UPDATE 3-Southwest grounds planes over new maintenance glitch

(Recasts with Southwest expecting normal operations Thursday)

By John Crawley

WASHINGTON, March 12 (Reuters) - Southwest Airlines Co (LUV.N: Quote, Profile, Research, Stock Buzz) grounded more than three dozen planes on Wednesday due to a new glitch in safety inspections, although it said normal operations would resume the following day after reinspection.

The groundings, which cut 4 percent of its scheduled flights, came after the Federal Aviation Administration last week proposed a record $10.2 million fine against the airline, charging it missed deadlines to inspect planes for structural flaws during 2006 and 2007.

Southwest said in a statement that it discovered "an ambiguity" in maintenance records on 44 planes and decided to "take a conservative approach" and take them out of service for reinspection.

Southwest normally operates 3,400 scheduled flights each day and only uses Boeing Co (BA.N: Quote, Profile, Research, Stock Buzz) 737 aircraft. It ended 2007 with a fleet of 520 737s.

Shares of Southwest fell 7.3 percent to close at $11.49 on the New York Stock Exchange. Shares of U.S. airlines were broadly lower on high fuel prices and a report cutting ratings on several airlines from JP Morgan analyst Jamie Baker.

Southwest did not disclose the specific problem but the FAA said it was related to an agency inspection requirement for window areas on 737-300 and 737-500 aircraft. The FAA said Southwest grounded the planes until it could verify that it correctly followed instructions for the inspections.

The airline said one plane was already retired, five more were in maintenance for scheduled work, and the remaining 38 were taken out of service.