U.S. federal aviation officials have known for years that cockpit window heaters in some Boeing planes catch fire. But they haven’t required airlines to fix the problem, even after dozens of incidents that unnerved pilots and, in some cases, forced emergency landings.

Pilots have complained about heaters that burned, smoldered or sent electric currents dancing across cockpit windows since at least 2002, according to an Associated Press search of a NASA aviation safety database. Safety investigators have traced the problem to a minor cause: a loose screw.

None of the reported incidents was deadly, but they were scary. Sometimes, flames would reappear after flight crews had blasted them with fire extinguishers. In many cases, the window heater would cause an inside ply of windshield to shatter into spidery cracks that obstructed pilots’ view. Sometimes, pilots and instrument panels were sprayed with glass.

Pilots reported having to remove their oxygen masks in smoky cockpits in order to reach circuit breakers or grab fire extinguishers.

The National Transportation Safety Board has prodded the Federal Aviation Administration to make airlines fix the problem, concerned that a major accident could happen if nothing is done. The FAA has yet to mandate the repairs, although it has promised the NTSB since 2004 that it would.

“There is no shortage of information. In fact, there’s no shortage of incidents,” NTSB Chairman Deborah Hersman said in an interview. “What’s the missing is the will to do something about it.”

The FAA did propose a safety fix in March 2008, two months after heavy smoke filled the cockpit of an American Airlines 757 flying from Puerto Rico to Philadelphia. The flight was diverted to Palm Beach, Florida, while terrified passengers were instructed on procedures for a rough ground or ocean landing.

“It was absolutely horrifying. There’s no other way to describe it,” said Rebekah Conrad, 23, who was among two dozen students who held hands, sang hymns and prayed through the ordeal.

More than two years later, the FAA’s safety fix still is not final. The regulator promised to expedite it after a cockpit fire last month forced a United Airlines 757 to make an emergency landing at Washington Dulles International Airport. In that incident, United pilots emptied one fire extinguisher on the flames, and sent a flight attendant for a second extinguisher after the fire reignited.

The FAA hopes to issue a final order next month, said FAA spokesman Les Dorr. He said the process has been slowed by the FAA’s obligation to respond to airlines who objected to the proposed remedy. Also, Dorr said, the FAA and Boeing have received new information about the extent of the problem.

Boeing has acknowledged at least 29 incidents globally involving cockpit window heaters in its 747, 757, 767 and 777 models since 2002. Their tally includes only those incidents where there was evidence of flame or smoke, not emergency landings due to electrical arcing or cracked windows.

NASA’s Aviation Safety Reporting System, a database to which pilots, flight attendants, airline mechanics and others voluntarily report safety concerns, contained 24 cockpit window heater incidents in those Boeing models over the same time period.

Twelve were fires, but the other 12 involved arcing — hot, lightning-like streams of electricity — along windows, as well as emergency landings and cracked or shattered windows, which were not included in Boeing’s count.

There appears to be a similar problem with the Boeing 737, the most widely used airliner in the United States. The NASA database contains 16 reports of fire, smoke, electrical arcing or cockpit window cracking in 737s since 2002. There also was one December 2007 report in the NASA database of arcing and a shattered window in a Boeing 727 that ultimately made an emergency landing.

Between 2004 and 2007, NASA sent four safety alerts to the FAA and the NTSB regarding cockpit fires, smoke or arcing in the 757. It sent a similar safety alert about the 737 in July 2003.

For their part, major carriers American Airlines, Delta Air Lines, United Airlines and US Airways are either replacing the old windows or stepping up inspections.

Boeing spokesman Peter Conte acknowledged electrical arcing in cockpit window heaters has led to cracked windows in the 737. He said the problem was not considered dangerous because the arcing happens between window layers, unlike the incidents in other Boeing models, and doesn’t cause smoke or flames. The 737’s heaters require less power to warm the plane’s smaller windows, he noted.

However, a May 2003 report filed with NASA describes a window fire in a 737 with “blowtorch” flames, smoke and sparking. The plane, which was on a test flight, made an emergency landing. Two other reports mention smoke as well as arcing and window cracking.

Two years ago, the FAA said its proposed remedy would affect 1,212 planes. That order does not include 737s.

The NTSB said it learned of the window heater problems after two 757 cockpit fires in January 2004. Safety investigators traced the fires to arcing caused by a loose screw that chafed power wires connected to heating wires in the windows, which were manufactured by PPG Aerospace.

The FAA’s 2008 proposal would require airlines to check for the loose screw and replace damaged windows — a task that some airlines say requires extensive window disassembly because the screws are difficult for mechanics to reach.