The Department of Transportation's new rule limiting the time passengers can be held on closed aircraft away from the gate goes into effect April 29, but Delta and JetBlue think construction at JFK may give them good reason to be excused from potential fines. The airlines, which operate a majority of flights at the airport, have petitioned for temporary exemptions from the rule because a four-month-long project at JFK will close the airport's longest of four runways (14,572-foot 13R/31L) as it is widened and repaved in concrete through July. Under normal conditions during peak hours, the FAA estimates the runway's closure may cause delays of about 50 minutes, and those delays will then ripple out to other airports and through affected carriers' schedules. Both Delta and JetBlue have made adjustments to their schedules but fear that may not be enough. Under the new DOT's new rule, an airline could be fined for every passenger held on a closed non-traveling airliner for more than three hours. Translated into dollars, it means that an airline that left passengers on a full Boeing 737 could be subjected to nearly $3.5 million in fines.