Fairchild Industries Inc., already hit by reduced demand for the aircraft it makes, received another setback yesterday when the credit-rating firm Standard & Poor's announced it was considering revising downward its ratings for the Maryland-based company.

The announcement came the same day Fairchild said it was laying off 76 employes at its Hagerstown plant because of a slowdown in the production schedule of its commerical aircraft.

Standards & Poor's said it was putting Fairchild on its "credit watch" list to determine whether its ratings of the company would be revised downward in light of adverse economic conditions that have hit Fairchild.

An S&P official noted that 70 percent of the companies put on the credit-watch have their ratings changed after a 90-day review.

A revision downward would mean S&P is less confident about Fairchild's finances and its outlook, the company representative said.

Specifically, S&P said that although the company's liquidity and debt-ratio currently is satisfactory, reduced demand for its commercial and military aircraft make it increasingly likely that earnings could decline enough to increase the company's debt ratio.

S&P said Fairchild's earnings will remain depressed through 1982, reflecting manufacturing problems on the U.S. Air Force's A10 close-air support plane made by Fairchild and soft demand for commercial and general aircraft whose parts are made by the company.

The A10, which has been "an important earnings contributor" for Fairchild, is being phased down, noted a statement by S&P analyst Edward Tyburczy.

On Thursday, Congress cut 20 A10s from the Air Force's budget, taking $360 million worth of business away from Fairchild.

Even though the company was recently awarded a contract to build the next-generation trainer aircraft for the Air Force, the sales and earnings from this program will not be signficant until the mid 1980s when production is under way, Tyburczy said.

Additionally, Tyburczy said, "sales for certain commercial segments are expected to remain weak over the near term."

It was the weak sales that forced Fairchild to lay off about 3 percent of its Hagerstown plant. Company officials said 59 hourly workers and 17 salaried employes were laid off.