* Involuntary layoffs set as part of cost-cutting plan
* Bigger sites to feel greatest impact from losses
* Shares up 1.8 percent
Sept 26 (Reuters) - The aeronautics unit of Lockheed Martin Corp (LMT.N), the Pentagon's No. 1 supplier by sales, said on Monday that it would cut 540 jobs to reduce costs as defense spending comes under pressure.
The involuntary layoffs are part of the unit's plan announced in June to cut about 1,500 jobs. Under that plan, more than 450 workers took voluntary layoffs, and the division also eliminated about 300 open positions. About 300 jobs were also cut in the sustainment services group.
The aeronautics unit, which produces the F-16 multi-role fighter jet, C-130 military transport aircraft and F-35 Joint Strike Fighter, said it expected the affected workers to depart by year end.
The greatest job losses would be felt by larger sites in Fort Worth, Texas, where about 370 jobs are being cut; Marietta, Georgia and Palmdale, California. Lockheed has cut jobs and held down discretionary spending in response to a flattening of U.S. defense spending.
The defense contractor has 126,000 workers worldwide, including 28,000 at aeronautics unit sites in the United States.
Shares of Lockheed were up 1.8 percent at $73.39 in midday trading. (Reporting by Karen Jacobs in Atlanta; Editing by Lisa Von Ahn)