Now his job and thousands of others are being done by outside maintenance contractors.
It's called outsourcing. It's cheaper for the airlines. But, Hafer says shoddy maintenance now is a threat to safety.
For example, one shop was paid to overhaul all circuit breakers on United's 727s.
"They just cleaned them up and made them look shiny but didn't fix any of them," Hafer says.
Hafer says United's computerized maintenance records prove a lack of oversight. By law certified mechanics must approve repairs.
But Hafer says he's found "hundreds" of instances in which secretaries signed off on maintenance records.
CBS showed the logs to NTSB investigator John Goglia, a mechanic himself for more than 30 years.
His reaction: "They're what!"
"Oh, that's a problem," says Goglia. "How in the heck do we know that this work was done properly!"
Mechanics at this Indianapolis maintenance facility tell an even more alarming story: of being pressured by United to look past potential safety problems, in an effort to cut costs. More than 170 of them got a copy of a letter threatening "termination" for "excessive write-ups." In other words, they could be fired for doing their jobs."
United did fire Mark Sassman, a mechanic who blew the whistle on the memo to federal regulators.
He believes he was fired for bringing the matter to the FAA's attention.
Some of the most critical "write-ups" involved structural cracks on United's 737s.
"Mechanics would find cracks in certain areas that required 3 to 4 extra days to fix. So they were pressured not to look in that area, '' said one veteran mechanic who said he was reprimanded for reporting the crack.
"For a member of management to discipline someone for finding a defect that's significant is crazy, that's what we're there for," says Goglia.
United would not discuss the memo and declined our request for an interview. But, United safety chief Hank Krakowski did speak with KCBS. On secretaries signing the logs, Krakowski said, "absolutely false."
He said it was just poor record-keeping, and under pressure from the FAA, United corrected the mistakes.
On those improperly repaired circuit breakers, Krakowski said: "It surprised us too, and I have to tell you that that's an unacceptable issue."
But, Krakowski defended outside maintenance.
"Outsourcing is one of the things that pretty much the industry is finding itself in right now," he said.
United has one of the strongest safety records. But industry-wide maintenance-related crashes have jumped some 30 percent.
"I think it has the potential of being catastrophic," says Sassman.
Now as cash-strapped airlines increasingly farm out repairs to cut costs, many worry they're also cutting corners on safety.