By the time the clock hit midnight Sunday, three United Airlines flights had made emergency landings over the weekend, stranding passengers and leaving others wondering whether the airline, which recently merged with Continental, was in the midst of a rocky marriage.

United Flight 96 to Berlin was forced to return to New Jersey's Newark Liberty International Airport after it experienced problems with its left engine, United spokesman Joe Micucci said in an emailed statement. The flight landed at about 8 p.m. Saturday night, two hours after taking off for Germany. Several news agencies have cited unconfirmed reports that debris from a blown tire may have been sucked into the engine causing the problem, however the FAA said the flight apparently overheated on takeoff.

Witnesses on the ground reported flames coming from the aircraft, but no passengers were injured in the incident, and they departed for Berlin on another plane about four hours later.

On Sunday, United Flight 409, also out of Newark, was forced to return to the airport after the crew "experienced an unusual smell in the cockpit," Micucci said. The Seattle-bound plane had "a possible problem with the air-conditioning system," according to the FAA.

United Flight 1124 out of George Bush Intercontinental Airport in Houston, meanwhile, returned to the airport "out of an abundance of caution" Sunday due to mechanical problems with one of the Boeing 737's engines. The Boston-bound plane was the third in what proved to be a tough weekend for United.

Though no passengers were reported injured on any of the aircraft, hundreds were delayed as the airline arranged alternative flights.

Micucci said United was in the midst of "conducting a thorough review on each."

The beleaguered airline has suffered from bad publicity during its rocky merger and holds a report card from the Transportation Department that's full of failing marks. United not only recorded the most delays (just 77.2 percent of its flights were punctual), but it had the highest rate of lost or mishandled baggage of any mainline carrier (3.5 per thousand) and bumped the most passengers (1.73 per 10,000).

Moreover, the airline tracked the highest number of consumer complaints among all U.S. airlines during the first half of 2012, according to the Department's Air Travel Consumer Report. Between January and June, United logged 1,740 complaints, more than three times higher than any other airline surveyed. That figure is up from just 502 complaints logged during the same period a year earlier.

The vast majority of issues likely derive from the company's switch to Continental Airlines' reservation system, as well as a single frequent-flier program and website. The sweeping changes, which happened on March 3, resulted in widespread flight delays, computer glitches that continued for weeks and hours-long wait times on jammed phone lines.

This weekend's incidents, however, highlight another concern: How well has the airline integrated its maintenance team?