NEW YORK (AP) _ Ronald Melott, a fire-protection consultant from Portland, Ore., hopes to forget a recent two-day business trip to Cleveland. His United Airlines flights via Denver were delayed, overbooked or canceled, leaving passengers sleepless and surly.

''I will have to say I saw people get extremely upset. Some became violent and started throwing things,'' he said, recounting the scene at Denver's Stapleton International Airport en route home. ''In my opinion, what I saw going on in Denver is not just something that's an isolated incident.''

Melott was lucky. United agents were able to put him on a Continental Airlines flight to Seattle and a connecting Trans World Airlines flight back to Portland, where he arrived more than seven hours late. He said 200 fellow passengers remained stranded in Denver.

Frequent flyer James Nelson, a broadcaster for the Christian Science Monitor's radio service in Washington, says he has endured increasing instances of overbooking, runway delays exceeding an hour and indifferent treatment by flight attendants.

''My experience suggests there has been a decline in personal service whether you fly first class, business or coach,'' he said. ''Specifically, the servicing of the planes, the physical cleaning has declined right across the board. I don't think any airline I fly is exempt from this.''

Their experiences reflect what many passengers, travel agents and industry consultants see as a sharp drop in the quality of service offered by U.S. airlines, once known for their courtesy, efficiency and glamorous image.

Many blame the 7-year-old era of airline deregulation, which has caused acute competition, cheaper fares and an explosion in the number of passengers, many of them erstwhile bus riders. In July, for instance, 90 percent of all passengers were flying at a discount, according to figures from the major carriers.

At the same time, there is a widespread belief that the federal air-traffic control system has failed to handle the increased load. Fear also is growing that some airlines are compromising safety standards.

In the last 20 months the Federal Aviation Administration has collected $4.6 million in safety fines from 16 airlines, quadruple the amount collected in the previous five years combined, for violations ranging from sloppy bookkeeping of maintenance records to leaky lavatory toilets.

The FAA levied a record fine of $9.5 million against Eastern Airlines in March for what the agency called 78,000 safety violations. Eastern is fighting the fine, but revamped the management of its maintenance division in August.

Complaints about long lines at airport counters, flight delays, dirty cabins, lost bags, no meals and rude service are increasingly common. Flight attendants claim they are forced to work more trips with less help, while pilots grumble about long waits to take off. An Eastern captain recently got so exasperated he quit while his aircraft was parked on a runway.

''What's happening is that air travel is becoming more like the bus that goes down Broadway,'' said Herbert Teison, head of Travel Smart For Business, a consumer publication about airline travel based in Dobbs Ferry, N.Y. ''As a consequence, the traveler is discovering he is not being coddled and babied, and very often amenities are decreasing.''

Eric Munro, owner of the Travelwise travel agency in San Diego and president of the regional chapter of the American Society of Travel Agents, blames lower fares, which have compelled many airlines to drastically reduce their budgets.

''We've got fares from San Diego to L.A. for $19. The airlines can't possibly make any money on those fares,'' he said. ''They've got to cut down somewhere. Where's the first place? Service.''

Others link the change to what they consider a decline in the work ethic nationwide, particularly among younger people. Airlines rely heavily on young recruits to fill positions for flight attendants and reservation clerks.

''The American mentality is less attuned to service than perhaps 25 years ago,'' said H. Wayne Berens, president of Revere Travel Inc., a travel agency based in Trenton, N.J. ''I think we find some of the younger people coming into the workstream are less attuned to what it means to give good service.''

Some say the airline industry's troubles have been aggravated by an unprecedented wave of mergers and acquisitions, preoccupying managers with takeover concerns and shareholder interests instead of running their business.

Over the past year, a wealthy investor took over Trans World Airlines; Pan Am sold its Pacific routes to United; Northwest Airlines agreed to buy Republic Airlines; and Texas Air Corp. agreed to buy Eastern and People Express, which itself had bought the now shut-down Frontier Airlines.

Airlines concede they have been forced to reduce some amenities to save money because competition and fare-cutting are so intense, leading to a collective loss of nearly $350 million in the first half of this year. But all say they haven't compromised safety standards or overall quality of service.

Some industry analysts say airline performance is admirable, considering that the number of passengers has grown from 292 million on 14.7 million commercial flights in 1980 to a projected 392 million on 19.2 million flights this year, while the number of airports has remained about the same.

''You've got a very significant pressure on physical facilities at a time when all these guys are facing problems to hold the line on cost,'' said James V. Cammisa Jr., a consultant and editor of the newsletter Travel Industry Indicators. ''In the context of that, I think they're doing remarkably well.''

Nevertheless, the number and type of complaints compiled by the Department of Transportation's consumer affairs unit show dissatisfaction with airline performance, particularly over delays.

In July, for example, the department recorded 304 complaints in the ''cancellations, delays, deviations'' category, 25 percent more than the same time last year. The department has calculated that for every complaint it receives, the airlines get 10.

Gripes vary widely from airline to airline. Transportation Department figures show the complaint rates for Delta, historically one of the most profitable carriers, is less than one per 100,000 passengers, while it has ranged as high as 10 per 100,000 for People Express.

To most seasoned air travelers, the declining number of complimentary items are obvious, ranging from magazines to lavatory soap.

Many airlines now serve more cold snacks and sandwiches instead of hot meals. American Airlines eliminated the black olives in salads and saves $100,000 a year. Pan Am reportedly saves $1 million a year by no longer serving macadamia nuts with drinks.

Other less obvious services also have decreased. For example, most airlines previously would replace lost tickets at no cost, but now they charge as much as $40. Courtesies such as allowing passengers to pick up tickets at a future destination now include surcharges.

''I think this is primarily a function of price,'' said Berens of Revere Travel. ''As long as there are air fares that barely pay the out-of-pocket expenses, I think the level of periphery services will continue to decline.''

Many flight attendants, who often receive the brunt of passenger anger, argue that cutbacks in the number of personnel aboard flights is a major reason for complaints about service deterioration.

At United, for example, the elimination of an extra flight attendant on some routes has forced the airline to serve cocktails and meals all at once, instead of having a pre-meal drink service.

''I think one passenger problem is they believe that somehow, even if they don't pay for it, they should have the same quality of service,'' said flight attendant Katherine Hutchens, an 18-year United veteran. ''You're not going to get it for $99. Your fare doesn't generate enough money to hire another body.''

Asked if flight attendants have become more insensitive to passenger complaints, she said: ''I think there's a hardening of attitude to some extent. If you tried to solve every problem given to you in one day, you'd be in an insane asylum.''

Many industry analysts believe that lower fares, which have made air travel affordable to millions, mean that the era of half-empty airplanes occupied mostly by well-dressed executives are over, probably forever.

''I guess I would call it a democratization of the airways,'' said Daniel T. Smith, spokesman for the International Air Passengers Association, a Dallas-based lobbying group that advocates passenger interests.

''It's pretty obvious when you get on the airplane that you have a wide section of the country on board,'' he said. ''The tank-top crowd is now sitting next to the fur-coat set.''

End Adv Weekend Editions Sept. 27-28