US prosecutors filed a civil complaint against Atlas Air on Thursday alleging that maintenance on at least one of its cargo planes was improperly done.

Atlas Air has a fleet of 37 Boeing 747 freighters that it leases to international carriers.

Atlas Air had no immediate comment on the complaint, filed in US District Court in Manhattan.

A Federal Aviation Administration inspector noted a crack 2 to 3 inches long on the cowling of one of the engines during a ramp inspection in May 2006 at JFK Airport in New York, according to the complaint.

Maintenance personnel welded the crack and returned the plane to service, but according to the complaint the repair was inadequate and done by a mechanic who "was not trained or qualified to perform" such repairs.

The complaint said Atlas Air maintenance workers noted that the crack had been repaired in its log but the job was not done in accordance with FAA rules about such cracks exceeding 1 inch.

Prosecutors said the plane was released into service "in an unairworthy condition" and flew several times until June 4, 2006, when the same part became cracked again and was properly replaced.

The FAA is seeking civil penalties.

Atlas Air's core business is leasing aircraft to other airlines, said a spokesman for the company, adding that it also offers a charter service and some limited scheduled service.

Atlas Air's shares shares closed up 56 cents, or 1 percent, at USD$58.51 on the Nasdaq.