LOCKHART, Texas (AP) — If Alfred "Skip" Nichols had been a commercial airplane pilot, he probably would have been grounded long ago.

Nichols, the pilot of a hot air balloon that crashed over the weekend in Texas, killing 16, was able to keep flying despite having at least four convictions for drunken driving and twice spending time in prison — pointing to gaps in oversight of hot air balloon pilots.

Whether the pilot's drinking habits had anything to do with the crash was unclear. A former girlfriend described Nichols as a recovering alcoholic. She said he had been sober for at least four years and never piloted a balloon after drinking.

The Federal Aviation Administration might allow a recovering alcoholic to fly commercial jets if the pilot could show that he or she was being successfully treated, said John Gadzinski, an airline captain and aviation safety consultant. But the agency is unlikely to accept an airline pilot with convictions for driving under the influence, he said.

The 49-year-old Nichols also had a long history of customer complaints against his balloon-ride companies in Missouri and Illinois dating back to 1997. Customers reported to the Better Business Bureau that their rides would get canceled at the last minute and their fees never refunded.

When pilots apply for a ballooning certificate with the FAA, they are not required to disclose any prior drunken driving convictions, only drug convictions, said Patrick Cannon, a spokesman for the Balloon Federation of America trade group, who called that a loophole in the law. He noted that the ballooning certificate specifically says not to include alcohol offenses involving a motor vehicle, as those are covered on the FAA's medical application.

But balloon pilots do not have to get regular medical exams from FAA-certified examiners. They are only required to write a statement certifying that they have "no medical defect" that would limit their ability to pilot a balloon.

Commercial plane pilots are required to fill out a form that includes questions on alcohol dependence or abuse and convictions for driving under the influence of alcohol.

A member of the National Transportation Safety Board, Robert Sumwalt, criticized what he called a "disparity" in the FAA requirements for balloon operators compared to plane or helicopter pilots.

"I was an airline pilot for 24 years, flew airplanes for 32 years, and I had to have a medical, an FAA medical every six months," he said.

Nichols got his commercial license to pilot hot air balloons in Missouri in July 1996. His first drunken driving conviction came in St. Louis County in 1990, followed by two more convictions in 2002 and a fourth in 2010, according to online court records.

He was also convicted of a drug crime in 2000 and spent about a year and a half in prison before being paroled. He was returned to prison in April 2010 after his parole was revoked because of his drunken driving conviction that year. He was paroled again in January 2012.

After they receive a license, all pilots are supposed to notify the FAA within 60 days of a drug or alcohol conviction. However, Cannon said, there is no oversight of that reporting requirement for balloon pilots.

Investigators said they do not yet know why the balloon operated by Heart of Texas Hot Air Balloon Rides, hit high-tension power lines before crashing into a pasture Saturday near Lockhart, about 60 miles northeast of San Antonio.

While there were patches of fog on the day of the flight, the ground crew said it was clear when the balloon took off, and there was no sign of any maintenance trouble with the balloon, Sumwalt said. An examination of the balloon after the crash found "no evidence of preexisting failures, malfunctions or problems," he said.

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Schmall reported from Fort Worth. Associated Press writers Reese Dunklin and Jamie Stengle in Dallas, Jim Salter in St. Louis and Joan Lowy in Washington, D.C., contributed to this report.