SeaTac’s Alaska Airlines says its December flights operated less full in 2014 than the year before

Alaska Airlines’ aggressive push into new markets and its increase in its fleet’s seating capacity has finally resulted in a drop in the percentage of seats filled on its planes in December.

Alaska, based in SeaTac, also faces fresh competition from Delta Air Lines on many of its most-traveled routes.

According to December results released this week, Alaska filled 85 percent of its seats with paying passengers last month compared with 87.5 percent a year earlier. The airline saw its passenger capacity jump by 12.3 percent in December compared with an increase in passengers of 9.1 percent.

The airline in recent months has pioneered a dozen new nonstop routes from Sea-Tac as well increasing the capacity of its existing jets by installing thinner seats that allow the airline to carry more passengers per aircraft.

For the full year 2014, Alaska’s load factor, the percentage of seats filled by paying passengers, dropped from 85.6 percent in 2013 to 85.1 percent last year.

The airline’s on-time performance also took a hit in December with its mainline flights (those flown by the airline’s full-size jets) falling to 80 percent from 85.4 percent in December 2013.

Some pilots report that increasing congestion at Sea-Tac because of the increase in flights by both Delta and Alaska has affected the ability of flights to meet their timetables.

Delta is making Sea-Tac a hub for its West Coast international flights. It has increased the number of domestic flights to and from Sea-Tac to support those international flights.