Nearly a dozen key senators formally asked the Civil Aeronautics Board last week to reconsider its recent decision giving the airlines unlimited freedom to raise fares on short routes while limiting increases on longer ones.

In their motion, the senators urged reopening the pricing issue to allow local officials and others who didn't expect the board to take such "sweeping" action to make their views known. Among the "others" would be some senators who believe the board has taken action "beyond what was envisioned" by the Airline Deregulation Act, they said.

The CAB's decision granted airlines a three-tiered system of fare freedom. Effective right away, the airlines were allowed to raise fares without CAB approval by an unlimited amount for routes of 200 miles or less, by 50 percent above the base level for routes of up to 400 miles and by 30 percent above the base for routes of more than 400 miles.

Following a hearing on the CAB action last week, Senate Commerce Committee Chairman Howard W. Cannon (D-Nev.) said he thought the CAB had justified giving the airlines some added flexibility to increase fares but hadn't justified uneven application of this flexibility.

Besides Cannon, petitioners included six other members of the committee that oversees the CAB, Majority Leader Robert C. Byrd (D-W. Va.) and Majority Whip Alan Cranston (d-Calif.).

Byrd and Jennings Randolph (D-W. Va.), another signer, have been holding up the Senate vote on the renomination of CAB Chairman Marvin S. Cohen. Both have been outspoken about their concern that West Virginia's cities and small towns may have lost some airline service under deregulation.

Byrd was the lead-off witness at a Commerce Committee hearing last week on the CAB's fare flexibility program, complaining both that West Virginia had lost 3,000 airline seats a week since deregulation and suffered "skyrocketing rates" in excess of the air fare increases in other, larger airline markets.

In an interview following the hearing, Byrd admitted he was holding up Cohen's confirmation but said he doesn't plan to hold it up forever. "I want to talk with him, " Byrd said. A meeting of the two hasn't been scheduled.

Meanwhile, airlines have been filing notices with the board outlining their intentions under the fare flexibility program. Most of the major airlines have said they will raise fares across the board next Sunday by between 4 percent and 6 percent with exceptions for some routes. Braniff had said it generally would take the maximum allowable increases but later withdrew its plan and has'nt announced a new one. USAir and Northwest Airlines haven't filed any increases. Republic Airlines has said it would raise fares 12 percent on most routes.

In a telephone interview, Cohen said he would look over the senators' petition and give it some thought. "We'll seriously consider it, of course," he said.

In another development, the board approved for one year an airline industry plan setting up an optional system to settle airline claims on tickets inssued by travel agents for travel on two or more airlines. At the same time, the board implemented its February order abolishing uniform travel agent commission rates. Effective today, each airline must compensate travel agents under its own plan, not according to an industrywide, agreed-on fixed-rate system.

The CAB last week also sent President Carter for his expected approval a proposed decision giving 11 U. S. airlines new authority between Bermuda and eight U.S. cities. New nonstop authority from Washington to Bermuda was granted to eight airlines.