Investigators have disclosed that electrical arcing was observed in the cockpit of an Icelandair Boeing 757-200 it diverted to the Canadian air force base at Bagotville with a badly damaged windscreen.
The twinjet (TF-ISJ) had been operating from Orlando to Reykjavik Keflavik airport on 19 October.
It had been cruising some 49nm south-east of Bagotville, at an altitude of 35,000ft, when the captain noticed electrical arcing in the right-hand corner of the left main windshield pane.
The crew shut down the windshield heater but, shortly afterwards, a “loud cracking sound” was heard, says the Transportation Safety Board of Canada.
It states that the crew put on oxygen masks, transmitted a ‘Mayday’ call owing to the cracking of the windshield, and began an emergency descent.
As the aircraft descended through 25,000ft the crew realised that the cabin pressure was remaining constant, and the pilots aborted the descent.
While the captain removed his mask, the first officer did not, as a precautionary measure. The crew requested a diversion to Bagotville where the aircraft landed without further incident. None of the 155 passengers and seven crew members was injured.
Several previous incidents linked to windshield-heating systems on Boeing jets, and particularly 757s, led US investigators to issue a formal recommendation in 2007 which sought to mandate replacement of windshield heat terminal blocks on a number of Boeing aircraft types.
Investigators have yet to reach conclusions on the circumstances of the Icelandair incident. Flight Fleets Analyzer lists the 757 involved as a 19-year old airframe originally delivered to Iberia.