It was an unusually bright, sunny day at Chicago O'Hare airport as passengers at Gate B9 of Terminal 1 waited to board the Boeing 757 that was docked at the gate.

The flight schedule monitor in the terminal said United Airlines Flight 629 to San Francisco would leave on schedule.

The monitor was wrong.

A gate attendant fired up the public address system. "We have a new departure time. They're taking this plane away," she said, offering no further explanation. The 12:17 p.m. flight would be delayed to 1:30 p.m., she said.

Passengers crammed into seating near the gate let out a collective groan.

The scene is one that has played out time and again on United Airlines flights since United and Continental combined in a US$3.2 billion deal to form the world's largest carrier, and especially since it had a trouble-plagued conversion to a new computer system March 3. The move was the biggest yet in trying to weave together two mega-airlines in the wake of a massive recession and in the midst of rocketing jet fuel prices.

United, Chicago's dominant carrier, is the most complained-about airline in America by far, according to the latest statistics from U.S. Department of Transportation.

During March, United's on-time performance was worst among the major legacy airlines in the U.S., according to those same statistics. Among large, full-fare airlines, United rates worst in mishandled bags and denied boardings, a term for passengers who hold confirmed reservations but are bumped from a flight because it is oversold.

To see what United's troubles might portend for the busy summer travel season, I took a two-day excursion from Chicago to San Francisco to Las Vegas to Houston and back to Chicago to witness firsthand the passenger experience at United Airlines. The trip involved some 25 hours of flying time and waiting at airports, including three of United's hub airports.

Overall, the trip was a microcosm of what United passengers say and what the industry statistics suggest: Flights on United Airlines nowadays can go relatively smoothly, but if there's a problem, things can go very wrong in a hurry.

Much of this trip was uneventful, even pleasant, but one flight was frustrating and exhausting. I experienced good and also lousy customer service along the way. Every flight was full, overhead bin space always ran out, and the only bag I checked -- and paid US$25 for -- didn't show up at the baggage claim. However, on one return flight I basked in a satisfying flying experience after splurging for roomier EconomyPlus seating and in-seat television service offered on a newer plane.

United officials know the recent industry stats aren't good and concede the airline is dealing with problems.

"We've got to improve," said Martin Hand, United senior vice president of customer experience. "The last year and a half to two years have been a challenge, no doubt about it."

Hand pointed out that other carriers had similar problems, and poor rankings, when they merged, but they later improved. "We are absolutely focused on improving the customer experience," he said. "We know that we have in many cases not been easy to do business with, from the customer perspective."

United officials are examining every way the customer interacts with the airline, from the first point of contact the customer has with the airline, such as searching for fares on the website and booking a ticket, through picking up their luggage at baggage claim after a flight, Hand said.

"I wish I could flip a switch and make it happen overnight," he said of United's efforts to fix problems. "This is going to be a journey."

My journey in mid-May did not start well at O'Hare.

After the delay announcement at Gate B9, a queue immediately formed at the gate agent counter, mostly passengers who would miss their connecting flights in San Francisco and needed to rebook on later flights.

Me included.

Among those waiting in line was Carole, a San Francisco-based lawyer and consultant who was en route from Syracuse, N.Y., back to the West Coast. (She didn't want her last name used because she sometimes works with airlines and didn't want to jeopardize business, she said.)

"I fly United all the time. I'm very loyal," Carole said. "But every time in the last six weeks that I've flown, the flight has either been canceled or delayed. Every. Single. Flight." She said that was five straight flights.

Patience in the customer-help queue, 15 people deep, wore thin as gate agents were slow in rebooking passengers.

My waiting time in line? One hour and 26 minutes.

Some people in line grew noticeably restless and irritable. Many tried, seemingly unsuccessfully, to rebook connecting flights by cellphone.

When it was finally my turn, the gate agent maintained a cheery disposition but clearly was uncomfortable with the new computer system. Her demeanor soured as she fumbled, reaching next to her keyboard for two pages of computer instructions, a mix of typed and handwritten notes.

On March 3 United switched to a computer system called Shares. In a rip-off-the bandage approach, United on the same day also merged websites and frequent-flier programs. The airline experienced technology glitches and rampant inefficiencies throughout the system. The results were widespread flight delays for several days and customer inconveniences, such as long phone hold times, for weeks afterward. Frequent fliers howled about problems with their rewards accounts and confirming their proper seat upgrades.

Continental employees had used Shares and knew the system. United gate agents generally do not, aside from the recent training they received.

Mine was a United agent. "I'm supposed to have a Continental agent beside me," she grumbled as she flipped through her instruction pages, searching for notes on the next computer command she needed.

Aviation consultant Robert Mann said he's heard similar stories about perplexed United agents.

"It's still a case where agents are not comfortable with the new software," Mann said. "It's kind of a learning curve, but you have to wonder how long this kind of thing can go on."

Hand, the United official, said it has been a challenge to retrain longtime workers on a new computer system. It's most apparent when things go wrong, such as flight delays and cancellations, something the industry calls "irregular ops." Now, more than two months after the conversion, United is conducting a second round of training to fill in gaps and help struggling employees.

"Trying to rebook passengers is probably one of the biggest challenges we've had at many of our airports," Hand said. "That's why we're focusing on that recurrent training and why it's the No. 1 thing we're doing."

United is also designing a new, more user-friendly computer interface that will make transactions easier to complete, he said.

"Think of it more as a Web-based tool to point and click to get things done, versus having to know string commands in a native host system," Hand said. United aims to have the first version of that new system out before the end of the year, he said.

Besides additional training, United is working on a priority system for routing phone calls through the reservations department. That's so people at airports who need to rebook because of a delay or cancellation can call on their cellphones and be sped through the call-in reservation system. That change started in mid-May, Hand said.

Back at Gate B9, I at long last received a new boarding pass for the next flight.

Four minutes later came another public announcement. The flight was oversold, the agent said, and United would offer a US$250 travel voucher for volunteers who would give up their seats and take a later flight.

In addition to flight delays, so-called bumping on oversold flights is another measure the U.S. Transportation Department uses to gauge consumer friendliness. All four legs of my trip were completely full, passengers were told by gate agents and flight attendants.

United Airlines stats on bumping are not good either. During the first quarter, United's so-called denied boarding rate was by far the worst among the large, full-fare airlines and nearly double the industry average.

(Although passengers on our flights were asked to volunteer to be bumped, nobody who wanted to fly was denied their flight, a United spokesman said. Gate agents and check-in kiosks will ask for volunteers to be bumped in anticipation of an oversold flight, but volunteers often aren't ultimately needed because of no-shows, he said. The DOT statistics refer to involuntary bumping.)

At 2:12 p.m., nearly two hours after the original departure time, the purser aboard Flight 629 announced over the plane's public address system, "We have finally closed the doors for departure."

Passengers broke out in a round of applause.

The 12:17 p.m. flight was wheels up at 2:35 p.m.

Delays and bad customer service might be memorable for passengers, but most United flights are on time. Indeed, 77 percent of United flights in March, a particularly challenging month because of the computer conversion, were on time, both at O'Hare and nationwide. On time means they arrived within 15 minutes of the scheduled time. The industry average was about 82 percent.

My experience reflected that; three of four flights arrived on time or early.

Greg Gillis, 40, of Oakland, was at O'Hare heading back to San Francisco after attending his uncle's funeral in Moline, Ill. He noticed problems during the first weeks of the computer conversion. During a trip to Mexico in March, a United agent was unable to fix a seating problem for him.

"But as far as everything else, it's been the same old thing as usual," he said. "Maybe I'm used to it and just have patience with it. The airlines are the airlines."

United officials don't relish ranking so poorly in DOT statistics, but they point to the number of passengers affected. For example, United ranked 13th of 15 large airlines in denied boardings. But that's fewer than 2 of every 10,000 United passengers involuntarily bumped from a flight.

Similarly, United had the most consumer complaints in March, triple the industry average. But numbers show that's about 3 complaints against United for every 100,000 passengers.

After connecting in San Francisco to Las Vegas, I faced one more major problem. At the baggage claim in McCarran International Airport in Las Vegas, the silver suitcase conveyor showed no signs of my ugly olive-green roll-aboard. At the nearby lost-baggage desk, an agent said the suitcase was on a later flight and would be delivered to my hotel about 1 a.m., which it was.

United also scores poorly in mishandled bags, ranking 11th out of the largest 15 airlines and worst among the majors. To be fair, its rate of mishandled luggage was 4 per 1,000 passengers. Expressed differently, that's a 99.6 percent success rate.

A further aggravation is that I had paid US$22 for an upgrade to a roomier seat in the EconomyPlus section of the plane to Vegas. But because of the delay at O'Hare, I was rebooked on a small regional jet that had no EconomyPlus. I didn't get a refund until later telling a public relations official at United about it. I was supposed to go online and request the refund.

"The customer shouldn't have to request anything," Hand conceded. "If we haven't fulfilled what you have purchased, we should automatically know that, especially if you're a MileagePlus customer, and be able to give that refund without you starting the process." He said the refund problem is a known issue with the new computer system and will be rectified soon, making refunds automatic.

The return trip from Las Vegas to Houston to Chicago went much smoother, an example of how United can function when it has no irregular ops. Flights were on time, and a checked bag showed up on the baggage claim carousel at O'Hare. The extra room in EconomyPlus was a joy after two recent flights jammed in a middle seat. And in-flight television, costing an US$8 swipe of a credit card, helped pass the time, allowing me to flip among several baseball games while seatmates watched movies and sit-coms.

United officials point out how the future is likely to be better. Not only is it addressing problems stemming from the computer conversion, but it also is investing US$550 million in upgrading its planes with such amenities as larger overhead bins and in-flight Internet access. It is scheduled to take delivery of 24 new planes this year, including five Boeing 787s, which have been praised for their passenger amenities.

Brad Kalmans, 46, of Houston, was traveling from Las Vegas to Houston after a lighting-fixture trade show. A platinum-level flier, he flies mostly United, uses a Continental-branded credit card and frequents United's exclusive lounges at airports.

He said he hasn't noticed a problem with flight delays, but he would complain about the attitude of United workers. "It's the overall atmosphere," he said. "They're a little more rigid now in the way they do things. The customer service dropped a little bit. ... I think the people aren't quite as friendly."

It can't help attitudes that United is in sometimes contentious negotiations with its major employee unions. United pilots, in particular, have been vocal about their displeasure with the slow pace of contract talks. They have periodically staged sign-carrying protests, including one in early May outside United's headquarters in Chicago, about continuing to operate under old, bankruptcy-era contracts.

Hand said that for customer agents, the computer conversion could be a source of surly attitudes. "I think our employees want to do the right thing and take care of the customers, but, candidly, the systems were hampering them from executing on that. And do they get frustrated? Sure."

Among the solutions is a new effort to empower employees, he said.

"Our job is to give the front-line employees the right tools and training to take care of the customer and get out of their way. They know what to do. ... Our goal is to push down that decision-making as far as possible in the organization."

On two of my return flights, curiously out-of-date preflight welcome videos played on monitors in the cabin. They featured United CEO Jeff Smisek talking about how great the airline would be once it combined computer systems, something that happened more than two months earlier.

"Our job is to bring two great carriers together and use technology to make the travel experience easier for customers and put the customers in control," Smisek said. "Everything will work together as a single airline should, so the entire travel experience is seamless for our customers."

So far, that promise of a better travel experience has not been fully realized.