What upsets airline passengers the most these days?"Insufficient room" in coach-class or economy flights, says Consumer Reports Travel Letter, a newsletter aimed at the frequent traveler that informally surveyed its readers on comfort aloft.

That response prompted the editors to investigate airline seating arrangements. In what it labels a "special research report," the newsletter rates 32 of the major U.S. airlines for comfort in coach class, based on width of seats, space between rows and the configuration of the seating (how many seats in a row).

Most passengers feel less crowded when the seat next to them is empty, the publication notes. But when a plane is full, or close to it, seat width and the space between rows (called "pitch") become important.

Establishing a "comfort index" from a combination of these factors, the newsletter cites Midwest Express (flying DC-9s) and Air Atlanta (727s) as "substantially" more comfortable than other airlines. Both are small airlines with limited schedules. (The study includes regional and low-fare airlines, as well as the major carriers.)

Ironically, Midwest Express, a Wisconsin-based carrier, was in the news most recently under less pleasant circumstances. One of its DC-9s crashed Sept. 6 after taking off from Milwaukee bound for Atlanta, killing all 31 people aboard.

Air Atlanta, a not-quite 2-year-old airline with five 727s, serves Atlanta, New York City, Miami and Memphis. Before inaugurating flights, the airline surveyed business travelers on what they wanted. The airline has targeted the frequent traveler as its most-valued customer.

"They told us they were looking for comfort. They felt cramped," says spokeswoman Carol Bizens.

"We removed 25 percent of the seats from our planes," she says, reducing seating to a total of 88, down from the 125 that is more common in other airlines. As a result, each row has only five seats, instead of the standard six; and Air Atlanta promises to keep the middle seat free if passengers reserve at least 24 hours in advance.

"We are competing on superior service," says Bizens, while charging the same fare as its competitors.

Also ranking high are three regional lines, Muse (Dallas-based), Ozark and PSA (California-based), offering "better-than-average comfort, at regular coach fares, on all their aircraft." These include DC-9s and MD-80s.

At the very bottom of the list is Hawaiian Air, flying DC-8s. Hawaiian operates within the Hawaiian Islands and between the West Coast and Hawaii. A Hawaiian Air spokesman declined to comment because he had not seen the newsletter article.

Each of the major U.S. carriers -- among them United, TWA, American, Delta, Eastern, USAir -- has aircraft placing from high to low on the comfort index. Not surprisingly, their long-distance, wide-bodied planes (DC-10s, L-1011s, 767s, 747s) tend to do best; their small planes flying shorter distances (727s and 737s) are clustered toward the bottom.

People Express, perhaps the most famous of the low-cost airlines, ranks near the bottom for its 727s and 737s, and not much higher for its wide-bodied 747s. Replies People Express spokesman Russell Marchetta: "Other airlines have the same pitch. What happens with People Express is that the prices are so good, the planes fill up. They feel more cramped."

"Seating comfort on airplanes has been decreasing steadily in recent years," asserts the November issue of Travel Letter, because airlines are squeezing more seats into coach cabins and more of those seats are being filled because of bargain promotional fares. Travel Letter, begun this year, is published monthly by Consumers Union, a nonprofit consumer-information organization approaching its 50th anniversary.

Seat space and pitch are determined by airline policy, "not by airplane manufacturers," contends the report. The result is that the degree of comfort differs on the same type aircraft flown by different airlines.

On domestic routes, the width between rows in coach class varies from 30 to 37 inches, the report says, "with most now between 31 and 33 inches." According to its findings, Air Cal, a midsized airline headquartered in Newport Beach, Calif., flies a 737-200 with a roomy 37-inch pitch. There's an almost-as-comfy 36-inch pitch on United's DC-8s. In contrast, several airlines fly 737s with only a 31-inch pitch, and Hawaiian's DC-8 is low with a 30-inch pitch.

Certainly as important to comfort is seat width, especially for the passenger given a middle seat. Seat width in coach class ranges from 18.5 inches to 22 inches, the report says. Midwest Express and Air Atlanta, at the top of the comfort index, are the only two airlines cited with the maximum width. Hawaiian's and Pan Am's L-1011s and World's DC-10s, all wide-bodied planes, are listed at the 18.5 figure.

Some airlines, aware of the middle-seat problem, have installed wider seats for the unfortunate passenger assigned one, Travel Letter says. To do so, these airlines have trimmed a bit of room from the aisle and window spots.

"Don't be misled by the apparently small difference of an inch or less," says the newsletter. "When as many as 10 seats are tightly packed together in each row, just a half-inch less per seat makes the seating noticeably more crowded, and an inch more per seat is a significant improvement."

Most travelers pick an airline for schedule convenience and for the fare available, says the newsletter. But on popular routes with a choice of competitive flights, "comfort seems to us a sensible basis on which to select your flight."

And what about in-flight meals, another passenger concern? Less than half of the readers surveyed consider food as significant a problem as their "discomfort."