FORT WORTH, Texas—Trying to ease employees' pension worries, a top AMR Corp. executive says most American Airlines Inc. employees will not lose any of their accrued benefits if the airline's massively underfunded pension plans are terminated.

“For those whose pension benefits are vested, if the pension plans are terminated, more than 90% of participants would see no reduction in their pension benefits accrued” as of the Nov. 29, 2011, bankruptcy filing by AMR, Jeff Brundage, AMR senior vp-human resources, wrote in a letter distributed Monday to employees.

Excluding upper management and pilots, fewer than 2% of pension plan participants, “in the remaining work groups, primarily those at the highest compensation levels,” would see a reduction in accrued benefits, Mr. Brundage wrote.

American Airlines did not disclose how much benefits earned by top paid employees—such as long-service pilots—would be reduced if the airline's pension plans were terminated and taken over by the Pension Benefit Guaranty Corp.

The maximum annual benefit the PBGC guarantees to a 65-year-old retiree in plans taken over in 2011 is $54,000 per participant if pension plan payments begin at normal retirement age.

American Airlines has not said yet whether it intends to terminate its four pension plans, which have about 130,000 participants.

However, an American Airlines spokesman said last month that the airline's pension plans are “very expensive” and it spends more on them than competitors spend on their retirement plans.

“Given American's plans to reduce its costs to a more reasonable level in line with industry norms, these costs and many other factors are considerations when deciding whether to continue the pension plans,” the spokesman said.

If American Airlines were to terminate the plans, the PBGC could be hit with its biggest loss ever.

According to preliminary PBGC estimates, the four plans have about $8.3 billion in assets and about $18.5 billion in promised benefits.

The PBGC said if the plans were to fold, the agency would be liable for about $17 billion in benefits, resulting in an $8.7 billion loss to the agency.

None of American Airlines' key competitors offer ongoing defined benefit plans.