The Washington Post Company reported yesterday that its net income for 1979 fell to $29.5 million ($1.89 a share) from $49.7 million ($3.06).

Most of the decline was due to a change in the method used to account for the cost of selling subscriptions for its magazine division, the company said. The accounting change required a write-off of $13.5 million (86 cents) and decreased 1979 profits by $4.5 million (29 cents).

Another major Washington area employer, Fairchild Industries Inc. reported its profits for the year soared from $24.5 million to $39.9 million, and several local financial institutions reported higher earnings.

The Post Company's revenues for the year increased 14 percent to $593 million from $520 million in 1978.

For the fourth-quarter, income fell slightly $14.7 million (98 cents) from $15.9 million (99 cents) while revenues increased to $173 million from $149 million.

[TEXT OMITTED FROM SOURCE] the company said; changes in income per share reflect the repurchase of 1.7 million shares of the company's stock during the year.

The Post Company said that without the accounting change, pre-tax operating income for 1979 would have been about the same as the prior year, although operating profit margins would have been reduced.

Operating income fell from $89.2 million to $80.4 million, a decline of $8.7 million, including a $9.2 million reduction due to the accounting change.

Revenues of the company's newspaper division increased by 13 percent, or $30.6 million, while operating income fell $2.4 million or 6 percent. Advertising linage for the year increased 4.5 percent at The Washington Post, 8.5 percent at the Everett (Wash.) Herald and declined by 2.7 percent at the Trenton (N.J.) Times.

The magazine and book division showed a $33.2 million (15 percent) increase in revenues, but a $8.1 million decline in operating income, including the $9.3 million reduction due to the accounting change.

Broadcasting division revenues increased by $9.1 million (14 percent) and operating income increased by $1.8 million or 9 percent.

Other income declined to $3.3 million in 1979 from $8.6 million in 1978, when the company sold a radio station and other property.

Fairchild Industries Inc. reported record income of $39.9 million ($6.43) for 1979, up from $24.5 million ($4.09) for the previous year. The Germantown aircraft manufacturer's sales soared to $717.8 million from $543.8 million.

Fairchild turned in fourth-quarter earnings of $11.9 million ($1.89) on sales of $196.1 million, compared with $7.3 million ($1.20) on sales of $160.5 million in the same period a year ago.

Fairchild said the gains were due primarily to manufacturing efficiencies and increased sales of military and commercial aircraft and airplane components.

Sales of Boeing 747 components were up 93 percent to $70.8 million. Sales of aircraft seats by the Fairchild Burns Company grew 60 percent to $26.3 million.

Fairchild's deliveries of the A-10 Thunderbolt close support plane to the U.S. Air Force increased to 129 units worth $425.8 million from 89 planes at $325.5 million in 1978. The company's year-end backlog of orders was $1.125 billion, up by $120 million.

W. Bell & Co. Inc., the Rockville-based catalogue showroom chain, reported its profits for the final quarter of 1979 increased 20 percent to $1.75 million ($1.67 a share) from $1.46 million ($1.40).

Bell's sales for the quarter -- which traditionally accounts for the bulk of business for catalogers -- increased to $41 million from $33 million.

For the last half of the year, the Bell company earned $1.83 million ($1.95) on sales of $56.8 million, up from $1.54 million and sales of $44.1 million in the same period a year earlier.

The Richmond, Fredericksburg and Potomac Railroad Co. said its earnings for 1979 hit a record $13.7 million ($38.39), up from $10.5 million ($29.58) in 1978.

Revenues of the railroad and its subsidiary, Richmond Land Corp., increased to $43.9 million from $37 million. President Stuart Shumate said there was a "substantial improvement" in the railroad's operating profits and large increases in earnings of its subsidiary, Richmond Land Corp.

Universal Security Instruments Inc. of Owings Mills, Md. reported earnings for the three months ended Dec. 31 of $42,816, (3 cents), compared with a loss of $158,000 (10 cents) in the same period a year ago.

Sales of the burglar and fire alarm maker increased to $2.6 million for the quarter from $1.3 million in 1978.

For the first nine months of its fiscal year, Universal Security showed a loss of $149,000 (9 cents), down from a $782,000 (50-cent) deficit the year before.

State National Bank of Rockville's 1979 income increased 29 percent to $1,023,000 from $791,000. Deposits of the bank increased during the year by $18 million to $106 million. State National opened a new office in Laurel during the year and will open branches in Silver Spring and Glen Burnie this year.

The bank announced a dividend of 13.5 cents a share to be paid to shareholders as of Jan. 31.