De-icing/anti-icing fluids cling to the plane after they are applied and prevent precipitation from freezing for a certain period of time, depending on weather conditions.

Rozelle insists that pilots should de-ice when it's snowing, because only this fluid can ensure that falling snow on takeoff won't freeze to the plane.

United, Delta and Frontier airlines all said they de-ice if it's snowing, period.

But the National Business Aviation Association, whose members operate corporate jets and charters like the one that crashed in Montrose, disagrees. Spokesman Dan Hubbard said that for charter operators, the standard process is: "If there is freezing weather, and there is visible precipitation accumulating on the aircraft, you de-ice."

But "the snow has to be accumulating, and it has to be sticking to the aircraft," added Joe Hart, manager of the NBAA's service operations group.

The Federal Aviation Administration regulation says "no pilot may take off an aircraft that has frost, ice or snow adhering to" critical surfaces such as the wings, stabilizers and instrument systems.

The FAA says it's up to the pilot to decide whether to de-ice. "The operator must make an observation to determine whether or not contamination has adhered to the aircraft surfaces," the agency said in an e-mail.

Some pilots say if the snow is not "adhering," as the rule states, then de-icing is not necessary.

The NBAA's Hart said, "If it's snowing and 35 degrees and as soon as it hits the surface, it melts, you don't need to."

"If it's extremely light powder snow, you don't have to," he said.

But the Flight Safety Foundation's Rozelle is adamant in the other direction.

"The accident reports speak for themselves," Rozelle said. "If you've got snow falling, you need to put something on that wing."

In the 1990s, a number of fatal crashes convinced the industry and regulators that planes sitting on the runway a long time after being de-iced

 could lose their protection. So now, air traffic control quickly sends planes to take off after de-icing, and the FAA publishes detailed charts showing how long each type of de-icing fluid will protect in different weather.

When the airlines tightened up their policies in the early 1990s, "A lot of pilots thought if they had snow on their wings, that snow would be blown off as they roar down the runway. And that isn't always the case," Rozelle said.

For example, sunlight might warm one wing of a plane awaiting takeoff, but not the other, Rozelle said. Snow hitting the warmer wing might melt and then freeze. Then more snow might cover the layer of ice. "The pilot moves his hand on it, and the snow blows off. He says, 'It'll blow away.' "

If he has ice on just one wing, "he may be rolling down the runway to an accident. The only way to ensure that you don't have contamination on the aircraft is to remove it," Rozelle said.

The FAA seems to be contradictory on the subject. Its regulations leave the de-icing decision up to the pilot. But a number of its guidelines are more strongly worded. For example:

• An FAA advisory on de-icing small aircraft, cited by the NBAA, says ice protection systems or procedures should be carried out whenever the temperature is below 50 degrees and there is visible moisture present. Those procedures don't necessarily mean de-icing, however.

• The FAA de-icing update for winter 2004-2005 has a section explaining how long anti-icing will last if there is light snow of just 0.2 mm per hour. That's 0.008 inches.

• Conversely, the same document advises against using de-icing fluid if dry snow is not adhering to the aircraft. It says that if there is a "significant" amount of dry snow, it should be removed by mechanical means that won't cause the snow to melt.

• The FAA has approved airline plans calling for de-icing whenever it is snowing.

FAA spokesman Mike Fergus refused to provide an FAA expert to explain the agency's advice, saying it does not allow comment on issues that may pertain to an ongoing accident investigation.

But Arnold Scott, chief investigator in the Denver office of the National Transportation Safety Board, said, "The FAA rules are minimums," and airlines can choose to have tighter in-house policies. The FAA's more strongly worded guidelines on icing have less weight than FAA regulations, Scott said. In 1996, the Department of Transportation's Office of Inspector General recommended the FAA tighten its de-icing rules after finding six accidents and 203 deaths caused by icing in the previous 14 years. The report found FAA safety inspectors "disagreed over whether a light dusting of snow on the wing constitutes ice contamination requiring a de-icing procedure."

It also said FAA inspectors were finding numerous violations of de-icing rules, but they were not looking for patterns or seeking policy changes. As a result, the inspector general suggested the FAA hire a de-icing expert and develop "best practices" in de-icing to provide to air carriers.

Neither the FAA nor the inspector general's office over the course of several days could say if the FAA adopted these recommendations. The FAA did post an opening for the icing expert in 1996, but it is not clear whether the position exists today.