Challenges for revitalizing the Colorado Springs Airport continue as passenger traffic declined in April by the biggest percentage in more than four years.

The number of people flying out of Colorado Springs dropped 15.6 percent in April 2013 from the same month a year ago, to 51,506 people. That is the biggest percentage decline since traffic fell 16.7 percent in February 2009.

From Jan. 1 to April 30, the number of people using the airport was down 8.4 percent from the same period last year, according to statistics announced Tuesday at the monthly Colorado Springs Airport Advisory Commission meeting.

Most of the decline in passengers is tied ot the departure of Frontier Airlines, which stopped all flights from the airport on April 8, said John McGinley, assistant director Operations and Maintenance. From Jan. 1 to April 8, 20,818 people had flown out of the Springs on Frontier.

But it wasn't just Frontier that led to April's drop. Three of the four airlines still flying out of the Colorado Springs Airport saw a drop in passengers that month and during the first four months of 2013.

Only United/United Express had an increase in passenger traffic: 6.6 percent in April of this year over April 2012. The airline also saw passenger traffic increase to 107,775 people, a 2.4 percent increase, from Jan. 1 to April 30 of this year over the same period last year.

Airport officials have been taking more aggressive steps, including advertising, to draw passengers back to the airport. The airport hopes to begin construction of a 1,700 square-foot first class/ premium lounge in August, and launch a customer loyalty program by late summer.

Airport officials also hope to bring Boeing back to the airport later this summer. The aircraft manufacturer's possible return to test some of its newly designed aircraft would increase revenues through landing fees.

But airport officials face an uphill climb to overcome Frontier's loss. The airline's departure hurt both the airport's budgeted revenue amounts for April and year-to-date.

"Frontier's leaving was about a $2.7 million revenue loss in this year," McGinley said, "and depending on what happens with air service in 2014, the absence of Frontier could equal a $3.5 million loss of revenue next year."

Revenues for the airport were 11 percent - $752,488 - below forecasted amounts for the four-month period ending April 30. The greatest blows came from lower landing fees (down $249,030), terminal building rent (down $263,981) and parking (down 120,590).