US investigators are urging the Federal Aviation Administration to pass long-delayed directives to fit redesigned windshield-heating systems on Boeing aircraft after the latest in a series of smoke and fire incidents resulted in an American Airlines Boeing 757 windshield's shattering in flight.

The National Transportation Safety Board has reiterated recommendations - some of which date back nearly four years - to install redesigned electrical terminals in the windshield-heating system on Boeing 747s, 757s, 767s and 777s.

The move follows the emergency diversion of an American Airlines 757, operating the San Juan-Philadelphia route on 30 January, to Florida's West Palm Beach after smoke from the first officer's windshield heater filled the cockpit.

The pilots put on oxygen masks and smoke goggles. During the descent to Palm Beach Airport, the inner pane of the first officer's windshield shattered, although the fragments remained in place. The aircraft landed safely but some of the 146 occupants needed hospital treatment for smoke inhalation.

Five other similar incidents on 757s were reported between 2004 and 2006 and, during the investigation into the earliest of these, Boeing stated that at least four similar incidents had previously occurred on 747, 757, 767 and 777 aircraft.

Analysis has identified inadvertent cross-threading of the screw attaching power wires to the heating terminal as a cause of electrical arcing. In 2004, Boeing developed a modification to the terminal, but the issuing of FAA ADs has been delayed.

Source: Flight International