SORRY," SIGHED the reservations agent. "We have no flights from Pittsburgh to Youngstown. Try Indiana Airways."

I'd never heard of Indiana Airways, but I soon discovered that the airline serves a tiny market of four cities from a base not in Indiana but in Pennsylvania -- Indiana, Pa., that is.

The little-known company is just one of 262 small commuter lines that operate around the country taking passengers short distances to smaller cities that the larger carriers frequently by-pass.

Their names may sound amateurism, such as Gold South Airlines and Chaparral Airlines or Air Sunshine but their service is professional. Their planes may be small and cramped, but their flights are usually direct and swift. They are hardly in the forefront of an industry dominated by about 15 major carriers. But if a traveler needs to get from one small city to another in a hurry, a commuter line can often meet that need perfectly.

Recently the commuter airline business has been booming. The airline Deregulation Act, which was passed a year ago, has freed many major carriers from serving small markets. Commuters have jumped in to pick up these discarded cities whose residents seem eager to take advantage if the new, more convenient airline service.

The first year after deregulation, the number of commuter passengers increased about 22 percent. FAA Administrator Langhorne Bond predicted this trend will continue. "The commuter industry is growing like a weed," he said. With more and more trunk carriers abandoning service to marginal markets, he added, "we're going to see more flights per day with smaller airplanes and therefore a better level of service."

Despite such optimism by Bond and within the industry, commuters still have trobules. One basic problem is simply how to overcome many potential passengers' fears of flying in a small plane.

Just listen to some of the comments at airports when a Cessna or Piper flies by: "You'd never get me in that thing" . . . "People sure are crazy to fly in such contraptions." And when a ticket agent recently led me and three other passengers to our plane to Youngstown, one woman eyed the small Piper aircraft that was dwarfed by the surrounding jets and gasped: "Oh my god, I've never flown in a toy airplane!"

However, our brief but smooth flight completely dispelled my fears of commuter flying and it proved to be a nice change from the impersonal ambiance of large jets.

To deal not only with passenger's doubts but with increased traffic as well, commuter airlines have been concentrating on another more serious problem -- safety deficiencies. Within the past year these carriers have experienced a dramatic increase in the number of fatal crashes. While 36 persons died in commuter accidents in 1978, as of October, 1979, 51 persons had died.

The crashes have prompted the National Transportation Safety Board to hold hearings, which are scheduled to begin Monday. The board will look into charges of repeated safety deficiencies and lack of safety surveillance enforcement by the FAA.

Seven weeks ago, the FAA issued for both commuter and air taxi lines new safety standards calling for more sfety equipment, additional piolet training and testing, and tougher maintenance requirements. Air taxis provide on-demand and some scheduled service, while commuters generally provide only scheduled service. Only 10 percent of the 3,085 air taxis and 262 commuters failed to meet the deadlines for submitting plans for the new standards, and those lines were grounded.

"There's a cost involved," explained FAA's Bond, "and some are not choosing to stay in the business." To help the industry adjust to the new rules, the FAA recently sponsored a two-day symposium on commuter safety. In addressing the commuter airline representatives, Bond said that while many commuters are performing responsibly, the overall safety record is "unacceptable."

He compared figures on regional and commuter accidents and noted that commuters had "nearly eight times as many accidents per hours flown." Bond promised the commuter industry that stiffer fines and heavier penalties will await operators "who repeatedly and willfully endanger safety. We will put greater emphasis on your accident prevention programs. Our budget next year will provide for more inspectors in addition to the 712 who now oversee the 262 commuter operators."

Safety Board chairman James King agrees with Bond that more inspections are needed. He said commuter carriers' maintenance practices are "not just bad or sloppy but non-existent." He said commuters must improve their safety records if they want to upgrade their public image. "The only way we can assure healthy growth is if we (the Safety Board, the FAA and commuters) make safety our first responsibility."

Another safety standard the FAA recently extended to commuters is the anti-hijacking security rules which commuters operating aircraft with 20 or more seats must follow. While many commuters objected to this rule because of its high cost, the FAA said it is necessary due to the rapid growth of the industry.

The security rule involves hiring police officers to monitor loading areas, screening passengers and their carry-on luggage, installing fences to prevent unauthorized access to aircraft and establishing procedures to keep suspect cargo off planes.

The FAA estimates the new security measures will cost the commuter lines $5.3 million the first year. Agency officials expect the full cost of the new security measures to be passed on to customers in an average $1.19 increase in ticket prices.

Despite increased revenues and passenger traffic, commuters still have difficulty making a profit. Many linesbegin service heavily undercapitalized, which makes it even thougher for them to meet rising costs. And one cost which has risen recently at an unprecedented rate is fuel. In fact, fuel bills now account for nearly 30 percent of an airline's operating cost.

Commuters operating costs are also more directly affected by bad weather and seasonal demand than those of major carriers. A canceled commuter flight is more difficult to reschedule than a canceled trunk carrier flight since trunk carriers have more planes and staff.

Most commuter companies are small outfits with limited staffs who do almost everything (take reservations, issue tickets, handle baggage and even fly planes), so operating costs are usually low. However, commuter fares are also low and the competition from a few of the larger carriers that still serve smaller markets can be stiff.

A few of the larger carriers do provide assistance to many commuter companies. United Airlines, for example, trains several commuters' personnel in ticketing and reservations and in some cases helps out with airport agent service. In addition, commuter lines help each other by lending planes or sharing personnel when necessary.

Air New England, a commuter that has branched into a regional carrier, has been losing money for several years in spite of loan agreements with a trunk carrier, an annual federal subsidy and financing by two major stockholders. Revenues and the number of passengers have increased since the line was founded in 1970, but the problem that plagues all commuters -- the need to serve low population areas which generate only so much revenue -- still hinders profits for the company.

Some commuters, on the other hand, have managed to overcome their initial debts and are beginning to invest in more equipment and to open new routes. Sky West, a commuter airline based in St. George, Utah, carried 29,000 pasengers in 1978, a 46 percent increase over 1977. Their annual growth rate has been between 35 and 45 percent, but for 1979 they project a 50 percent rate. The airline serves Utah cities St. George, Cedar City and Salt Lake City; Page, Ariz., Las Vegas, Nev., and just recently Phoenix.

Rocky Mountain Airways, another Western commuter line, increased their revenue passenger miles last year by 51.3 percent. Passenger boarding in 1978 was up 45.6 percent. The airline, based in Denver, serves a number of ski resorts and Colorado cities such as Pueblo and Colorado Springs.

While the majority of commuter lines serve only four or five cities, a few do fly between more than a dozen. Air New England, for example, serves 16 cities in seven states.

Some commuter companies' names give an indication of where the airline flies. About 30 commuter lines are named after the state in which they are based, such as Air Illinois, Iowa Airlines, Tennessee Airways and Airways of New Mexico.

A few commuters are named after cities, such as Las Vegas Airlines and New Haven Airways. Other lines have dubbed their companies with descriptive names like Gem State Airlines, Big Sky Airlines, Costal Plains Commuter, Desert Pacific Airways and Lake State Airways, to name a few. There even is a commuter line called Polar Airways, which serves Alaska, and one called Realwest Airlines, based in Fargo, N.D.

Some small cities that have little more than a few farms and fields to boast commuter service. Blue Bell, Pa., has its own airline called Wings. Las Cruces, N.M., is home to Zia Airlines.

Commuters have come a long way. While many are still fighting to gain recognition and to allay passengers' fears, this past year has marked the evolvement of many into professional operations. Passengers who'd like to take advantage of such sevice can check with the major carriers, who frequently take reservations for many commuter companies. However, if a major carrier does not list the commuter, the passenger will have to phone the company directly.

Locally, commuters are listed in the yellow pages along with the major carriers. However, in trying to book a reservation on an out-of-state commuter, it is best to try a major carrier for a phone number or base city. If that fails, as it did in my case, one can always try directory assistance in several cities in hopes of eventually tracking down the carrier.

The Commuter Airlines Association of America is in the process of compiling a commuter airline brochure which may help at least major carriers and travel agents inform the public about commuter services.

"Due to the recent growth of the industry," said an association spokeswoman, "we are compiling data about all commuter carriers, but it's still only in the beginning stages."