June 10 (Bloomberg) -- U.S. air safety regulators have decided, after almost 50 years, that it's no longer safe for private and cargo aircraft to fly with ``polished frost'' on their wings.

Since 1960, the Federal Aviation Administration has allowed some planes -- not commercial airliners -- to fly with ice on the wings as long as it was smooth. That left it up to operators to clean off the wings and decide whether it was safe to go.

On May 8, the FAA proposed to remove that language from its regulations. That was two years after it issued two safety alerts advising against the practice and after complaints from safety groups.

``We would have no pain whatsoever if `polished frost' disappeared from the language,'' said Bruce Landsberg, executive director of the Air Safety Foundation at the Aircraft Owners and Pilots Association, of Frederick, Maryland. ``We absolutely recommend completely uncontaminated wings. Spray the things down and be done with it.''

The regulatory action comes after several fatal crashes of business aircraft attributed to ice on the wings, and an increase in the use of corporate and fractional-ownership jets over the last decade.

``It's a pretty strange rule,'' said Jim Burkin, director of technical programs for the Flight Safety Foundation, an Alexandria, Virginia-based nonprofit international safety organization. ``We don't know why they even allowed it.''

Commercial aircraft operate under rules that call for a ``clean wing'' before take-off and de-icing baths on the runaway under certain weather conditions. Passenger jets also have access to an array of sophisticated technology to respond to ice build- up on the ground and in flight, as do some private aircraft.

Level of `Criticality'

John Allen, deputy director of the FAA's flight standards service, said that only in the last few years has the icing issue risen to the level of ``criticality'' that required a change.

While the old rule said pilots could take off ``with frost adhering to the wings, or stabilizing or control surfaces, if the frost has been polished to make it smooth,'' the consensus now is that ``you don't want polished frost, you don't want anything on the wing,'' Allen said.

Under the current rule, pilots are provided no guidance on how to actually polish frost or reach an ``acceptable level'' of smoothness.

Thus, the FAA discovered numerous inventive --- though sometimes disastrous --- ways to clean ice, snow and frost off the surfaces of aircraft. Some pilots tried to ``polish frost'' by running a rope against the wing surface, brushing it off by hand or with a broom, using a paper towel, or using a credit card as a scraper.

Any `Contaminants' Unsafe

In its proposal, the agency said extensive research now shows that any amount of ``contaminants'' on the wings ``increases the risk of unsafe flight.''

Others have been saying that for years. The National Transportation Safety Board has asked the FAA since the 1990s to revise its rules on icing conditions and the certification of planes to fly in those conditions.

``We have said in past investigations that wings should be free of ice, frost or snow,'' said Dan Bower, chief of the NTSB's vehicle performance division. ``We recommend pilots get out and feel the wings. There are small imperfections and things you may not be able to see.''

A January 2002 crash in Birmingham, England, that killed two business executives and three pilots illustrated the dangers of ice on aircraft. British investigators pointed out that the plane had been flying under the FAA ``polished frost'' regulation.

In 2004 and 2005, fatal crashes in Colorado, attributed to ice on the wings, provided impetus for the change.

Studies 11 Accidents

In the current rulemaking, the FAA studied 11 accidents between 1982 and 2006 involving general aviation and air taxis and found that inadequate de-icing led to fatalities and injuries.

Chris Dancy, spokesman for the aircraft owners group, said most general aviation aircraft don't have equipment, such as de- icing ``boots'' on the wings that break up ice in flight. Most aren't certified to fly in icy conditions. He said many pilots of private planes just wait out the weather or pay for de-icing spray.

The agency recommends four options to meet the new rule: Use wing covers made of light fabric that keep snow and ice off; wait for the weather to warm up; park the aircraft in a heated hangar, or de-ice the plane.

After the accident in Birmingham, British investigators recommended the FAA ``delete all reference to polished frost.''

In 2006, the FAA issued the two safety alerts. Allen said this was done to get the word out quickly because rulemaking takes so long. Public comments on the proposed deletion of the language are due Aug. 6.

To contact the writer of this column: Cindy Skrzycki at cskrzycki@bloomberg.net