PHILADELPHIA — If the nation’s airports get their way, passengers could see an $8 increase on a round-trip airline ticket.

Airports now receive a facility-use passenger fee of $4.50 per flight, and want Congress to increase the cap to $8.50 a ticket for departing flights — that’s $8.50 when fliers get on a plane to leave on a trip, and $8.50 when they board a flight to return home.

The proposed increase was approved in a Senate appropriations bill in July and is now before a House conference committee considering budget legislation.

Airports say the higher fee is needed for construction projects and to improve aging facilities. Airlines and the trade association Airlines for America, oppose the increase, saying it will discourage families and budget-sensitive air travelers.

“The increase gets hidden within advertised airfares, leading passengers to believe it is a fare increase when in actuality it is a tax increase,” said Nicholas Calio, president and CEO of Airlines for America.

In Philadelphia, the usage fee generated about $61.8 million last year, and has helped pay debt on bonds to build the international Terminal A West, Terminal F, the expansion to Terminals D and E, as well as airfield and other improvements.

Airlines contend that airports can get money in other ways, including airline leases and charges, rents and fees paid by rental car companies, concessionaires, parking, and federal grants.

Delta Air Lines said that airports will rake in a record $3.6 billion in passenger facility charge taxes this year, and an additional increase “is not only unnecessary but harmful.” The Atlanta-based carrier cited studies by the U.S. Government Accountability Office that for every $1 increase in the passenger facility charge, passenger demand declines by more than 1 percent.

Airports Council International-North America, representing the nation’s airports, said the $4 fee increase on a one-way ticket is modest compared with the gouging by airlines with add-on fees to check a bag, for seats with extra legroom, and $200 to change a ticket reservation. Some now charge to put a carry-on in the overhead bin.