An excellent discussion of safety issues is available from the Federal Aviation Administration. Detailed data on domestic aviation accidents and incidents is available from the National Transportation Safety Board and its Aviation Accident/Incident Database.
A brief history of why we're concerned with United Airlines' attitude toward the safety and security of its passengers and employees:

Although we have long had plenty of concerns about safety standards at United, the most worrying evidence of safety in decline at the "new" (merged) United Airlines comes from the pilots themselves. In a report issued to Congress by the United chapter of the Air Line Pilots Association (ALPA), the pilots warn of spikes of safety-related reports, which had increased by a dramatic 98% between September 30 and October 5, 2011, reaching an alarming level of more than 6000 reports in 2011, with 95% of the increase attributed to Phase 2 training (page 13 of the report), related to the new procedures being adopted as Untied and Continental were merging.

Specific issues included the training methodology itself, which the pilots found to compromise safety, inadequate training, inaccurate training information, and a recommendation that pilots who were uncomfortable with their training should "learn" the new procedures while flying passengers (page 14).

More shockingly, United's own Principal Operations Inspector, John Martin, dismissed the pilots' safety concerns (page ii), was unaware of spikes in the safety reports (page 10), and threatened airline captains with "FAA violations" for attempting to uphold higher safety standards than those adopted by Continental (page 11).

And if that doesn't give you pause, consider Mr. Smisek's flippant passing of the buck when testifying before Congress on June 16, 2010 regarding safety implications of the proposed merger between United and Continental, related to a fatal accident on a Continental regional carrier (Colgan) the previous year:

Mr. Boccieri. Mr. Smisek, was Colgan Air training to your satisfaction?
Mr. Smisek. No, it was not.
Mr. Boccieri. And why did you keep them as one of your carriers?
Mr. Smisek. We were not aware of that training deficiency. That is the responsibility of the Federal Aviation Administration. We expect all of our regional carriers----
Mr. Boccieri. That is your responsibility. That is your responsibility.
Mr. Smisek. Let me tell you that we are very concerned with safety. We did not train those pilots, we did not maintain those aircraft, we did not operate the aircraft. We expect them to be safe, we expect the Federal Aviation Administration to do its job, we expect that you do your job----
Mr. Boccieri. Well, we expect you to do your job too, sir.
Mr. Smisek. And I expect me to do my job.

As a United pilot related, "in the past there was always a very thick layer of safety over and above the minimum FAA requirements but now there is a zero safety margin as we sink right to those FAA minimum standards."

Earlier safety issues

In 1997-98, I received a number of letters alleging serious problems with UAL's safety procedures. In the first two weeks of January 1998 alone, I received one letter discussing an emergency landing at Heathrow due to an electrical fault that was not repaired prior to takeoff and another dealing with a faulty aircraft flap that was inadequately repaired.

That same month, a former UAL pilot described a number of safety violations that United was unwilling to correct, including at least one case of direct violation of FAA rules. While there may be good explanations for the incidents described in the previous two letters, one cannot help but be troubled by the account of UAL's attitude toward safety issues when coming from someone with extensive personal experience with the company.

When the pilot's article was posted, gave United Air Lines an opportunity to respond:

Here's my challenge to the brass at UAL: I'm willing to open up my web site and give you the opportunity to respond. Tell us how you've corrected the problems described in the letters and assure your passengers that you value their safety more than their money. Describe how you monitor the performance of your pilots and ensure that all of your crew members, including reserve pilots, have adequate rest as required by the FAA. Finally, explain what you've done to prevent your pilots from taking off with undiagnosed or unrepaired electrical or mechanical problems.

And please, this time, spare us the ad campaign.

Unfortunately, United's reply simply denied the allegations without providing any details, and suggested that the former pilot "left [the] airline ... for reasons quite different from [those stated]." Since then, the pilot's supervisor from his Air Force days, an active duty Lt. Colonel, has vouched for the integrity and professionalism of the former UAL pilot.

Most interestingly, a UAL Coordinator under the supervision of Mr. Soliday, UAL's former Vice President of Corporate Safety and Security, received a letter of reprimand on August 4, 1992, from the National Transportation Safety Board, regarding UAL's withholding of information related to the investigation into the fatal air crash of United flight 585 (that crashed in Colorado Springs). Could it be that UAL was more interested in covering up their questionable practices than fixing them?

As we wrote back in 1998, "If United wishes to whitewash over its glaring lapses in customer service, poor training procedures, incompetence of senior employees, and so on, with a laughable public relations campaign, that's one thing. However, turning a blind eye to serious safety issues is inexcusable. Once again, we offer UAL an opportunity to explain what it is doing to correct these problems -- we remain happy to publish the news here. Many readers have commented that they appreciate the candor of those pilots who give passengers honest information rather than excuses. Isn't it time for the airline to do the same?"

Since that time, there have been numerous additional charges of inattention and disregard for safety, many by current UAL employees.

At the end of May 2001, UAL terminated the employment of Tim Hafer, a former Warranty Coordinator with the airline, following his informing the FAA of maintenance safety issues related to the servicing of aircraft by third party vendors. These concerns for maintenance issues at UAL were subsequently reported by Forbes magazine.

In Jaunary 2002, we reported on a case involoving a whistleblower, who was reprimanded after reporting a crack in an aircraft frame and another regarding poor training for security staff.

In April 2002, Forbes magazine ran a feature on airline safety, specifically citing incidents of maintenance oversight with subcontractors used by United. Not surprisingly, UAL took exception to the article, denying the problem and further claiming that the "Forbes story cites several maintenance errors that are not substantiated in any of United's extensive maintenance records or quality- assurance reports." This was apparently an outright lie, given that a UAL mechanic was able to send us copies of the maintenance reports in question.

In September 2002, as UAL was spiraling into Chapter 11, we featured a discussion concerning the maintenance of U.S. Air Force aircraft at the Charleston Air Force Base by United Airlines' mechanics. By this time, numerous UAL mechanics had raised allegations of violations of FAA, Air Force and airline policies, and each time, UAL's response was the same denial of all wrongdoing. At present, this case is proceeding in the courts.

Of course, now that UAL is in bankruptcy protection, many of these issues are moot, but as the airline continues to fly planes with passengers on board, we should remain aware of United's attention to safety issues.

In December 2002, the Federal Aviation Administration announced a $805,000 civil penalty against UAL for making improper wing repairs using duct tape on three Boeing 727s. UAL's spokesman Joseph Hopkins says the airline will contest the fine because "the planes were not unsafe and no passengers were ever in danger." That would be easier to believe if not for the evidence of years of UAL's deliberate inattention to safety while hiding behind insulting slogans of "Safety First."

In May 2007, a commercial vehicle inspector wrote: "I was at O'Hare a couple of weeks ago and saw a United flight come in. One of the tires on the nose gear was completely bald. The tire next to it looked fine. I asked a retired USAF pilot about acceptable tire wear on airplanes. He said pilots compensate for tire wear on nose gear by keeping the weight off the nose as much as possible when landing. I showed him some photos of the plane at Chicago. He said the tire looked bad and should have been changed." The inspector legitimately wonders what the acceptable tread depth minimums are for tires on airplanes...