were still running and creating considerable noise, the aft exits were not used. Passengers initiated the evacuation through the four overwing window exits and the forward galley door, completing it in three to four minutes. There was no fire.

### What the investigation found

The aircraft remained generally intact, though the fuselage was split open circumferentially in two places. Investigators found no evidence of a failure or malfunction in the aircraft's systems, structure, or powerplants before it struck the ground.

The flight data recorder showed that the airspeed trace oscillated irregularly throughout the takeoff. About 19 seconds after rotation, the airspeed decreased from 157 knots to 116 knots in about five seconds. The aircraft crashed 6.6 seconds later at an airspeed of 126 knots.

The Board examined the meteorological conditions and found that a thunderstorm had developed west of the airport and moved over the northern portion of the airport. As it began to dissipate, the thunderstorm generated numerous downdrafts. Because of low relative humidity, much of the rain evaporated before reaching the ground—a condition known as virga. This evaporation further cooled the descending air, causing it to descend more rapidly and creating severe downdrafts near ground level.

An independent wind analysis indicated that the thunderstorm contained centers of divergence that significantly affected surface winds. The Board compared Continental 426's encounter with those of the two preceding flights. Braniff 67 lost 23 knots of airspeed in 15.6 seconds, and Frontier 509 lost 36 knots in 10.8 seconds. When Continental 426 began its takeoff, the main center of divergence had moved eastward and was dominating the surface wind flow on the northern portion of the runway. Shortly after liftoff, Continental 426 encountered a rapid change from a headwind to a tailwind, losing 41 knots of airspeed in 5.0 seconds. The analysis calculated that the tailwind at impact was between 60 and 90 knots.

The Board noted that this wind flow was not officially recorded because the airport's sole official anemometer was located about 1,800 feet southeast of the runway threshold.

The Board concluded that the accident was unavoidable after the aircraft encountered the wind shear. At the altitude and airspeed at which the encounter occurred, the aircraft was performing near its maximum capability. Because the engines were already producing maximum thrust, the flightcrew had no way to increase the aircraft's total energy to overcome the descent induced by the wind shear.

The Board found that the weather information available to the crew was adequate except for the wind information, and that the captain's decision to take off could not be faulted. However, the Board noted that neither the Federal Aviation Administration nor Continental Air Lines had acted in a positive and timely manner to provide wind shear training for flightcrews.

Regarding the evacuation, the Board concluded that the flightcrew's performance did not conform to the standards of professional crewmembers because they failed to perform their assigned evacuation duties. The Board noted that the evacuation was successful largely because there was no fire.

### Probable cause

The National Transportation Safety Board determined that the probable cause of the accident was "the aircraft's encounter, immediately following takeoff, with severe wind shear at an altitude and airspeed which precluded recovery to level flight; the wind shear caused the aircraft to descend at a rate which could not be overcome even though the aircraft was flown at or near its maximum lift capability throughout the encounter. The wind shear was generated by the outflow from a thunderstorm which was over the aircraft's departure path."