Passengers with window seats on Continental Airlines' Flight 1713 as it rolled toward a disastrous takeoff attempt Sunday would have seen little but the snowstorm that reduced visibility to three-eighths of a mile.
From the Stapleton International Airport control tower, the twin-engine DC9 would have been part of a familiar sight in normal aviation practice -- a shadowy parade of commercial airplanes lifting off in snowy weather.
Airline pilots commonly operate in snow, deciding whether to take off or land based on safety rules established by the Federal Aviation Administration and the air carriers, and their own judgment and experience. By tradition, no one, from an airline president on down, would question a pilot's decision not to take off.
Operating in weather such as the 6-to-8-inch snowfall with moderate winds at Denver yesterday is common in the North and West.
"It was kind of a normal snow day for us. There was no reason to close the airport because of the weather," Norm Avery, a spokesman for the Denver airport, told The Associated Press.
Wayne Sand, deputy general manager of the National Center for Atmospheric Research in Boulder, said it is "not uncommon" for the Denver airport to operate in heavy snow such as Sunday's "several times a year."
The FAA establishes minimum visibility and cloud ceiling standards for takeoffs, which vary according to the airport, the specific runway and the type of aircraft. Airlines establish their own minimum standards, which may be stricter.
Continental Flight 1713 took off with visibility down the runway of 2,000 feet, as measured by electronic runway instruments, more than the 1,600-foot minimum requirement, FAA spokesman Fred Farrar said. The cloud cover at the time also was above the 500-foot minimum ceiling, he said. "It appears to be legal," Farrar said of the takeoff.
Federal officials also said the plane had been de-iced 26 minutes before its attempted takeoff. The National Transportation Safety Board recommended after the Jan. 13, 1982, crash of Air Florida Flight 90 into the Potomac River that the FAA require airplanes to be sprayed with a glycol de-icing solution at least 20 minutes before departure, but the FAA refused, arguing that no time limit would be safe. The safety board withdrew the recommendation.
Federal investigators said they do not know if weather was a factor in the Continental crash, pointing out that several airplanes lifted off successfully before Flight 1713. Weather that was acceptable for one aircraft might have been unacceptable for another, or the weather might have changed rapidly, federal and aviation sources said.
Commercial aircraft are equipped with various devices to handle cold weather, including anti-skid brakes and heaters to melt snow and ice on the airplane windshield, wing edges and engines. The equipment varies among the airlines and among different aircraft, Sand said. Some of the devices work automatically, while others must be activated by the pilot, he said.
Some of the FAA weather rules reflect lessons learned from the Air Florida crash. That accident, in which 78 people died, occurred as the Boeing 737, its wings heavy with ice and snow and its onboard de-icing equipment not activated, tried to take off from a slushy runway at National Airport.
Some officials also noted that weather was not listed as the primary cause of the Air Florida crash. Other planes had lifted off ahead of the Air Florida plane. The safety board blamed pilot error, including the decision to take off despite ice or snow on the wings.
The FAA has always required that pilots not take off with snow or ice on their wings, Farrar said. Tapes from the Air Florida cockpit recorded the pilot and copilot joking nervously about snow on their wings, apparently trying to melt the buildup by turning their wings toward the engine exhaust of the airplane in line ahead.
Ground crews last sprayed the Air Florida aircraft with a glycol de-icing solution 49 minutes before takeoff, which allowed extra snow and ice to build up.
United Press International quoted one passenger on the Continental flight, Robert Linck of Green Pond, N.J., as saying he saw ice on the wings before takeoff. Investigators are certain to question him, but such eyewitness accounts often prove inaccurate, and there was no other evidence of ice.
After the Air Florida crash, the FAA followed the safety board's recommendation to emphasize to the airlines the importance of proper maintenance for ground equipment. The board ruled that a nozzle on a de-icing fluid hose diluted the glycol mixture, diminishing its effectiveness.
Following another safety board recommendation, the FAA has stressed to airlines that their ground-service contractors must understand maintenance procedures for specific types of aircraft under different conditions. The safety board report found that the personnel who de-iced Flight 90 did a "deficient" job.
After the Air Florida crash, the safety board recommended that in icy weather pilots use wing anti-ice devices while the plane is on the ground, increase runway speed beyond standard levels and use different wing flap settings to provide extra lift to compensate for slush, which slows acceleration on the runway.