But air traffic controllers apparently did not receive the warning and gave the pilot permission for an instrument landing at about 7 p.m. Thursday. Moments later, the twin-engine Gulfstream III crashed a few hundred yards short of the runway, killing all 15 passengers and three crew members.
Investigators have not yet determined how large a part the mistake played in the crash, the cause of which was still unknown.
The ban, which officials admit was confusing, was supposed to be distributed by facsimile by Denver's FAA office.
"The FAA says there is no record of a fax being sent or received from the Denver center to Aspen," Carol Carmody, acting chairwoman for the National Transportation and Safety Board, said Sunday.
The pilot had received the warning when the plane, which departed from Burbank, Calif., stopped in Los Angeles, Carmody said. But the warning, she said, was not written clearly and the pilot may not have understood it.
Investigators revealed Sunday that the flight was delayed by 45 minutes. That lead to speculation the pilot originally thought he would be landing before nightfall, but the NTSB said it would draw no conclusions from the late departure.
"We don't really know what was in his head and we have to find that out," NTSB investigator Al Dickinson said.
On Monday, investigators move to Oklahoma City to meet with the FAA pilots who issued the ban on nighttime instrument landings at Aspen.
The airport's single runway is surrounded by mountains that force approaching aircraft to make steep descents, making landings tricky even on clear days.
During an instrument landing, a pilot is constantly scanning instruments on his panel showing the airplane's speed and position until the runway is in sight.
Following the crash, the FAA issued a new notice explicitly banning nighttime instrument landings at the airport.
The airport had rules in place banning all nighttime landings by noncommercial jets until 1994, when several groups convinced the FAA to reverse the policy.
Some Aspen residents, including the late singer-songwriter John Denver, had said safety would be compromised at the airport if the restrictions were lifted. Denver was killed in a 1997 plane crash in California.
"Even in my great desire to hurry home after weeks on the road, I have no problem honoring the present curfew," Denver wrote in a 1994 letter to Pitkin County commissioners. "I am quite happy to land in Grand Junction or Denver and drive the extra distance for safety's sake."
Investigators planned to develop a minute-by-minute account of the weather at the airport at the time of the crash. The National Wether Service reported that visibility dropped from 10 miles to just under two miles over about 20 minutes.
Also Monday, other investigators expected to finish clearing the wreckage of the jet, while some were to fly to Washington, D.C., where the contents of the airplane's cockpit voice recorder will be analyzed.
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