Check out the links to photos of the window and the heating block at the bottom...
The National Transportation Safety Board is investigating an
incident that occurred on January 30, 2008, in which an
American Airlines B757-200, en route from San Juan, Puerto
Rico to Philadelphia, diverted to West Palm Beach, Florida,
and made an emergency landing after the cockpit filled with
smoke.
Of the 139 passengers and 7 crewmembers, several were
transported to the hospital for smoke inhalation. All have
since been released. No other injuries were reported.
According to reports from the crew, while at cruise altitude
over the Atlantic Ocean, smoke began emanating from the
window heating system connected to the first officer's
windshield. The crew donned oxygen masks and smoke goggles
and diverted to Palm Beach International Airport. During
the descent to land, the inner pane of the first officer's
windshield shattered. The crew continued the descent and
landed without further incident.
The digital flight data recorder (DFDR) was downloaded and
sent to the NTSB laboratories in Washington. The affected
windshield, which remained in one piece, and the heating
unit were removed from the aircraft and will undergo a
detailed analysis.
While the cause of this particular incident is unknown and
remains under investigation, the NTSB is aware of five
events between 2004 and 2006 in which smoke, and in some
cases fire, were reported to have originated from window
heating systems in B-757 aircraft.
Based on these incidents, in September 2007 the NTSB issued
two Safety Recommendations to the Federal Aviation
Administration (FAA) asking the agency to require the
installation of redesigned window heating systems in all
Boeing 747, 757, 767, and 777 series aircraft. These Safety
Recommendations have yet to be implemented by the FAA.
The Safety Recommendations are available at
http://www.ntsb.gov/Recs/letters/2007/A07_49_50.pdf
View of cracked windshield from inside flight deck
http://www.ntsb.gov/Pressrel/2008/im...57-cockpit.jpg
Heating unit on affected windshield
http://www.ntsb.gov/Pressrel/2008/im...ater_block.jpg