President Ford has sent Congress a legislative proposal designed to help the nation's airlines pay for replacing or altering aircraft which otherwise will not meet federal noise standards.

The carriers have contended that their current financial conditions - albeit better than a year ago - leave them without the cash or credit- generating ability to come up with the $5 to $8 billion the Department of Transportation estimates it will take to bring the carrier's fleets into compliance with the noise requirements.

Some three-quarters of the carriers' existing fleet - about 1,600 planes - cannot meet the federal noise standards that will be phased in over the next eight years.

The President proposes authorizing the Civil Aeronautics Board to place an "aircraft noise reduction" surcharge of up to 2 per cent on air passenger tickets. At the same time, the proposal would allow a 2 per cent reduction in the current airline ticket tax from 8 to 6 per cent, so consumers would pay the same amount.

The proposal contemplates that the board would place the revenues from the environments surcharge in trust and that they would be disbursed by CAB-appointed trustees to individual carriers in accordance with board guidelines.

Existing air passenger ticket and waybill taxes have created a surplus of $1.5 billion in the Airport and Airway Trust Fund, which now is used to build airports and improve facilities. Under the outgoing administration's proposal, the Secretary of Transportation would be authorized to make grants not exceeding $300 million from the surplus for the modification of aircraft if he determines that replacement is not feasible or if the fund established by the CAB is not available.