A report provided to the recently created Ontario International Airport Authority indicates that trend is expected to continue at least through spring 2013.
Addressing authority members on Monday morning, Robert Hazel of Oliver Wyman, a global management consulting firm, discussed ONT's performance and expectations in the years ahead.
Passenger traffic in 2012 is estimated to total 4.2 million, a 5.5 percent drop from 2011, Hazel said. That's the same number of passengers who went through ONT in 1986.
According to flight schedules through April, ONT will also have 6 percent fewer seats on flights than in the previous year.
At least one reason for the difference is the number of departures from ONT. There were an average of 111 departures each day in 2005, but there are only about 59 now, Hazel said.
"I think we will continue to lose traffic until something is done to change that," Hazel said to authority members.
The struggles have led to an effort by Ontario officials to regain control of the airport from its owner and operator, Los Angeles World Airports. LAWA also operates Los Angeles International and Van Nuys airports.
The Los Angeles City Council in October gave its final approval to a plan to negotiate the transfer of ONT to a new regional authority.
The Ontario International Airport Authority was set up to manage ONT if it went to local control. Monday was the group's second meeting.
Hazel said ONT has suffered more in the down economy than similar airports. The airport lost 37 percent of its passenger traffic from 2007 to 2011. The average airport of its size lost 12.7 percent.
Hazel said most Southern California airports have started to see their passenger traffic figures increase.
ONT and Burbank's Bob Hope Airport are the only exceptions and are both projected to lose passengers this year, Hazel said.
"If this rate continues, Burbank will equal or exceed Ontario's passenger traffic level," said Hazel, who referenced data from the U.S. Department of Transportation as well as the Official Airline Guide, a United Kingdom- based company that provides global flight information.
Rather than comparing average airfares between medium-hub airports, Hazel said he looked at the average fare in selected markets for a more accurate comparison.
For example, the average airfare from ONT to Denver on Southwest was $134, but it was only $109 at LAX, $119 in Burbank and $114 at John Wayne International Airport in Santa Ana, he said.
There are some instances where ONT's airfares are higher than other airports, and there are certain markets where ONT is lower than the rest. These figures change from quarter to quarter, Hazel said.
"Ontario's fares are roughly comparable with the other competitor airports, which also makes it important that Ontario's costs be comparable," Hazel said.
Traditionally, the costs to do business for airlines at ONT has been higher than other regional airports.
A couple of authority members expressed interest in learning more about the cargo figures out of the airport. Hazel's report did not address the cargo number, but they will be analyzed at a future meeting.
"Unlike other airports, ONT has a capacity for ancillary opportunities to create cash to underwrite these operational costs. Huge part of that might be on the cargo side," said Jim Bowman, an authority member and Ontario councilman.
Glendora residents Daniel and Betsy Matthews attended Monday's meeting and said they were concerned when they started to see a decline in traffic at ONT.
"We noticed, and we wrote to (LAWA) making our own personal appeal for reversing the evident decline in service at Ontario," Daniel Matthews said.
The couple previously lived in Thousand Oaks and flew out of LAX, but changed that nine years ago when they moved to Glendora and considered ONT their airport.
For years, the couple did not go to LAX because they did not want to deal with the hassle or traffic, Daniel Matthews said. But flight availability and tickets costs at ONT meant that was no longer an option.
"This weekend, we're going to go out of Los Angeles to get a trip we formerly would have gotten out of Ontario," he said.
Through Facebook, the couple became aware of the Set ONTario Free campaign - the city's public-relations effort to drum up support for local control of ONT - and decided to attend the authority's meeting.
"We endorse local control ... we wanted to let them know their influence goes beyond the borders of Ontario," Matthews said.
Reach Liset via email, call her at 909-483-8556, or find her on Twitter @DBOntarioNow.