US operators of more than 700 Boeing airliners will have to replace insulation blankets covered with metallised Mylar as US Federal Aviation Administration proposals become rules. But the FAA has extended by five years the time allowed for completion of the work.

The airworthiness directive (AD) was prompted by reports of in-flight and ground fires on certain aircraft with metallised Mylar coated insulation blankets. FAA safety officials fear they may contribute to the spread of a fire if ignited by electrical arcing.

The AD, which was proposed a year ago, covers 719 US-registered McDonnell Douglas DC-10s and Boeing MD-80s, MD-90s and MD-11s. US carriers will do the work during normal maintenance cycles, but another 800 affected aircraft are operated by foreign airlines, many of which are expected to carry out the work voluntarily.

After considering comments from affected US airlines, the FAA has revised the estimated cost of fleet-wide replacement from $255 million to $368 million. This had influenced the agency's judgement that a four-year deadline "would have a significant impact on scheduling and cost and might result in hurried accomplishment of the required replacement, which could result in potential damage to associated wiring".

The AD results from the crash of a Swissair MD-11 off Nova Scotia on 2 September, 1998. While the cause of the fatal accident is still being investigated, the possibility of a faulty wiring induced electrical fire propagated by flammable insulation remains under investigation. The outbreak of several other aircraft fires had prompted Boeing to recommend in 1977 that metallised Mylar be replaced. Alternative insulation blanket materials include non-metallised Mylar, metallised Tedlar (polyvinyl fluoride) and a polyester-covered foam known as Teril 34.

• The FAA has ordered repetitive inspections of wiring and Teflon sleeves for fuel tank boost pumps and override/jettison pumps on 250 US-registered Boeing 767s. The AD is prompted by reports of chafing of Teflon sleeves that cover electrical wires running through fuel tanks.

Source: Flight International